WORKING SPACE FOR ELECTRICAL EQUIPMENTS / PANELS


Electrical Equipment Space (As per NEC 110.26)

(A) Working Space:

  • Equipment that may need examination, adjust­ment, servicing, or maintenance while energized must have working space which is measured from the enclosure front, must not be less than the distances con­tained in Table 110.26(A)(1).

 (1) Depth of Working Space.

Table 110.26(A)(1) Working Space

Voltage-to-Ground

One side of working Space having Pnel Exposed live parts and other side of Working Space having no live or grounded parts (including concrete, brick, or tile walls)

One side of working Space having Panel Exposed live parts and other side of Working Space having live or grounded parts (including concrete, brick, or tile walls)

Exposed live parts on both sides of the working space.

0 To 150V

3 Foot (900MM)

3 Foot (900MM)

3 Foot (900MM)

151V To 600V

3 Foot (900MM)

3.5 Foot (1000MM)

4 Foot (1200MM)

 601V TO 1000V

3 Foot (900MM) 

4 Foot (1200MM) 

5 Foot (1500MM)

A

(a) Rear and Sides. Working space isn’t required for the back or sides of assemblies where all connections and all renewable or adjustable parts are accessible from the front.

(2) Width of Working Space.

  • The width of the working space must be a minimum of 760MM (30 in) but in no case less than the width of the equipment.
  • The width of the working space can be measured from left-to-right, from right-to-left, or simply centered on the equipment, and the working space can overlap the working space for other electrical equipment.
  • In all cases, the working space must be of sufficient width, depth, and height to permit all equipment doors to open 90 degrees.

B

(3) Height of Working Space (Headroom).

  • The height of the working space in front of equipment must not be less than 2 Meter (6½ ft) measured from the grade, floor, platform, or the equipment height, whichever is greater.
  • Equipment such as raceways, cables, wireways, cabinets, panels, and so on, can be located above or below electrical equipment, but must not extend more than 6 in. into the equipment’s working space.

D

(B) Limited Access

  • Where equipment is installed above a lay-in ceiling, there shall be an opening not smaller than 559 mm × 559 mm (22 in. × 22 in.), or in a crawl space, there shall be an accessible opening not smaller than 559 mm × 762 mm (22 in. × 30 in.).
  • The width of the working space shall be the width of the equipment enclosure or a minimum of 762 mm (30 in.) whichever is greater.
  • All enclosure doors or hinged panels shall be capable of opening a minimum of 90 degrees.
  • The space in front of the enclosure shall comply with the depth requirements of Table 110.26(A)(1).
  • The maximum height of the working space shall be the height necessary to install the equipment in the limited space. A horizontal ceiling structural member or access panel shall be permitted in this space.

 

 (C) Entrance to and Egress from Working Space.

  •  (1) Minimum Required: At least one entrance of sufficient area must provide access to and egress from the working space.
  • (2) Large Equipment: An entrance to and egress from each end of the working space of for electrical equipment rated 1,200A or more and over 6 ft wide is required an entrance of Not Less than 600MM Wide and 1800MM Height at each end of Working Place.

E

 

  • A single entrance to and egress from the required working space is permitted where either of the following conditions is met:
  • (a) Unobstructed Egress. Only one entrance is required where the location permits a continuous and unobstructed way of egress travel.
  • (b) Double Workspace. Only one entrance is required where the required working space depth is doubled, and the equipment is located so the edge of the entrance is no closer than the required working space distance.

F

  • (3) Personnel Doors: If equipment with overcurrent or switch­ing devices rated 1,200A or more is installed, personnel door(s) for entrance to and egress from the working space located less than 25 ft from the nearest edge of the working space must have the door(s) open in the direction of egress and be equipped with panic hardware or other devices that open under simple pressure

G

 

Difference between PVC- LSF-LSHF- FR- FRLS -FRLSH Cables. (PART-2)


(B) Fire Rated Cable (Retardant / Resistance Cable)

  • Fire is one of the biggest risks in factories, public place and a majority of them occur due to electrical faults.
  • The terms Fire Resistant and Fire Retardant (both are commonly referred to as FR) terms are very similar and misused or confusing a lot.
  • Both are different in structure, in materials, in Application and react even differently in the event of a fire. If we required one but select other can lead the problem.

(1) Fire  Retardant Cables 

  • Insulating Material of Fire Retardant Cable is chemically treated to Retard or Slowdown ignition or Burning of Fire hence slow down the spreading of fire. It also actually self-extinguishes when exposed to an open flame.
  • Flame-retardant Cable is characterized by delaying the spread of flame along the cable so that the fire does not expand.
  • Fire-resistant cables and flame-retardant cables are different in structure and materials.
  • The basic structure of the flame retardant cable is:
  • The insulation layer uses flame retardant.
  • The inner sheath and outer sheath are made of flame retardant.
  • The tape and filling use of flame retardant material.

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Advantage:

  • Low Cost compared to Fire Resistance Cable.
  • Produce Low Smoke

Disadvantage:

  • By Adding Fire Retardant Material / Filler in PVC it decreases insulation property at least 10% compare to normal PVC, however its conductor temperature withstanding capability (during overload) remains only at 70 deg C same as ordinary PVC cables.

Applications:

  • Control Wiring of Building
  • Fire Alarm Circuit

(2) Fire Resistant cables

  • The Fire resistance materials (non-flammable.) are designed to prevent / Resist the spread of fire (self-extinguishing) and will not melt or drip when in close proximity to a flame.
  • Because it self-extinguishes once the source of ignition is removed and does not melt or drip. Fire-resistant cables can maintain normal operation for a certain period under flame burning conditions and maintain the Circuit integrity and continue to work for a specified period of time under defined conditions hence improving the chances of escape and survival.
  • Because of Fire resistant fabrics are not usually made from 100% flame resistant materials, they will burn, but will do so very, very slowly and are often self-extinguishing.
  • A Fire-resistant cable is a cable that can maintain safe operation for a certain period under flame-burning conditions. Fire-resistant wires are widely used in high-rise buildings, subways, underground shopping malls, power stations, and important industrial and mining enterprises related to fire safety and fire rescue. For example, power supply wires and control wires for firefighting facilities.
  • Fire-resistant cable is divided into class A and class B.
  • Class B: Class B cable can be in 750 ℃ to 800 ℃ flame and rated voltage to withstand burning for at least 90min, and the cable is not broken.
  • In the refractory layer to improve the manufacturing process and increase the refractory layer and other methods based on
  • Class A: Class A fire rated cable can be 950 ℃ to 1 000 ℃ flame and rated voltage to withstand burning for at least 90min and the cable is not punctured.
  • Class A fire-resistant cable fire performance is better than class B.
  • Mineral Insulated Cable (MI): mineral insulated cable is a better performance of fire-resistant cables made of copper core, copper sheath, magnesium oxide insulation material processing, referred to as MI (mineral insulated cables) cable.
  • MI cable has good fire resistance characteristics and can work for a long time under 250 ℃ high temperature, but also explosion-proof, strong corrosion-resistance, high flow rate, radiation resistance, high mechanical strength, small size, lightweight, long life, and smokeless. However, the price is high. The process is complicated, the construction is difficult in the oil irrigation area, important public buildings, high-temperature places, and other fire-resistant requirements, and the economy can accept the occasion and use fire-resistant cable.

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Advantage:

  • Produce Low Smoke compared to Fire Retardant Cable.

Disadvantage:

  • High Cost compared to Fire Resistance Cable.
  • By Adding Fire Retardant Material / Filler in PVC it decrease insulation property at least 10% compare to normal PVC, However its conductor temperature withstanding capability (during overload) remains only at 70 deg C same as ordinary PVC cables.

