4160/120v control transformer blowing primary fuses 1

[Snoopnasty]

Hey guys first post here! so today at my job as a maintenance electrician we had a problem with one of our 4160 breakers blowing the primary 1/2a 4160v fuse on the control transformer causing the breaker to open and cut power to the buildings. so we replace the fuses and try to re energize the circuit. as soon as we did this the fuses blew again. anyone have any idea why this could be happening? we have taken the transformer out and bench tested it in the shop and it is working as it should. we did find a latch relay on the control side that was stuck closed so im thinking maybe its a transformer inrush causing the fuse to blow? anyone else have any ideas? could a short on the secondary side cause the primary side to blow? thanks guys! any help is very appreciated

 

[thermionic1]

"could a short on the secondary side cause the primary side to blow"-yes. Sounds like a 4160 contactor, not a breaker, but these devices are typically held in closed by an AC circuit. Use a megger to check the medium voltage and low voltage sides of the control transformer circuit for shorts or grounds.

 

[oldfieldguy]

When you bench-tested the transformer, did you use full voltage on the primary? Absent a full voltage test because a lot of shops don't have 4160 easily available, did you perform a reverse excitation test?

In this test, using a variable voltage source and taking safety precautions because you have the ability to generate 4160 volts, apply low voltage AC to the LV terminals and slowly raise it while monitoring current. If, as you're raising the voltage, you see a sudden spike in current, that indicates you have shorted turns in the HV winding. The transformer is toast.

Normal ratio and insulation tests quite often miss the shorted turns.

old field guy

 

[dpc]

These low amp primary fuses are known to be sensitive and somewhat unreliable. Inrush can certainly cause a failure. I'd go up a couple of sizes. It's mainly there for short circuit protection anyway.

 

[moa1]

"we did find a latch relay on the control side that was stuck closed". You may be dealing with a faulty shunt trip circuit or module relating to the 4160 breaker. Good luck.

 

[Snoopnasty]

hey all thanks for the reply's just an update. we decided to pull the control transformer and megger it out with the 5kv megger and we found out that the primary had a short in it once it reached around 1000v. So we replaced it and put in new fuses and it all works perfectly now. thanks for the help guys!