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# Maximum fault current that the cable can tolerate?

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## Maximum fault current that the cable can tolerate?

A 70 sqmm copper cable can withstand a short circuit current of 8.05kA for 1 second. If the protection relay operates in 0.2 second, then what is the maximum fault current that the cable can tolerate?

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Originally Posted by epenland
A 70 sqmm copper cable can withstand a short circuit current of 8.05kA for 1 second. If the protection relay operates in 0.2 second, then what is the maximum fault current that the cable can tolerate?
The maximum fault current that the cable can handle is only limited to it's alloy physical characteristics and size. Unless this is worded wrong, I don't see how the speed of the relay affects what the cable can withstand.

If the question asked how much energy was dissipated, and having tables of resistivity, one could compute the 5X reduction in energy due to the 0.2 second relay operation.

3. Originally Posted by epenland
A 70 sqmm copper cable can withstand a short circuit current of 8.05kA for 1 second. If the protection relay operates in 0.2 second, then what is the maximum fault current that the cable can tolerate?

I^2t= Available energy

( 8.05kA )^2 * 1 = 64.8025

lets now try to obtain the same energy at less time:

i^2 * 0.2 = 64.8025
i= 18kA