Blowing fuses

[texie]

Have a 6 unit meter center with main fusible switch class T JJN-400 fuses (Siemans meter pack), each meter/breaker feeder is 100 amp 3 pole Siemans QP (10K). The load on each feeder is minimal (it is not overloaded). I'm told that on 3 occasions over 6 months the same 2 phases/400 amp fuses blow. It seems there is an intermittant phase to phase short. Is it possible that a phase to phase short on one of the feeders could open the 400 amp fuses without opening the 100 amp feeder breaker?

 

[jim dungar]

Yes, it is possible that the 400A Class T fuses operates before the 100A main. Their protection curves overlap around 2500A.

What are the loads?

Line-ground faults are more common than line-line ones.

 

[rt66electric]

CUt open the fuses

CUt open the fuses

Knaw/cut/smash the fuses open. Inspect the blown elements. Some times you can tell if the fuses have been slowly melted apart. Sometimes is is very obvious that the fuse elements have been completely obliterated due to direct short. Some times , you just can't tell.
DENNIS

 

[texie]

Yes, it is possible that the 400A Class T fuses operates before the 100A main. Their protection curves overlap around 2500A.

What are the loads?

Line-ground faults are more common than line-line ones.

I would agree line to ground faults are more common but they tell me it is always blowing 2 phases at once and always the same 2.

 

[texie]

Knaw/cut/smash the fuses open. Inspect the blown elements. Some times you can tell if the fuses have been slowly melted apart. Sometimes is is very obvious that the fuse elements have been completely obliterated due to direct short. Some times , you just can't tell.
DENNIS

They did give me a blown fuse and I have it on my desk. I may cut it open for grins. I don't think this is an overload though. These are just small office/retail spaces and the fact they blow 2 at once. Of course at this point I'm just going by what has been reported to me.

 

[jim dungar]

The 400A fuse will open, before the 100A QP breaker, around 2,500A. Do you have several AC units starting up at the same time?

 

[texie]

The 400A fuse will open, before the 100A QP breaker, around 2,500A. Do you have several AC units starting up at the same time?

It looks like there are 4 a/c condensing units-appear to be about 3 tons each. There is no way these would blow a 400 amp fuse even if they all started at once without the pressure being equalized. Besides its winter and they are not on. If what you say about the trip curves is correct (I suspect you are) that means I can't rule out a problem in one of the feeders.

 

[GearMan]

The 400A fuse will open, before the 100A QP breaker, around 2,500A. Do you have several AC units starting up at the same time?

I agree with Jim. He is pointing you toward motor loads. As indicated, 400a T fuses do not selectively coordinate with the 100a tenant cb. Depending on how well (or poorly) the 6-100a panels are phase balanced, it wouldn't take much inrush to open the fuses on the imbalanced legs.

 

[broadgage]

And of course the 100 amp circuit breaker might in fact be tripping as well as the 400 amp fuses blowing.
If there is a fault in one of the tenancies, then they might well reset the breaker and deny so doing in the hope of avoiding any blame for the inconvieience caused to other tenants.

 

[texie]

And of course the 100 amp circuit breaker might in fact be tripping as well as the 400 amp fuses blowing.
If there is a fault in one of the tenancies, then they might well reset the breaker and deny so doing in the hope of avoiding any blame for the inconvieience caused to other tenants.

Yes, I agree on both points! Seen that before. I did question this as well and they assure me that no feeder breakers tripped. You never know what the real facts are.