Motor Short Circuit Ground Fault Protection

[mbrooke]

This reads as if Paul and some of the people responding to him are talking about 2 different things. Isn't the Ground Fault Protection of the circuit overcurrent protection named in relation to the possibility of a hard fault rather than a much lesser level of current LEAKAGE?

--
Tom Horne

Yes!

 

[paulengr]

This reads as if Paul and some of the people responding to him are talking about 2 different things. Isn't the Ground Fault Protection of the circuit overcurrent protection named in relation to the possibility of a hard fault rather than a much lesser level of current LEAKAGE?

--
Tom Horne

Yes but it’s not easy to detect.

Bender and MegAlert makes offline ground fault relays that activate and start checking when the motor is offline. It’s essentially an automated “Megger”.

Online you can use a digital meter to calculate I1+I2+I3 in vector form. It is subject to the errors of all three CTs and noisy but usually set to 10-20% of FLA.
G
Another approach is to use a single C.T. on all three phase legs. It works OK but if your FLA is say 100 A and you are looking for currents in the range of 5-10 A. That means a very low C.T. ratio or none at all but measurement gets pretty error prone. The other approach is the high impedance approach so say using a 20:0.025 C.T. rather than the traditional 1 or 5 A. This can be fairly accurate but still measures Amos if ground fault currents, not milliamperes. This is much more reliable and

 

[hornetd]

Yes but it’s not easy to detect.

Bender and MegAlert makes offline ground fault relays that activate and start checking when the motor is offline. It’s essentially an automated “Megger”.

Online you can use a digital meter to calculate I1+I2+I3 in vector form. It is subject to the errors of all three CTs and noisy but usually set to 10-20% of FLA.
G
Another approach is to use a single C.T. on all three phase legs. It works OK but if your FLA is say 100 A and you are looking for currents in the range of 5-10 A. That means a very low C.T. ratio or none at all but measurement gets pretty error prone. The other approach is the high impedance approach so say using a 20:0.025 C.T. rather than the traditional 1 or 5 A. This can be fairly accurate but still measures Amos if ground fault currents, not milliamperes. This is much more reliable and

paulengr

I still think that you are misunderstanding the intent of the language of the code. In the cited context "Ground Fault Protection" is the same ground fault protection which any circuit breaker, fuse, or other Over Current Protective Device (OCPD) provides when an energized conductor faults to ground rather than when there is a current leakage to ground that could be small enough so as not to melt the link of the fuse nor trip the circuit breaker. What I suspect that the code making panel was saying was that the overcurrent protection of the circuit conductors need not protect them from overload but only against a hard fault; or, if you would prefer to think of it that way, a short circuit which is using the Ground Fault Current Path as one of the shorted circuit conductors.

 

[paulengr]

paulengr

I still think that you are misunderstanding the intent of the language of the code. In the cited context "Ground Fault Protection" is the same ground fault protection which any circuit breaker, fuse, or other Over Current Protective Device (OCPD) provides when an energized conductor faults to ground rather than when there is a current leakage to ground that could be small enough so as not to melt the link of the fuse nor trip the circuit breaker. What I suspect that the code making panel was saying was that the overcurrent protection of the circuit conductors need not protect them from overload but only against a hard fault; or, if you would prefer to think of it that way, a short circuit which is using the Ground Fault Current Path as one of the shorted circuit conductors.

I think you are missing what happens with article 430 and ground faults in general.

430 protection generally has two devices (MMS and a few others excepted). The fuse/breaker/MCP/MMS is responsible for short circuits. It is generally set to somewhere between 10x and 17x FLA. Low peak fuses are relatively “tight” while breakers are not. If a short circuit is close, say 20x FLA, the short circuit protection works fine on a phase to phase fault. But assuming very good grounding, ground fault in a solidly grounded neutral will be 58% of that or 11.5x FLA. That is well below an MCP set to 13-17x FLA to avoid inrush. Not only that but the most common ground fault, damage to the ground wall insulation of the motor itself, sees significant impedance in the undamaged turns, rendering ground faults well below the trip range of the OCPD.

This is why the overload relay, not the OCPD catches most motor circuit ground faults.

 

[mbrooke]

paulengr

I still think that you are misunderstanding the intent of the language of the code. In the cited context "Ground Fault Protection" is the same ground fault protection which any circuit breaker, fuse, or other Over Current Protective Device (OCPD) provides when an energized conductor faults to ground rather than when there is a current leakage to ground that could be small enough so as not to melt the link of the fuse nor trip the circuit breaker. What I suspect that the code making panel was saying was that the overcurrent protection of the circuit conductors need not protect them from overload but only against a hard fault; or, if you would prefer to think of it that way, a short circuit which is using the Ground Fault Current Path as one of the shorted circuit conductors.

