Where should 3ph motorÆs body be grounded?

[hassankh]

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

 

[dpc]

You can do both, but the equipment grounding conductor is required since it represents a low impedance path for fault current.

 

[nkn5]

First is TN-S System and second is TT System. So it depends on which earthing system your electrical network uses.

 

[squeeky]

Dear nkn5. I had a look at the definitions before answering this. The type selection of your earthing seems to be tidied very much with the tranformer side and the earthing to the consumers earth terminal and not within the installation. TT Systems in South Africa are deemed to be impractical in most parts due to high soils resistivities.This backs up the initial statement. TN-S brings the utility earth into the consumers eearth terminal. "All exposed conductive parts of a consumer's installation are connected to the protecte conductor via the supply earth terminal.

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

 

[dpc]

In the US it is fairly uncommon (and not required) to find ground fault protection on a typical motor feeder. Solidly-grounded wye systems are most common. When a ground fault occurs at the motor, the equipment grounding conductor (green wire) provides the necessary low impedance path for fault current to flow at sufficient magnitude to trip the phase overcurrent protection. Relying on an external ground connection to a plant grounding grid drastically increases the zero sequence impedance of the circuit, due to the increased inductance. Bonding the motor frame to the ground grid in addition to the equipment grounding conductor will not cause any problems and may provide additional shock hazard protection, but during a ground fault, virtually all of the fault current will flow in the ground wire and not through the ground grid.

The equipment grounding conductor is a legal requirement in the US.

 

[squeeky]

Thanks dpc. Legal requirement in South Africa are the same. Selection of the Earth Conductors are selected from a chart where the distance and fault current are known. This cross references to the safety earth conductor. My company requires that we bond everything. On LV we maximize the fault current to ensure the protection disconnects as fast as possible by ensuring the earth has multiple paths back to the star point.

 

[waross]

The equipment grounding conductor ensures fast tripping in the event of a ground fault in the motor. However, the supply conductor and the equipment grounding conductor form a voltage divider and a lethal voltage may be present on the motor frame until the protection clears the circuit.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

dpc,

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?

 

[dpc]

Scotty,

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.