Applications:

  • In Fire Fighting System,
  • In Fire Alarm Circuit

(3) FRLS (Fire Retardant Low Smoke)

  • To overcome these deficiencies of FR Cable, FRLS Cable was developed.
  • FRLS has special flame retardant, low smoke emitting and toxic fumes suppressing properties.
  • In FRLS Cable, inner sheath and/or outer sheath is made material of Polyethylene Material having Fire Retardant Properties.
  • In the Case of fire, convectional PVC insulated wires give out thick black smoke and toxic fumes of hydrochloric acid gas. This impairs visibility and hampers rescues operations. But in FRLS Cable not only emits very little smoke and toxic gases, but also retards the spreading of fire. It is thus ideal of concealed and conduit wiring in multi-storied high-rise buildings such as hotels, banks, hospitals, factories, commercial complexes and residential apartments, etc

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Advantages

  • Excellent flame-retardancy
  • Low smoke generation
  • Low toxic gas emission

(4) FRLSZH/ NHFR / ZHFR (Fire Retardant Low Smoke Halogen Free)

  • FRLSZH, Halogen Free Flame Retardant non-toxic smoke house wires for building wiring.
  • FRLSZH Wires are recommended especially in a situation where high degree of safety of personnel and equipment are obligatory like Hotels, Theatres, Hospitals, High-rise buildings, Commercial complexes, Centrally A.C. offices, Residential properties etc.
  • Owing to its special insulation characteristics the wires continue to provide uninterrupted power supply even during fire – keeping alive fire alarm circuits, exit lights, Lifts & other emergency Circuits.
  • As part of sustainable green building technology, to bringing down the use of hazardous PVC from green building. Normal PVC cables will be replaced with Green Cable.44

Advantages

  • Excellent flame-retardancy
  • Halogen Free
  • Low smoke generation
  • Low toxic gas emission
  • Better visibility help easy for people escape
  • Environment friendly
  • Benefit to environment
  • PVC is not only hazardous during the manufacturing process but also potential risk in case of fire. Green Cable is superior performance cable with utmost quality which is replacement for PVC cables.
  • Generally used where green environment and higher safety is expected for Human life and valuables

Disadvantages

  • Costly Compare to FR and FRLS Cables

Applications:

  • Airports
  • Centrally A. C. Buildings
  • Complexes
  • Educational Institutions
  • General House wiring
  • Green Buildings
  • High Raise Building
  • Hospitals
  • Hotels
  • Public Places, Theaters

Flame retardant (FR) or FRLS compounds are not suitable for building wires for the following reasons:-

  • FR & FRLS PVC compounds are said to be flame retardant because they have better LOI (Limiting Oxygen Index) and TI (Temperature Index) than ordinary PVC, but, only better LOI & TI does not guarantee better flame retardant properties. LOI & TI are only quality control tests and flame retardant testing is incomplete without finished cable testing as per IEC 332-1 &3.
  • Moreover, all the FR and FRLS PVC compounds contain Antimony Trioxide which is a probable carcinogen. When inhaled, ATO can cause irritation of the respiratory track, mouth, nose & stomach. It may also cause the heart to beat irregularly or even stop.
  • The use of FR & FRLS PVC compounds does not solve the issues of dense black smoke and HCL acid gas emitting from burning which are the main cause of loss of human lifes during fire accidents.
  • All PVC,FR & FRLS compounds also contain phthalate plasticizers. These plasticizers leach out of PVC compound after some time and results in PVC loosing its flexibility and other properties. Moreover most of the phalates presently used have been identified as suspected endocrine disrupters and reproductive toxicants

Difference between Fire Resistant vs. Fire Retardant Cable

  • Fire Resistant and Fire Retardant cables are being used increasingly due to their usefulness in the event of fire. However, though they both sound similar, they have vastly different uses and react differently in the event of a fire.
  • Heat resistant: It will operate as normal at high temperatures, but may not operate as normal in the event of a fire.
  • Fire retardant: It will not operate as normal within fire conditions, but will actively prevent the fire from spreading.
  • Fire resistant: It can operate as normal within fire conditions.
  • Conclusion:
  • In brief, Fire retardant cables are designed to resist the spread of fire into a new area. It would not maintain circuit continuity for Work.
  • Fire resistant or fire rated cables are designed to maintain circuit integrity and continue to work, allowing power to be transferred through it under defined for a specified period of time and conditions.
  • The distinction between the two is crucial when it comes to maintaining critical circuits required for life safety or for a safe and immediate plant shutdown.
  • Fire resistant cables are used in critical electrical circuits, such as safety circuits and life support circuits which are required to function in the case of emergencies.
  • Flame retardant cables on the other hand are used in all other circuits so if there’s a fire, they can curb its spread. A flame resistant cable will be passed as per IEC 60331 and are encased in a red outer sheathes. Flame retardant cables behavior under fire is predefined as per passing the IEC 60332 and are encased in a grey or black outer sheathe.
Fire resistant (fire rated) cables Continues to operate in the presence of fire, hence their reference as Circuit Integrity cables.
Flame retardant cables Fire performance limited to not propagating fire

Difference between FR vs PVC vs LSF vs LSHF Cables

  • FR Cables:
  • Fire resistant and fire retardant cable sheaths are design to resist combustion and limit the propagation of flames.
  • Fire Retardant (FR): Designed for use in fire situations where the spread of flames along a cable route needs to be retarded
  • Fire Resistant (FR): cables are designed to maintain circuit integrity of those vital emergency services during the fire
  • FR is for essential services such as fire alarms, emergency lighting, life safety and firefighting applications.
  • These systems have to operate during a fire to detect the fire, alert people and help them evacuate and also help emergency services do their job.
  • These circuits need to function fully and retain circuit integrity in the event of fire.
  • In case of fire, it does not emit toxic or corrosive gases, thereby protecting public health and avoiding any possible damage to electronic equipment
  • LSF, LSHF and PVC Cables
  • Low smokes cables have a sheath designed to limit the amount of smoke and toxic halogen gases given off during fire situations
  • Low Smoke and Fume (LSF): burns with very little smoke and fumes compared to standard PVC, fumes may contain halogens
  • Low Smoke Zero Halogen (LSZH): when burns there is very little smoke and fumes contains no Halogen (compared to standard PVC)
  • LSHF and PVC cables are used for non-essential services that do not need to operate in a fire.
  • These include all the usual power circuits in buildings for services such as general lighting or kitchen and office appliances like cookers or photocopiers.
  • These circuits are not essential for the safety of the public; they can fail in a fire with no increase in danger so they do not need to be fire resistant.
  • For public buildings however, all cables need to be low smoke and zero halogen type but in domestic premises and for buried cables they do not, so PVC is acceptable
  • Both LSZH and LSF are used to limit smoke, fumes and halogen given off in fire conditions.
  • In the event of a fire, both types will emit very low levels of smoke. LSF cable will emit toxic gases while LSZH will limit the emission of these (typically under 0.5% hydrogen chloride emission). In addition to being toxic, hydrogen chloride is corrosive to equipment. The use of LSZH cables protect both people and limit the amount of equipment damage during a fire situation.

Comparison of various cable

Comparison of various cable

Feature Normal PVC Wire Heat Resistant HR PVC Fire Retardant FR – PVC Fire Retardant Low Smoke FRLS Zero Halogen Low Smoke ZHFR
Insulation Material PVC PVC Special PVC Special PVC Special Polymer
Insulation Property Normal Good Good Good Very Good
Temperature Rating 700 C. 850 C. 70C. 700 C. 850 C.
Thermal Stability Normal Very Good Good Good Very Good
Flam Retardancy Good Good Very Good Very Good Excellent
Safety During Burning Average Average Good Good Excellent
Requirement of Oxygen to Catch Fire > 21% > 21% > 30% > 30% > 35%
Temperature Required to catch fire ( with 21% oxygen) Room Temp. Room Temp. > 2500 C. > 2500 C. > 3000 C.
Visibility during Cable burning < 20% < 20% < 35% < 40% < 80%
Release of Halogen Gas during burning < 20% < 20% < 20% < 20% 0%
Abrasion Resistance during Installation Good Good Good Good Good

Comparison various Specification’s of Cable

Test Function Specification Values of FRLS Compound Values of Halogen Free Compound Values of PVC
Critical Oxygen Index To Determine % of Oxygen Required For Supporting Combustion of Insulating Material at room temperature. ASTM–D-2863 > 29% More than 29% 23%
Temperature    Index To determine at What Temperature Normal Oxygen Content of 21% In Air will Support  Combustion of Insulating Material ASTM–D-2863 > 2500 C. More than       2500 C. 1500 C.
Smoke density Rating (Light Transmission) To Determine the visibility ( Light Transmission ) under Fire of Insulating Material ASTM-D-2843 > 40 % More than 80 % 10-15 %
Acid Gas Generation To ascertain the amount of Hydrochloric Acid Gas Evolved from insulation of Cable Under Fire. IEC – 754 – 1 < 20 % Less than 0.5 % 45-50 %

