Where should 3ph motorÆs body be grounded?
Where should 3ph motorÆs body be grounded?
(OP)
3ph motors usually have their metallic body grounded either to the PE conductor of their power supply or to the earthing system terminal locally established in the building.
I wonder which of the above two methods is the most appropriate?
Thanks in advance
I wonder which of the above two methods is the most appropriate?
Thanks in advance





RE: Where should 3ph motorÆs body be grounded?
RE: Where should 3ph motorÆs body be grounded?
RE: Where should 3ph motorÆs body be grounded?
Once within the consumers permisis where do we go from there? The earth mat / spikes are connected to the LV ostar point of the transformer. We have an earth bar that seperates the earthing system from the transformer earths and bonding earths by a removable link for test purposes. The earth then connects to the earth bar in the MCC via earth bonding conductors. The cables that go out to the motors have a seperate earth that goes back to the earth bar in the MCA. This is normally within the terminal box of the motor. The MCA earth is bonded to the MCC earth bar.
The motor is also bonded to the adjasent steel work. This is an external earthing point normally near the foot of the motor. The steelwork is bonded to the earth mat / spikes.
I welcome an input to this discussion
RE: Where should 3ph motorÆs body be grounded?
The equipment grounding conductor is a legal requirement in the US.
RE: Where should 3ph motorÆs body be grounded?
RE: Where should 3ph motorÆs body be grounded?
Grounding to the local ground grid forms an equi-potential zone and the touch voltage is much lower.
For long motor feeders, I prefer both types of motor grounding.
Bill
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"Why not the best?"
Jimmy Carter
RE: Where should 3ph motorÆs body be grounded?
Do you never find that larger starters without E/F protection tend to be slower to clear an earth fault than the E/F element of the MCC incomer, with the result being loss of the MCC?
RE: Where should 3ph motorÆs body be grounded?
Yes, that is a common problem in the US and not just for motors. Our code requires ground fault protection for any service 1000 A or larger (below 1000 V). This requirement was added due to fires caused by ground faults that did not clear. But there is no code requirement for GF protection below the main overcurrent device. Since the GF protection is delivered set at minimum, miscoordination for ground faults is pretty common.
Bill,
Of course the motor frame must be grounded regardless. I agree that bonding to the ground grid (if there is one) can reduce shock hazards.