Abstract of over current Protection of Transformer (NEC 450.3)


Introduction:

  • The over current protection required for transformers is consider for Protection of Transformer only.Such over current protection will not necessarily protect the primary or secondary conductors or equipment connected on the secondary side of the transformer.
  • When voltage is switched on to energize a transformer, the transformer core normally saturates. This results in a large inrush current which is greatest during the first half cycle (approximately0.01 second) and becomes progressively less severe over the next several cycles (approximately 1 second) until the transformer reaches its normal magnetizing current.
  • To accommodate this inrush current, fuses are often selected which have time-current withstand values of at least 12 times transformer primary rated current for 0.1 second and 25 times for 0.01 second. Some small dry-type transformers may have substantially greater inrush currents.
  • To avoid using over sized conductors, over current devices should be selected at about 110 to 125 percent of the transformer full-load current rating. And when using such smaller over current protection, devices should be of the time-delay type (on the primary side) to compensate for inrush currents which reach 8 to 10 times the full-load primary current of the transformer for about 0.1 s when energized initially.
  • Protection of secondary conductors has to be provided completely separately from any primary-side protection.
  • A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons will monitor and service the transformer installation.
  • Over current protection for a transformer on the primary side is typically a circuit breaker. In some instances where there is not a high voltage panel, there is a fused disconnect instead.
  • It is important to note that the over current device on the primary side must be sized based on the transformer KVA rating and not sized based on the secondary load to the transformer

Over current Protection of Transformers > 600 V (NEC 450.3-A)

 1) Unsupervised Location of Transformer (Impedance <6%)

  • Over Current Protection at Primary Side (Primary Voltage >600V):
  • Rating of Pri. Fuse at Point A= 300% of Pri. Full Load Current or Next higher Standard size. or
  • Rating of Pri. Circuit Breaker at Point A= 600% of Pri. Full Load Current or Next higher Standard size.
  • Over Current Protection at Secondary Side (Secondary Voltage <=600V):
  • Rating of Sec. Fuse / Circuit Breaker at Point B= 125% of Sec. Full Load Current or Next higher Standard size.
  • Over Current Protection at Secondary Side (Secondary Voltage >600V):
  • Rating of Sec. Fuse at Point B= 250% of Sec. Full Load Current or Next higher Standard size. or
  • Rating of Sec. Circuit Breaker at Point B= 300% of Sec. Full Load Current.

Example: 750KVA, 11KV/415V 3Phase Transformer having Impedance of Transformer 5%

  • Full Load Current At Primary side=750000/(1.732X11000)=39A
  • Rating of Primary Fuse = 3X39A= 118A, So Standard Size of Fuse =125A.
  • OR Rating of Primary Circuit Breaker =6X39A=236A, So Standard Size of Circuit Breaker =250A.
  • Full Load Current at Secondary side=750000/ (1.732X415) =1043A.
  • Rating of Secondary of Fuse / Circuit Breaker = 1.25X1043A=1304A, So Standard Size of Fuse =1600A.

 2) Unsupervised Location of Transformer (Impedance 6% to 10 %)

  • Over Current Protection at Primary Side (Primary Voltage >600V):
  • Rating of Pri. Fuse at Point A= 300% of Primary Full Load Current or Next higher Standard size.
  • Rating of Pri. Circuit Breaker at Point A= 400% of Primary Full Load Current or Next higher Standard size.
  • Over Current Protection at Secondary Side (Secondary Voltage <=600V):
  • Rating of Sec. Fuse / Circuit Breaker at Point B= 125% of Sec. Full Load Current or Next higher Standard size.
  • Over Current Protection at Secondary Side (Secondary Voltage >600V):
  • Rating of Sec. Fuse at Point B= 225% of Sec. Full Load Current or Next higher Standard size.
  • Rating of Sec. Circuit Breaker at Point B= 250% of Sec. Full Load Current or Next higher Standard size.