International Standards

Halogen & Smoke Emission, Corrosively & Toxicity Standards

  • IEC 60754-1/BS6425-1 – emission of halogen
  • IEC 60754-2 – corrosivity , Acid gas emission
  • IEC 61034-1/ ASTM E662 – emission of smoke
  • ISO4589-2/ BS2863 – oxygen index LOI
  • ISO4589-3/ BS2782.1 – temperature index TI
  • ASTM – D – 2863- Oxygen index
  • ES713 – toxicity index

Flame Retardant Standards

  • IEC 60332-1 / BS 4066-1 – flame test on single vertical insulated wires/cables
  • IEC 60332-3 / BS 4066-3 – flame test on bunched wires/cables
  • UL Standard for Fire Retardant Cable
  • NFPA -262 =CMP (Plenum Flame Test/ Steiner Tunnel Test)
  • UL1666=CMR (Riser Flame Test)
  • UL 1581=CM (Vertical Tray Flame Test)
  • UL1581=CMG (Vertical Tray Flame Test)
  • UL1581=CMX (Vertical Wire Flame Test)

Fire Resistance Standards

  • IEC 60331 – fire resistance test
  • BS 6387 / BS 8491: BS 8434/2 – fire resistance test (more stringent than IEC 60331)

Difference between PVC- LSF-LSHF- FR- FRLS -FRLSH Cables. (PART-1)


Introduction:

  • Due to lack of standardization and lack of awareness. While selecting of Cable, there is a lot of confusion and misunderstanding regarding the terminology associated with cables in terms of “LSF / LS” (Low Smoke), “LSZH / LSHF (Low Smoke Halogen Free),” FR” (Fire Retardant),”FR” (Fire Resistance) “FRLS” (fire resistant, low smoke), “FRLSZH” (Fire retardant Halogen-Free).

Cable / Wire Terminology  

  • According to type of Insulation Material around the conductor, we can classify Cables / Wire in Three main Categories PVC, Zero Halogen and Fire Retardant.
  • According to application we can mainly classified in to Two categories  

(A) Non-Fire Rated Cable

  1. PVC = Polyvinyl Chloride
  2. LS / LSF = Low Smoke / Low Smoke Fume
  3. LSHF / LSZH / LSNH = Low Smoke Halogen Free / Low Smoke Zero (No) Halogen
  4. LH / HF = Low Halogen / Halogen Free

(B) Fire Rated Cable

  1. FR =Fire Retardant
  2. FR =Fire Resistance
  3. FRLS = Fire Resistant, Low Smoke
  4. FRLSH= Fire Resistant, Low smoke, Low Halogen
  5. FRLSZH / NHFR / ZHFR / HFFR = Fire Retardant Low Smoke Zero Halogen / Non (Zero) Halogen Free, Fire retardant
  6. HRFR=Heat Resistance Fire Retardant
  • PVC, FRLS and FP cables, have conductors and insulation to manage the electrical current and voltage. Some also have extra physical protection, like steel wire armour.
  • PVC and FRLSH cables are different insulating materials around conductors for different application and performance.
  • The properties that distinguish one electrical insulation from the other are
  • (1) dielectric strength or break down voltage
  • (2) maximum permissible temperature
  • (3) dielectric loss
  • (4) permittivity; and some special properties to suit the application.
  • FRLS / FRLF is the quality of insulating material. It may be PVC or XLPE.

(A) Non-Fire Rated Cable

(1) PVC  Cable:

  • PVC (Polyvinyl Chloride) cables is usually made up of a PVC compound as an insulating Material.
  • PVC insulation has a temperature limit of about 70°C. From the point of view of maximum permissible temperature, it belongs to the lowest class of insulation, yet it serves the purpose as the voltages and power ratings involved are relatively low.
  • While burring of PVC in case of Fire produces dense of black smoke and produce large amount of toxic gas and cocktail of harmful chemicals.
  • Smoke:
  • Burning PVC has been reduced visibility in the surrounding area by 50% within 10 minutes. After 30 minutes, visibility can be reduced by as 90%
  • This reduced visibility could make it very difficult to escape a burning Area / Building.
  • The smoke and fumes produced during a fire can be more dangerous to people than the fire itself.
  • Toxic Chemicals:
  • Burning PVC produces a number of toxic chemicals, but the most problematic is hydrogen chloride (HCI). PVC emits approximately 28% of Hydrogen Chloride (HCI).
  • In natural state HCL is a pungent, almost colorless gas, which forms into white vapor clouds on contact with air.
  • Furthermore, when mixed with water it changes state yet again to form Hydrochloric Acid, whether it’s in gaseous, vaporized or liquid state it’s a highly toxic and corrosive substance.
  • There are numerous harmful effects that HCl can have on a person. If inhaled the lining of the throat can be irritated to such an extent that it swells, making breathing extremely difficult.
  • Contact with the eyes can be responsible for anything from severe irritation to permanent damage to the corneas. Similarly, lips and mucous membranes may be burned or even ulcerated, the severity dependent on the concentration of HCl and length of exposure.
  • Taking into account the combined effects on someone of the smoke and HCl produced during the burning process, it’s difficult to see and the victims have been rendered unconscious long before the flames have reached them.
  • Some extent Fire Retardant property:
  • PVC is resistant to Fire ignition.
  • PVC (polyvinyl chloride) is naturally Fire Retardant due to chlorine base. It contains a large number of chlorine ions in the molecular structure and these are particularly difficult to break off when exposed to heat.
  • If it does catch fire, PVC has a particularly slow spread of flame. PVC has one of the lowest flames spread ratings, meaning that it won’t typically contribute to the spread of a fire
  • The temperature required to ignite rigid PVC is more than 150 deg C higher than that required to ignite wood. The ignition resistance of common flexible PVC formulations is lower, but with specialized formulations it may be significantly increased.
  • The fire in the gets extinguished immediately on removal of the fire source.
  • In the Plant or Building, PVC cables are bunched in the cable shaft or on cable trays. In case of fire in these cables the fire becomes self-sustaining.
  • Moreover, due to the burning of PVC a dense corrosive smoke is emitted which makes firefighting very difficult, due to poor visibility and toxic nature of the smoke. HCL content of the smoke, not only damages other costly equipment lying nearby, but also penetrates the RCC and corrodes the steel reinforcement.
  • PVC have some Fire retardant Property due to halogen even though it may create an extensive damage to the property and harmful for human.

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Advantage:

  • PVC is Cheap.
  • PVC offers greater flexibility and robust
  • PVC have a relatively long working life

Dis Advantage:

  • When PVC insulated cable burns it gives off a cocktail of chemicals and dense black smoke.

Application:

  • PVC cables are used for non-essential services that do not need to operate in case of fire
  • Mostly use for Domestic, Office for general lighting.
  • They are ideal for low-risk buildings, not generally for public or large commercial buildings.

(2) LS / LSF (Low Smoke & Fume) Cables:

  • LSF is also manufactured using PVC compounds.
  • LSF cables are usually made up of a modified PVC compound (varying degrees dependent on the manufacturer’s) which produces somewhat less HCI gas and smoke on burning than PVC.
  • However, it still produces 15% to 22 % (depending on quality) of HCI gas and due to the presence of PVC can still emit dense black smoke and HCl emissions.
  • It does contain halogen, so it shouldn’t be confused or similar with Low Smoke Halogen Free (LSHF) cables.
  • The amount of PVC present in these cables can differ from manufacturer to manufacturer which makes installing LSF cables in public places.

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Advantage:

  • These cables are often purchased to cut cost or in confusion with LSHF cables.
  • They should be considered to be a small improvement over PVC cables.

Dis Advantage:

  • These cables are not recommended for public, large commercial buildings, near sensitive electronic equipment and where escape is limited in case of fire.

Application:

  • Mostly use for Domestic, Office for general lighting.