I think this comes from the fact that people don't know that GFCIs were originally put into the code to deal with 2 prong tools, where an internal ground fault often resulted in bad outcomes.

 

[hornetd]

I think you are missing what happens with article 430 and ground faults in general.

430 protection generally has two devices (MMS and a few others excepted). The fuse/breaker/MCP/MMS is responsible for short circuits. It is generally set to somewhere between 10x and 17x FLA. Low peak fuses are relatively “tight” while breakers are not. If a short circuit is close, say 20x FLA, the short circuit protection works fine on a phase to phase fault. But assuming very good grounding, ground fault in a solidly grounded neutral will be 58% of that or 11.5x FLA. That is well below an MCP set to 13-17x FLA to avoid inrush. Not only that but the most common ground fault, damage to the ground wall insulation of the motor itself, sees significant impedance in the undamaged turns, rendering ground faults well below the trip range of the OCPD.

This is why the overload relay, not the OCPD catches most motor circuit ground faults.

I am not arguing that what you are saying is wrong. Especially since I lack the engineering education to evaluate that. I am assuming that what you are saying is correct. That said I think that you are arguing best practice or some standard required approach. None of that effects what the US NEC actually requires whether that is good practice or not. We've all had the boiler plate quoted to us at one time or another but here it goes again.

90.1 Purpose.
(C) Intention. This Code is not intended as a design specification or an instruction manual for untrained persons.

What I and others have tried to say is that the US NEC does not require any ground leakage protection in these circuits. It does not matter one whit which system of device would be best for providing that protection if the US NEC does not require Ground Fault Protection of Equipment it is far less likely to be installed. You can continue to argue what you believe the likely response of various kinds of protection would be when a ground fault occurs but that isn't the point. The NEC requires circuit overcurrent protection on the supply conductors to protect against Short Circuits and Ground Faults. There is no requirement that you have as yet pointed out for protection from current leakage to ground that is below the trip point of the breaker or the melting point of the fuse link. If such a requirement is actually in the NEC point that out and we will all learn something. Arguing the quality of an orange against an apple is a hell of a way to waste time and it accomplishes nothing. I'm Clear of this discussion.

 

[kwired]

I think you are missing what happens with article 430 and ground faults in general.

430 protection generally has two devices (MMS and a few others excepted). The fuse/breaker/MCP/MMS is responsible for short circuits. It is generally set to somewhere between 10x and 17x FLA. Low peak fuses are relatively “tight” while breakers are not. If a short circuit is close, say 20x FLA, the short circuit protection works fine on a phase to phase fault. But assuming very good grounding, ground fault in a solidly grounded neutral will be 58% of that or 11.5x FLA. That is well below an MCP set to 13-17x FLA to avoid inrush. Not only that but the most common ground fault, damage to the ground wall insulation of the motor itself, sees significant impedance in the undamaged turns, rendering ground faults well below the trip range of the OCPD.

This is why the overload relay, not the OCPD catches most motor circuit ground faults.

Probably true for MCP's or other instantaneous trip devices sized at/near max allowed for the conditions.

My experience is mostly with inverse time breakers or standard time delay fuses, either of those usually catch ground faults before the motor overload responds.

 

[ptonsparky]

Probably true for MCP's or other instantaneous trip devices sized at/near max allowed for the conditions.

My experience is mostly with inverse time breakers or standard time delay fuses, either of those usually catch ground faults before the motor overload responds.

My experience is that the overloads were reset so many times, the breaker or fuses finally had to respond.

 

[mbrooke]

What is the current draw of a faulted stator?

 

[ptonsparky]

What is the current draw of a faulted stator?

Did you include enough information somewhere in this thread to determine that? Or is ‘all it possibly can’ the correct answer?

What is the color theme of our Guest Room?

 

[kwired]

What is the current draw of a faulted stator?

I'm guessing you referring to one with a ground fault. I would think that depends on where in the winding the fault occurs.

With a fault near a supply end of the coil there isn't much impedance and fault would be much more like a fault in the supply conductor.

Take a 240 volt coil operating on grounded 120/240 and fault it at the exact midpoint, you may not even know you have a fault.

Anywhere between would depend on how much impedance is in series between the supply conductor and the fault point.

 

[topgone]

I'm guessing you referring to one with a ground fault. I would think that depends on where in the winding the fault occurs.

With a fault near a supply end of the coil there isn't much impedance and fault would be much more like a fault in the supply conductor.

Take a 240 volt coil operating on grounded 120/240 and fault it at the exact midpoint, you may not even know you have a fault.

Anywhere between would depend on how much impedance is in series between the supply conductor and the fault point.