Example: 10MVA, 66KV/11KV 3Phase Transformer, Impedance of Transformer is 8%

  • Full Load Current At Primary side=10000000/(1.732X66000)=87A
  • Rating of Pri.  Fuse = 3X87A= 262A, So Next Standard Size of Fuse =300A.
  • OR Rating of Pri. Circuit Breaker =4X87A=348A, So Next Standard Size of Circuit Breaker =400A.
  • Full Load Current at Secondary side=10000000/ (1.732X11000) =525A.
  • Rating of Sec. Fuse = 2.25X525A=1181A, So Next Standard Size of Fuse =1200A.
  • OR Rating of Sec. Circuit Breaker =2.5X525A=1312A, So Next Standard Size of Circuit Breaker =1600A.

 3) Supervised Location (in Primary side only) of Transformer:

  •  Over Current Protection at Primary Side (Primary Voltage >600V):
  • Rating of Pri. Fuse at Point A= 250% of Primary Full Load Current or Next higher Standard size.
  • Rating of Pri. Circuit Breaker at Point A= 300% of Primary Full Load Current or Next higher Standard size.

 4) Supervised Location of Transformer (Impedance Up to 6%):

  • Over Current Protection at Primary Side (Primary Voltage >600V):
  • Rating of Pri. Fuse at Point A= 300% of Pri. Full Load Current or Next Lower Standard size.
  • Rating of Pri. Circuit Breaker at Point A= 600% of Pri. Full Load Current or Next Lower Standard size.
  • Over Current Protection at Secondary Side (Secondary Voltage <=600V):
  • Rating of Sec. Fuse / Circuit Breaker at Point B= 250% of Sec. Full Load Current or Next higher Standard size.
  • Over Current Protection at Secondary Side (Secondary Voltage >600V):
  • Rating of Sec. Fuse at Point B= 250% of Sec. Full Load Current or Next Lower Standard size.
  • Rating of Sec. Circuit Breaker at Point B= 300% of Sec. Full Load Current or Next Lower Standard size.

Example: 750KVA, 11KV/415V 3Phase Transformer having Impedance of Transformer 5%

  • Full Load Current At Primary side=750000/(1.732X11000)=39A
  • Rating of Primary Fuse = 3X39A= 118A, So Next Lower Standard Size of Fuse =110A.
  • OR Rating of Primary Circuit Breaker =6X39A=236A, So Next Lower Standard Size of Circuit Breaker =225A.
  • Full Load Current at Secondary side=750000/ (1.732X415) =1043A.
  • Rating of Secondary of Fuse / Circuit Breaker = 2.5X1043A=2609A, So Standard Size of Fuse =2500A.

 5) Supervised Location of Transformer (Impedance 6% to 10%):

  • Over Current Protection at Primary Side (Primary Voltage >600V):
  • Rating of Pri. Fuse at Point A= 300% of Pri. Full Load Current or Next Lower Standard size.
  • Rating of Pri. Circuit Breaker at Point A= 400% of Pri. Full Load Current or Next Lower Standard size.
  • Over Current Protection at Secondary Side (Secondary Voltage <=600V):
  • Rating of Sec. Fuse / Circuit Breaker at Point B= 250% of Sec. Full Load Current or Next higher Standard size.
  • Over Current Protection at Secondary Side (Secondary Voltage >600V):
  • Rating of Sec. Fuse at Point B= 225% of Sec. Full Load Current or Next Lower Standard size.
  • Rating of Sec. Circuit Breaker at Point B= 250% of Sec. Full Load Current or Next Lower Standard size.

Example: 750KVA, 11KV/415V 3Phase Transformer having Impedance of Transformer 8%

  • Full Load Current At Primary side=750000/(1.732X11000)=39A
  • Rating of Primary Fuse = 3X39A= 118A, So Next Lower Standard Size of Fuse =110A.
  • OR Rating of Primary Circuit Breaker =4X39A=157A, So Next Lower Standard Size of Circuit Breaker =150A.
  • Full Load Current at Secondary side=750000/ (1.732X415) =1043A.
  • Rating of Secondary of Fuse / Circuit Breaker = 2.5X1043A=2609A, So Standard Size of Fuse =2500A.

