What might have caused this safety switch to fail?

[GaryGrogel]

I have a 50 HP Quincy rotary screw air compressor. It was running fine and I shut it down to change filters yesterday.

After locking out the disconnect switch, A Square D, 100 amp fused switch, I found the machine was still powered. I went to the 80 amp breaker, 25 feet away, and locked out the power. Inside the disconnect I found the top of the switch melted at one pole (far right) and the switch stuck in the on position. the Number 2 wire coming into that leg had the insulation melted off 1.5 inches above the connecting terminal.

When I pulled the 70 amp FLSR RK5 fuses from the holders, two of them pulled apart at the end and the body of the fuse was charred inside. The fuse on the leg that was melted at the switch(right most fuse) came apart at the top. The fuse in the middle came apart at the bottom. I find a little heat discoloration on the wire below it when I removed it but no melting.

Ive replaced the disconnect and its running fine. Normal amperage from the motor. Thoe only thing I found that didnt seem correct were the terminal screws, not loose but not they didnt take much to loosen. What else should I look for?

 

[telectrix]

terminals can loosen over time causing exactly the problems you describe. arcin causes heat, which in turn damages the terminals and the cables connected to them. if you've replaced the disconnect and repaired the damaged cable ends, ensured all terminations are tight, should have solved the problem.

 

[GaryGrogel]

Thank you. I'll get into the box and check everything. I have had theachime running for an hour and opened the disconnect to check terminal temps with a laser thermometer. They are 90 to 100 on the Line side and 85 to 90 on the load. I've never checked these before. Is that high? 480v btw

 

[telectrix]

Thank you. I'll get into the box and check everything. I have had theachime running for an hour and opened the disconnect to check terminal temps with a laser thermometer. They are 90 to 100 on the Line side and 85 to 90 on the load. I've never checked these before. Is that high? 480v btwView attachment 55736

are you talking fahrenheit or centigrade?

 

[GaryGrogel]

That is Fahrenheit

 

[GaryGrogel]

That is Fahrenheit

And After talking with my Compressor company they tell me that because of the cycling of the compressor the service sees inrush amperage frequently and that 100F is not unusual.
I appreciate the info on terminals getting loose over time. I called my electrician and he tells me they offer a maintenance service where they check all boxes and services with a IR camera and he is coming in next week to give me more information, In the mean time, I will add checking terminals to the things I do as PM on the equipment. Thank You. Gary

 

[Megawatt]

I have a 50 HP Quincy rotary screw air compressor. It was running fine and I shut it down to change filters yesterday.

After locking out the disconnect switch, A Square D, 100 amp fused switch, I found the machine was still powered. I went to the 80 amp breaker, 25 feet away, and locked out the power. Inside the disconnect I found the top of the switch melted at one pole (far right) and the switch stuck in the on position. the Number 2 wire coming into that leg had the insulation melted off 1.5 inches above the connecting terminal.

When I pulled the 70 amp FLSR RK5 fuses from the holders, two of them pulled apart at the end and the body of the fuse was charred inside. The fuse on the leg that was melted at the switch(right most fuse) came apart at the top. The fuse in the middle came apart at the bottom. I find a little heat discoloration on the wire below it when I removed it but no melting.

Ive replaced the disconnect and its running fine. Normal amperage from the motor. Thoe only thing I found that didnt seem correct were the terminal screws, not loose but not they didnt take much to loosen. What else should I look for?

Welcome to the forum and that sounds like an old bulldog switch which was notoriously bad about maybe 2 phases disconnect and the other switch breaks. Anytime I encounter that style of switch and bulldog is the name brand I would change it. They are dangerous and good for you checking to see if voltage was present where some people would assume all the power is off without checking and they are going to have a bad day