Hoping for an opinion on arcing problem with switchgear

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gwright

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Long Beach, Ca
We recently sent out a transformer (4160v 208/120v) to be rewound. Got it back and re-installed it. We then switched on at the switchgear and checked voltage. Little high so we went to the switchgear and threw the switch to off. Big arc and blew a fuse. Surprised us. There was no load. We bought another fuse, installed it, switched on - no problem. We still had to make a couple of adjustments to the transformer so we again switched off the gear. Same thing happened, arc and fuse blown. Anyone have this happen? Energizes fine, but doesn't like being turned off. And no load... Appreciate any thoughts.
 
An unloaded transformer is just a giant inductor. When you open the primary at any point in the cycle except at a current zero you can get an arc across the switch. If the arc spreads to involve grounded parts you can have an overcurrent trip.
 
To give a better idea of how this is set up, the switchgear panel (Westinghouse 4160v type 12/61) is located about 150' from the oil switch. The transformer (dry-type interior) is 10' from the oil switch. Transformer (single unit 3 phase) Delta/Wye 4160v 120/208v.

The arcing looks to have happened at the tip of the copper contact arm when the switch was opened.

Thanks for your time.
 
Assuming that gear is rated to break the load, it sounds like a problem with the arc suppression in that switch. Did the arc start on the same pole of the switch each time? Is the gear assembled properly internally, and are all the parts in good condition? It's either defective or is being used outside its ratings.
 
mpoulton said:
Assuming that gear is rated to break the load, it sounds like a problem with the arc suppression in that switch. Did the arc start on the same pole of the switch each time? Is the gear assembled properly internally, and are all the parts in good condition? It's either defective or is being used outside its ratings.
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Yes, same pole both times. Gear is assembled correctly. I thought the problem was with the switch opening too slow, but that was just a guess. Thanks for your help on this. Appreciated.
 
rlundsrud said:
I'm just taking a WAG, but are the arc chutes installed? Where they removed? Perhaps they didn't line up correctly or were damaged when reinstalled.
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The chutes weren't removed, but they might be dirty. The switchgear is old and hasn't been cleaned in a long time. Anyway, thanks, I'm going to make sure they are working properly.
 
If this is a regular knife switch and not spring-loaded then it is not designed to be opened under load, even though the xfmr is unloaded. Either open a breaker upstream of this switch or replace the switch with a circuit breaker or spring-loaded switch that is designed for this application. It's all in the speed of the disconnect.
 
gwright said:
The chutes weren't removed, but they might be dirty. The switchgear is old and hasn't been cleaned in a long time. Anyway, thanks, I'm going to make sure they are working properly.
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If the arc chutes have splitter plates, soot or blobs of copper from the arcing contacts can attach themselves to the bottom of the plates. These reduce the effectiveness of the arc quenching.

If you can get in with a needle file remove any blobs of copper, blow any soot out with compressed air. I don’t recommend taking the arc chutes to bits, you need three hands and 20 fingers to get the damn things back together!
 
gwright said:
The chutes weren't removed, but they might be dirty. The switchgear is old and hasn't been cleaned in a long time. Anyway, thanks, I'm going to make sure they are working properly.
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What model switch? Most of them have adjustments to be made to ensure proper pressure
 
Sounds very much like their is a failure with the arc suppression within the switch and this is not related to the load.

Unless the arc is directly contacting the fuse and causing it to blow due to thermal stress, it sounds like the arcing is so severe that it's flashing to ground.
 
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