Large Motor Grounding

[mikesami]

I have a motor grounding question. Our specifications state that large motors should have grounding pads on both sides. This particular application is for a 2000HP 4160v motor. We have two size 500 ground wires going there, but this motor came with only one grounding pad in one side. I couldn't find anything in the NEC book regarding the 2 sides requirement, but would a single 500 size cable connected to only one side of the motor frame be sufficient in this application? Thank you for your help...

 

[mikesami]

Correction to my previous post: motor is only 900HP.

 

[dpc]

It's probably OK. The 4160 V system is likely to be grounded through a resistor and this will limit the fault current and any resulting touch potentials.

The two ground pads are more of a belt and suspenders thing than a physical necessity.

The equipment grounding conductor that is run with the phase conductors will provide the lowest impedance return path anyway.

Make sure to ask the motor supplier for a credit.

David Castor

http://www.cvoes.com

 

[rockman7892]

I worked in a plant with several similar size motors on a 4160V system and often wondered what is required.

From an NEC standpoint I believe all that is required is the equipment ground conductor and that this EGC will carry most of the fault current as dpc mentions. I dont believe a connection to earth is required on a motor but I've seen it done in several cases as a supplimentary protection.

dpc brings up a good point about the 4160 system being resistance grounded. How much would this HRG system limit the step potential at the motor compared to a solidly grounded system? I guess this would be a funcion of the reduced fault current that would flow. In the HRG case would the wire between the motor frame and earth even do anything to limit touch potential if most of the current flows through the EGC?

 

[mikesami]

Thanks guys, I appreciate your feed back on this. IEEE Std 665 recommends this dual grounding configuration. It provides a reliable ground fault current return path and a good limiting of the touch potential.

 

[Hoxton]

Having a pad either side allows the freeedom to run the earth conductor where it goes best, rather than either looping the conductor round the motor to reach the pad, or drilling the motor base on site to screw on a pad. Local codes and the custom of using two pads as a belt and braces approach (in case one becomes loose) may supercede this idea!

As we all know, terminal boxes are always the wrong side and motors always rotate the wrong way on start up!