grounding medium voltage motors

[cmelguet]

Hi all,

I have two underflow pumps (552 kW each, 4.16 kV) for the thickeners in a mining project. For this size of motors I normally use a two 2/0 AWG bare copper cable to ground the motor directly to the earthing grid. In this case we dont have a grouding grid under the motors because of possible leaks from the tanks. Civil enginering doesnt want any perforation on the underflow tunnels. So the solution to ground the motors is to run a 2/0 awg bare copper cables on a cable ladder to the grounding grid which is outside of the tunnel.

Is this correct? My understanding is that we ground the motors locally to have an equipotencial bonding between all metal structures in the area. Also in case there is a electrical fault on the motor, and someone is standing next to it, we can control the mesh and step voltages that person will be subject to.

Please I want some comments on the design. Attached a file with the propose solution.

Regards,
cmed

 

[dpc]

I don't think that approach will accomplish too much when you consider the impedance of the grounding path you are creating. You should certainly run grounding conductors with the main phase conductors to the motor. Regardless of the connection to the ground grid, the motor should be well-bonded to any metal nearby.

 

[cmelguet]

Hi dpc, thanks for the feedback. We are running a grounding conductor next to the phase conductors. I was w

http://www.eng-tips.com/orry

about the bonding of the motor to the groundgrid.

regards

 

[waross]

You may want to consider installing a 4" x 4" or 6" x 6" welded, galvanized mesh on the ground so that a worker must be standing on the mesh in order to touch any part of the motor or pump. This will create an equi-potential grid so that workers touching the motor or pump will be at the same potential as the motor. A conductor must be run from the motor to the mesh and not continue any further. DO NOT run the conductor from the ground grid to the mesh and then to the motor. Two separate 2/0 cables from the motor to the mesh, so that if one becomes damaged the other will still provide protection. THE ONLY CONNECTIONS to the mesh are to be THESE TWO CABLES. Some-one who doesn't know better always thinks that they know better and wants to run from the ground grid to the mesh and then to the motor. Don't.
This is similar to the mats installed below manually operated high voltage distribution switches to provide protection for the line-men operating the switch handle.

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

 

[cmelguet]

Hi Waross

I understand the purpose of the mesh platform next to the motor, but why you dont recommend connecting the mesh to the ground grid and from the mesh to the motor?

regards

 

[waross]

If you connect to the mesh and then to the motor, there will be current in the conductors between the mesh and the motor in the event of a fault. Current means voltage drop. When the current is at fault levels that voltage drop may cause unacceptable touch voltages. In the event that the fault current "blows" a connection open, there may easily be lethal touch voltages and a workers body may become part of the grounding path.

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