Posted by John T on December 13, 2008 at 06:04:44 from (66.244.97.31):
ELECTRICAL GROUNDING 101, THE VERY BASICS
I’ve never seen so much confusion in my life as lately posted on these boards regarding Neutral and Grounds and Grounding when it comes to our typical home and shop 120/240 volt single phase 3 wire services, so I will try me best to educate yall in hopes to keep you safe. It’s IMPOSSIBLE to convey an a few paragraphs what I spent much of a lifetime in learning (and what can fill books) when I practiced as a secondary power distribution engineer and it’s been years since I retired and I’m a lil rusty, but I will give it my best shot.
1) NEUTRAL: The neutral is a GROUNDED CONDUCTOR. Its carrys current PLUS at the service entrance panel its bonded to the Equipment Ground Buss, soooooooo it’s a “Grounded Conductor”. 2) The Neutral is tied to the center mid point of a 240 volt transformer and that’s how you get two legs of 120 volts (L1 to N and L2 to N) service PLUS 240 volts (L1 to L2) 3) If all loads were perfectly balanced and there was the exact same current draws on L1 as L2 there wouldn’t be any neutral current, however, if not (as typical) the Neutral also carries that imbalance current . 4) GROUNDING: Actually what gets grounded BOTH at the outdoor utility transformers secondary and your service entrance, be it up on the riser or the meter base or the main panel, IS THE NEUTRAL. It gets bonded to mother earth (driven ground rods or metal water pipes etc) which is the best available single common zero volt reference plane. That way the grid and all the services and transformers etc all have a single common zero volt reference. Once more, the Neutral is grounded to mother earth on the utilitys high voltage primary (if it has a neutral) ,,,,,,,,,,on its secondary out at the pole or pad mount,,,,,,,,,,,,once more where it enters your home. IT’S THE NEUTRAL THAT NEEDS GROUNDED TO MOTHER EARTH NOTTTTTTTTTTTTTT ALL YOUR TOOLS AND APPLIANCES (they need to be tied to the equipment ground buss in your panel, see below) 5) ONCE AND ONLY ONCE at the main panel, the Neutral Buss is bonded to the Equipment Ground Buss. Those two big buss bars are tied together. You MUST NOT at other downstream sub panels in the building re bond N and G again. At a sub panel the N and G busses are separate and isolated, i.e. if you run to a sub panel you carry 4 wires, 2 hots, Neutral, Equipment Ground (and UNLIKE the main service entrance panel, the N and G busses are NOT tied together). The reason for all this is given a certain set of unfortunate circumstances like an open of one conductor yet a short at other places, it my be possible to energize the equipment ground buss system with live hot voltage and if you touch an appliance with a metal case (which is required to be tied to the Equipment Ground Buss) YOU CAN DIEEEEEEEEE 6) The sole purpose of the equipment ground (the bare/green wires) is to provide a dedicated low impedance return path for fault current back to the panel to clear a fault and trip the breaker so you don’t dieeeeeeeeeee if a hot phase conductor comes in contact (shorts out) with an appliances metal case. The bare/green equipment ground conductor MUST NOT BE USED AS A NEUTRAL nor rebounded to the N at any place downstream again from the main panel. 7) The safety DOES NOT come because you tie any appliance to any mother earth ground i.e. if you were to drive a rod in the ground and wire the appliance to it, that WOULD NOT carry sufficient return current to trip the breaker as the earth is a poor high resistance conductor. The safety comes because the appliance it tied to the bare/green equipment ground conductor, which happens to be tied to the N buss at the main panel so the equipment Ground Buss is the path for that fault current so the breaker trips to clear the fault and de energize the appliances case. THERES NO HELP OR SOLICE OR ADVANTAGE IF THE APPLIANCE IS GROUNDED TO MOTHER EARTH, It needs bonded to the equipment ground to trip the breaker and save your life, grounding to mother earth is no help, you can still dieeeeeeeee if that’s the ONLY path for fault current back to the panel, its high resistance and the breaker don’t trip. 8) ONCE MORE it’s the Neutral that gets tied to mother earth, at the transformer and your service entrance, that has NOTHING to do with the Equipment Ground Buss. Even if the Neutral was NOT tied to mother earth, yet at the panel the N and G busses were properly bonded and there’s a short form L1 or L2 to to an appliance case, the equipment Ground Buss saves your life by tripping the breaker NOT BECAUSE OF ANY BOND OR NON BOND TO MOTHER EARTH ANYWHERE…The appliance case bonded to mother earth only dont help you if theres a shot of a live conductor to the appliance metal case……What saves your life is the appliance bonded to the equipment ground system REGARDLES OF ANY GROUND RODS OR CONNECTIONS FROM THE APPLIANCE TO MOTHER EARTH AND REGARDLESS IF N IS TIED TO EARTH
In closing longggggggg ago before I understood all this I thought like many here, “Hey at the Panel the N and G are tied together, whats all the fuss and difference, they’re basically the same” BUTTTTTTTTT in years of practice and taking NEC courses from the likes of McPartland and Holt I’ve learned these NEC rules are wellllllllll thought and reasoned out by experts and there’s darn good reasons why there are in place regardless if a lay person don’t understand why THEY ARE THERE TO AND WILL INDEED SAVE YOUR LIFE OR PREVENT A FIRE.
God Bless n keep yall safe and although the atheist and secular humanists cant stand this, I wanna say Merry Christmas, it’s a celebration of the birth of Jesus Christ and despite their objections, it even happens to be a Federal Holiday……………
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