ECN Forum Forums Photo Discussion Forums Photos for General Discussion Meter Fire

What are they protecting from?

Larry

To elaborate a bit on what Don said:

Larry, the cutouts are used to help reduce the possibilty of violent failure of a faulted transformer/capacitor, to prevent the primary conductor burning down from a fault, to provide isolation of a faulted tx/cap/feeder; and to provide in most cases a visible means of locating a fault. (Most cutouts drop open or otherwise provide a visual cue that they've opened.)

The cutouts on a transformer are rarely fused to protect the transformer from overloads, even sustained ones. The utility practice is to allow the trans to operate to failure, usually no more than a burnout resulting in primary current in excess of the fuse cutout rating, which then opens. But sometimes the short circuit current available on the primary is very high, or the fuse rating is accidently too high, then the transformer can fail anyway by explosion or fire.

That's why faults like this one and some others we've seen here can happen without taking out a primary fuse/cutout.


[This message has been edited by mxslick (edited 09-18-2006).]

OK, that makes sense to me now.

Thanks

Wow.

I was always taught to assume that service conductors have no overcurrent protection whatsoever. Here is a stark illustration of this fact.

1-Don't you need the transformer impedance to figure out the short circuit amps?

2-The transformer sees this type of short as a load and nothing trips - I think!

Please correct me if I'm wrong
I'm not sure about either of my comments!

Yes, you need to know the transformer rating, transformer impedance and line impedance to figure out the fault current. At higher currents, the transmission cables (especially the poco ones that are undersized by NEC standards) act as resistors, further limiting the fault.

The transformers themselves are stupid- they don't know a fault from a load until they burn up. The fuse on the primary is only slightly smarter, but apparently not quite smart enough in this instance...

[This message has been edited by SteveFehr (edited 11-30-2006).]