 Difference in C.B between Supervised & Unsupervised Location 

  • Here we see two notable conditions while we select Fuse / Circuit Breaker in Supervised Location and Unsupervised Location.
  • First notable Condition is Primary Over current Protection. In Unsupervised Location Fuse in Primary side is 300% of Primary Current or Next Higher Standard size and in Supervised Location is 300% of Primary Current or Next Lower Standard size. Here Primary Over current Protection is same in both conditions (300%) But Selecting Size of Fuse/Circuit Breaker is Different.
  • Lets us Check with the Example for 750KVA, 11KV/415V 3Phase Transformer.
  • Full Load Current At Primary side=750000/(1.732X11000)=39A
  • In Unsupervised Location: Rating of Primary Fuse = 3X39A= 118A, So Next Higher Standard Size =125A
  • In Supervised Location: Rating of Primary Fuse = 3X39A= 118A, So Next Lower Standard Size =110A
  • Second notable Condition is Secondary Over current Protection increased from 125% to 250% for unsupervised to Supervised Location.

 Summary of over current Protection for more than 600V:

Maximum Rating of Over current Protection for Transformers more than 600 Volts
LocationLimitations TransformerRated Impedance Primary Protection (More than 600 Volts) Secondary Protection
More than 600 Volts Less than 600 Volts
Circuit Breaker Fuse Rating Circuit Breaker Fuse Rating C.B or Fuse
Any location Less than 6% 600%(NH) 300%(NH) 300 %( NH) 250%(NH) 125%(NH)
6% To 10% 400%(NH) 300%(NH) 250%(NH) 225%(NH) 125%(NH)
Supervisedlocations

only

Any 300%(NH) 250%(NH) Not required Not required Not required
Less than 6% 600% 300% 300% 250% 250%
6% To 10% 400% 300% 250% 225% 250%
NH: Next Higher Standard Size.

 Over current Protection of Transformers< 600 V (NEC 450.3-B)

 1) Only Primary side Protection of Transformer:

  • Over Current Protection at Primary Side (Less than 2A):
  • Rating of Pri. Fuse / C.B at Point A= 300% of Pri. Full Load Current or Next Lower Standard size.
  • Example: 1KVA, 480/230 3Phase Transformer, Full Load Current at Pri. Side=1000/(1.732X480)=1A
  • Rating of Primary Fuse = 3X1A= 3A, So Next Lower Standard Size of Fuse =3A.
  • Over Current Protection at Primary Side (2A to 9A):
  • Rating of Sec. Fuse / C.B at Point A= 167% of Pri. Full Load Current or Next Lower Standard size.
  • Example: 3KVA, 480/230 3Phase Transformer, Full Load Current at Pri. Side=3000/(1.732X480)=4A
  • Rating of Primary Fuse = 1.67X4A= 6A, So Next Lower Standard Size of Fuse =6A.
  • Over Current Protection at Primary Side (More than 9A):
  • Rating of Pri. Fuse / C.B at Point A= 125% of Pri. Full Load Current or Next Higher Standard size.
  • Example: 15KVA, 480/230 3Phase Transformer, Full Load Current at Pri. Side=15000/(1.732X480)=18A
  • Rating of Primary Fuse = 1.25X18A= 23A, So Next Higher Standard Size of Fuse =25A.

 2) Primary and Secondary side Protection of Transformer:

  • Over Current Protection at Primary Side (Less than 2A):
  • Rating of Pri. Fuse / C.B at Point A= 250% of Pri. Full Load Current or Next Lower Standard size.
  • Over Current Protection at Primary Side (2A to 9A):
  • Rating of Sec. Fuse / C.B at Point A= 250% of Pri. Full Load Current or Next Lower Standard size.
  • Over Current Protection at Primary Side (More than 9A):
  • Rating of Pri. Fuse / C.B at Point A= 250% of Pri. Full Load Current or Lower Higher Standard size.
  • Example: 25KVA, 480/230 3Phase Transformer, Full Load Current at Pri. Side=125000/(1.732X480)=30A
  • Rating of Primary Fuse = 2.50X30A= 75A, So Next Lower Standard Size of Fuse =70A.
  • Over Current Protection at Secondary Side (Less than 9A):
  • Rating of Pri. Fuse / C.B at Point B= 167% of Sec. Full Load Current or Lower Standard size.
  • Example: 3KVA, 480/230 3Phase Transformer, Full Load Current at Sec. Side=3000/(1.732X230)=8A
  • Rating of Primary Fuse = 1.67X8A= 13A, So Next Lower Standard Size of Fuse =9A.
  • Over Current Protection at Secondary Side (More than 9A):
  • Rating of Pri. Fuse / C.B at Point A= 125% of Pri. Full Load Current or Higher Standard size.
  • Example: 15KVA, 480/230 3Phase Transformer, Full Load Current at Sec. Side=15000/(1.732X230)=38A
  • Rating of Primary Fuse = 1.25X38A= 48A, So Next Higher Standard Size of Fuse =50A.

