Cable Construction & Cable Selection- Part:4
June 2, 2016 11 Comments
Cable Selection Parameters:
(1) Voltage of Cable:
- The Nominal voltage is to be expressed with two values of alternative current Uo/U in V (volt)
- Uo/U : Phase to earth voltage
- Uo : Voltage between conductor and earth
- U : Voltage between phases (conductors)
- (i ) Low-tension (L.T.) cables — upto 1000 V
- (ii ) High-tension (H.T. ) cables — upto 11,000 V
- (iii ) Super-tension (S.T.) cables — from 22 kV to 33 kV
- (iv ) Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV
- (v ) Extra super voltage cables — beyond 132 kV
- A low-voltage system usually has a solidly earthed neutral so that the line to earth voltage cannot rise higher than (line volts) ÷ √ 3. Cables for low-voltage use are insulated for 600V rms score to earth and 1000V rms core to core.
- High-voltage cables used in Shell installations are rated 19000/3300V or 3810/6600V or 6600/11000V, Phase/Phase.
- In selecting the voltage grade of cable, the highest voltage to earth must be allowed for. For example, on a normal 6.6kV unearthed system, a line conductor can achieve almost 6.6kV to earth under earth-fault conditions, to withstand this, a cable insulated for 6600/11000V must therefore be used.
(2) Current carrying capacity:
- The current carrying capacity of a cable is called Ampacity. Ampacity is defined as the maximum amount of electrical current a conductor or device can carry before sustaining immediate or progressive deterioration and is the rms electric current which a device or conductor can continuously carry while remaining within its temperature rating
(3) Short Circuit values:
- the “short-circuit current rating” is the maximum short-circuit current that a component can withstand. Failure to provide adequate protection may result in component destruction under short circuit conditions.
- Short circuits and their effects must be considered in selecting cables. These cables should have a short circuit rating which is the highest temperature the cable can withstand during an electrical short circuit lasting up to about half a second.
(4) Type of Conductor:
- Type of Conductor Material Copper or Aluminum is main criteria for selection of Cable
(5) No of Core:
- No of Core selection is depends upon Power System.
- For Single Phase Power Supply We can use 2 core Cable for Three phase supply we can use 3.5 Core or 4 Core Cable for HV supply We may be use Single Core Cable.
(6) Voltage drop:
- It is a primary concern when installing lengths of cables is voltage drop. The amount of voltage lost between the originating power supply and the device being powered can be significant. All cables have resistance, and when current flows in them this results in a volt drop.
(7) Type of Insulation:
- Type of Cable Insulation Material like, PVC, XLPE, Rubber
- PVC Cable is cheaper than XLPE Cable
(8) Method of Installation:
- If we lay cable in Ground Armor cable is required but If we lay cable in cable tray We may be used un armor cable to reduce cost of cable.
- I we lay cable on cable tray than shielded cable is required.
- Mutual heating effect due to cable group laying is also consider while selecting a cable. When multiple cables are in close proximity, each contributes heat to the others and diminishes the amount of external cooling affecting the individual cable conductors. Therefore cable de rating is necessary consideration for multiple cables in close proximity.
(9) Shielded Cable or un shielded Cable
- The choice of a shielded cable or non-shielded cable is depend upon some criteria.
- An area such as a production/factory floor where heavy equipment is being used is a prime example of a place where we might consider a shielded cable.
- Grounding can also be a concern in some installations. If shielded cable is used to connect equipment from two different circuits, a ground loop can occur causing noise on a network line. If the ground voltage difference is great enough it may even cause damage.
- Terminations of the shielded cable must also be made with care, to provide for a smooth dielectric transition from the shielded condition to the unshielded condition
- the substantial space required if shielded cables were used. Shielded cables require a significant amount of space at each end of the cable for installation of the stress cone kit. Also, the minimum bending radius for shielded cables is twelve times cable outside diameter, whereas the minimum bending radius for unshielded cables is only eight times outside diameter (and even less with extra-flexible appliance connection cables used in controllers).
- The two factors, high cost and large space requirements, preclude use of shielded cable in switchgear
- It is also an important factor for selecting the type of cable.
- It is to be kept in mind that the cost of the cable should not be such large that it causes loss and another cable may fetch the same results in low cost and loss.
(11) Environmental conditions:
- Cable operates at its best when it is installed in its optimum environmental conditions.
- For example, Elastomeric Cable is applied in trailing, coal cutter, wind mill, panel wiring, battery cable and such other areas. XLPE cables work good in areas where moisture content is good. Thus, proper cable should be selected so that the system becomes more efficient.
- Low voltage cables with both PVC and XLPE insulation are suitable for indoor and outdoor applications.
- Armored cables are not recommended for tray applications, as they are heavy in weight and extra loads are exerted on the tray.
- Unarmored cables are not recommended for direct buried applications, except if the quoted cables are designed and produced to pass direct burial test requirements (example, direct burial tests described in UL 1277 and UL 1581).
- A PVC jacket is a very stable material against a wide range of chemicals, while HDPE jacketed cables can serve better in wet locations.
Cable Core Colors Identification
- Single core – Natural
- Two core – Red, Black
- Three core – Red, Yellow and Blue
- Four core – Red, Yellow and Blue and Black
- Five core – Red, Yellow and Blue and Black and Green
Abbreviation for PVC & XLPE Cable
- A = Aluminum Conductor.
- Y = PVC Insulation or PVC Sheath
- 2X = Cross-linked Polyethylene Insulation
- W = Round Steel Wire Armoring
- WW = Double Round Steel Wire Armoring
- F = Formed Steel Wire (Strip) Armoring
- FF = Double Formed Steel Wire (Strip) Armouring
- C = Metallic Screening (Usually of Copper)
- CE = Metallic Screening (Usually of Copper) over each individual core.
- Gb = Holding Helix Tape (of Steel)
- Wa = Aluminum Round Wire & Aluminum Formed Wire (Strip) Fa Armouring.
- AYY- Aluminum Conductor, PVC Insulated, PVC Outer Sheathed Heavy Duty Cables.
- AYWY- Aluminum Conductor, PVC Insulated, Round Steel Wire Armored and PVC Outer Sheathed
- AYFY- Aluminum Conductor, PVC Insulated, Flat Steel Wire (Strip) Armored and PVC Outer Sheathed
- AYCY- Aluminum Conductor, PVC Insulated, Metallic Screened and PVC Outer Sheathed
- A2XCY- Aluminium Conductor, XLPE Insulated, Metallic Screened and PVC Outer Sheathed
Cable Application Standard:
- IEC 60502 (Part 1)”PVC/ XLPE insulated cables” single core /multi-core
- BS 5467 for XLPE insulated armored cables
- BS 7889 for XLPE insulated single core unarmored cables