Jeff000

Here's an interesting one.
Disconnect off the 600v bus feeding 125kva 347/600 transformer to lighting panel. Fill it with 125A fuse (non time delay) and it holds the transformer inrush, but then will lose phase C at 17, so next time skip 17, 30 trips it, start from the bottom and go up 17 nor 30 trip but 11 or something else will. After 4 fuses being blown from 4 different circuits we put the 100 back in and all is good.

Put a 100A fuse in B and C and fire it up no problem. Fuses are the same outside current rating. Could just be 4 bad fuses in a row.... but I hope not likely as the first two came from a different supplier then the next two.

Current is about 38 amps on each phase, balanced quite nicely, inrush from transformer is hardly anything as we load it after it's powered up.

So any ideas?

Techy

are the 100a fuses time delay?

Jeff000

Nope, fuses are identical except for the current rating. That was my first thought too.
But even then there is no spike, unless my meter can't read it, that comes even close to the fuse rating.

Service Call

A digital meter sometimes can't fully read inrush current.

jza

Try another set of 100a fuses, maybe the ones you're using are defective.

Jeff000

I need 125's, not 100's.
The thing that gets me is that it isn't the same circuit that causes the phase to trip.
Makes it hard to think it could be an inrush issue.

Wireman191

Loose connection somewhere?

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bobelectric

How can a 125 amp fuse fit in a 100 amp fuseholder?

varmit

Could the fuses be different class or voltage ratings? This sounds like an inrush trip to me. Both 100 or 125 amp fuses are too small for this transformer. It should be fused at 150 amp minimum. Also, a transformer with no load will have a slightly higher inrush. How are 125 amp fuses fitting a 100 amp holder?
I would think that the branch circuits have nothing to do with the main fuses blowing.
(125 kva x 1000) /(600 vac x 1.73) = 120.4 x 1.25 =150.5 min circuit rating

Jeff000

Where? lugs have all been torqued and marked. Why would 100a hold and 125a not?


It is a 100A fitting in a 125A holder.


Same class, same voltage rating.
Transformer may be a 100kva or smaller even, it's been many months since install, and there are a LOT of transformers.

And again, never said it was a 100 amp holder. It has always been a 125a disconnect, just didn't have any 125a fuses when we powered up.

HARRY304E

Are they the same available fault current rating?

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Big John

You have any info on the fuses? It'd be interesting to see the trip curves on each. And I agree, without a fast capture on your multimeter, you're probably not getting the right inrush value.

-John

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Jeff000

hmm, I'd think so, but never actually checked. I'll get the info off the fuses tomorrow.
I will check this in the AM

On Friday we are trying a different brand fuse, I'll get the office to bring out a meter capable of recording the proper value. And I'll get the values off the new fuses.

joethemechanic

Is it possible that the 125s had something happen to them in your storehouse?

JRaef

Transformer inrush, the current that flows to initially set up the magnetic fields, is only going to last a cycle or so*, way WAY too fast for the average DMM to capture.

The magnetude of the inrush current is also dependent upon the phase angle of the incoming sine wave at the exact moment of connection and any residual flux in the transformer. Depending on that, the spike can be anywhere from 10 to 50 times the FLC rating of the transformer. So that alone often results in the "sometimes it blows the fuse, sometimes it doesn't" scenario. That's exxactly why they make Time Delay fuses.

What you are experiencing is not that uncommon, WHEN YOU USE NON-TIME DELAY FUSES.

* (depending on the size, the larger the transformer the longer the inrush lasts.)

Jeff000

We don't store anything, it comes from the wholesaler and then for things like pipe and fittings we put it out of the way on site, fuses would go into the office or right into the enclosure.


My suggestion of just ordering time delay was turned down. I'm not sure why.

jmsmith

I remember with motors having to use time-lag type fuses along with the percentage increase to hold for inrush. In the cases where a hand would use a non-delay type, we would invariably wind up on replacement calls as soon as someone would hit a start button. Just throwing that out there...

JRaef

Well, you made the correct suggestion, but as to the "why?"

I'll guess because you can't fix stupid?

I've had people tell me they don't want time delay fuses because they are afraid the "delay" will kill someone or start a fire! I'm serious.

Sometimes it's just a matter of education to dispel some myth they heard from some idiot brother-in-law once a long time ago. People get weird ideas in their heads when they don't really understand something, and sometimes they will stick to the stupid choice just because they don't want to admit it or show that their previous notion was hair-brained.

So keep replacing the 125A NON fuses and charge them for every call (assuming you are a contractor). Sooner or later a bean counter will question the expense and the idiot behind that decision will have to justify it.

Jeff000

Only lighting, no motors on the disconnect.


This is a brand new 300 million dollar building, partially occupied as things are still being worked on.
This is with the company I work for and not my company. I am sure it is on our tab and not the owners at this point.

jmsmith

Sorry about that... However, the same principal applies here. A transformer is an inductive load, also, and firing one up can be just as hard on a fuse as