 Summary of over current Protection for Less than 600V:

Maximum Rating of Over current Protection for Transformers Less than 600 Volts
ProtectionMethod Primary Protection Secondary Protection
More than 9A 2A to 9A Less than 2A More than 9A Less than 9A
Primary only protection 125%(NH) 167% 300% Not required Not required
Primary and secondary protection 250% 250% 250% 125%(NH) 167%
NH: Next Higher Standard Size.


Difference between Bonding, Grounding and Earthing


Introduction:

  • One of the most misunderstood and confused concept is difference between Bonding, Grounding and Earthing. Bonding is more clear word compare to Grounding and Earthing but there is a micro difference between Grounding and Earhing.
  • Earthing and Grounding are actually different terms for expressing the same concept. Ground or earth in a mains electrical wiring system is a conductor that provides a low impedance path to the earth to prevent hazardous voltages from appearing on equipment. Earthing is more commonly used in Britain, European and most of the commonwealth countries standards (IEC, IS), while Grounding is the word used in North American standards (NEC, IEEE, ANSI, UL).
  • We understand that Earthing and Grounding are necessary and have an idea how to do it but we don’t have crystal clear concept for that. We need to understand that there are really two separate things we are doing for same purpose that we call Grounding or Earthing.
  • The Earthing is to reference our electrical source to earth (usually via connection to some kind of rod driven into the earth or some other metal that has direct contact with the earth).
  • The grounded circuits of machines need to have an effective return path from the machines to the power source in order to function properly (Here by Neutral Circuit).
  • In addition, non-current-carrying metallic components in a System, such as equipment cabinets, enclosures, and structural steel, need to be electrically interconnected and earthed properly so voltage potential cannot exist between them. However, troubles can arise when terms like “bonding,” “grounding,” and “earthing” are interchanged or confused in certain situations.
  • In TN Type Power Distribution System, in US NEC (and possibly other) usage: Equipment is earthed to pass fault Current and to trip the protective device without electrifying the device enclosure. Neutral is the current return path for phase. These Earthing conductor and Neutral conductor are connected together and earthed at the distribution panel and also at the street, but the intent is that no current flow on earthed ground, except during momentary fault conditions. Here we may say that Earthing and grounding are nearly same by practice.
  • But In the TT Type Power Distribution System (In India) Neutral is only earthed (here it is actually called Grounding) at distribution source (at distribution transformer) and Four wires (Neutral and Three Phase) are distributed to consumer. While at consumer side all electrical equipments body are connected and earthed at consumer premises (here it is called Earthing). Consumer has no any permission to mix Neutral with earth at his premises here earthing and grounding is the different by practice.
  • But in both above case Earthing and Grounding are used for the same Purpose. Let’s try to understand this terminology one by one.

