Upstream protection

[joshfromoshoto]

I cant seem to get a straight answer on this. Will a fuse protect what is upstream from it? I have a customer who had a splice burn up in the power companies utility box. The service drops from a pole, hits a splice and then runs to a set of 200a CTs, and from there into a 400a main fused disconnect. On the night that the splice burned up, the power company said that he pulled over 600a all night on the leg that burned up, and blamed his fuses, saying that they were faulty. My understanding is that while the splice was burning, it was pulling excess current, and since its upstream from the fuses, that they would not blow, as nothing on the load side was pulling that much current and that the splice was solely to blame. When I questioned the accuracy of the CTs that arent rated for the service, they said that its worked fine and never had issues before. I will also add that there are no signs of excess current or heat anywhere at all, aside from the splice.

 

[LarryFine]

Welcome to the forum.

On the night that the splice burned up, the power company said that he pulled over 600a all night on the leg that burned up, and blamed his fuses, saying that they were faulty. My understanding is that while the splice was burning, it was pulling excess current, and since its upstream from the fuses, that they would not blow, as nothing on the load side was pulling that much current and that the splice was solely to blame.

If that last statement is correct, then so are you.

Excess current on only one line implies a line-to-ground (or -neutral) fault.

 

[ptonsparky]

600 amps "all night" seems high for 400 amp fuses to carry.

 

[LarryFine]

600 amps "all night" seems high for 400 amp fuses to carry.

The fuses were not exposed to that current. It was a L-G fault ahead of them.

 

[ptonsparky]

The fuses were not exposed to that current. It was a L-G fault ahead of them.

I agree. Stating 600 amps most likely will not flow thru a 400 amp fuse all night, regardless of where the fault is. It also indicates the POCO splice was poorly done. I would think that fault should be easy to find

 

[Joethemechanic]

I can't think of any time I've seen a splice in good condition fail from a 150% overload. And I've been in plenty of sketchy places where someone installed much larger loads than the conductors were rated for and just overfused

 

[winnie]

A normal fuse cannot detect or trip on an upstream fault. The fuse will open because of _heat_, usually heat caused by the current flowing _through_ the fuse.

All fuses have a 'time current curve'. It would not surprise me at all for a 400A fuse to hold a 600A current for an extended period of time, especially if it were in an outdoor disconnect on a cold night. Remember that fuses trip on heat, and if you have better cooling the trip current will be higher.

It is very unlikely that the splice was drawing the excess current. Sure you could have a high resistance fault causing a bit of excess current and localized heating...but you have a 600A 120V fault you are talking about 72KW of heating. If this were 'all night' then your entire service equipment would be a melted puddle.

For the splice to be the culprit in the excess current, you need to consider where all the heat is going. For example if the failed splice were underwater, and the water was dirty enough to allow 600A of current to flow, then you'd be boiling away about 25 gallons per hour.

IMHO the 600A of excess current (if real, and not some sort of measurement error) would have to be caused by a load in the facility, not by the failed splice.

-Jon

 

[wwhitney]

If the splice is upstream of the CTs, the CTs would not see the current through a fault at the splice. So what is the source of this 600A measurement?

Cheers, Wayne