Breaker did not trip

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designer82

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50 HP submersible pump motor with a reduced voltage starter.

protected by overload relay at the starter and MCCB at the main panel.

So apparently there was damage to the cable (it was nicked) which caused a short that made contact with the water and destroyed the motor.


The breaker did not trip. Wondering if anyone has any ideas why.

Pic below of the breaker settings

 
Was the housing of the pump well grounded? If not perhaps the leakage from one phase of the damaged cable through the water elevated the voltage of the housing relative to one of the other phases. And that might have damaged some of the motor windings. Kind of a stretch but that's what I have.
Of course, there might have been a surge/overvoltage that was unrelated to the nicked cable and it caused the failure.
Have any electrical measurements or tear-down of the pump been done to help determine the failure mechanism?
 
synchro said:
Was the housing of the pump well grounded? If not perhaps the leakage from one phase of the damaged cable through the water elevated the voltage of the housing relative to one of the other phases. And that might have damaged some of the motor windings. Kind of a stretch but that's what I have.
Of course, there might have been a surge/overvoltage that was unrelated to the nicked cable and it caused the failure.
Have any electrical measurements or tear-down of the pump been done to help determine the failure mechanism?
Click to expand...
They're going to tear down the pump to try to figure out the root cause.
 
The breaker will not trip unless there is enough current for a long enough time to make it trip. It can often carry 150% almost indefinitely.

In general, the breaker is there for short circuit conditions and the overload for overcurrent conditions.

My guess is the nicked wire did not cause the motor to fail. I would guess more likely some kind of surge.

But, without knowing more it is hard to say much of anything with any degree of certainty.

Possibly a brownout caused motor to overload and the overload relay setting was not appropriate so did not shut it down.
 
This is far out, but we have had water wick down inside the cable into the motor causing enough current to do damage, yet not trip the c.b. This was typically in sewer/storm lift stations where the explosion proof seal was too much trouble to deal with; so there were flying splices near the top.
 
designer82 said:
Is that basically an open phase condition? Also, the breaker would not open during open phase?

Lastly, it can be enough to ruin a motor?
Click to expand...

Yes, loss of phase.

It will definitely toast a motor if the condition persists long enough - how long depends on the load.

A breaker sized for short circuit protection will likely not trip on a loss of phase condition.
 
I'm wondering if there is an issue with the overload relay. I didn't think to check that is was set properly. I just checked the LSI breaker settings.
Would the starter cabinet vendor be responsible for providing and setting the overload relay per the motor requirements or would the electrical contractor do this?
 
designer82 said:
I'm wondering if there is an issue with the overload relay. I didn't think to check that is was set properly. I just checked the LSI breaker settings.
Would the starter cabinet vendor be responsible for providing and setting the overload relay per the motor requirements or would the electrical contractor do this?
Click to expand...
This would need to be set based on the nameplate of the motor so typically it would be the responsibility of the installer
 
designer82 said:
50 HP submersible pump motor with a reduced voltage starter.

protected by overload relay at the starter and MCCB at the main panel.

So apparently there was damage to the cable (it was nicked) which caused a short that made contact with the water and destroyed the motor.


The breaker did not trip. Wondering if anyone has any ideas why.

Pic below of the breaker settings

Click to expand...
Why did you expect the circuit breaker to trip? As you have explained, one of the supply cables was nicked and open-circuited. A single-phasing supply results into "overloading" of the remaining two lines by about 2.4 times its normal load amps! The breaker mostly likely won't trip as you have chosen 250% factor for breaker sizing (CB P35 is set at 150A). You should instead inspect why your thermal overload relay OL-P35, why it didn't catch the problem when it was set at 85A (140%)
 
topgone said:
Why did you expect the circuit breaker to trip? As you have explained, one of the supply cables was nicked and open-circuited. A single-phasing supply results into "overloading" of the remaining two lines by about 2.4 times its normal load amps! The breaker mostly likely won't trip as you have chosen 250% factor for breaker sizing (CB P35 is set at 150A). You should instead inspect why your thermal overload relay OL-P35, why it didn't catch the problem when it was set at 85A (140%)
Click to expand...

A standard class 20 overload relay will not trip fast enough to save a motor attempting to start while missing a phase.
 
retirede said:
A standard class 20 overload relay will not trip fast enough to save a motor attempting to start while missing a phase.
Click to expand...
Whatever class you have there, if the TCCs tell you the OLR/ CB will respond to an overcurrent, it would trip at a slight time difference from published figures.
Back to the topic, the CB-P35 is set at 250% (60*2.5=150A). Granting the motor experienced single-phasing, the overload current will be = 2.4 X 60 = 144A! The CB-P35 won't trip as the setting is 150A. What should trip will be the OL-P35 which set at 85A.
 
Retrofit a Symcom 777 or M8000 electronic overload. It will sense single phasing among other problems and shut the pump down.

a merger test on the sub wire leads will find a bad wire. I normally Meghan the wires at the control panel, if bad we move to the top of the well and Megg again. If it’s still bad the pump is pulled and the motor leads are megged. A single phased motor can megg good, but be ruined. But a Meg get test will definitely find that nicked wire under wster.
 
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