Bonding:

  • Bonding is simply the act of joining two electrical conductors together. These may be two wires, a wire and a pipe, or these may be two Equipments.
  •  Bonding has to be done by connecting of all the metal parts that are not supposed to be carrying current during normal operations to bringing them to the same electrical potential.
  • Bonding ensures that these two things which are bonded will be at the same electrical potential. That means we would not get electricity building up in one equipment or between two different equipment. No current flow can take place between two bonded bodies because they have the same potential.
  • Bonding, itself, does not protect anything. However, if one of those boxes is earthed there can be no electrical energy build-up. If the grounded box is bonded to the other box, the other box is also at zero electrical potential.
  • It protects equipment & Person by reducing current flow between pieces of equipment at different potentials.
  • The primary reason for bonding is personnel safety, so someone touching two pieces of equipment at the same time does not receive a shock by becoming the path of equalization if they happen to be at different potentials.
  • The Second reason has to do with what happens if Phase conductor may be touched an external metal part. The bonding helps to create a low impedance path back to the source. This will force a large current to flow, which in turn will cause the breaker to trip. In other words, bonding is there to allow a breaker to trip and thereby to terminate a fault.
  • Bonding to electrical earth is used extensively to ensure that all conductors (person, surface and product) are at the same electrical potential.  When all conductors are at the same potential no discharge can occur.  

Earthing:

  • Earthing means connecting the dead part (it means the part which does not carries current under normal condition) to the earth for example electrical equipment’s frames, enclosures, supports etc.
  • The purpose of earthing is to minimize risk of receiving an electric shock if touching metal parts when a fault is present. Generally green wire is used for this as a nomenclature.
  • Under fault conditions the non-current carrying metal parts of an electrical installation such as frames, enclosures, supports, fencing etc. may attain high potential with respect to ground so that any person or stray animal touching these or approaching these will be subjected to potential difference which may result in the flow of a current through the body of the person or the animal of such a value as may prove fatal.
  • To avoid this non-current carrying metal parts of the electrical system are connected to the general mass of earth by means of an earthing system comprising of earth conductors to conduct the fault currents safely to the ground.
  • Earthing has been accomplished through bonding of a metallic system to earth. It is normally achieved by inserting ground rods or other electrodes deep inside earth.
  • Earthing is to ensure safety or Protection of electrical equipment and Human by discharging the electrical energy to the earth.

Grounding:

  • Grounding means connecting the live part (it means the part which carries current under normal condition) to the earth for example neutral of power transformer.
  • Grounding is done for the protections of power system equipment and to provide an effective return path from the machine to the power source. For example grounding of neutral point of a star connected transformer.
  • Grounding refers the current carrying part of the system such as neutral (of the transformer or generator).
  • Because of lightening, line surges or unintentional contact with other high voltage lines, dangerously high voltages can develop in the electrical distribution system wires. Grounding provides a safe, alternate path around the electrical system of your house thus minimizing damage from such occurrences.
  • Generally Black wire is used for this as a nomenclature.
  • All electrical/electronic circuits (AC & DC) need a reference potential (zero volts) which is called ground in order to make possible the current flow from generator to load. Ground is May or May not be earthed. In Electrical Power distribution it is either earthed at distribution Point or at Consumer end but it is not earthed in Automobile( for instance all vehicles’ electrical circuits have ground connected to the chassis and metallic body that are insulated from earth through tires). There may exist a neutral to ground voltage due to voltage drop in the wiring, thus neutral does not necessarily have to be at ground potential.
  • In a properly balanced system, the phase currents balance each other, so that the total neutral current is also zero. For individual systems, this is not completely possible, but we strive to come close in aggregate. This balancing allows maximum efficiency of the distribution transformer’s secondary winding

Micro Difference between earthing & Grounding:

  • There is no major difference between earthing and Grounding, both means “Connecting an electrical circuit or device to the Earth”. This serves various purposes like to drain away unwanted currents, to provide a reference voltage for circuits needing one, to lead lightning away from delicate equipment. Even though there is a micro difference between grounding & earthing.

(1) Difference in Terminology:

  • In USA term Grounding is used but in UK term Earthing is used.

(2) Balancing the Load Vs Safety:

  • Ground is a source for unwanted currents and also as a return path for main current some times. While earthing is done not for return path but only for protection of delicate equipments. It is an alternate low resistance path for current.
  • When we take out the neutral for a three phase unbalanced connection and send it to ground, it is called grounding. Grounding is done to balance unbalanced load. While earthing is used between the equipment and earth pit so as to avoid electrical shock and equipment damage.

