# Fused disconnect feeding step down transformer keeps blowing?



## myles (Aug 13, 2009)

Hey guys, so I was out last night until about 10PM hooking up the new transformer and testing to see if i could find the problem and i wanted to see if anybody else had any suggestions or ideas on what might be the problem...










So what we have is a "240/480" service coming from poco that goes into a non fused disconnect that feeds the meter can that feeds the fused disconnect.

From the fused disconnect the run is about 1000+ feet to the step down transformer where i hooked it up to step down to 120/240 but i also have the option to change it to 120 since only one leg would be used as we only have one breaker in the breaker panel that is fed from the transformer for the time being.

The problem we're having is the service works fine for about 3 hours and then for some reason the fuses keep blowing. We're using 10a fuses but the service only pulls about 1.5-2a.

When I hooked up the new transformer I tested the feed and was getting 480 no problem...hooked up the transformer 120/240 and got 120 on both legs no problem. But like i said, after about 3-6 hours the fuses blow again.

I tested for continuity on the lines and didn't get anything between ground and the feed...which this is where is confuses me.

Testing for continuity between the two legs when disconnected from the fused disconnect and i don't see any problem.

Now I did notice that after we had our conduit installed a contractor came out on site and did some underground directional boring and this has become the only thing i can think that would be causing our problem. Maybe they nicked the wires or pinched them?

Anybody got any ideas on what i could do to test this circuit to try and figure out exactly what the problem is? Our master electrician is out of state until next week so I've been trying to get a hold of him but haven't heard back yet.

Thanks for any help guys!

Also, there aren't any electric motors being powered off this service i've already told my boss we need to replace the fused disconnect with a main breaker but that still has yet to be done..


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## brian john (Mar 11, 2007)

High resistance connection and fuses are blowing due to thermal issue.

PERFORM

A FOP (Fall of Potential) millivolt drop test.

See earlier post in the electrical section for information.

What size transformer?


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## brian john (Mar 11, 2007)

myles said:


> Also, there aren't any electric motors being powered off this service i've already told my boss we need to replace the fused disconnect with a main breaker but that still has yet to be done..


In my opinion to randomly replace something until you know the reason is foolish and doing your customer and yourself an injustice.

Though I would bet there is an issue with the disconnect.


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## BBQ (Nov 16, 2010)

I suspect you have an underground fault, do you have a mega meter?


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## Zog (Apr 15, 2009)

Have you done any insulation resistance testing on the 1000' feed? How about the transformer? What were the results?


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## brian john (Mar 11, 2007)

BBQ said:


> I suspect you have an underground fault, do you have a mega meter?


 
Bad on me....I skipped that part, BUT I would still do a FOP first.


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## BBQ (Nov 16, 2010)

brian john said:


> Bad on me....I skipped that part, BUT I would still do a FOP first.


It sure would not hurt and takes little effort. 

We do lot of site lighting work and circuits that run a few hours and trip always seem to track down to damaged under ground conductors.


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## tsr341 (May 22, 2012)

If it were me i would get the feeder meggered. To see if you have a fault. And is you didnt know a megger is a high resistance meter for insulation and motors.


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## frenchelectrican (Mar 15, 2007)

I know this is mention couple time but really., Seriously get a megger that can able tell if you have underground conductor fault or not.

I know BBQ have done this many time with parking lot luminarie as myself as well.

As soon you say it start after few hours running time it trip out that useally a conductor fault useally a nick on the conductor coating will do it.

So do the FOP as Brian John suggest to make sure you are clear on that part then do the megger test.

Merci,
Marc


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## Aussielec (Apr 1, 2012)

1+ for the IR tester...a multimetre will tell you buggerall about IR breakdowns in the cable especially over that distance..and as mentioned do an IR test on your stepdown transformer while your at it..

Also I know you said you tested for voltage with a metre but what about amp draw? You say it pulls 1.5-2amps...Is that something you have calculated using the VA of the transformer or have you actually taken that reading using an ammeter?


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## Greg (Aug 1, 2007)

Like everyone else, I say megger the xfmr feeds. Where is this install? Your profile says Orlando and Government Contractor.


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## brian john (Mar 11, 2007)

Aussielec said:


> 1+ for the IR tester...a multimetre will tell you buggerall about IR breakdowns in the cable especially over that distance..and as mentioned do an IR test on your stepdown transformer while your at it..
> 
> Also I know you said you tested for voltage with a metre but what about amp draw? You say it pulls 1.5-2amps...Is that something you have calculated using the VA of the transformer or have you actually taken that reading using an ammeter?


Are you saying a FOP/millivolt test is inefficient?


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## noarcflash (Sep 14, 2011)

did you disconnect the 480--> 240 x, and see what happens.


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## Aussielec (Apr 1, 2012)

brian john said:


> Are you saying a FOP/millivolt test is inefficient?


Nope, never said that...Myles was trying to use the multimeter for IR testing...that is what I was getting at...:whistling2:


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