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Cable Insulation -- Temperature

Cable Insulation -- Temperature

Cable Insulation -- Temperature

(OP)
We have to follow a specification which asks that cables shall have XLPE insulation, but at the same time it asks that the maximum conductor short circuit current shall be 200C. I understand that conductor short circuit temperature for XLPE insulation is inherently 250C. This means that if we use XLPE insulation, then the maximum short circuit temperature of the conductor will automatically be 250C which is more than 200C. I am wondering is there a way that we use XLPE insulation and still limit the maximum conductor short circuit current to 200C? Thanks for help!

RE: Cable Insulation -- Temperature

I don't quite follow you.

The maximum short-circuit temperature is not determined by the insulation, but by the temperature rise before protection or fuses cut the current.

So, you can design your system to have a maximum short-circuit conductor temperature being 180, 200, 230 or whatever temperature you want. It is all about the maximum short-circuit current, the conductor area, conductor resistivity, thermal time constant and the trip time.

I see no problem with the 250 C specification. You can still limit maximum temperature to 200 C.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Cable Insulation -- Temperature

(OP)
The short circuit temperature can definitely be controlled by protective equipment like fuses, but as you see on page 6 of the link below, the conductor short circuit temperature is also controlled by insulation. It is inherently 250C for XLPE and 150C for thermoplastic insulation. I am trying to figure out that is it possible that XLPE cable have a short circuit temperature of 250C (without manipulating protection and fuses) ?

http://www.stabiloy.com/NR/rdonlyres/61EB3E0F-97DD...

RE: Cable Insulation -- Temperature

(OP)
Sorry in last post I meant to say: is it possible that XLPE cable have a short circuit temperature of 200C (without manipulating protection and fuses) ?

RE: Cable Insulation -- Temperature

I still don't see your problem.

Take a car for example: If one car can run 250 km/h and another one can run 200 km/h and the speed limit is 130 km/h, then both cars can be run below or at the 130 km/h speed limit. The maximum speed doesn't say what the actual speed has to be. The maximum allowed temperature doesn't say what the actual short-circuit temperature has to be. The protection can be designed to keep it at 200 C.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Cable Insulation -- Temperature

How is it "inherently 250C for XLPE" ?

Faults are very short duration so they are classed as adiabatic, i.e. no energy input is lost and all of it contributes to raising the temperature. The temperature rise is dependent on the energy input to the cable and the specific heat capacity of the conductor material. Insulation material plays no part, other than to define the maximum temperature the conductor can safely reach.

RE: Cable Insulation -- Temperature

To limit the temperature to 200°C, just substitute 200 for T2 in the formula on page 6 and calculate the maximum allowable fault current. You may have to manipulate protection and fuses to limit the number of cycles n to get the allowable fault current equal to the available fault current.

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