Separate Ground Beds at Mine

[msheets]

I am working on the design of the electrical system of the surface infrastructure of an underground metal mine. I recently received an inquiry about the requirements of keeping grounding beds separate from each other: the Substation Ground Bed and the Safety Ground Bed. As I am new to this type of system, I did some research and found many papers discussing this requirement from the 1980's and 1990's, but nothing in the last decade. One of the papers referenced the 1996 NEC Article 250, section 154, which is no longer printed in the current code book. I have been unsuccessful in locating the code sections (CFR or NEC) detailing this requirement and am suspicious as to whether it is still in effect. Does anyone know where I can verify this?

Thanks in advance.

 

[don_resqcapt19]

That section would be 250.188 in the 2011 code.

 

[msheets]

Thanks. Is there a resource that I can reference to track changes in the NEC like this? My Google skills failed me this time.

I don't think 250.188 will apply in this case as none of the equipment would be classified as portable or mobile, so I am still searching for an answer.

 

[jcormack]

Coal

Coal

That is a requirement for coal mines - found in CFR 30, parts 75 & 77 - while a good idea, it is not specifically referenced in regards to metal/non-metal mines. The only thing you will find in the CFR for metal mines is for the the lightning arrestors to have a separate ground field. If someone is asking for a separate ground field for a resistance grounded system, look in CFR 75 for undergound mines and/& in coal references for guidelines. Primarrily, what you are doing is providing a remote ground bed for the neutral resistor connection to ground - so that it is not connected to the station lightning arrestors nor to the station equipment frames - the thought was to reduce the influence of lightning strikes or faults onto the neutral to be fed underground - the separation distance is a minimum of 25 ft, with the distance becoming greater depending on size of the main substation. It is a requirement of all underground Coal mine power feeds. http://www.msha.gov/S&HINFO/TECHRPT/GROUND/IC8835.pdf Wils Cooley (one of the authors in the pdf) was my college advisor & professor in a 4 of my classes...............