(3) LSHF / LSZH / LS0H (Low Smoke Halogen Free) Cables

  • LSHF cables are made up of halogen free compounds that are good fire retardants but emit less than 0.5% hydrogen chloride gas and smoke when burnt.
  •  In case of fire, LSHF cable produces only small amounts of light grey smoke and miniscule amounts of HCl, which as a result greatly increases a person’s chances of escape from a burning building in which it’s installed.
  • The reason LSHF products react so differently when exposed to fire in comparison to PVC & LSF cables is the complete absence of PVC.
  • The outer sheath / Jacketing and conductor insulation of these products are often made from polyethylene which contains little by way of chlorine, and low chlorine means low HCl and Low nontoxic gases emissions.
  • It emits <0.5 % of HCL gas thus providing a safer environment in the event of a fire.
  • There’s no PVC in these cables, hence no harmful fumes or dense black smoke are given off in case of fire and generation ensures evacuation routes and signage remain visible during a fire.
  • In Some Manufacture’s LSHF Cable use standard PVC cables over-sheathed with an LSHF jacket or cables with PVC insulation. When the jacket burns through, the PVC inner sheath or insulation will give off poisonous gases in just the same way as PVC Cable.

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Advantage:

  • LSHF cables use in applications where smoke emission and toxic fumes could a risk to human health and essential equipment in the event of a fire.

Dis Advantage:

  • Costly compare to PVC and LSF
  • Not Flexible compared to PVC

Application:

  • Because of their low smoke and toxicity benefits, LSHF cables are often chosen for various Public, non / Poor ventilated Place and Essential applications.
  • Public space, Building like Railway and subway stations and cars, buses and bus stations, airplanes and airports, Carrier Ships, other mass transit facilities.
  • Any public underground or poorly ventilated location like elevators, subways
  • Public entertainment and sports facilities
  • Apartment buildings and hotels
  • Hospitals
  • Computer/data centers

Difference between PVC vs LSF vs LSZH

  • LSF cables are flexible and low-cost alternative to PVC cables but made from a modified version of PVC and can still produce a dangerous amount of toxic gas and large amounts of black smoke and hydrogen chloride gas when burned.
  • Black smoke can obscure exit routes in the event of a fire and hydrogen chloride gas can be deadly to both people and sensitive equipment.
  • Whereas LSHF cables are less flexible and a higher cost but due to absent of PVC reduce significant of toxic gas, smoke and no more than 0.5% HCL when burned. So in a high risk populated area where escape is limited, LHSF cables are strongly recommended.
  • But in low-risk areas where the evacuation is easy and high flexibility is required, PVC could still be a good choice.
  • One common misunderstanding is that LSF or LSHF cable is also flame retardant.  This is not necessarily true. The cables may spread the fire even though minimal fumes are being emitted

Forms of Separation for Panel (PART-2)


(C) Form 3

  • This is more complicated but safer than Form 2.
  • In form 3a, each device is isolated in a compartment that protects it from the effects of any incidents that may occur on another Part / Switchgear.
  • Busbars and functional units are segregated. Functional units are also separated from each other in cubicles, and terminals are then separated from functional units, but they are not segregated from other functional units’ terminals.
  • Busbar and Switchgear: Bus bars are separated from the Switchgear units,
  • Busbar and Termination: Bus bars are not separated from any incoming or outgoing terminations.
  • Switchgear and Switchgear units: Switchgear units are separated from each other.
  • Switchgear and Termination: Switchgear units are separated from any incoming or outgoing termination.
  • Termination and Termination: Incoming and outgoing terminals are not separated from each other
  • This is further classified into 2 categories.

FORM 3A

  • External cabling terminals are not segregated from busbars.

FORM 3B

  • External cabling terminals are separated from busbars

FORM 3B TYPE 1

  • As from 3 but: Busbar separation is achieved by insulated coverings, e.g. PVC sleeving, wrapping or coating.
  • Terminals are therefore separated from the busbars, but not from each other.

FORM 3B TYPE 2

  • As form 3 but: Busbar separations is achieved by metallic or non-metallic rigid barriers or partitions.
  • Terminals are therefore separated from the busbars, but not from each other. 1

 Advantages:

  • The advantages include safety, ease of maintenance and reliability because it’s possible to isolate and perform maintenance on each starter without having to power down the whole switchboard.
  • Serious faults within a starter are also more likely to be contained within a cubicle meaning adjacent starters are unaffected and can operate normally.

Electrical Safety:

  • More reliable and safer than Form-2 due to separation between live parts (Busbar and Switchgear, Switchgear and Switchgear).

Cost:

  • All these advantages come at a cost as a Form 3 board is significantly bigger and more expensive than a Form 1 or 2 board.

Application:

  • Form 3 segregation is typically used for Big projects and larger operations that have a greater number of loads, motors and critical processes.
  • They are utilised when safety, reliability and limited downtime are crucial.

(D) Form 4

 This is the highest form rating, as specified by AS/NZS / IEC 61439.1.

  • Busbars are separated from functional units
  • Functional units are separated from each other
  • Terminations to functional units are separated from each other
  • Busbar and Switchgear: Bus bars are separated from the Switchgear units,
  • Busbar and Termination: Bus bars are separated from any incoming or outgoing terminations.
  • Switchgear and Switchgear units: Switchgear units are separated from each other.
  • Switchgear and Termination: Switchgear units are separated from any incoming or outgoing termination.
  • Termination and Termination: Incoming and outgoing terminals are separated from each other
  • This is further classified into 2 categories.

FORM 4A

  • External cabling terminals are within the same cubicle as the corresponding functional unit.

FORM 4B

  • The external cabling terminals are not in the same cubicle as the corresponding functional unit, and they are separated from the terminals of other functional units.

CLASSIFICATION OF FORM 4B

TYPE Busbar Separation Termination Location Cable Gland
FORM 4B TYPE-1 PVC sleeving, wrapping or coating. Termination is within the same compartment as the functional unit. Common Gland Plate
FORM 4B TYPE-2 Rigid Barriers Termination is within the same compartment as the functional unit. Common Gland Plate
FORM 4B TYPE-3 Rigid Barriers Termination is within the same compartment as the functional unit. Individual Gland Plate
FORM 4B TYPE-4 PVC sleeving, wrapping or coating. Terminals are external to the functional unit and separated by insulated coverings, e.g. PVC Boots Common Gland Plate
FORM 4B TYPE-5 Rigid Barriers Terminals are external to the functional unit and separated by insulated coverings, e.g. PVC Boots Common Gland Plate
FORM 4B TYPE-6 Rigid Barriers Terminals are external to the functional unit compartment and enclosed in their own compartment by means of rigid barriers or partitions Common Gland Plate
FORM 4B TYPE-7 Rigid Barriers Terminals are external to the functional unit compartment and enclosed in their own compartment by means of rigid barriers or partitions complete with integral glanding facility Individual Gland Plate

2

  • The major difference between Forms 3 and 4 is the separation of the terminals of each functional unit the terminals of other units.

Advantages:

  • The main advantage of this model is the ability to safely connect and disconnect outgoing cables while the rest of the switchboard remains in operation.
  • In Large Panel access is required for inspection, to reset an auxiliary function. If the point of isolation and termination are each in their own individual box this can be deemed safer than if all the devices and connections are behind a single door

Electrical Safety:

  • More reliable and safer than Form-4 due to separation between live parts (Busbar and Switchgear, Switchgear and Switchgear, Termination and Termination).
  • Due to internal segregation is to limit the effects on adjacent circuits if something goes wrong. An external fault should cause a device to trip but this must not have any effect on any other circuits.

Cost:

  • Form 4 board is significantly bigger and more expensive than a Form 3 board.

Applications:

  • used in hospitals or for critical industrial processes.

