Need help guys

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brentp

Senior Member
I've got a 400a 480v 3ph GE Spectra Series breaker feeding the primary of a 112.5 kva transformer for temp power to jobsite (breaker feeds 125a fuses then transformer). The 400a breaker is gfci and sits in a hospital main switchboard. Pickups and gfi are set at minimum so as not to trip main. As the coordination study is not in yet, I'm afraid to 'up' the breaker settings.

My problem is that the transformer was shut down today and upon reenergizing the 400a breaker started tripping. This happened repeatedly under no secondary load (secondary conductors disconnected). After alot of time searching for a primary ground fault the breaker finally decided to hold.

I have used this type setup many times and have never had this problem. Inrush current to transformer? Ground fault? Any ideas?

BTW, I didn't have an ammeter. :)

Thanks in advance.
Brent
 
ron

ron

Senior Member
Location
New York, 40.7514,-73.9925
Re: Need help guys

Depending on how low the instantaneous setting on the 400A breaker is, it very well may be inrush. Inrush is unrelated to the secondary load.
Inrush average is 10-12 times full load. That does not mean that it can't be higher or lower, depending on where in the 480V sin wave the transformer is energized, at either extreme.
 
bob

bob

Senior Member
Location
Alabama
Re: Need help guys

Brent
It may have been a primary ground fault that burned in the clear upon re-energizing the transformer. If it was inrush why didn't it cause a problem before now? Also the primary full load current is 135 amps. Usually the fuses are set a 125% of FLA or 175 amps. Any reason for the 125's?
 
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brentp

Senior Member
Re: Need help guys

Originally posted by bob:
Brent
It may have been a primary ground fault that burned in the clear upon re-energizing the transformer. If it was inrush why didn't it cause a problem before now? Also the primary full load current is 135 amps. Usually the fuses are set a 125% of FLA or 175 amps. Any reason for the 125's?
Click to expand...
Bob,

I might as well been a carpenter when this happened. No amprobe, no megger. My ohmmeter did not pick up a grd fault.

I wish I could tell you why this didn't happen before, but I can't, that's why I'm here.

The 125a fuses were used because they were available. The transformer is oversized, but it too was available. This is for temp power.

Maybe I'll do a little better tomorrow with the input I've gotten here, and some better test equipment.

Brent
 
brian john

brian john

Senior Member
Location
Leesburg, VA
Re: Need help guys

In addition to instantaneous trip of the primary CB it is possible that the GF protection may be tripping culprit if it is set two low (current and time) the transformer inrush in a zero sequence type GF protection scheme can trip the CB/switch. In addition when resetting the CB it may trip sometimes and not others depending where in the cycle the CB is reset, something the operator cannot account for.
 
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jschultz

Member
Re: Need help guys

Originally posted by charlie tuna:
breaker is tripping on instanious current due to the magnetising current drawn by the transformer.
Click to expand...
how can that be since we typically protect a transformer of that size with a lot smaller cb and don't have problems.
 
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brentp

Senior Member
Re: Need help guys

First I'd like to say thanks for everyone's responses!

BUT, I'm still confused about the trip settings on these Spectra Series breakers. If I'm understanding the breaker instruction manual correctly, the instantaneous trip is based on multiplying the setting(1.5) times the rating of the rating plug (400). So if the instantaneous trip was set at 1.5, the lowest setting, does this mean the inrush current exceeded 600 amps with no secondary load? Am I understanding this correctly? Is it typical for a 112.5 kva transformer to have this much inrush?

Also, can someone tell me where to find more info on the long time, short time, instantaneous, gfi...settings? The instruction manual gives a short explanation without details.

I fall way short of understanding engineering principles or equations, but I really feel the need to understand what happened.

Brent
 
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brentp

Senior Member
Re: Need help guys

brian john,

Thanks for the link. I know the info is "there", I just have a hard time understanding most of it. My mother told me to go to college, but I didn't listen. :)
 
ron

ron

Senior Member
Location
New York, 40.7514,-73.9925
Re: Need help guys

If the full load of the xfmr is 135A, the average inrush, with or without load, due to magnetizing current is 1620A.
Most nonadjustable circuit breakers have their instantaneous rating at 10 times the handle rating of the breaker. So if it was a nonadjustable 400A breaker, the instantaneous trip would be approx 4000A, and not trip with the transformer inrush. Of course a 400A breaker is not code compliant for use on the primary of a 112.5kVA transformer. Your adjustable 400A breaker with the inst set at 1.5, will trip on an inrush of 600A, which probably happens almost every time you energize the xfmr.
The 125A fuse, depending on its characteristic, is more or less similar in performance.
For your adjustable breaker, the long time pickup is essentially the trip value (handle value). The long time delay is the programmed delay in seconds for the long time pickup. The short time pickup is midway (in time) up the characteristic, and also has a delay setting. The IsqrT is a setting for the knee of the curve between short time and instantaneous. Instantaneous is just what it sounds, it is the immediate reaction of the breaker characteristic.
 
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brentp

Senior Member
Re: Need help guys

I got it Ron! thanks for breaking it down for me! I apologize to all others for being so dense.

Brent
 
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