Breaker did not trip

[Designer_82]

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

https://ibb.co/j8NvjP0

 

[dpc]

"A short that made contact with the water" - not sure what you are describing. If the cable insulation was damaged, causing bare conductor to be exposed and this conductor contacted water, then you might have a short circuit. But this is presumably in the cable that feeds the motor, so the motor would not have be subjected to any fault current.

If one phase was lost to the motor, the resulting loss of phase could have destroyed the motor if this condition was not detected by the overload relay. The breaker is there primarily for short circuit protection.

 

[LionelHutz]

Did anything trip? How did the cable damage and the cable conducting current into the water destroy the motor?

 

[itsmoked]

Nicked cable conveyed water into the motor?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[FacEngrPE]

If the cable damage resulted in one phase carrying a significantly different current than the other two, you can get significant motor heating without having the indications of overload at the breaker. The only way to provide protection I am aware of for this would be a ANSI No, 43 Phase-Balance Current Relay, which I have never actually seen applied as a motor protector.

 

[itsmoked]

Well, or any current balance based phase protection module.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Did the grounded motor housing become one electrode of a liquid rheostat? (The nicked cable being the other electrode.)


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Designer_82]

Thanks for your feedback. I'm wondering if there is an issue with the overload relay. Would the overload relay be provided with the starter by the manufacturer and set accordingly or by the electrical contractor?

 

[RRaghunath]

The reason for breaker not tripping on cable fault was because the current was below 750A (MCCB setting for short circuit) considering its is a single phase-to-ground fault.
Does the MCCB not have separate ground fault protection?? If yes, what are the settings? I suppose it is this protection which should have acted.
Other than that, I don't see any problem with the settings of MCCB you indicated.
With regard to motor failure, rotor of the induction motor gets severely (thermally) stressed by negative phase sequence currents when only two phases (out of three) are being supplied to motor. I suppose that is the case here.
Traditional electromechanical overload relays have differential bar mechanism that opens the contactor quickly under single phasing.
What is the type of overload relay here??

 

[che12345]

Dear Mr. Designer_82 (Mechanical)(OP)11 Nov 21 18:55
" .... 1) a 50 HP submersible pump motor with a reduced voltage starter. 2) protected by overload relay at the starter and MCCB at the main panel. 3) 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. 4) The breaker did not trip.... why? 5) Pic below of the breaker settings "
Please furnish more detail
1) what is the source [voltage and frequency] ? Which type of reduced voltage starter e.g. SD, AT, soft-starter ... etc.
2) what is the i)over-load relay; ii) MCCB , [current settings]?
3) i) please explain how can the (nick on the cable) [destroyed the motor?]. ii)Does the cable nick lead to water leaking into the motor?
4) The breaker would NOT trip as long as the line currents are = or < the setting; even when there is a fault or the motor is completely destroyed, e.g. high resistance fault. This also applies to the over-load. These are current monitoring devices.
5) what is the breaker current setting?
6) the MCCB and the over-load did not trip, but motor stopped?
7) It is likely that (water seeped into the motor that caused (low insulation resistance between windings or to the earth); that result to [high resistance fault], where the fault current is = or < then the MCCB and the over-load current settings.
Che Kuan Yau (Singapore)

 

[waross]

Without complete information we can keep guessing all day.
Submersible motor designs don't have the same heat rejection characteristics as air cooled motors.
What does "nicked" mean? was the cable skinned but substantially intact as a conductor or was one conductor cut through?
I am unable to zoom your link. What voltage is the motor?
There is a good chance that a fault through the sea water may not draw enough current to trip the breaker.
The current through fresh water would be even less.
Doing a field load est on a generator, using 50 gallon plastic barrels and salt water I had trouble maintaining 200 Amps.
You may have had much less current. (I poured a lot of salt into the barrels while watching an ammeter.)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

I believe a Micrologic 3.2S trip unit is a LSI unit, not a LSIG so no ground trip.

If the cable damage resulted in one phase carrying a significantly different current than the other two, you can get significant motor heating without having the indications of overload at the breaker. The only way to provide protection I am aware of for this would be a ANSI No, 43 Phase-Balance Current Relay, which I have never actually seen applied as a motor protector.

A nick in the insulation would cause current flow to ground so the currents at the source will be imbalanced with one phase being high. But, the current flow to the motor would not be affected by this very much. Every soft-starter we sell has phase imbalance protection and the overload is biased via the current imbalance so it's not that rare to have protecting a motor.

Thanks for your feedback. I'm wondering if there is an issue with the overload relay. Would the overload relay be provided with the starter by the manufacturer and set accordingly or by the electrical contractor?

Who knows, we don't know the relay model and we don't know what your contractor did. I would give a high probability it was provided by the manufacturer but I doubt they set it. That is the installers job.

As Bill just stated, we could keep guessing all day but not enough information has been supplied for anyone to answer your questions.

 

[Designer_82]

Motors are 460-3Ph 60Hz.

I did not check if the overloads are set to at least 1.25% of the motor FLA. (1.15 service factor motor)

Would the breakers be required to be LSIG? The engineer shows only LSI on the drawings.

 

[che12345]

Dear Mr. Designer_82 (Mechanical)(OP)12 Nov 21 18:11
" .... 1)Motors are 460-3Ph 60Hz. 2) I did not check if the overloads are set to at least 1.25% of the motor FLA. (1.15 service factor motor). ... 3)Would the breakers be required to be LSIG? The engineer shows only LSI on the drawings ..."
A. Please refer to my earlier post (12 Nov 21). Without clear detail, any proposal may not be inaccurate/misleading.
1) motors are 460V-3Ph 60Hz is fine. If possible please also state the rated current per the name-plate.
2) Please tell us what type of reduced-voltage starting (e.g. SD, AT, soft-starter etc). The over-load setting is dependent on the type of stating/starter. BTW: the overloads is NOT necessary set to at least 1.25% of the motor FLA. Service factor 1.15 is NOT a critical factor in the over-load setting.
3) Check up the NEC or any other code/regulation in your location. In most locations, LSI is acceptable. Though LSIG is (generally not mandatary) but useful on detecting any earth-faults which are generally far much lower than the breaker current setting. It adds some cost, but not very much if you had chosen it at the beginning.
Che Kuan Yau (Singapore)