SUMAMRY OF FORM OF SEPERATION OF PANEL

SUMAMRY OF FORM OF SEPERATION OF PANEL

SEPERATION BETWEEN FORM-1 FORM-2 FORM-3 FORM-4
BUSBAR–SWITCHGEAR NO YES YES YES
BUSBAR–TERMIATION NO NO / YES YES YES
SWITCHGEAR—SWITCHGEAR. NO NO YES YES
SWITCHGEAR–TERMIANTION NO NO NO YES
TERMIANATION–TERMIANTION NO NO NO YES

Forms of Separation for Panel (PART-1)


Introduction:

  • Forms of segregation have great importance in electrical Panel designs.
  • Form of segregation is the rule for provide separation from a one energizes function part to other energize function pant and access to a part of the assembly while other parts may remain energized. This can be achieved by using metallic or non-metallic physical barriers or insulation.
  • The form of segregation provides protection against four objectives.
  1. Protection against direct contact with live dangerous parts of adjacent functional units.
  2. Protection against the entry of solid objects from one unit of an assembly to an adjacent unit.
  3. Limitation of the effects of the spread of electric arcs.
  4. Facilitation of panel maintenance operations.

Type of Separation:

  • As specified by AS / NZS / IEC 61439, There are four main categories outlined by the standard for internally separating the switchgear units and busbars of a Panel are
  1. Form 1 (No segregation between busbar, terminals and Switchgear units)
  2. Form 2 (Separation between switchgear units and the busbar)
  3. Form 3 (Separation are between switchgear units and the busbar and Separation between Switchgear unit to Switchgear Unit)
  4. Form 4 (Segregation between busbar, terminals and Switchgear units)
  • The complexity of the forms increases with the numbers.

1

(A) Form 1:

  • A Form 1 Panel has no internal separation among busbar, switchgear and outgoing Cable Terminations.
  • All functional units are installed in one central section to provide protection against contact with any internal live parts.
  • Busbar and Switchgear: Bus bars are not separated from the Switchgear units,
  • Busbar and Termination: Bus bars are not separated from any incoming or outgoing terminations.
  • Switchgear and Switchgear units: Switchgear units are not separated from each other.
  • Switchgear and Termination: Switchgear units are not separated from any incoming or outgoing termination.
  • Termination and Termination: Incoming and outgoing terminals are not separated from each other

2

Advantage:

  • Simple Design and Less Space Required.

Electrical Safety:

  • Less due to No separation between live parts.
  • This form construction is rarely used.

Cost:

  • Less Cost

Application:

  • For small, low power switchboards.

(B) Form 2

  • Form 2a is the simplest for protecting against accidental contact with any internal live parts or components like the busbars, which are considered to be the most dangerous components.
  • In FORM-2, Busbar is Separate from the Switchgear units but may or may not be separate from Cable terminal.
  • Busbar and Switchgear: Bus bars are separated from the Switchgear units,
  • Busbar and Termination: Bus bars may or may not separate from any incoming or outgoing terminations.
  • Switchgear and Switchgear units: Switchgear units are not separated from each other.
  • Switchgear and Termination: Switchgear units are not separated from any incoming or outgoing termination.
  • Termination and Termination: Incoming and outgoing terminals are not separated from each other
  • This is further classified into 2 categories.

FORM 2A

  • Terminals are not separated from the busbars or each other.

FORM 2B

  • Terminals are separated from the busbars

FORM 2B TYPE 1

  • As form 2 but Busbar separation is achieved by insulated coverings, e.g. PVC sleeving, wrapping or coating.
  • Terminals are the therefore separated from the busbars, but not from functional units or each other.

FORM 2B TYPE 2

  • As from 2 but Busbar separation is achieved by metallic or non-metallic rigid barriers or partitions
  • Terminals are therefore separated from the busbars, but not from functional units or each other

3

Advantages:

  • There are several advantages to segregating functional units and busbars.
  • This model allows circuit breakers to be reset when the switchboard is live because the operator is not exposed to a live busbar.

Electrical Safety:

  • More than Form-1 due to separation between live parts (Busbar and Switchgear).

Cost:

  • More Costly than Form-1

Application:

  • For small, low power switchboards.

Measurement of LUX Level and Uniformity at Indoor and Outdoor Lighting (Part-3)


(3) Grid Method to measure illumination on the road

  • The arrangement of the measuring points depends on the distance between the Illumination Pole and the width of the Road.
  • The measurement of illuminance should be performed on the area in longitudinal direction two consecutive luminaires in the same row and in transverse direction the width of the area with the same illumination class, i.e. if the road and adjacent pavement or bicycle path have the same illumination class, they may be considered as one area during the measurements. The measuring points should be distributed evenly within the measuring field.
  • The distance between the measuring points (D in Meter) in the longitudinal direction should be calculated using the formula
  • The distance between the measuring point in longitude ( D)=S / N
  • where:
    S= the distance between the luminaires in [m],
    N= the number of measurement points in the longitudinal direction,
    for S ≤ 30 m, it is N = 10,
    for S > 30 m, the smallest integer giving D ≤ 3 m.
  • The distance between measurement points (d in Meter) in the transverse direction should be calculated with the formula:
  • The distance between the measuring point in transverse (d) = Wr / n
  • where: Wr= the width of the road or the area under consideration in Meter.
  • n = the number of measurement points in the transverse direction equal to 3 or more and being an integer giving d ≤ 1.5 m.
  • The distance between the points and the edges of the surface under consideration should be D/2 in the longitudinal direction and d/2 in the transverse direction. The location of the measurement points in the measuring field is shown in Figure.

1

(4) Equal Space Method

  • In this Method at least10 equal measuring Points are taken between two lighting Pole on one side of the Roadway.
  • These measurement points cannot be spaced more than 5 meters apart. Two lines of measurement points are needed per driving lane, one-half lane width apart.
  • Once you have taken all of your illuminance measurements, you can calculate an average illuminance for the section of roadway you have measured.

2

What is Lighting Uniformity

  • light uniformity refers to the uniformity of lighting in an environment. It is necessary to maintain the uniformity of light in order to make sure that everything is perfectly visible in the room.
  • Uniformity is the ratio of the minimum lighting level to the average lighting level in a specified area.
  • U1 = E Min / E Average
  • U2 = E Min / E Maximum
  • U & E stands for uniformity & illuminance respectively.
  • Uniformity is a quality parameter for the overall illuminance distribution.
  • It is quite useful to use this uniformity ratio to describe how the lights are evenly distributed on the ground. If the difference between minimum and average lux is small, then the ratio is high, which gives better light uniformity.
  • The maximum lighting uniformity is 1, which means the lux levels in all the sampling points are the same. However, it is very unlikely to achieve this maximum value for artificial lighting.
  • If the uniformity is very low for the outdoor or indoor lighting, the citizens, workers, or athletes might feel uncomfortable, and thus their vision is affected.
  • The more uniform the light distribution, the better the illuminance and the more comfortable the visual experience.  The closer the illuminance uniformity is to 1, the better, otherwise the smaller the more visual fatigue.

How to improve Lighting Uniformity

  • Adjust the aiming angle of the floodlight,
  • The lights irradiated by the floodlights should overlap each other,
  • Use pole lights, high-power floodlights, street lights, etc. to supplement lighting.

Light Uniformity Standard

  • There are different light uniformity standards that need to be followed depending on the nature of the environment
  • Most focus-intensive tasks require a uniformity index of around 0.6, whereas, technical drawing and other demanding tasks require a ratio of at least 0.7.
  • Uniformity value greater than 0,60 is recommended in working areas. Because, above this level, the change in light levels cannot be sensed by people and that makes them comfortable. Proper lighting of the environment also helps employees work more comfortably when looking at the computer screen.
  • Due to low uniformity in road lighting, the homogeneity of lighting will be distorted. So, very bright and very dark spots will occur on the road. If brightness changed very often, this will cause eye strain and stresses the drivers
  • In order to avoid these situations, average uniformity value greater than 0.35 or 0.4 is required according to road lighting class.
Standard Area Ratio of Minimum/Average Illumination
UK CIBSE and German DIN guidelines The general lighting scheme 0.6 and 0.8
NBC-2005, page no 759 Working Area Not Less than 0.7

Table-6: Recommended Levels of Illumination (BIS, 1981)

Type of Road Road Characteristics Ratio of Minimum/Average Illumination
A-1 Important traffic routes carrying fast traffic

0.4

A-2 Main roads carrying mixed traffic like city main roads/streets, arterial roads, throughways

0.4

B-1

Secondary roads with considerable traffic like local traffic routes, shopping streets

