Equipment Grounding Conductor Vs Frame Grounding 2

[NickParker]

We have already grounded the Motor through the cables supplying the motor, which is Equipment Grounding conductor(EGC) and the size of the grounding conductor is based on NEC 250.122
My question is, Which code governs the sizing of frame grounding of the motor to the grounding grid?(this is in addition to the equipment grounding conductor)

I ask this question because we propose the same size conductor as EGC, for frame grounding, but the engineer proposes a larger size than the EGC.

 

[waross]

Consider a solidly grounded system.
The motor is fed by a cable with a grounding conductor included with the phase conductors.
The motor faults to ground internally.
The current path is from the supply to the faulted motor and back to the supply via the grounding conductor.
The phase conductor and the grounding conductor are of similar impedance and form a voltage divider.
For the duration of the fault, there may be a voltage rise to about one-half of line to neutral voltage on the motor frame.
Some industry codes and some engineering standards call for a much heavier grounding conductor than the code minimum to reduce possible touch voltages on skids and motor frames.
In the petrochemical industry it is common to see specs calling for grounding conductors larger than the phase conductors.
In the petrochemical industry there is a concern that the possible voltage rise on the frame of a faulted motor may cause sparking that may lead to fires or explosions in a hazardous area.
Trust your engineer. The code minimum will trip the breaker, but the code minimum is not always adequate for life safety and property protection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Power0020]

Would the answer be local grounding of ground continuity conductor near to the equipment? this will create a parallel path to source reducing the impedance considerable (given it is a good soil around, low rod resistance)

 

[waross]

That is a valid question Power 0020.
The answer to that depends on specific site conditions and possible codes in addition to the basic electrical code.
Ground conductivity may change with the weather.
Frozen ground is a poor conductor,
A hot, dry spell may raise the ground impedance.
In some instances a local ground may give rise to touch and step potentials under fault conditions.
Despite that, there may be specific, isolated skid mounted equipment where local grounding may be deemed to be adequate.
Such an exception would be rare in the petrochemical industry.
Any governing standards such as the API code, insurance underwriters and/or plant or industry engineering practice should be followed.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[buzzp]

The idea is to have a low impedance path back to the source. Using the earth as the return path for the bulk of the fault current is not a good idea nor is it allowed. Yes, there will be current flowing there in the event of a fault, but it should be minimal as compared to the current flowing in the EGC. In fact, using only the earth could in fact cause the breaker to fail to operate since the current may not raise to an adequate level to cause the breaker to trip.