Equipment grounding conductor in high resistance grounded system

[jy121]

I've searched the threads and found nothing on this topic, so forgive me if its been asked before.

Is it necessary to run an equipment grounding conductor with the phase conductors in a low voltage high resistance grounded system assuming that all conduits/trays are electrically continuous?

All HRG systems I've seen designed do not have dedicated equipment grounding conductors... NEC does not require, Red Book seems to say that regardless of the system grounding, equipment grounding conductors should be run...

What's the answer?

 

[rcw retired EE]

The metallic tray and conduit does provide an adequate Equipment Grounding (Bonding) Conductor, if sized and installed per the NEC. Our company has had problems with poor conduit connections or bonding of trays, so we also run an EGC with the larger power circuits. HRG or solidly grounded makes no difference.

With HRG, ground fault current is limited so an EGC may not seem as necessary but it is needed to keep the equipment enlcosures at ground potential or to carry the line-line fault current when the second ground fault occurs. The HRG could keep a motor enclosure at full Line-Neutral voltage during a motor winding fault if there was no EGC.

 

[burnt2x]

Agree wth rcwilson.
If you are designing, try to ask a lot of "what-if..". What if your men will remove bonding jumpers at tray joints? What if the trays vibrate a lot and bonding terminal nuts loosen, etc...
So, you will run an EGC and forget everything about losing grounding!

 

[jy121]

Ok... here's what I'm confronted with:
1. Multiple installations (utility, industrial) of low voltage HRG systems with no dedicated EGCs run with the phase conductors.
2. The NEC which does not require dedicated EGCs run with the phase conductors.
AND
3. The IEEE Red Book which states that a dedicated EGC should be run with the phase conductors REGARDLESS of system grounding type.

I'm trying to reconcile all... and I think the answer is that it's just good practice to install an EGC with the phase conductors regardless of system grounding type, and the engineers for installations noted in #1 were either trying to save money, or they know something I don't.

 

[dpc]

If you are in NEC-land - an "equipment grounding conductor" could include steel conduit, tray, etc. So if you just want to meet NEC requirements, an actual green ground wire may not be required.

However, I would always run a dedicated ground wire regardless of the method grounding for all the reasons mentioned above. It would be very easy to convert this HRG system to solidly grounded in the future and the grounding system should be able to deal with this. For solidly-grounded systems, the technical debate on this really ended about 40 years ago, but the conduit lobby on the NEC code panels have been able to retain use of conduit as a grounding conductor.

David Castor

http://www.cvoes.com