0.3

B-2 Secondary roads with light traffic

0.3

EUROPEAN STANDARD- EN 12464-1:2011

Space

 Uniformity U0 (Emin / Em

Areas with traffic and corridors

0.4
Stairways, escalators, and travelators

0.4

Lifts

0.4
Loading bays

0.4

Coffee-break rooms

0.4
Technical facilities

0.4

Storage spaces

0.4
Electronics workshops, testing, and adjustments

0.7

Ball-mill areas and pulp plants

0.4
Offices and writing

0.6

Check-out areas

0.6
Waiting rooms

0.4

Kitchens

0.6

Parking areas

0.4
Classrooms

0.6

Auditoriums

0.6

EUROPEAN STANDARD- EN 12464-1:2011

Task illuminance ≥ 0.7
Illuminance of immediate surrounding areas ≥ 0.5

Football Field Lighting Design

Nature of the Sports Field Required U1 Light Uniformity
Class I such as for a National Competition ≥ 0.7
Class II such as for a League ≥ 0.6
Class III such as for a Training Ball Field ≥ 0.5

Industrial and Commercial Lighting Uniformity Requirement

The Area The Light Uniformity Standard
Highway 0.4-0.6
Sports field 0.5-0.8
Office 0.4-0.6
Parking Lot 0.4-0.5
Warehouse 0.4-0.6
Running Track 0.3-0.5
Airport 0.2-0.3

Measurement of LUX Level and Uniformity at Indoor and Outdoor Lighting (Part-2)


(3) AS per Deutsch Norm DIN 5035

  • In this Method the working plane divide into a number of sections which are at least rectangular, of ratio of length to side not less than 1: 2 but which are preferably of square shape.
  • A square grid of minimum size 1 meter is established within each section with a measurement point at the centre of each square.
  • The grid module defining the measurement points is selected so as not to coincide with the luminaire grid in either principal direction.
  • In exceptionally large interiors the grid size may be up to 5 meters. there is not any mention of accuracy limits of the method, but this is not surprising given the flexibility which the user of the method is allowed in choice of grid size.
  • The DIN system is the only one of the three methods studied to give any advice concerning illuminance measurements in obstructed interiors. Areas of the working plane located between large obstructions are treated for measurement purposes as separate spaces.

1

OUTDOOR ILLUMINATION (LUX LEVEL) MEASUREMENT

 (1) Nine Point Method for Determining Lux Levels in Street Lighting

  •  The Lux Level of Street Light is measured by 9-point method.
  • We need to make two equal quadrants between two light poles and between Pole and Rode edge.
  • Two Measuring Points below Light Pole (A1,A2)  and Two opposite side of Pole at Road Edge (A3,A4).
  • Two Point between Pole and Road edge (B1,B3).
  • One Point Between Pole  (B2) and on One Point between opposite side of Pole at road edge (B4)
  • One Point is at centre (C1).
  • Average Lux = (A1+A2+A3+A4)/16 + (B1+B2+B3+B4)/8 +C1/4

2

  • Solution
26 Lux 27 Lux 13 Lux
12 Lux 15 Lux 14 Lux
26 Lux 32 Lux 22 Lux
  • Average Lux = (A1+A2+A3+A4)/16 + (B1+B2+B3+B4)/8 +C1/4
  • Average Lux = (26+26+13+22)/16 + (12+27+14+32)/8 +15/4
  • Average Lux =20Lux 
MIN 12 Lux
MAX 32 Lux
AVG 20 Lux
U1=MIN/AVG 0.58
U2=MIN/MAX 0.38

 (2) As per Grid Point Set Up Measurement

  • Identify a horizontal grid of measurement points on the Illumination Measurement site surface. Locate measurement points on gridlines covering the test measurement area.
  • Ensure that the spacing between measurement points is uniform in both directions and is less than one-half the pole height or less than 4.5 Meter, whichever is smaller.
  • For installations with lights spaced less than 4.5 Meter apart, locate measurement points no farther apart that one-half the pole height, with at least three points between poles in both directions.
  • Record the location of all measurement grids and point layouts with dimensions from surrounding poles or other structures. Provide this information, including a sketch or rendering of the grid layouts.
  • For open areas such as main parking, make the measurement grid large enough to cover at least four poles of this Area layout and at least two Pole are covered.
  • For site perimeter open areas or areas adjacent to a building edge establish the test area measurement grid in a typical perimeter or building edge area. The depth of the test area should extend from the paved site boundary or building edge inward to the nearest line of light poles that are at least 4.5 Meter from the boundary or building edge.
  • The width of the test area must cover at least two of the poles in the line that is at least 4.5 Meter from the boundary or building edge.

(A) In Open Area

3

(B) In the Area of Site Perimeter

4

(C) Near Site Boundary Area:

5

Measurement of LUX Level and Uniformity at Indoor and Outdoor Lighting (Part-1)


Introduction:

  • Working plane illuminance (Lux Level) need to be measured in the field for cross check of whether the existing installation meets a design requirement or not.
  • Field surveys may also be useful to identifying the causes of complaints about lighting, hence the results of field surveys may be useful for the designer, installers and end users.
  • There are various methods are developed for field measurement of Interior Lighting and External Lighting.
  • The Measurement Methods recommended by the various national lighting bodies are generally similar or slightly derivatives to each other. The most common method / Standard is BEE, CIBSE, IES and DIN code
  • The most of methods require to measurement of illuminance at points on a grid at working-plane height or at Floor, but the grid size and position of the measuring points may be differed from various standard to standard.
  • The IES method and its derivatives use the position of the grid according to the luminaire locations.
  • The CIBSE and DIN methods use a position of grid according to the room size.
  • The techniques of analysis of the field measurement results also differ

Basic Requirements for Exterior & Interior Light Level Measurement

  •  The following Points should be considered for accurate measurement of interior and exterior lighting Lux level.
  • Where possible, use the same calibrated illuminance measurement meter (LUX Meter) If the same meter is not available, use the same make and model of calibrated meter to minimize error.
  • When taking measurements, verify that any objects/materials are not blocking any light to the meter head. The use of a remote meter head cabled to the meter body is recommended to prevent the operator from blocking the meter’s “view” of the lighting system being measured.
  • In Outdoor Lighting it is essential to measure of illuminance should be done in night (proper dark).
  • For indoor lighting, measurements with lights ON and Lights OFF technique can be followed and the daylight variation is not too much and the survey time is not too long.
  • In an installation of fluorescent discharge lamps, the lamps must be switched on at least 30 minutes before the measurement to allow for the lamps to be completely warmed up.
  • In many situations, the measuring plane may not be specified or even non-existent. Hence it is necessary to define measurement height, typically 0.8 to 1 meter from the ground or floor level.
  • The lux measurement procedure simply requires positioning a meter’s sensor on the surface or location where you wish to measure the incident light.
  • The sensor should face the light source at a right angle. If the sensor is not perpendicular to the light, the measurement will be incorrect, though some lux meters have a cosine correction to account for the angle.
  • Meters that require a colour correction factor may have a means of inputting the CCF to adjust the result for LEDs or fluorescent lights; otherwise, you will have to manually multiply the measured lux by the CCF.

 INDOOR ILLUMINATION (LUX LEVEL) MEASUREMENT.

 (1) As per Room Index Method (as per BEE Code / CIBSE Code):

  •  This methos is more suitable where measuring Plan / Points for an interior is more rectangular than square. First, we need to be found Room Index.
  • Based on the room index, the minimum number of illuminance measurement points is decided by Room Index Number
  • Room Index (RI) = (L x W) / H x (L+ W)
  • Where L = Length of Room
  • W = Width of Room
  • H= Height of the luminaires above the plane of measurement 

Table 4-2: Number of points for measuring illuminance

Room index

Minimum number of measurement points

 

For ± 5% accuracy

For ± 10% accuracy

RI < 1

8

4

1 < RI < 2

18

9

2 < RI < 3

32

16

RI > 3

50

25

 Sample calculation

  • Measure Illumination Level of an office room have length, L = 7.5 m and width W = 5 m,
  • Solution:
  • Suppose Height of Illumination from Floor is 2 Meter
  • Room Index RI = (L x W) / H x (L+ W)
  • Room Index RI = (7.5 x 5) / 2 x (7.5+ 5)
  • Room Index RI = 1.5
  • From Table 4.2 minimum Illumination Measure Points should be 18 No’s
  • The illuminance measurements Points with Measured Value in Lux are marked on the grid.  

