Circuit Protection will Not Trip

[Masterman]

I am working at a pump station, 3 phase 208, that drives a 40 hp motor/pump. The service from the power company comes in, goes through a 200 amp fused disconnect, a 200 amp breaker and then separates to multiple pumps. The path to Pump #4 goes through a 150 amp breaker and an Allen Bradley starter with 130 amp thermal overload disconnect. This is the second time that the pump motor had issues in the last year, and the B phase shorted to ground in the pump.

What is confusing though, Phase B is drawing 285 amps, none of the circuit protection has tripped. We are trying to determine why its not tripping, any suggestions?

 

[ptonsparky]

I am working at a pump station, 3 phase 208, that drives a 40 hp motor/pump. The service from the power company comes in, goes through a 200 amp fused disconnect, a 200 amp breaker and then separates to multiple pumps. The path to Pump #4 goes through a 150 amp breaker and an Allen Bradley starter with 130 amp thermal overload disconnect. This is the second time that the pump motor had issues in the last year, and the B phase shorted to ground in the pump.

What is confusing though, Phase B is drawing 285 amps, none of the circuit protection has tripped. We are trying to determine why its not tripping, any suggestions?

It takes time for those items to react. You would need to look at the curves to know.

Obviously something is tripping eventually. The trip time is not causing the original problem. IDK what the FLA of your motor is. That is where the overloads should be set. Not the SFA. Not 125%. Generally Class 10 for pumps.

 

[Dennis Alwon]

What was the issue the first time with the pump? Was it replaced or rebuilt? The motor should e somewhere around 100 amps I suppose.
@ptonsparky -- if the motor has a dead short then it appears it should trip the breaker. Perhaps this isn't a dead short-idk

and the B phase shorted to ground in the pump.

 

[Masterman]

The first issue with the pump/motor was similar but I was not involved in troubleshooting it, it was turned in under warranty. But it was phase B that went out on it and drew high current as well.

I am measuring a dead short with the Ohm meter, when I measure the current I am getting 285 amps on phase B, I have used three different current meters, two Kliens, one southwire, all reading about the same. The phase A and C there is no current draw.

Each phase of the motor normally runs about 85 to 92 amps.

I think I am dealing with two different problems here, first there is the motor burning up issue and what is causing it, but secondly why are the disconnects not reacting. When this first happened it ran like this for close to three hours (it is a remote site) with this high current. a couple of days later I was called to the site where I observed this current for over 3 minutes without any breaker tripping or fuse blowing.

 

[ptonsparky]

OP.

A typical 208 40 HP FLA is about 114 amps. Start current about 6 times that. Yes, lots of variables.
IDK why two motors have faulted phase to ground but it is not caused by the SC or OC protection ahead of it. An overloaded motor with improperly sized OC protection could eventually allow the failure but the initial cause is the mechanical overload.

130 amp overloads are too high.

 

[Masterman]

OK, I can accept the overloads are to high, but that should not matter for the problem of why the circuit protection is not operating, if I have four protection devices (200 amp, 200 amp, 150 and 130 amp) and I am drawing 285 amps, shouldnt I be tripping the protection devices? I have seen a breaker fail, but to have one fuse, two breakers and a motor overload not trip is really strange...I have never seen this before

 

[ptonsparky]

How long are you allowing this to run? Personally, if I see a motor drawing twice its FLA, I would be shutting it down before the overloads could react and they are your first line of defense. The 200 & 150 could take quite a while to trip. The trip curves.

A number years ago I was called to a community water system that had lost several motors. The electrician that had been working on it had basically bypassed the overloads because they were tripping too often.

 

[Masterman]

No one was around to shut it down as it was a remote station, it dumped 285 amps for 3 hours. I turned it on to test and saw it continue to dump current for close to 3 minutes.

 

[ptonsparky]

No one was around to shut it down as it was a remote station, it dumped 285 amps for 3 hours. I turned it on to test and saw it continue to dump current for close to 3 minutes.

PP design. Start over and put a different electrician on it.

