Fault level on tie bus

[Parkpower]

Gooday,

We have a 30kV switchgear lineup with tied section A and B. The tie breaker is normally open. However, it will be closed for a paralled incoming operation not more than 20 seconds before switching for maintenance . The worst case of fault level on 30kV bus is parallel incoming operation with closed tie breaker. What is the fault level when we design? The parallel operation with closed tie breaker or separate operatin of section A and B with open tie breaker?

Thanks,

Parkpower

 

[lansford]

20 seconds is a long time so the safe answer is to use the highest level of fault current. Even if there is serious documentation based on engineering review, I would use the hihger level.

JIM

 

[VTer]

20 sec is the eternity in electrical world. Use the higher sc rating.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[dpc]

Ideally, you should base the design on the maximum fault current with the tie breaker closed. This is what the NEC basically requires.

In the real world, there are sometimes exceptions, but if you are designing new equipment, I'd assume the worst-case, especially if you are stuck on the 20 seconds. These transfers are often done with 1 or 2 seconds instead of 20.

You have to also consider that the arc-flash level will nearly double when the two sources are in parallel, even if the breakers are adequately rated.

 

[davidbeach]

20 cycles would be a long time for closed transition, as stated above, 20 seconds is the same as for ever. Closed transition can be reliable done with the two sources paralleled for less than 100ms, but even then I'd rate the gear for parallel operation.

 

[Parkpower]

Thanks all,

worst case shall be my choise. Switchgear short current is rated for 1s or 3s. 20 second is really a long time.

Parkpower

 

[kssschsekhar]

Switchgear short circuit current withstand time and switchgear change-over time are not related each other. The first one depends on fault clearing time with back-up protection.