Motor "Equipment Grounding" question (one-line attached)

[tankfarms]

Recently, I noticed that certain motors in my plant do not have the "equipment grounding" (i.e. grounding electrode/cable connection between the motor frame and the earth ground); upon further looking in to the matter, I found that all motors like these are fed by transformer with a 3-phase, 4-wire system where the transformer neutral is grounded, and each motor has a grounding cable that is connected all the way back to the switchgear. I just checked the code and I want to make sure my interpretation of my field observation is correct. My understanding is that NEC (2011 edition) 250.24 permits the grounding electrode conductor to be connected at different locations, and in this case, it is the transformer (via switchgear), thus I don't see a grounding cable on the outside of the motor frame to ground. Is this understanding correct? A one-line is shown below is illustrate the existing setup.

 

[ron]

On the oneline it appears that the equipment ground conductor runs from the motor to its source and on up to its transformer. The transformer will take care of the connection to the grounding electrode system.

Only separately derived systems and electrodes are connected to the grounding electrode system directly.

 

[david luchini]

You are looking for equipment grounding for your motors, but you have referenced 250.24. That section is for grounding Service Supplied Systems. It would apply to your motors. It would appear your system is a separately derived system, and the grounding for that system would be per 250.30.

For grounding of the motor frame, you should be looking at 250.112(C), which says to connect the motor frame to an equipment grounding conductor, which as Ron pointed out, your motor circuits appear to contain. So you would be OK under 250.112(C).

As an aside, if your system is a Separately Derived System, the #2G with each set of 500MCM would seem to be too small per 250.30(A)(2), 250.102(C) & 250.66. From Table 250.66, you would need a #1/0AWG with each set of 500MCM.

 

[rbalex]

From your discription it appears you are expecting to see connections from the motor frame to auxiliary grounding electrodes. (Section 250.54) While they are OK, they are neither required nor sufficent by themselves - and never have been. Ron & David's comments are also valid. In reality, other than the questionable equipment grounding conductor sizing David mentioned, what you have have found is the proper equipment grounding method.

 

[tankfarms]

You are looking for equipment grounding for your motors, but you have referenced 250.24. That section is for grounding Service Supplied Systems. It would apply to your motors. It would appear your system is a separately derived system, and the grounding for that system would be per 250.30.

For grounding of the motor frame, you should be looking at 250.112(C), which says to connect the motor frame to an equipment grounding conductor, which as Ron pointed out, your motor circuits appear to contain. So you would be OK under 250.112(C).

As an aside, if your system is a Separately Derived System, the #2G with each set of 500MCM would seem to be too small per 250.30(A)(2), 250.102(C) & 250.66. From Table 250.66, you would need a #1/0AWG with each set of 500MCM.

Thanks. when you say "it would appear your system is a separately derived system..." how do we normally tell? Since I think we have to physically inspect if the motor's power wire neutral is directly connected to the system ground.

 

[rbalex]

See the definitions of Separately Derived System and Grounding Conductor, Equipment (EGC) in Article 100.

The EGC is not a neutral nor a grounded circuit conductor; it isn't intended to carry current under normal conditions - only ground fault conditions.

It is highly unlikely the motors have a neutral connection and the one-line supplied would indicate they don’t. See Section 430.12(E) for the appropriate connection means to the EGC that originates in the MCC.

 

[skeshesh]

I think David was referring to the neutral being derived at the secondary of the transformer you showed.

Please pay close attention to Bob's comment and references in the previous post. Understanding these concepts is very important and violating proper grounding, i.e. connecting neutral and ground at a motor, could have some pretty ugly results.