Methinks it is safer to assume a ground fault at the motor's coil end. The current would be lower if we choose that location. We will be setting the ground fault slightly higher than that ground fault level and avoid unwanted fires in the area!

 

[kwired]

Methinks it is safer to assume a ground fault at the motor's coil end. The current would be lower if we choose that location. We will be setting the ground fault slightly higher than that ground fault level and avoid unwanted fires in the area!

The question was what is current draw of a faulted stator. My answer was I believe it depends where in the coil the fault occurs. Plus on top of that it still can depend on available fault current from the supply conductors at that point in the circuit.

 

[mbrooke]

I'm guessing you referring to one with a ground fault. I would think that depends on where in the winding the fault occurs.

With a fault near a supply end of the coil there isn't much impedance and fault would be much more like a fault in the supply conductor.

Take a 240 volt coil operating on grounded 120/240 and fault it at the exact midpoint, you may not even know you have a fault.

Anywhere between would depend on how much impedance is in series between the supply conductor and the fault point.

True, but the shorted windings will turn into a transformer where large current will flow until the stator starts to burn up.

 

[mbrooke]

Did you include enough information somewhere in this thread to determine that? Or is ‘all it possibly can’ the correct answer?

What is the color theme of our Guest Room?

Typically when no value is given I assume infinite source.

The walls are egg shell white, the rest is light colors with biscuit furniture from the 80s. I know, that room needs a change.

 

[paulengr]

Depends on failure mode. See this for examples.

Typical Failures in Three-Phase Stator Windings — Alexandria Armature Works

Good stator winding Unfavorable operating conditions—electrical, mechanical or environmental—can dramatically shorten the life of a three-phase stator winding. The winding failures illustrated below typify what can happen in such circumstances. They are shown here to help you identify the causes of

www.aawva.com

There is an outer ground wall insulation and separate winding insulation. In say an overload you get uniform cooking. Turn failures happen first but the heat at the turn to turn short cooks the ground wall insulation and burns through. Same with contamination, moisture damage, and single phasing or unbalanced voltages. The final failure is typically in the highest temperature area, deep in the slot. Readily detected by ground fault sensing but without that overload trips first. On the ends the short circuit protection will trigger eventually.

A motor destroyed by say surges for instance will short out the first few turns but the slot wiring is intact. Rarely shows ground fault, trips overload or short circuit.

 

[kwired]

True, but the shorted windings will turn into a transformer where large current will flow until the stator starts to burn up.

But won't the amount of current drawn (which you asked about) still depend on where in the coil the fault occurs? Higher the fault current the more likely it will take out the SC/GF protection. As far as further damaging the winding - most the time motor needs rewinding anyway even if damaged was minimized by fast overcurrent device response.

 

[kwired]

But won't the amount of current drawn (which you asked about) still depend on where in the coil the fault occurs? Higher the fault current the more likely it will take out the SC/GF protection. As far as further damaging the winding - most the time motor needs rewinding anyway even if damaged was minimized by fast overcurrent device response.

ADD (too late to edit): The SC/GF protection isn't there to protect the motor, it is there to keep the branch circuit conductors from overheating and as a result introducing fire and shock hazards if the fault were to persist.

 

[paulengr]

ADD (too late to edit): The SC/GF protection isn't there to protect the motor, it is there to keep the branch circuit conductors from overheating and as a result introducing fire and shock hazards if the fault were to persist.

Also to protect upstream systems (coordination).

As an example about 8 years ago a liar Ge mine had 5 kilns which were being operated in a way where if they shut down suddenly they were in danger of sintering the product and turning the.entire kiln into a brick, causing massive downtime and expense to clean it out. They had all kinds of redundancy, redundant fans, redundant ductwork, two discharge conveyors, you name it

So one day one of the kilns is down for regular routine maintenance. The other four are running. A small 5 HP motor in the kiln that is down develops a fault. It should trip the MCP but the MCP is so old it doesn’t. So instead the main MCC breaker trips. Now it so happens one of the discharge conveyor starters is in this MCC and control power for a second kiln. This shuts down both. It gets better. Control power for some isolation relays used for interlocks one the second discharge conveyor are also on this same MCC. So when the main tripped it took down another kiln outright and disabled discharging on the second discharge conveyor, effectively locking up the whole operation. The MCP is just the start of a bunch of 1965 vintage molded case breakers we found defective and no longer tripping.

So yes, a single small motor can cause major issues just from not tripping.

 

[mbrooke]

But won't the amount of current drawn (which you asked about) still depend on where in the coil the fault occurs? Higher the fault current the more likely it will take out the SC/GF protection. As far as further damaging the winding - most the time motor needs rewinding anyway even if damaged was minimized by fast overcurrent device response.

Yes, you're right about this.