Posted by John T on September 04, 2010 at 13:01:22 from (66.244.97.31):
As the whole electrical Neutral versus Ground issue has again raised its ugly head, as a public service I copied a couple long winded explanations I posted here or on Johnnypopper a long time back and based on the electrical post below feel its good it be posted again for yall.
ELECTRICAL GROUNDING
Over the years on many many various Tractor Board postings concerning general home n farm AC wiring, I’ve observed a common misunderstanding regarding “grounding” n ground rods etc. which I hope to correct in the interest of safety education and am posting this all over the place.
First of all, it’s the service NEUTRAL that gets bonded to Mother Earth via made grounds like rods driven into the earth or metallic water or gas pipes etc. Out at the electrical 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, its again the Neutral that gets tied to Mother Earth. That’s to keep the services n the grid etc. at one single 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 sole purpose of the third wire safety equipment green/bare ground is not so that circuit is tied to Mother Earth n all those ground rods n water pipes, buttttttttt it’s to provide a dedicated low impedance return current path back 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 any bond to earth that can save their life, it’s the drill case or stove being bonded to Neutral back at the panel that’s critical. The safety equipment green/bare ground wire is tied to the drills case so if there’s 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 NOOOOOOOOOOOOOOOOOOOOO YOU DIEEEEEEEEEEE. 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 impedance return current path (like the equipment ground) to trip the breaker.
Sooooooooo it’s the Neutrals that get tied to Mother Earth and driving rods into the earth to “ground” that equipment grounding conductor isn’t the thing, it’s the fact that it’s bonded to the Neutral and provides a current path back to the panel in case of a short/fault to trip the breaker n save your life that’s important.
Dave, The electrician was correct. Not so many years ago they just used 3 conductors (some appliances like stove etc) but nowadays 4 are required.
Heres the deal in theory:
The Equipment Grounding Conductor (EGC) is intended to conduct FAULT CURRENT ONLY and is there as a dedicated always premanently attached low impedance current path so fault current has a path back to the panel to trip the protective circuit breaker.
The Neutral (a grounded conductor) is supposed to carry ONLY the normal return current.
In the old days an appliance such as a stove that had 220 heating elements plus say a 120 volt clock and a convenience outlet but (hey they existed and still do) was only served by three conductors (two hots and a green/bare EGC i.e. NO Neutral) had to be using the EGC as a Neutral and that same green/bare EGC was tied to the metal frame. THATS NOW A NO NO If an appliance has 220 ONLY, its okay to run just 3 wires, 2 hots and an EGC tied to frame, but if it has BOTH 110 and 220 the Neutral is obviously required. YOUR FRIEND IS WRONG you only have to run 3 wires to a pure 220 load like a motor, the 2 hots and the EGC thats tied to the motors frame, its ONLY those appliances (maybe a stove or dryer) that have BOTH 110 and 220 volt internals that require 4 wires so the Neutral carries 120 volt return current and the EGC ONLY carries fault current.
Its NOT so much the appliances dont still work fine, its the safety issue that drives this 4 wire service thing. Its similar to why at sub panels the Neutral and EGC are NOT re bonded cuz the situation might arrive where a combination of wrong things like an open in the wrong place coupled with a short in the wrong place could cause the EGC to become HOT and you touch the stove n dieeeeeeeeeeee.
Clear as mud lol John T Nordhoff, retired Electrical Engineer in Indiana
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