Motor Grounding

[cmelguet]

Hi, If I have a 3 phase induction motor grounded through a 3c+E cable connected to the terminal box of the motor. Do I need to connect the motor frame also to a ground grid in the motor area? Isnt the motor already grounded through the cable that provide the power to the motor?


Thanks. Regards.

 

[ScottyUK]

You don't state the voltage you are operating at. If an HV motor then you run the risk of introducing a dangerous transfer voltage if a fault occurs on the HV system.

You don't state where you are but many codes require local equipotential bonding for LV installations. You need to check the regulations for your country.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

Some industries such as the petro-chemical industry operate under industry specific codes which may require motor grounding methods in addition to electrical code grounds.

Bill
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"Why not the best?"
Jimmy Carter

 

[electricpete]

I remember reading somewhere that motor frames should be grounded at 2 diagonally opposite corners. I wish I could remember where... Now it's going to bug me until I figure out where I read that.

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(2B)+(2B)' ?

 

[electricpete]

It obviously was not NEC - perhaps something limited to the power industry (?)

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(2B)+(2B)' ?

 

[rockman7892]

Scotty

Is the "transfer voltage" you are referring to the potential that exists on the motor case/frame to ground if an internal winding fault occured? Wouldn't this voltage be cleared by the equpment ground conductor connected to the source?

 

[rbulsara]

It depends on the local conditions as well. Beside providing a path for ground current, bonding of non-current carrying metallic parts in a area is also necessary to create an equi-potential plane within reach of a person's arm spread or steps. Plus on larger equipment you do not want to rely on only one connection for safety.

I would not argue against too much grounding or established safety practices in your work place, which may appear to exceed minimum code requirements.

Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

Hi rockman,

If an HV substation suffers an earth fault then the substation earthing system and the entire substation will rise above the potential of the general mass of earth due to the earth current passing through the resistance of the earthing electrode system. The potential of anything connected to that earthing system will also rise above that of a 'remote' earth, where 'remote' is far enough away to be outside the sphere of influence of the substation earthing system. If equipment isn't locally bonded to earth then it can rise to a dangerous potential relative to other structures and equipment in the vicinity. Transfer voltages are often more severe than touch or step voltages in the immediate area of the substation.


ePete,

The diagonally opposite earthing connections quite a common requirement in both power and oil industries in the UK. It's certainly typical practice for MV and big LV machines, maybe less so for small LV ones.


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If we learn from our mistakes I'm getting a great education!

 

[rockman7892]

Scotty

Are the additional grounds or bonds to earth only for HV motors and equipment. I have seen some facilities get carried away with this and spend lots of money on providing earth connections on every skid and piece of equipment even with small 480V motors.

I'm not saying this additional earthing is a waste or should be removed but obviouly it is costly and time consuming so I was wondering where and how you draw the line to provide reasonable but safe installations?

 

[waross]

Consider a typical motor in an oil refinery. The motor may be several hundred feet from the MCC. For the sake of simplicity let's say that two phases have burned free and one phase is shorted to ground. The motor is fed with Teck cable. (A type of armored cable in common use in Canada.) With code grounding, the current will flow through one phase conductor and return through the grounding conductor. In Teck cable and also in several other power cable constructions the ground conductor is two gage sizes smaller than the power conductors. As a result, the voltage to ground on the motor frame may rise to over 50% of line to neutral voltage. There will probably be a parallel ground current path through piping and structures but this path may include high impedance areas where a high voltage drop may exist or heat and/or sparks may develop. Heat and sparks in unexpected places are never a good thing in an oil refinery. Hence the requirement in the API codes for supplementary motor grounding conductors much larger than the NEC requires.

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Hi rockman,

I'd say it's frequently a necessity for HV motor installations to stay within acceptable step/touch/transfer voltage limits, and as Bill says it's good practice / industry requirement in high hazard environments like refineries, power plants, and chemical works.

For LV a lot depends on your code requirements - the UK bonding requirements are pretty strict and at times can feel like overkill, but they are the result of the collusion of some very knowledgable people - easier to comply with the Regs than fight them!


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If we learn from our mistakes I'm getting a great education!

