Fault clearing

[mpparent]

Gents,

I just want a sanity check. Say I have a 3ph bolted fault on the secondary of a unit substation ahead of it's secondary main. The primary is protected by a current limiting fuse. Will the fault be cleared by the primary fuse(this is assuming of course there is enough fault current beyond the fuse's clearing curve)? I'm just checking some worst case scenarios. I am thinking it will clear, even if the fault was on the load side of the secondary main, as typically, the main has maximum time delay built in for downstream coordination.

Thanks in advance,

Mike

 

[DanDel]

Typically, the primary transformer device is responsible for faults(short circuits), and the secondary device(or devices) are responsible for overloads. Check IEEE Std 242, section 10.8.3.2.

 

[dpc]

Maybe I'm not clear on where the fault is - if it is "ahead" of the main, I take that to mean between the transformer and secondary main breaker. If this is the case, the secondary breaker won't see the fault and it has to be cleared by the primary protection.

If it is downstream of the secondary breaker, it just depends on the coordination between the fuse and main breaker protection. There is no assurance that the breaker will trip first unless someone has made sure this will happen. For very high fault currents, it is difficult to fully coordinate with fuses.

 

[wbd]

The fault may or may not be cleared by the primary fuse. It would depend on the fault level as seen on the primary side. Check that against your fuse TCC and txf damage curve to see if it will clear in time to prevent further damage.

 

[mpparent]

dpc,

Your assumption is correct as to where the fault is on the secondary. That's exactly what I was thinking, but wanted to be 100% sure.

Thanks guys,

Mike

 

[busbar]

For secondary faults in front of or behind secondary overcurrent protection, wouldn't IEEE Std C57.109-1993 {Liquid-Immersed Transformer Through-Fault-Current Duration} usually apply?

 

[rashieda]

mpparent,
If this is an existing installation, a check of the short-circuit & co-ordination study for your installation will indicate whether the fuse will clear before the main protective device operates. This should be apparent from the co-ordination curves for the transformer feeder.

 

[peebee]

Your concern is valid. Selectivity is lost with current-limiting fuses. This is about the same thing that happens when using "series-rated" breakers or insulated-case breakers. A high-level fault can open the main protection before opening downstream devices.

 

[jbartos]

Suggestion: Please, notice that the transformer primary protection is dependent whether or not there is a transformer secondary protection. A properly and mainly sensitively set transformer primary protection should protect the transformer. See NFPA 70-2002 (NEC) Art 450 Transformer and Transformer Vaults for details.