EGC serving as only GEC on SDS transformers- 250.121 ex issues?

[housemoney]

A facility has a number of 75 kVA 480V:120/208V step down XFMRs located in close proximity to high load concentration areas. All these XFMRs are using the EGC as the only grounding electrode conductor. Most of these EGC's have only one connection to a grounding electrode back where they originate, hundreds of feet away in an electrical room. I understand this is OK per 250.121-exception.

However- There are no other GECs connected to the XFMRs ground bus locally- building steel, metal process piping or cable tray ( which is bonded to building steel in areas) is available near all of them.

This facility has a history of electrical disturbances associated with these SDS sources (which are all grounded correctly other than my concern above) in a high lightning prone area (building is equipped with a lighting protection system but needs to be inspected). Would it be advisable/ good practice to install a dedicated GEC from a local grounding electrode to the ground bus of these transformers to potentially limit / stabilize the voltage imposed by line surges or other disturbances?

 

[housemoney]

Title should read: EGC serving as the only GEC. Should also be noted that all potential local grounding electrodes for the XFMRs are all interconnected as part of the grounding electrode system at the main service equipment ground bus 200-500' from each XFMR.

 

[tom baker]

250.121 does allow the EGC to be used as a GEC but it needs to meet all the requirements of Part III, in particular 250.64 E Raceways and Enclosures for GEC, basically it requires bonding at each end to create a parallel path. The objective is to lower the impedance and prevent an inductance choke. It is important that the SDS GE be bonded to the building GE system. is there interconnected building steel, could it be used, and of course there is already a bonding connection to it and water pipes in the area served per 250.104 D
You could add GE at each transformer but that may make the issue worse. All the GEs need to be bonded together .

 

[housemoney]

Below is a snip out of my 2017 grounding and bonding book- Circled is the GEC I'm questioning to be installed that is absent at all these step down transformers. All the potential grounding electrodes in the vicinity of the transformers (which I mentioned above) are tied together back at the services some 200-500 feet way from the transformers. The EGC/GEC is terminated to at least 1 ground bus termination point along this path upstream. I'm thinking adding this "local GEC" will ensure voltage stability to ground and establish a redundant effective ground fault current path in the event the EGC is disconnected upstream somehow?

 

[petersonra]

Below is a snip out of my 2017 grounding and bonding book- Circled is the GEC I'm questioning to be installed that is absent at all these step down transformers. All the potential grounding electrodes in the vicinity of the transformers (which I mentioned above) are tied together back at the services some 200-500 feet way from the transformers. The EGC/GEC is terminated to at least 1 ground bus termination point along this path upstream. I'm thinking adding this "local GEC" will ensure voltage stability to ground and establish a redundant effective ground fault current path in the event the EGC is disconnected upstream somehow?

I think you should stop worrying about grounding stuff. do the bare minimum. it is of dubious value in most cases anyway.

The GEC is not required to be part of the ground fault clearing path.

 

[housemoney]

I think you should stop worrying about grounding stuff. do the bare minimum. it is of dubious value in most cases anyway.

The GEC is not required to be part of the ground fault clearing path.

Probably right. I’m just scratching my head as the Inspector is saying this "local GEC" will ensure transformer voltage stability to ground and establish a redundant effective ground fault current path in the event the EGC is disconnected upstream. Engineer says I don't need it as long as I'm using grounding bushings on the RMC all the way back to the service. Thoughts ?

 

[petersonra]

Probably right. I’m just scratching my head as the Inspector is saying this "local GEC" will ensure transformer voltage stability to ground and establish a redundant effective ground fault current path in the event the EGC is disconnected upstream. Engineer says I don't need it as long as I'm using grounding bushings on the RMC all the way back to the service. Thoughts ?

the rmc is a perfectly good egc.

 

[housemoney]

I agree, just trying to see if anyone has experienced this previously and if there was any sense in adding the additional GEC at the XFMRs

 

[petersonra]

I agree, just trying to see if anyone has experienced this previously and if there was any sense in adding the additional GEC at the XFMRs

I can't think of any good reason to do so. I also do not think that on the whole it is likely to hurt anything. The example of how it might hurt something shown in the MH diagram posted above could just as easily happen by having a piece of metallic equipment sitting on concrete outside. There is then a conductive path from the earth through the concrete and to the equipment just as if someone had put a ground rod in and ran a wire to it.