Question about High Resistance to Ground Systems

[RWZIMM]

I am working on a project with a High Resistance to Ground System and not being that familiar with this type of grounding system I would like some pointers.
First the High Resistance to Ground System has been designed by others and as I have researched in the NEC this system can only be used on 3 phase 480-1000V systems per NEC 250-36 and then only if it meets the criterial listed. Well my first issue is that it has been proposed for a high voltage system with transformers at 34.5 KV Primary and the secondary side range from 480/277V to 4160/2400V Also a Generator Unit at 38KV. I do not see how this is code compliant.
Secondly they have requested tapping my Grounding Ring System into their High Resistance to Ground System at multiple locations from each transformer and the generator system which also has me confused. Per the NEC 250-36(E) the equipment bonding jumper shall be connected via an unspliced conductor run from the first system disconnecting means or overcurrent device to the grounded side of the grounding impedience and the grounding electrode (grounding ring) per NEC 250-36(F) shall be connected at any point from the grounded side of the grounding impedience to the equipment grounding connection at the service equipment or first system disconnecting means. Which I believe allows me to tie my Grounding Ring System into the main Grounding Bus in the service equipment MDP with a single grounding conductor to the grounded side of the Grounding Impedience in the High Resistance to Ground System cabinet.
Please let me know your thoughts. Thanks

 

[buldogg]

Basically, the high resistant grounding system that I am familiar with uses a resistor bank (55 ohms) to ground on the 480V substations secondary. The 13,800V primary ground of the substations are tied into the ground grid (ring). If there is a ground fault on the system, a ground fault alarm will come in at the substation but there won't be a fault trip because the fault is limited to 5 amps and the loads will keep operating, thus giving you time to find the fault without incurring downtime.

 

[jim dungar]

You need to have a 480Y/277V transformer in order to install a High Resistance Ground (HRG) system. But, you are not allowed to have any loads connected 277V to ground on an HRG. Most voltage systems, especially generators, above 1000V and up to 34.5kV use a Low Resistance Ground (LRG) instead. The LRG usually lets about 400A flow during a line-ground fault.

The NEC is primarily only concerned with the conductor from the wye center point to the grounding resistor.

 

[dbuckley]

I cant answer the question directly, but I know that some of the AOL datacentres use HRG on the MV distribution, theres a paper about it somewhere... and through the magic of Google I've found a reference to it here.

 

[skeshesh]

Also take a look at 250.186. The NEC provides some wiggle room for special operations such as substations, and I guess, in many cases, for any operation that has qualified persons managing the system. As the previous posters pointed out HRG is used for reduction of available fault current in order to allow for continuous operation while ground fault detection (they have some pretty accurate devices) is used by the maintenance crew to find and eliminate the fault. If you want a good referrence about what is proper (not necessary enforced code but good practices) take a look at IEEE 80-2000: IEEE Guide to Safety in AC Substation Grounding.