 

[ptonsparky]

Newer overloads have a lot of options that could have helped.

Is there a chance they were placed in Auto Reset?

 

[Masterman]

Auto Reset is a new idea that we never thought of.... I will look into that, thanks

 

[petersonra]

I am measuring a dead short with the Ohm meter, when I measure the current I am getting 285 amps on phase B, I have used three different current meters, two Kliens, one southwire, all reading about the same. The phase A and C there is no current draw.

What two points are you measuring when you get a dead short?

If you are only getting current on one phase the motor would not have been turning.

 

[Dennis Alwon]

Disconnect all the phases remembering the order that they were connected. Then run your ohm meter to ground on each phase and see what you get

 

[petersonra]

Disconnect all the phases remembering the order that they were connected. Then run your ohm meter to ground on each phase at the motor and see what you get

 

[Dennis Alwon]

Yes... sorry I thought that was obvious.

 

[Jraef]

The 150A breaker for the starter is likely a “MCP” aka Instantaneous Trip (only) type breaker, so it only protects against a dead short at 10-17x the FLA. so unless the current gets to 1500A or more, it’s not going to trip.

It is supposed to be the OL relay that trips on thermal overload. That uses an “I2t” trip curve, typically based on what’s called Class 10 on pumps. So Class 10 means it has to trip in 10 seconds at 600% of the setting, 2 hours at 125% and a parabolic curve between those two values. So if set at 130A, 285A represents a value of 219% of the setting. Looking at a typical I2t curve, the trip time could be anywhere from 30-50 seconds. But if the current dropped as resistance increases for some reason, it could take even longer. Likewise for the 200A breaker and fuses, it’s a similar curve but 285A is only 142%, so the trip time would be significantly longer, as in 2-10 minutes.

By the way, had the OL relay been set for 114A, the 285A would have been 250%, so the trip time would have been 15-30 seconds on that same curve, so by setting it too high, you doubled the amount of time it took to trip.

 

[petersonra]

Yes... sorry I thought that was obvious.

One never knows here

 

[Dennis Alwon]

The 150A breaker for the starter is likely a “MCP” aka Instantaneous Trip (only) type breaker, so it only protects against a dead short at 10-17x the FLA. so unless the current gets to 1500A or more, it’s not going to trip.

It is supposed to be the OL relay that trips on thermal overload. That uses an “I2t” trip curve, typically based on what’s called Class 10 on pumps. So Class 10 means it has to trip in 10 seconds at 600% of the setting, 2 hours at 125% and a parabolic curve between those two values. So if set at 130A, 285A represents a value of 219% of the setting. Looking at a typical I2t curve, the trip time could be anywhere from 30-50 seconds. But if the current dropped as resistance increases for some reason, it could take even longer. Likewise for the 200A breaker and fuses, it’s a similar curve but 285A is only 142%, so the trip time would be significantly longer, as in 2-10 minutes.

By the way, had the OL relay been set for 114A, the 285A would have been 250%, so the trip time would have been 15-30 seconds on that same curve, so by setting it too high, you doubled the amount of time it took to trip.

He stated the was 3 hours with the condition of 285 amps. Something ain't right- lol

 

[tom baker]

I’ve designed installed and worked on water pump stations for most of my 40 year career. I my recommendations are get the nameplate data for the motors and the pumps. If there’s submersibles there’s typically a class 10 overload. Allen Bradley overloads often have small dip switches on the end so get the instructions for the overloads and go through and set the overload according to the motor nameplate data. It’s possible that you have the wrong setting on the older loads for example maybe a class 30 instead of class 10 or 20. It’s not unusual for overload settings to be incorrect when a contractor does it they get in a hurry so it’s always a good idea to verify the settings. Also if the motors have MCPS breakersmake sure those have the right settings . MCPs typically have a small dial that you set the overload trip point on. Dials tend to be adjusted by somebody with a screwdriver that doesn’t know what they’re doing. Check the settings on all pumps

 

[petersonra]

My experience is that most of the time nobody bothers to set the dials at all unless something trips and then they open them up all the way.