Breaker Meltdown

[wirebender]

Sunday, we had a customer who had a breaker catch fire and melt. This breaker is at a sewage lift station with 2- 5HP 230v 3phase grinder/pump motors. A converter supplies the high leg at around 195 volts.

I replaced the breaker and the pumps worked fine. Took some readings (true RMS) to insure no overload issues. No problems but I am not sure what the current should be in this situation as I don't work with motors or converters. The current on one motor was around 18 on one leg, 17 on another and 9 on the third. The other pump was similar 16,17 and 8. Is that you would expect?

I am going to replace the panel where the meltdown occurred tomorrow because the bus is a mess. I can take more accurate readings then as the above values are by memory.

Oh, the breaker that melted is a two pole 50A and I assume it was a breaker breakdown that caused the fire, but then I don't do a lot of service work and have never seen a breaker do this.

 

[nakulak]

is the only overload device the breaker (no fuses or heaters ?) ? what size breakers ?

 

[wirebender]

is the only overload device the breaker (no fuses or heaters ?) ? what size breakers ?

It was a 50 amp 2pole with a 30 or so amps current. This is ahead of the converter and the control station. I'm not sure what all it supplies and there is another breaker. I didn't check it all out too close because it was like a swamp of raw sewage overflow around all the equipment and replacing the breaker got everything running. I will check closer tomorrow when I go to replace the panel.

 

[augie47]

breaker failure

breaker failure

I didn't notice "bolt-in" or "plug-in". Is it possible that the breaker was loose on the buss? That would be my first suspicion.

 

[wirebender]

I didn't notice "bolt-in" or "plug-in". Is it possible that the breaker was loose on the buss? That would be my first suspicion.

It's a plug-in. I was not the first one there and the breaker was already pulled. But I will check on that. Thanx.

 

[code_compliant]

It's a plug-in.

Sounds like you've identified the problem.

I find it doubtful this application didn't include thermal overloads. If those are sized properly and didn't trip, the motors aren't the problem.

 

[wirebender]

Sounds like you've identified the problem.

I find it doubtful this application didn't include thermal overloads. If those are sized properly and didn't trip, the motors aren't the problem.

That's the I was looking it, too. Just wanted to get y'all's opinons.

Thanks

 

[brian john]

High contact resistance, at the bus, at the load connection or in the circuit breaker interior.

 

[don_resqcapt19]

Tony,

The current on one motor was around 18 on one leg, 17 on another and 9 on the third. The other pump was similar 16,17 and 8. Is that you would expect?

I know that the current is often not balanced on the output of a phase converter, but that is way too far off in my opinion. The motor won't like that.
Don

 

[electricman2]

Tony,

I know that the current is often not balanced on the output of a phase converter, but that is way too far off in my opinion. The motor won't like that.
Don

Agreed, some imbalance is expected from a static converter (which I am assuming it is) and will vary with load but this seems a little much. Also The 50A breaker feeding the converter and pumps seems a little low for 2 5HP pumps.

 

[micromind]

Is this panel sealed? If so, are the EYs filled? What I'm getting at is if sewer gas is allowed around copper or aluminum, it corrodes rather quickly. This could be the cause of the meltdown. Especially if the breaker is a plug-in type.

I'm assuming that the service here is 240V single phase, and a 2 pole breaker (the one that melted) feeds a phase converter, then there are 3 pole starters w/overloads that control the pump motors. If so, is the phase converter static or rotary? The static ones are nothing more than a capacitor bank, the rotary ones look alot like a motor.

If it's static, then that much current imbalance in somewhat normal. The motors are obviously not loaded very much, or they wouldn't still be operational. With a static converter, you can only load a motor to 50 or 60%, and expect severe current imbalance.

If it's rotary, that much current imbalance is cause for concern. They do a much better job of generating the third phase. With a rotary unit, you can load the motor to 80 or 90%, and expect some, but not severe current imbalance. In this case, check the capacitors, one at a time. There's usually several in parallel. Remember to discharge them first. They are tuned to the reactance of the rotating part, and if they fail, they'll cause a serious current imbalance.

 

[Jraef]

Is this panel sealed? If so, are the EYs filled? What I'm getting at is if sewer gas is allowed around copper or aluminum, it corrodes rather quickly. This could be the cause of the meltdown. Especially if the breaker is a plug-in type.

I'm assuming that the service here is 240V single phase, and a 2 pole breaker (the one that melted) feeds a phase converter, then there are 3 pole starters w/overloads that control the pump motors. If so, is the phase converter static or rotary? The static ones are nothing more than a capacitor bank, the rotary ones look alot like a motor.

If it's static, then that much current imbalance in somewhat normal. The motors are obviously not loaded very much, or they wouldn't still be operational. With a static converter, you can only load a motor to 50 or 60%, and expect severe current imbalance.

If it's rotary, that much current imbalance is cause for concern. They do a much better job of generating the third phase. With a rotary unit, you can load the motor to 80 or 90%, and expect some, but not severe current imbalance. In this case, check the capacitors, one at a time. There's usually several in parallel. Remember to discharge them first. They are tuned to the reactance of the rotating part, and if they fail, they'll cause a serious current imbalance.