(3) Equipment Protection Vs Human Safety:

  • Earthing is to protect the circuit elements whenever high voltage is passed by thunders or by any other sources while Grounding is the common point in the circuit to maintain the voltage levels.
  • Earth is used for the safety of the human body in fault conditions while Grounding (As neutral earth) is used for the protection of equipments.
  • Earthing is a preventive measure while Grounding is just a return path
  • The ground conductor provides a return path for fault current when a phase conductor accidentally comes in contact with a grounded object. This is a safety feature of the wiring system and we would never expect to see grounding conductor current flow during normal operation.
  • Do not Ground the Neutral Second time When It is grounded either at Distribution Transformer or at Main service Panel of Consumer end.
  • Grounding act as neutral. But neutral cannot act as ground.

(4) System Zero Potential Vs Circuit Zero Potential:

  • Earthing and Grounding both is refer to zero potential  but the system connected to zero potential is differ than Equipment connected to zero potential .If a neutral point of a generator or transformer is connected to zero potential then it is known as grounding. At the same time if the body of the transformer or generator is connected to zero potential then it is known as earthing.
  • The term “Earthing means that the circuit is physically connected to the ground and it is Zero Volt Potential to the Ground (Earth) but in case of “Grounding” the circuit is not physically connected to ground, but its potential is zero(where the currents are algebraically zero) with respect to other point, which is also known as “Virtual Grounding.”
  • Earth having zero potential whereas neutral may have some potential. That means neutral does not always have zero potential with respect to ground. In earthing we have Zero Volt potential references to the earth while in grounding we have local Zero Volt potential reference to circuit. When we connect two different Power circuits in power distribution system, we want to have the same Zero Volt reference so we connect them and grounds together. This common reference might be different from the earth potential.

Illegal Practice of interchange Purpose of Grounding & earthing wire

  • Neutral wire in grid connections is mandatory for safety. Imagine a person from 4th floor in a building uses Earth wire (which is earthed in the basement at Basement) as neutral to power his lights. Another Person from 2nd floor has a normal setup and uses neutral for the same purpose. Neutral wire is also earthed at the ground level (as per USA practice Neutral is Grounded (earthed) at Building and as per Indian Practice it is Grounded (earthed) at Distribution Transformer). However, ground wire (Neutral wire) has a much lower electrical resistance than Earth Wire (Earthing) which results in a difference of electrical potential (i.e. voltage) between them. This voltage is quite a hazard for anyone touching a Earth wire (Metal Body of Equipment) as it may have several tens of volts.
  • The second issue is legality. Using ground wire instead of neutral makes you an energy thief as the meter uses only the Phase and neutral for recording your energy consumption. Many Consumers make energy theft by using Earthing wire as a Neutral wire in an Energy meter.

Conclusion:

  • Ground is a source for unwanted currents and also as a return path for main current. While earthing is done not for return path but only for protection of delicate equipments. It is an alternate low resistance path for current. Earth is used for the safety of the human body in fault conditions while Grounding (As neutral earth) is used for the protection of equipments.

 

Safety Clearance for Transformer


Safety Clearance for Transformer:

 Clearance from Outdoor Liquid Insulated Transformers to Buildings (NEC):

Liquid Liquid Volume (m3) Fire Resistant Wall Non-Combustible Wall Combustible Wall Vertical Distance
Less Flammable NA 0.9 Meter 0.9 Meter 0.9 Meter 0.9 Meter
<38 m3 1.5 Meter 1.5 Meter 7.6 Meter 7.6 Meter
>38 m3 4.6 Meter 4.6 Meter 15.2 Meter 15.2 Meter
Mineral Oil <1.9 m3 1.5 Meter 4.6 Meter 7.6 Meter 7.6 Meter
1.9 m3 to 19 m3 4.6 Meter 7.6 Meter 15.2 Meter 15.2 Meter
> 19 m3 7.6 Meter 15.2 Meter 30.5 Meter 30.5 Meter

 Clearance between Two Outdoor Liquid Insulated Transformers (NEC):

Liquid Liquid Volume (m3) Distance
Less Flammable NA 0.9 Meter
<38 m3 1.5 Meter
>38 m3 7.6 Meter
Mineral Oil <1.9 m3 1.5 Meter
1.9 m3 to 19 m3 7.6 Meter
> 19 m3 15.2 Meter

