Posted by John T on February 19, 2010 at 10:31:46 from (71.55.31.109):
I dug this up from a post I made on another board years ago to help some folks undestand all the electrical ground talk that has been here lately, sorry its longgggggg and may confuse some, but its my best shot and I didnt want to have to do it all over again lol
ELECTRICAL GROUNDING 101
Over the years on many Tractor Board postings concerning general home n farm AC wiring, I’ve observed a common misunderstanding regarding “grounding” and ground rods etc. which I hope to correct in the interest of safety.
First of all, it’s the service NEUTRAL that gets bonded to Mother Earth via a grounding electrode conductor (that bare No 4 copper wire) to made grounding electrodes such as copper rods driven into the earth or metallic water or gas pipes etc. Out at the electrical service pole the Neutrals (if a Y service) on the high voltage primary side are tied to Mother Earth,,,,,,The Neutrals on the low voltage (120/240) secondary side of the transformer also get bonded to Mother Earth,,,,,,,,,And finally at the electrical service entrance meter base or the main service entrance panelboard or even up on the riser (depends on local practice), its again the Neutral that gets tied to Mother Earth. That’s to keep the services and the grid etc. at one common voltage reference which Mother Earth provides, albeit not perfect.
So what about the equipment safety ground, the bare/green GROUNDING conductor that is used on three wire appliances and is wired to the outer metal cases on an electric drill or skill saw etc., isn’t it “grounded” ??? Well, it is, but just because at the main service entrance the Neutral Buss and the Safety Equipment Ground Busses are bonded together, therefore, the safety equipment Ground is also tied to Mother earth but notttttttt for reasons some might think. Since the Neutral conductor is an ordinary current conducting path which happens to be grounded, it’s referred to as a GROUNDED CONDUCTOR while the safety equipment ground (green/bare) is referred to as a GROUNDING CONDUCTOR. The purpose of the third wire safety equipment green/bare ground is not only so that circuit is tied to Mother Earth and all those ground rods n water pipes, buttttttttt it’s to provide a dedicated low resistance return current path back (for fault current ONLY) to the Neutral (Remember at the panel Neutral and Ground busses are bonded together) in case of a fault (like a hot wire gets shorted to the drill case) so the breaker trips de-energizing the circuit and you don’t die hanging onto the drill. I hear people talking about driving more ground rods and making sure that drill case or appliance or stove etc. is bonded to Mother Earth thinking that alone somehow makes it safe, while its NOT so much that bond to earth that can save their life, it’s the drill case or stove being bonded to Neutral back at the panel (via the equipment grounding bare/green conductor) that’s critical. The safety equipment green/bare ground wire is tied to the drills case so if there’s a short there’s a dedicated current return path back to the panel to trip the breaker. If you had a drill with a metal case and only a two wire circuit serving it with no third equipment grounding conductor and say you drove a ground rod and attached it to the drills case, you think that would save your life if a hot wire got shorted to the drill case MAYBE NOTTTTTTTTTTTTTT. The reason is the earth (depends on moisture n mineral content etc. etc.) is a poor high resistance conductor so there wouldn’t be enough return current back to the panel to trip a 20 amp circuit breaker butttttttttttttt it only take like 50 milliamps through your old ticker to kill you which that short can continue to supply since there’s no low resistance return current path (like the equipment ground) to trip the breaker.
Sooooooooo it’s the Neutral that gets tied to Mother Earth and driving rods into the earth to “ground” that equipment (saw etc) isn’t the thing, it’s the fact that the equipment grounding conductor (bonded to metal saw case) is bonded to the Neutral at the panel, and as such it provides a return current path back to the panel in case of a short/fault (hot wire to saw case) to trip the breaker n save your life that’s important.
Clear as mud ???? lol John T, toooooooo longggggggg retired Electrical Engineer in Indiana
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