Excellent response. I have done hundreds of "Brown Trout" lift stations in my time and they all used Static Phase Converters (SPCs) because Flygt and ABS, the largest mfrs of the submersible pumps, are European companies who only make 3 phase motors. Rather than design 1 phase motors, they chose to de-rate their 3 phase motors by about 50% and the SPCs are really there just to get them started. Once running they are severely imbalanced but as I said, the motors are seriously de-rated.

But that will have nothing to do with causing the breaker to melt down. I agree it's either a bad decision on using a plug-in load center, and/or sewer gas (H2S) leaking in and causing rapid corrosion, especially if they used a load center with silver plated bus, thinking that was better for outdoor use (most are Tin plated, but some offer silver as an upgrade option). In any H2S environment, silver grows "whiskers" and they are conductive, so shorts are common and if on the plug-in bus, they would be on the line side; resulting in meltdown. Been there, seen that.

 

[220/221]

I replace dozens of burned up breakers/bus every year. Usually on A/C loads.

 

[wirebender]

Is this panel sealed? If so, are the EYs filled? What I'm getting at is if sewer gas is allowed around copper or aluminum, it corrodes rather quickly. This could be the cause of the meltdown. Especially if the breaker is a plug-in type.

The panel is not sealed. It is plain old 3R Square D QO.

This is out in the open air and I really don't think there is that much sewer gas present but I could be wrong (I was once before ). The sewage is from a trailer park and it doesn't stay there long. Even when it was overflowing there wasn't any odor to speak of. What should look for on the bus to see if this was the problem?

I will try to sell the customer on a sealed panel with bolt in breakers if I can show him what the sewer gas does to the bus.

Thanks , everyone, i'll post more after I go out there tomorrow.

Happy New Year!

 

[templdl]

When there is a catastrophic failure as you have described its difficult at best to reinvent the crime.
I would focus on the connections especially the line side connections, and rule that out first.

If you have a compromised line side connection the heating that it caused is conducted from the point to the stationary contacts. The heat can take the temper out of the springs that hold the moving contacts firmly against the stationary contacts. As such you end up with poor contact pressure and heat to be generated between the moving and stationary contacts which can escalate to arcing.
Should a load side connection start to fail causing heating that heat is commonly conducted up to the trip unit where the thermal element is able to sense it and trips the breaker thermally.

Remember that when you have a compromised connection that lease to heating when the load is reduced such that the connections cools and then the load resumes that connection commonly will loosen further causing more heating.

 

[quogueelectric]

heavy pump motors

heavy pump motors

In my experience have always had problems with plug in style breakers. A small chemical refinery I worked In once all of the pump loads turned the entire 600a service to junk in 5 yrs. Brand new service. Plug in breakers in a commercial industrial application is always a bad Idea unless we are talking I-line or equivalent.

 

[iaov]

Tony,

I know that the current is often not balanced on the output of a phase converter, but that is way too far off in my opinion. The motor won't like that.
Don

I agree. The phase currents should be a lot closer than that. This unbalance should cause an over load to trip not a breaker melt down.

 

[augie47]

seal-OFFS

seal-OFFS

The panel is not sealed. It is plain old 3R Square D QO.

This is out in the open air and I really don't think there is that much sewer gas present but I could be wrong (I was once before ). The sewage is from a trailer park and it doesn't stay there long. Even when it was overflowing there wasn't any odor to speak of. What should look for on the bus to see if this was the problem?

I will try to sell the customer on a sealed panel with bolt in breakers if I can show him what the sewer gas does to the bus.

Thanks , everyone, i'll post more after I go out there tomorrow.

Happy New Year!

Tony, I see a possible miscommunicatiion (and, of course I may just be reading incorrectly) but your "3R" statement leads me to believe you might not have checked for "sealing" in the conduit runs by way of an explosion proof seal-off. I have inspected some "lift stations" where the mfg. does not require them, but most I've seen have a poured expl. seal-of in the conduit runs to the pump pit. Although you may not smell the gas, it can enter your cabinet and reek havock with your components.
My vote leans more to just plug in panle associated problems, but I wanted to make sure you knew the "seal" msot of us are talkking about.

 

[ptonsparky]

I replace QO breakers frequently that have failed as you describe. I am not sure if it is the QO breaker being installed outside in a 3R panel or just because there are so many of those installations in this area that are 20+ years old.

Check the Static or Rotary phase converter for capacitor failure.
Rotate the leads going to the load. If the amperages rotate with the leads, it is a load problem.

Some current imbalance is normal for the Rotary (what I am familiar with) but that seems high.

 

[wirebender]

My vote leans more to just plug in panle associated problems, but I wanted to make sure you knew the "seal" msot of us are talkking about.

Thanks, Augie

You're right , I wasn't thinking of seal offs because this panel does not have anything going to the pit. It only feeds the converter and a control center. the control center contains the 3 pole breakers, alarm circuit and I don't know what all else. I didn't look at it all that close but I will when I get back out there. Might not be today, I'm starting a large horse arena this morning.

Thanks everyone for your responses.