 Dry Type Transformer in Indoor Installation (NES 420.21):

Voltage

Distance  (min)
Up to 112.5 KVA 300 mm (12 in.) from combustible material unless separated from the combustible material by a heat-insulated barrier.
Above 112.5 KVA Installed in a transformer room of fire-resistant construction.
Above 112.5 KVA with Class 155 Insulation separated from  a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

 Dry Type Transformer in Outdoor Installation (NES 420.22):

Voltage

Distance  (min)
Above 112.5 KVA with Class 155 Insulation separated from  a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

 Non Flammable Liquid-Insulated Transformer in Indoor Installation (NES 420.21):

Voltage

Distance  (min)
Over 35KV Installed indoors Vault (Having liquid confinement area and a pressure-relief vent for absorbing any gases generated by arcing inside the tank, the pressure-relief vent shall be connected to a chimney or flue that will carry such gases to an environmentally safe area
Above 112.5 KVA Installed in a transformer room of fire-resistant construction.
Above 112.5 KVA (Class 155 Insulation) separated from  a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

 Oil Insulated Transformer in Indoor Installation (NES 420.25):

Voltage

Distance  (min)

Up to 112.5 KVA

Installed indoors Vault (With construction of reinforced concrete that is not less than 100 mm (4 in.) thick.

Up to 10 KVA & Up to 600V

Vault shall not be required if suitable arrangements are made to prevent a transformer oil fire from igniting

Up to 75 KVA & Up to 600V

Vault shall not be required if where the surroundingStructure is classified as fire-resistant construction.
Furnace transformers (Up to 75 kVA) Installed without a vault in a building or room of fire resistant construction

 Transformer Clearance from Building (IEEE Stand):

Transformer

Distance from Building  (min)
Up to 75 KVA

3.0 Meter

75 KVA to 333 KVA

6.0 Meter

More than 333 KVA

9.0 Meter

 Transformer Clearance Specifications (Stand: Georgia Power Company):

Description of Clearance

Distance  (min)

Clearance in front of the transformer

3.0 Meter

Between Two pad mounted transformers (including Cooling fin)

2.1  Meter

Between Transformer and Trees, shrubs, vegetation( for unrestricted natural cooling )

3.0 Meter

The edge of the concrete transformer pad to nearest the building

4.2 Meter

The edge of the concrete transformer pad to nearest  building wall, windows, or other openings

3.0 Meter

Clearance from the transformer to edge of (or Canopy) building (3 or less stories)

3.0 Meter

Clearance in front of the transformer doors and on the left side of the transformer, looking at it from the front. (For operation of protective and switching devices on the unit.)

3.0 Meter

Gas service meter relief vents.

0.9 Meter

Fire sprinkler values, standpipes and fire hydrants

1.8 Meter

The water’s edge of a swimming pool or any body of water.

4.5 Meter

Facilities used to dispense hazardous liquids or gases

6.0 Meter

Facilities used to store hazardous liquids or gases

3.0 Meter

Clear vehicle passageway at all times, immediately adjacent of Transformer

3.6 Meter

Fire safety clearances can be reduced by building a suitable masonry fire barrier wall (2.7 Meter wide and 4.5 Meter Tall) 0.9 Meter from the back or side of the Pad Mounted Transformer  to the side of the combustible wall

 

Front of the transformer must face away from the building.

 

Clearance of Transformer-Cable-Overhead Line (Stand: Georgia Power Company):

Description of Clearance

Horizontal Distance (mm)

to pad-mounted transformers

to buried HV cable

to overhead HV Line

Fuel tanks

7.5 Meter

1.5 Meter

7.5 Meter

Granaries

6.0 Meter

0.6 Meter

15 Meter

Homes

6.0 Meter

0.6 Meter

15 Meter

Barns, sheds, garages

6.0 Meter

0.6 Meter

15 Meter

Water wells

1.5 Meter

1.5 Meter

15 Meter

Antennas

3.0 Meter

0.6 Meter

Height of Antenna + 3.0 Meter

 

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