1

Measurement Reading Details

107 Lux

99 Lux

85 Lux

65 Lux

65 Lux

45 Lux

73 Lux

130 Lux

105 Lux

110 Lux

86 Lux

87 Lux

59 Lux

50 Lux

58 Lux

99 Lux

75 Lux

106 Lux

115 Lux

76 Lux

         

Min

45 Lux

     

Max

130 Lux

     

Average

85 Lux

     
         

U1=MIN/AVG

0.5 Lux

     

U2=MIN/MAX

0.3 Lux

     

 

 (2) As per Point Layout Method

  • For office and other task areas, identify a set of measurements points on desktops and other work surfaces that best represents lighting conditions in the space.
  • It may not be possible to develop a uniform spacing grid, but points should be chosen that represent the various lighting conditions across the space.
  • For each separate horizontal grid, identify a vertical plane representative of the lighting in the area (typically the gridline directly between two light fixtures).
  • On this vertical plane, set a grid (line) of points at 1.5 Meter above the site surface at each of the corresponding horizontal measurement points.
  • The following figures provide sample layouts for selecting horizontal measurement points for typical areas where lighting measurements are taken

2

Measurement Reading Details

107 Lux

80 Lux

100 Lux

75 Lux

100 Lux

65 Lux

73 Lux

70 Lux

75 Lux

60 Lux

99 Lux

87 Lux

95 Lux

58 Lux

98 Lux

60 Lux

65 Lux

63 Lux

66 Lux

78 Lux

75 Lux

78 Lux

62 Lux

99 Lux

87 Lux

95 Lux

58 Lux

98 Lux

         

Min

58 Lux

     

Max

107 Lux

     

Average

80 Lux

     
         

U0 or U1=MIN/AVG

0.7 Lux

     

Ul or U2=MIN/MAX

0.5 Lux

     

Typical Earthing Resistance Value


  • The resistance offered by the earth electrode to the flow of current into the ground is known as the earth resistance or resistance to earth.
  • Ideally a ground resistance should be of zero ohms but It is always greater than Zero .System ground resistances can be reduce by the use of a number of individual electrodes connected together.
  • Total earthing resistance is the sum of the resistance of earth lead wires, Contact resistance between the surface of the earth electrode and the soil and The resistance of the body of the soil surrounding the earth electrode.
  • The value of earthing resistance varies on the Type of Soil, Soil characteristic, soil resistivity and the climatic condition. Moisture content in soil plays a vital role in the soil resistivity. value of individual earthing pit resistance is not so important. Different codes specifies the required value of earthing system.
  • Electrical Systems can work with earth resistance of 20 ohms, though generally 10 ohms is the specified Maximum limit.
  • But communication systems need very stringent limit, typically one ohm. This is because the higher the ground resistance, higher would be noise interference in the systems.

USAID

a) Power stations (generating station)

0.5 ohms

b) EHT Sub-station

1.0 ohms

c) 33 KV Stations

2.0 ohms

d) D/t Structure

5.0 ohms

e) Tower Foot resistance

10.0 ohms

IEEE STANDARD 142

Chapter: 4.1.3 , page 164
For industrial plant substations and buildings and large commercial installations.

1Ω to 5 Ω

Resistances of less than 1 ohm may be obtained using a number of individual electrodes connected together. Such a low resistance is only required for large substations, transmission lines, or generating stations.

National Electric code (NEC) 2011, (IS SP30 Chapter 14 -India)

Chapter: 3.0.9
unless otherwise specified ,It is recommended that the value of any earth system resistance shall not be more than

IS 3043 (India)

Chapter: 22.2.3
The continuity resistance of the earth return path through the earth grid should be maintained as low as possible and in no case greater than

This applicable for main earth grid connected with the transformer/return path

Oil Industry Safety Directorate Government of India (OISD STANDARD – 137)

Chapter: (7. ii. b) Allowable earth-Resistance Values
Allowable earth-Resistance Values The resistance value of an earthing system to general mass of the earth should not exceed.
For electrical systems and metallic structures.

4Ω

For storage tanks.

7Ω

for main earth grid, and bonding connections between joints in pipelines and associated facilities.

1Ω

for each electrode to the general mass of the earth

2Ω

IS 2309 (india) / BS 7430:1998

Clause:12.3.1 Page 32,Resistance to Earth
Lightning arrestors ground resistance for  Protection of buildings and allied structures is

10Ω

An earth electrode should be connected to each down conductor. Each of these earths should have a resistance not exceeding the product given by 10 a multiplied by the number of earth electrodes to be provided.
The whole of the lightning protective system, including any ring earth, should have a combined resistance to earth not exceeding 10 Ω without taking account of any bonding.
If the value obtained for the whole of the lightning protective systems exceeds 10 Ω, a reduction can be achieved by extending or adding to the electrodes or by interconnecting the individual earth terminations of the down conductors by a conductor installed below ground, sometimes referred to as a ring conductor

IS 2689:1989

Table 4 page 28 (Reaffirmed March 2010)
Lightning arrestors ground resistance for Protection  of buildings and allied structures is

10Ω

NEC 250.56

Clause: 250.53 Grounding Electrode System Installation.
The maximum resistance for a single electrode consisting of a rod, pipe, or plate.

25Ω

If a higher resistance is obtained for a single electrode, a second electrode of any of the types specified in the NEC is required.
This should not be interpreted to mean that 25 ohm is a satisfactory resistance value for a grounding system.

IEEE Std 80-2000 (Revision of IEEE Std 80-1986)

the evaluation of ground resistance for the most transmission and other large substations, the ground resistance is usually about

1Ω or less

In smaller distribution substations, the usually acceptable range is

1Ω to 5Ω

NFC 17-102, July 1995

that the resistance value measured using conventional equipment should be

1Ω or less

This resistance should be measured on the earthing termination insulated from any other conductive component.

IEC 62305-1

edition 2.0 – 2010-12
the conventional earthing impedance related to the earth termination system is (*for the soil resistivity less than or equal to 100 Ω)

4 Ω

Ministry of Railways- Government of India

 
The acceptable Earth Resistance at earth MEEB bus bar shall not be more than

1Ω

For achieving this value more than one earth pits can be installed if necessary depending upon the soil resistivity. In places where space is not available to provide parallel earth pits then longer earth rods may be provided.
The longer earth rods thus provided should be in multiples of three meters.
The combined resistance of the earthing system shall be not more than the following values
Traction substation

0.5Ω

Switching station

2Ω

Booster transformer station

10Ω

Auxiliary transformer station

10Ω

Maximum values of earth resistances specified for earthing of Signaling and Telecommunication equipment’s are as under
Telegraph and Block Instrument using earth return circuit 10 Ω
Earths for surge arrestors/ lightening dischargers

10Ω

Earthing of Signalling equipment

10Ω

Earthing of signalling cable screen in AC electrified areas

10Ω

Earthing of Telephone Exchange

Earthing of aluminum sheathed telecom cable in AC electrified area

Earthing of equipment in VF repeater stations and cable huts.

Axle counter cable screened in AC electrified area

Electronic Interlocking installation

Integrated Power Supply System & its individual modules

Digital Axle Counter EJB and its apparatus case connected to same earth All cable armors connected to same earth.

Reset box of Digital Axle Counter connected to earth (indoor) near SM’s Room.

Railway Vikas Nigam Limited

RVNL/Elect/GS/11
The earth continuity test of metallic envelopes shall be done for electrical continuity. Electrical resistance of the same, along with the earthing lead, excluding any added resistance of earth leakage circuit breaker, measured from the connection with the earth electrode to any point in the earth conductor in the completed installation, shall not exceed

No earth electrode shall have resistance greater than

In rocky soil, the resistance may be up to

Locations having more than one electrode shall be connected in parallel to reduce the resistance.

