Equipment Grounding Conductor Vs Frame Grounding
Equipment Grounding Conductor Vs Frame Grounding
(OP)
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.
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.






RE: Equipment Grounding Conductor Vs Frame Grounding
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
RE: Equipment Grounding Conductor Vs Frame Grounding
RE: Equipment Grounding Conductor Vs Frame Grounding
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
RE: Equipment Grounding Conductor Vs Frame Grounding