Medium voltage grounding

[cjhut]

Hello,
I currently have a 2,300v 250hp squirrel cage motor being fed from an across the line starter. No VFD's- nothing fancy. My company wants me to reroute the motor feed, which is now in steel rigid conduit running across the building structure to the motor. They want me to take it underground,because we are going to demolish the old structure that is supprting the existing conduit. The problem I face is, this is an old steel mill. We don't exactly have a great grounding system. Most devices are bonded to building steel. In my situation here, theres no ground coming with the feed. There was an external ground bonded to the starter and run outside of the building to the motor base. The motor has a homemade PVC junction in leau of the original metal one. This machine is a large industrial baghouse that's comprised of three main parts. The blower assembly, the large baghouse, and the starter inside the building. I wanted to atleast bond these three pieces of equipment and the building steel, and provide a good ground path. I have bonded foundations for similar sized baghouses in the past in which the Engineer specified 4/0 copper and 3/4" x 10' ground rods. If I say put 2 ground rods in, and bond the starter, building steel, blower unit and baghouse using exothermic welds, would that be adequate?

 

[dpc]

It would not be adequate for me. But, is the 2300 V system grounded?

The main purpose of the ground wire is to provide a low impedance path for return current during a ground fault in the motor. This means the ground wire needs to be inside the conduit. Relying on building steel, earth, etc results in higher impedance and possibly high touch potentials at the motor during ground faults. As a worst-case, the ground fault may not produce sufficient current to operate the protective devices.

If the 2300 V system is ungrounded or resistance grounded things are a little different.


David Castor

http://www.cvoes.com