HV cable sizing - 1 second fault level rating

[mjmarkey1979]

The contractor on a project has submitted the cable sizes for 11kV cables. One of the parameters used in sizing the cables is the fault level withstand. The contractor has sized the cables to withstand the prospective fault level for 1 second.

However, the client has challenged us on why 1 second is used. He feels that unless there is a standard or a law that requires 1 second fault rating he should be able to use whatever time he feels appropriate, for example the protection operation time. The disconnection time for the smaller cables under a fault will be significantly under 1s, and the upstream protection operation will still be under 1s. Obviously this makes the cables smaller and cheaper. We have been having our own arguments with him about our feelings on this.

The question I have is, does anyone have a water tight argument why a one second fault rating should be used. Specifically, is there a British, European or international standard or a British regulation/law that specifies a 1 second rating for HV cables?

Cheers in advance.

 

[Silja]

You may check some standards for the fault withstand of switch gears. for instance, in some areas, the breakers are designed to withstand 40kA for 3s and 100kA for 1s and be tested for this(I can not recall the exact numbers). so if the breaker is designed to withstand only 1s, why the cable shall be longer than that?

 

[Marmite]

The short circuit withstand has to be calculated for both the phase conductors and the screen. The 1 second rating is just a convenient figure used to compare cables objectively against one another. It comes from standards such as IEC60949. It doesn't mean you have to use 1 second as the basis for sizing cables. The heating of the cable during a fault is proportional to I squared T.
Based on the reference values, the short-circuit currents for other durations can be converted with using the I squared T formula.
If you have a faster fault clearance time (Relay + C/B operate time) than 1 second then your cable can take more fault current than stated in the 1s rating table, or put another way you can get away with a smaller cable if the protection operates faster than 1 second. Also bear in mind that the 1 second rating assumes that the cable is already operating at its maximum temperature when the fault occurs.

Regards
Marmite