Grounding and bonding motors and equipment

[Shane Platt]

I work for a mining company, and common practice is to run a ground wire with all phase conductor feeding a piece of equipment thats normal. But they also run a external ground and attach it to the equipment I.E. motor, machine, etc etc. This EGC is not in the same pipe nor is it always coming from the same grounding source. Our safety dept thinks driving a ground rod and bonding the equipment is the solution for static shocks that sometimes occur(Even though there already is a ground that is bonding the cabinet and the equipment with the phase conductors). On the other hand I have some concern about parallel paths to ground, seperate grounding systems, and ground loops. Is there any safety concerns with this? And does the code or some other literature directly address this issue?

Thank you

 

[Dennis Alwon]

I don't believe a ground rod will do much and I would not worry about parallel path for the equipment grounding conductor. Parallel paths are not wanted for the current carrying conductor's but it is not an issue for the equipment grounding conductor. Of course mining is not in the NEC and I don't pretend to know anything about it.

 

[Shane Platt]

I don't believe a ground rod will do much and I would not worry about parallel path for the equipment grounding conductor. Parallel paths are not wanted for the current carrying conductor's but it is not an issue for the equipment grounding conductor.

Thank you for the quick response Dennis! So you dont think the ground rod is a concern, but is it alleviating the problem? Basically is it doing any good or is it a waste of time and material?

 

[Shane Platt]

True Underground and Mobile Mining equipment isnt in the code but i do believe open pit and the processing of the material is covered. I think lol

 

[Dennis Alwon]

I am not familiar with using rods to alleviate static shock. Perhaps one of our engineers can help here. Unfortunately it is late and many members have gone nite nite.

 

[Dennis Alwon]

Check out this article http://www.iaei.org/magazine/2004/05/static-protection-through-bonding-and-grounding/

This is from that article

Addition of grounding rods and similar grounding systems will not reduce the hazard associated with static electric charges apparent in the liquid [NFPA 77 7.5.2.2].

 

[tish53]

I work for a surface mining company ( stone aggregate) and we run a ground with the power feed as you described. In addition we ground our structures( screen towers, crusher structures, large conveyor towers) to ground rods or a ground mat, typically tied into the rebar in the concrete base. Sometimes it is hard to drive a ground rod into solid rock. Our purpose for this additional ground in lightning protection

 

[kwired]

I really can't answer whether or not this will change the static shock conditions significantly enough to be worthwhile. It would seem to me if you are connecting to something already connected to the equipment grounding conductor and it is earthed someplace, you probably do not accomplish much. But NEC doesn't prohibit additional supplemental grounding electrodes to be connected to the equipment grounding conductor at pretty much any point either.

 

[J. Hofstetter]

Shane Platt,
I also work in the Mining Industry and this sounds like a similar problem that I had and what I found was that the EGCs that were run were not large enough. They offered too much resistance which caused a difference in potential at the equipment than ground. You will always have some difference due to the resistance in the wire, but what I ran into was that a 10 AWG wire used as an EGC for a piece of equipment that was some 700' away from the source wasn't adequate. I do not like to drive extra ground rods in the field because, 1) it is ridiculously hard and expensive to drive a ground rod in solid stone and 2) solid stone doesn't offer a good ground so you would have to drive several to make much of a difference. We always excavate the area around the transformer and MCC control house and install a grid and everything is tied back to this grid. Maybe it is dumb luck, but since I started working for the company and we started doing this we haven't had any real lightning issues. I was told when I started with the company that several times a year they would suffer major hits that would cause days and sometimes weeks of downtime. Hope this helps.