Circuit Breakers

Well it is really going to depend on your upstream protection elements. Will the SF6 circuit breaker be subjected to a 3 second fault or will it trip out before then. Are you using a 1 second or a 3 second fault rating when designing (or retrofitting to an existing power system)?

If you really have 50KA short circuit current, a circuit breaker having short circuit current rating of 40KA for 3 Seconds, maybe can not interrupt the 50KA Short Curcuit current. I think magnetude and duration of short circuit current are two different parameters and selected circuit breaker should meet both of them individually.

One of the supplier claims that 50 KA is more robust having 1 Sec withstand time than 40 KA having 3 second withstand. Obviously 50 KA arc will produce high temperature than 40 KA, but in the former case the withstand time is only 1 second, thus fault will be interrupted earlier without additional temp. rise, whereas in the later case the contacts will be subject to lesser thermal stress but for a longer duration. In the scenario, which one is the better choice?

50Ka for 1 sec and 40Ka for 3 sec are ratings these do not define the interruption time.!

In which of the two cases the thermal stress on the contacts would be higher?

One of the supplier claims that 50 KA is more robust having 1 sec withstand time

He is right.

Buy his.

Thermal stress can be defined by I²t;

50KA @ 1 sec. produces 2500 Kamp².sec

40KA @ 3 sec. produces 4800 Kamp².sec

Therefore, physically, the contacts of 40KA @ 3sec would be more robust.

However, electrically, it is not a point at all, but:

• how fast is your protection?
• how much is your time grading (time coordination) in the protection system?
• how many amps is prospected to flow in such a CB in the worst case of fault according to a right short circuit calculation?

If the max. short circuit level is less than 40 KA, then 40KA @ 3sec is preferable as it gives you a wider time margin to play time coordination.

If NOT, you are forced to select 50KA @ 1sec.