 

[Kiribanda]

cmelguet,
1)The PEN conductor per IEC terms or the bonding conductor per NEC terms,running along with the motor feeder cable provides a low impedance path for the ground fault return current (if properly sized)thereby ensuring timely operation of ground fault protection.Therefore it is the system ground.It has nothing to do with the safety ground mentioned in (2)below.
2)All exposed conductive parts such as a motor frame should be grounded with a low impedance ground electrode,to ensure no over voltages are present on the enclosure during a ground fault or due to lightning etc etc.This is the safety ground and it is not sized according to the available ground fault at the location.
Therefore,whether you are from IEC or ANSI world,and whether your network is high resistance or low resistance gorunded,you should provide both system ground and safety grounds to your motor.

 

[rockman7892]

Scotty

I am in NA so I am regulated by the NEC. As I saw you had pointed out in the other thread what about concrete rebar acting as a grounding electrode? Is it possible that for a large motor sitting on a motor base that is embedded a reinfored rebar and concrete strudture that this connection through the concrete and rebar will provide an equalpotential plane so therefore no additional grouding is required? For instance in this case would there be an equal potential between the motor and the concrete where someone was standing by the motor? Is there a way to measure this accurately to make the determination if any additional gounding is needed.

I've always thought of an equipment grounding conductor as having the purpose of clearing a fault at the motor and never really gave much thought to a fault occuring at another location on the system. So if I understand what you are saying, it is that if I am standing at a motor that is remote from its substation and there is a fault elsewhere on this system then the ground or case of the motor can be elevated to a potential above the remote earth at the motor causing a dangerous potential for someone standing at the motor? Is this becasue the groud grid at the substation will be energized to the potential of the fault and therefore be the same potential as the local earth but not at the same potential at a remote earth?

 

[ScottyUK]

First paragraph - I'll let someone who knows NEC answer this one. But possibly yes if the resistance to earth is sufficiently low and you're allowed to take credit for the rebar and concrete.

Second paragraph - yes, that's about it. The GPR depends on how good the earthing system at the substation is: if it is in sand or rock then the voltage rise can be quite large. If it is a transmission sub then even a small percentage rise can result in disturbingly large transfer voltages appearing if care is not taken. Fortunately most transmission subs don't feed remote loads like motors, but it can cause problems in places like power plants if the GSU transformer is close to the generator and remote from the sub.

Have you ever noticed all the crushed rock around substations? It's not cosmetic - it allows a much higher step voltage to be tolerated than if it were bare earth.


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If we learn from our mistakes I'm getting a great education!

 

[cmelguet]

Can someone specify where in the NEC is stated that the motor should be grounded through a ground electrode.?

If we ground the motor frame through a ground electrode to control posible transfer potencial from the substation GPR, shouldnt we also ground with an electrode any metallic frame?

Thanks for the help.

 

[dpc]

As far as the NEC is concerned, the grounding conductor provided with the motor feeder is adequate. There is no requirement for additional grounding. And there must be a grounding conductor regardless of whatever other supplemental grounding methods are used.

It is customary, in my experience, to provide a direct connection (or two) from the motor frame to the local ground grid for medium-voltage motors. But I do not think it is ever required by the NEC.

The NEC does require that non-current carrying electrically conductive materials that are "likely to become energized" must be bonded to the ground system.

David Castor

http://www.cvoes.com

 

[cmelguet]

One more question, if we run an earth conductor to provided a low impedance return path for earth fault at the motor, arent we on purpose connecting a LV ground grid (at the motor location)with a MV ground grid (at the utility substation).

Do this mean i need to design the earth electrode, or grounding grid, at the motor location to disipate the total fault current available at the feeding substation? This could mean a quite big grounding grid if the feeding substation is a 15kV/400V 50 MVA substation.

 

[Skogsgurra]

No. It is the actual (or rather, the prospective) fault current at the actual location that you need to design for. And not even that, because most codes include wording like "fault must be cleared in x milliseconds" and it is usually not necessary to design for low fault voltage during that time.

But there are reasons not to let the fault voltage rise too much - bearings being damaged in the machines is one reason. That problem is usually coped with using bonding between motor frame and driven equipment.

Gunnar Englund

http://www.gke.org

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