MANUAL OF STANDARDS & SPECIFICATIONS FOR RAILWAY ELECTRIFICATION

RDSO/SPN/197/2008
Equipment’s with solid state components which are more susceptible to damage due to surges, transients and over voltages being encountered in the system due to lightning, sub-station switching such as Electronic Interlocking, Integrated Power supply equipment, Digital Axle counter, Data logger etc. shall Value of earth resistance shall not be more than

For conventional signaling equipment’s the earth resistance shall not be more than

10Ω

DHASHIN HARYANA BIJALI VITRAN NIGAM (DHBVN)

Specification no CSC-140 / DH/UH/P&D
Hose hold Earthing (3KA)

<8Ω

Commercial / Industrial Buildings (5KA)

<2Ω

Transformer / LT Line Earthing  (15KA)

1Ω to 2Ω

Transformer / Substation /HT Line ,HT Switchgear (40KA)

<1Ω

Lighting Arrester /Extra High Current appliances (50KA)

<1Ω

UPS / Data center / ATM

<0.5 Ω

*Earthing may be Single or Multiple Electrode.

Earthing Value

Earthing Condition Earthing Value
Best

0.1Ω to 2Ω

Good

2.1Ω to 5Ω

Need to be Maintenance

5.1Ω to 10Ω

Need to be Replacement

>10Ω

Difference between Fault Current and Short Circuit Current


Introduction:

  • There is a difference between “Fault Current” and “Short Circuit Current” in electrical system. Both parameters are important while selecting an Equipment or designing a Network, however both terms are misled in Electrical engineering.
  • In very simple language “Short” means less (shortest distance, time or circuit), Short circuit Fault means least resistance or no resistance in circuit and Current is high due to less resistance. This high current convert into heat energy. The opposite of a short circuit is an “open circuit”, which is an infinite resistance between two nodes.
  • While Fault means wrong. Fault Current means Current pass in to wrong path.

What is Fault Current

  • A fault current is a current which takes the wrong path instead of using the normal conducting path during Fault condition.
  • Under normal condition, the electric equipment operate at normal voltage and current ratings. Once the fault occurs in a circuit or device, voltage and current value deviates from their nominal Value. This may be high or Low Values.
  • The fault may be occurred due to insulation failures, Wrong Connection or conducting path failures, which further convert in Open Circuit, Short Circuit and Ground Fault.
  • A fault current can either current being more or less than the normal rated current.
  • In Three phase power system, there are basically three types of Fault Current.
  • Open Circuit Faults
  • Short Circuit Faults (L-L / L-L-L)
  • Ground Circuit Faults (L-G / L-L-L-G)

What is Short Circuit Current:

  • When a two or more conductors of differential potential comes to contact with each other (one phase comes in contact with other Phase, Neutral or Earth) gives the electricity to a path of less resistance hence a large current flow in the un-faulted phases, such current is called the short circuit current.
  • When Short circuit occurs, current returns to its source without passing to the load. It caused zero or very little resistance and No Voltage drop in that circuit.
  • This Current will be the maximum that the source can deliver for a very small time before the protection device operates. The current is limited only by the resistance of the rest of the circuit.
  • We know that V (Voltage) =I (current) x R (resistance of Circuit).
  • When short circuit occur, resistance is very small and can be considered as negligible. We can consider R=0. This means I = V/0, which means infinite current will Flow so the conductor must have the capacity to allow this huge current to flow. In most of the cases breakdown happens.
  • The resistance when short circuit occur is very small and can be considered as negligible. We can consider R=0.
  • This means V=Ix0, which means Voltage at Short circuit is very Less.
  • V(drop)=0 and current(I)=infinite
  • Short circuit gives thousands time larger Current than the normal current and Zero Voltage at Fault Point. This will produce more heat and result in burns and fires.
  • Short circuit faults are also called as Shunt faults.
  • Causes:
  • Over Loading of Equipment: Overloading of equipment and insulation failure due to lighting surges and mechanical damage.
  • Loose Connections:Due to Loose Connections, Sometimes Neutral and Phase wires to touch.
  • Faulty or Wrong Connections: Wrong Connections make Short circuit in Circuit.
  • Failure / Ageing of Insulation:Old or damaged insulation makes neutral and Phase wires to touch, which can cause a short circuit. Punctures in Insulation can damage insulation and makes short circuit.
  • Harmful Effects:
  • The short-circuit produces the arc that causes the major damage of equipment such as transformers and circuit breakers.
  • The short circuit causes a heavy current in the power system which produces excessive heat and hence results in fire or explosion.
  • The short circuit affects the stability of the network which disturbs the continuity of the supply.
  • The operating voltages of the system can go below or above their acceptance values that creates harmful effect to the service rendered by the power system.

Open Circuit Faults:

  • Open Circuit Faults occur due to the Failure / Open of one or more Phase Conductors in Circuit.
  • In Open Circuit Fault, Current cannot flow hence Current is Zero and Voltage become Infinite.
  • V(drop)=infinite and current(I)=0
  • Open circuit faults are also called as series faults. These are unsymmetrical or unbalanced type of faults except three phase open fault.
  • Causes:
  • Broken Conductor, Failure of Conductor Joints and malfunctioning of circuit breaker in one or more phases.
  • Harmful Effects:
  • Abnormal operation of the system.
  • Danger to the Human and Animals.
  • Exceeding the voltages beyond normal values in certain parts of the network, which leads to insulation failures and developing of short circuit faults.

Difference between Fault Current and Short Circuit Current:

Circuit Resistance:

  • A short circuit has zero resistance between two Wires / Circuits / Systems, on the other hand a Fault current has a resistance that draws current. The amount of resistance decides how much current is drawn and is usually caused by a breakdown in the insulation of a system.

Amount of Current:

  • Fault Current: it is the current exceeding the equipment current rating e.g. motor rated 25A, then more than this will be the fault current.
  • Short Circuit current: it is the maximum current which can flow when the equipment is short circuited & it can withstand. above this the current will damage the equipment.
  • Fault current is the current that flows during an Open Circuit or Short Circuit Fault condition so each time it is not necessary that Fault Current is a Short Circuit Current (It may be Open Circuit Fault).
  • A short-circuit current will flow when there is short-circuit in the system, and it will represent the highest possible fault current that a system can experience.
  • Therefore, a fault current can be less that the short-circuit current, and a short-circuit current will represent the highest fault current in the system.
  • A fault current can either current being more or less than the normal current while Short Circuit Current is higher than Normal Current.

  • A Fault Current is not necessary a short circuit Current but Short Circuit Current is always a Fault Current.

Comparison of Fault Current -Short Circuit Current

Basis For Comparison Fault Current (Open Circuit Fault) Short Circuit Fault Overload
Meaning In the Open circuit the voltage at the fault point is high up to infinite and current is zero through the faulty point of the network. In the short circuit the voltage at the fault point decreases to zero and current of irregular high value flow through the faulty point of the network. The overload means that load greater than the desired value have been imposed on the system.
Resistance High Zero
Current Zero High Low as compared to short circuit.
Voltage High Zero The voltage becomes low, but cannot be zero.
Occur It occurs when the neutral and live wire Break or Open. It occurs when the neutral and live wire touch each other. It occurs when a large number of devices are joint in a single socket.

Importance of Fault Current and Short circuit Current for designing of System or Panel.

  • The safety of the system is decided by short-circuit current rating (SCCR) of the Equipment with the reference of the available fault current where the Equipment is installed. 
  • The short circuit current rating gives a baseline for the fault current that an equipment can withstand for a specific amount of time, or until it clears the circuit with opening of a circuit breaker.
  • The short circuit current rating of a panel is the amount of energy, usually expressed as a value in kilo-Amperes (kA), that the panel can handle without causing fire, a shock hazard, or explosive danger.
  • In equipment with higher short-circuit current ratings compared to Fault Current is not an issue.
  • The available fault current of panel can be decided by the size of the upstream transformer, size of the electrical conductors / Cables up to the Equipment.
  • If the System Fault Current at the Location is 20KA to 50KA and if we use Equipment having short circuit current of 5KA to 10KA may cause damages of equipment or network in fault condition.
  • If the System Fault Current at the Location is 5KA to 10KA and if we use Equipment having short circuit current of 65KA to 100KA will not create any issue but it will unnecessarily increase the price of equipment hence short circuit level of the equipment is not too much high with respect of fault current.
  • We have to ensure that the Short Circuit Current is equal or more than Fault Current available at the point of Equipment.

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