Ground rods? How many is too many?

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Old 08-29-15, 10:50 AM
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Ground rods? How many is too many?

1. I built my workshop in 1996 and over the years got some advice for various things that I want to clarify if it was good or bad advice. I have a powder coating spray booth that is made of wood but for the electrostatic spray to work, the parts need to be grounded. This brought up many opposing and interesting discussions. In the end, the conclusion was that it was best to have a separate isolated ground rod connected to the rack to hang the parts from. Is that ok and if so is there a minimum distance from another building ground rod that must be kept?

2. On another piece of equipment, my welding table, the Miller installation sheet shows an isolated ground that they refer to as a "drain" that is to be connected to the steel welding table. It show in the pic a single wire going to a ground rod. Could that be an isolated ground rod and if so, would there be a minimum distance from the main building ground rod(s)? It is easier to connect to an isolated ground rod and it is not clear if they mean just using the equipment ground in the main panel.

Thanks,
G
 
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Old 08-29-15, 11:25 AM
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There is no maximum number of ground rods allowable. The maximum required is two unless certain complex electrical tests show you can get by with just one.

All ground rods used for anything that is powered by the house electrical system must be interconnected (bonded) using 6 gauge copper wire, run outdoors as much as is practical. Exception: A leg of the interconnection that must cross a lawn, walkway, etc., say, to get to a shed, may be left out.

There is no minimum distance except that if you do not qualify to have just one rod, then out of all the ground rods that you have, there must be at least two that are interconnected that have at least a six foot separation between them.

When something needs to be grounded so it or associated electrical equipment works properly then that means being bonded* to the equipment ground in the panel. If you are not sure whether the building wiring has proper equipment grounding conductors, a separate equipment grounding conductor may be strung from the panel up the stairs and across the floor if needed, and rolled up when you are done painting or whatever.

* If A is bonded to B and B is bonded to C then A is bonded to C. So from the spray paint gun frame to the power cord ground wire to the plug ground pin dada dada to the panel neutral bus means the spray paint gun is bonded to the service ground. However certain bondings must be accomplished with specific or more robust materials e.g. 6 gauge copper wire.
 

Last edited by AllanJ; 08-29-15 at 11:48 AM.
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Old 08-29-15, 11:30 AM
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As far as the powder coating I thought it relied on parts having opposite charges, not actually a ground to earth. The resistance of earth is high so I don't see there being enough potential difference . A breaker connection directly to a ground rod will not trip a 15 amp breaker.
 
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Old 08-29-15, 12:28 PM
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As far as the powder coating I thought it relied on parts having opposite charges, not actually a ground to earth. The resistance of earth is high so I don't see there being enough potential difference . A breaker connection directly to a ground rod will not trip a 15 amp breaker.
Powder Coating: The Complete Guide: How to Properly Ground Your Parts

The link above is just some info I pulled off of the web so you can see what some discussions include. I have no problem with either method but in the case of people with cheap low quality equipment they usually blame the ground. I was more concerned if adding one (which I did) just to the booth, if it would ever cause an issue like a ground loop, even though it is not electrically bonded to any electrical source.
Thanks,
G
 
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Old 08-29-15, 12:32 PM
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Allan,
Thanks for the info. I was mainly concerned if add some separate "isolated" ground rods could create any problems. I think in the case of the welding table I could just ground that to the existing building ground (as opposed to having a separate rod). The main issue is for AC High Frequency welding. In that case it seems not to many people follow the rules in the case of a steel building. Look at section 8-3 of this manual for my welder. I have all kinds of conduit but never welded my steel building panels together or added braided jumper straps. I also do not do too much AC HF welding.
Thanks
G

added link to pdf:
http://www.millerwelds.com/om/o363k_mil.pdf
 
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Old 08-31-15, 07:48 AM
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I was mainly concerned if add some separate "isolated" ground rods could create any problems.
Yes, in the event of a nearby lightning strike, a high voltage gradient can develop between unbonded grounding electrodes causing high current to flow through the building. It can also cause an electrocution hazard if for example your welder shorts out on the metal welding table and you touch it with one hand while also touching a part of the "other" grounding system (structural steel). Finally, it can cause problems with RF communications and analog audio systems introducing a buzz or hum.
 
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Old 09-01-15, 08:55 AM
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Yes, in the event of a nearby lightning strike, a high voltage gradient can develop between unbonded grounding electrodes causing high current to flow through the building. It can also cause an electrocution hazard if for example your welder shorts out on the metal welding table and you touch it with one hand while also touching a part of the "other" grounding system (structural steel). Finally, it can cause problems with RF communications and analog audio systems introducing a buzz or hum.
Thanks. That is exactly what I needed to know. I will remove the isolated ground for the pc spray booth as I never noticed any difference anyhow, and have just a single ground setup. I appreciate it.
G
 
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