Fuse blowing

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ghelec

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Texas
One of my customers (a center for disadvantaged youth) is having a problem with fuses blowing in their 400 amp disconnect. It is served by 277/480 y bank. They have a 480 panel behind the disconnect which has a 90 amp 3 phase breaker feeding a 480/120-208 delta/y step down transformer. I have just started working for them and customer said bldg. was built 6 years ago and they have had one of the 400 amp fuses blow several times but it is always a different one. There is minimal load and voltages all check good. I thought maybe was a problem with the step down transformer but looks like it would trip the 90 amp breaker. Any suggestions would be appreciated.
 
If there is not enough load to blow the fuse, the most likely problems are poor connections at the wire terminations, or poor contact pressure on the fuse holders. Both create excess heat that can blow the fuse. If either of these are the case you should be able to see some evidence of damage that was caused by the heat.
 
I would take current readings, maybe even set up a recorder and let it run for a week.
Do you know what loads it serves?
Do you have access to a infared camera?
Keep us informed please:)
 
Friend that works for AEP is going to provide a recorder. I do not have access to an infrared camera. The customer asked if fuses wore out and I told him I had never heard of that happening. What seems odd is that it does it on different phases.
 
I have seen a pizza restuarant that would lose a fuse about every two weeks.
We found that the service was running at capacity and there was an intermittent short in one of the ovens.
Took a while to figure out.
 
sawdust454 said:
I have seen a pizza restuarant that would lose a fuse about every two weeks.
We found that the service was running at capacity and there was an intermittent short in one of the ovens.
Took a while to figure out.
Click to expand...

I thought about some type of intermittent short like that maybe in the step down transformer but wondered why the OCPD did not open up. If load was near capacity that may account for it not opening but unless we they have some load that comes into play that we missed that theory does not fly.
 
The fuses may occasionally be clearing on transformer inrush after a brief interruption event, such as a grid switch. If the fuses are not time delay that can happen. Even if they are and the spike is too fast and high enough, the breaker may not react before the fuse blows. It may need better coordination.
 
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