"Rock ‘n' roll is like a circus today." -Ray Manzarek, keyboardist for The Doors
Once upon a time, early in my career, I was at the top of my game. At the time, my game was a 16-foot ladder, and I working on a light that was rigged on a truss. As I was holding on to the truss for stability, I reached out and grabbed a second truss and that's when I got a shocking lesson in grounding and the potential differences between two metal structures.
The lesson was a hurtful one. As I lost my balance and began my long descent to the floor 16 feet below me, I held on to the first truss with all of my might. The force of the near fall did its level best to pull my arm out of its socket as I hung from the truss with one arm. But the ligaments in my shoulder said, "Oh, no you don't," and they stretched like the bellows of a cheap accordion. To this day, the knuckles on my right hand still drag the ground when I walk.
Had the two trusses been at the same ground potential, I might not be left-handed today. I don't know what caused the problem, but I knew there had to be a solution. For a long time, I wondered how two big metal structures that aren't in contact with each other could have a difference in potential. There is much to learn about it, and I'm still learning, but it has to do with grounding.
Grounding and Bonding
We often run into situations where we have power feeding loads in or on a structure like a truss, a tent or a building. And the power we use should have a green grounding wire somewhere in the mix. How that green grounding conductor is eventually bonded to the metal parts of the remote structure is what determines whether or not everything is at the same ground potential. In case you're wondering, "bonding," according to the National Electrical Code (NEC), is "the permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed."
In most electrical systems, there is a grounding electrode, usually a rod that is driven into the earth, whose job it is to conduct ground fault current (a short to ground) into the biggest sink we can find – the ground under our feet. By bonding the green grounding conductor to the grounding electrode and to all of the metallic parts in the electrical system, we make sure that there is a low impedance path from any metallic enclosure through the green grounding conductor to earth. That insures that, in the event of a ground fault, a huge current will flow, and it will trip the overcurrent protection device(s). Whether these devices are circuit breakers or fuses, they are inverse time related devices, meaning that the higher the overcurrent, the faster they trip. And we want them to trip as fast as possible to get the voltage off of the affected parts and minimize the damage and danger.
There is a lot of information about grounding and bonding, but two of the best sources on grounding that I know of are the National Electrical Code and Soares Book on Grounding, Eight Edition by the International Association of Electrical Inspectors (IAEI). The latter is a 384-page tome all about the practice of grounding. The original book was written by Eustace C. Soares, who has been described as "one of the most renowned experts in the history of the National Electrical Code in the area of grounding electrical systems." In the NEC, Article 250 deals with grounding and bonding.
NEC 250.32(B)(1)
According to 250.32(B)(1) in the NEC, when an equipment grounding conductor (the green wire) is run with the feeder(s) or branch circuit(s), then the grounding conductor "shall…be connected to the building or structure disconnecting means and to the grounding electrode(s)." But, it goes on to say, "Any installed grounded conductor (the neutral or white wire in most cases – ed.) shall not be connected to the equipment grounding conductor or to the grounding electrode(s)." [emphasis added].
{mosimage}What this means is that there should be a high impedance air gap (no connection!) between the grounding terminal bar and the neutral terminal bar inside of the disconnecting means (company switch, disconnect, etc.) in the tent or building. (See Fig. 1.) This is because we want to make sure the normal path for return current from the tent or building back to the power source is through the neutral conductor and not through the earth.
If there was a main bonding jumper between the neutral terminal bar and the grounding terminal bar, then the return current to the remote source would be divided between the feeder neutral and the feeder grounding conductor. The grounding conductor is not intended to be a current-carrying conductor, so we don't want this to happen. But we do want all of the metal parts in the electrical system to be bonded to the grounding conductor so that the return path for ground fault current is through the grounding conductor and not the earth.
A Rule of Thumb for Tents
I have not often worked in tents, mostly because there is no room for more clowns in most of those tents, but I am told that the people who erect those tents will often place a piece of plywood under the metal legs or towers in order to isolate the metal parts of the tent from the earth. I think this makes sense if they are not using the metal structure of the tent as an electrical enclosure or part of an electrical system. After all, it's your job, not theirs, to set up the electrical power distribution. But when you bring power into the tent and the equipment is within arm's reach of the metal parts of the tent, then they should be bonded.
Why?
I'm glad you asked. Suppose you were to rig a light to the tent pole, and somehow the hot conductor got loose and came in contact with the metal pole. Unless there is a low impedance path to complete the circuit back to the source, then the pole will stay "hot," meaning it has a voltage on it. So the next person who comes along and touches it will complete the path to ground through their body, and they will be shocked.
In Earth Grounding and Bonding Pamphlet: A Guide to Proper Earth Grounding and Bonding Methods for Use with Tactical Systems, published by the U.S. Army Communications-Electronics Command (CECOM, 1998), it says, "Personnel can sustain much worse injuries when contacting two metal surfaces at different potentials with bare hands (a low resistance path provided across the chest) than by contacting a surface energized to ground while wearing boots (a high resistance to earth). For this reason, equipment and shelters located within arm's reach of each other (6-8 feet) must be bonded together to eliminate any hazardous voltages that may develop between them should a fault occur."
So the answer is that any metal part within arm's reach of the electrical system should be bonded to insure it remains at ground potential.
There are times when you might want to take chances, and there are times when you should play it safe. If you find yourself in Las Vegas at the gaming tables, then you might want to take chances and that's okay. But if you're in Las Vegas under the big top and you're dealing with electricity, don't take chances. Be sure your grounding systems are safe.
