Static Discharge vs. Lightning Protection

[BDBoard]

I have been tasked with designing the electrical distribution system in a mine application. One of the concerns brought up by my client is static build-up due to the use of rubber flex hose in a classified area. They agreed to purchase flex hose with metal braiding such that I can bleed off the static by bonding the ends of these hoses to the system ground.

After a little more thought I realized that such a bonding scheme would then make the system vulnerable to lightning strikes as the lighting could then travel down my bonding connections into the mine. Is this a legitimate concern or would my system ground be sufficient protection from a lightning strike?

What about inclusion of a Transient Voltage Surge Suppressor (TVSS)? Do they provide effective protection from lighting?

 

[bphgravity]

Re: Static Discharge vs. Lightning Protection

Here's a pretty good paper on this topic from the IEEE. It's a PDF file.

http://www.ieee.org/portal/cms_docs/xplorefiles/xplore0306.pdf

Here's another:

http://www.qp.gov.sk.ca/documents/english/Regulations/Regulations/O1-1r2.pdf

A good website:

http://www.msha.gov/30cfr/75.0.htm

[ January 19, 2006, 03:33 PM: Message edited by: bphgravity ]

 

[BDBoard]

Re: Static Discharge vs. Lightning Protection

Good information that does seem to address one question but brings up another. It would seem isolation is the key to lightning protection. Any conductor from the surface to the mine is a hazard that must include an arrestor, including system ground.

IEEE standards indicate that anything less the 1 M-ohm resistance to ground is enough to bleed off the static. So it would seem the solution is to bleed the static off through an independent and isolated ground system in the mine. Does this mean I should isolate all my motors from the mine ground system? After all, wouldn't a ground fault in a motor then put a voltage on my flex hose fittings even if it only lasted half a cycle?

And since the motors will be placed on a concrete foundation in the mine, do I bond the rebar in this foundation to the mine (static) ground or does code require it to be grounded to the power system ground?