Another ground question

On Wed, 16 Mar 2005 20:34:55 -0600 Brian Dugas wrote:

| Ok, I've been doing some reading to help myself out on all this theory | stuff. :) Most of the macro-level concepts are making sense. I am trying | to learn just because I am curious by nature. | | Anyway, my question is in regards to the grounding wire. The | grounding(green) wire of an appliance is usually connected to the case. | This way, if the hot wire comes loose in a device and touches the case, the | grounding wire carries the current back to the breaker panel where the | grounding wire is bonded to the neutral, and a short circuit is allowed to | trip the breaker. During the short time the grounding wire is energized, do | the rest of the grounding wires in the house carry a current (or voltage) | until the breaker trips? I mean, all the grounding wires are connected in | the breaker panel right? | | My instincts tell me that the current will take the path of least | resistance, which would be up the neutral wire back up to the transformer on | the pole. But, let's say an earthquake hits and breaks the neutral wire | coming into my house. At the same time an appliance falls off a shelf, | breaks internally, and now the hot wire is touching the case. Well, the | circuit can't complete back to the transformer, and from what I understand, | a ground rod at the main service entrance is not gonna carry enough amps to | trip a breaker. So are all the grounding wires now energized? | This may not seem like a real practical hypothetical, but it illustrates my | point a little better. | Under this hypothetical, if all of the grounds are now carrying a voltage, | then doesn't that mean any appliance I touch with a metal case will zap me? | Thanks to anyone who wishes to enlighten me on this. | Brian

Yes, in this case the grounding wires will be energized along with the neutral. Loads on the phase side the energizing comes from should see their voltage drop to near nothing. Loads on the other side will now see upwards of 240 volts. It's open neutral made worse.

Just how much you get zapped depends on how good a path to ground you are making. If you are well isolated from ground, you might not get much. If you are barefoot on wet concrete, you could get 120 volts and some number of milliamps depending on ground path resistances, etc.

So is this a good enough reason to become Amish?

Not necessarily. It depends on the voltage on the appliance you are touching with respect to ground, and the resistance through you to ground. The voltage on the appliance you are touching could range anywhere from 0 to 120 volts, and will be different on various appliances throughout your house, depending on where they are on the circuits in your house, and their resistance to ground via the equipment grounding conductor and the grounding electrode system, as well as the resistance to ground of the broken appliance that contains the short.

Ed

transformer on

That will happen only if the is no earth ground at the meter. Assuming the returns are still grounded though the ground stake at the meter, the breaker will trip.

Are you saying that given my original scenario, the breaker will trip with no neutral connection to the transformer, and only the ground rod at the meter? I read somewhere that the ground rod wouldn't even conduct 1 amp. How would it conduct enough to allow a 15A breaker to trip? Guess if you had some really rich soil it would work.

If you have a good ground (low resistance), the dangerous voltages at the fault point will be near zero. The voltage drops distribute themselves by Kirchoff's Voltage Law which sums to zero and individual drops depend on the individual resistances in the circuit. The key resistance point must be set low enough to allow enough current to flow to trip the breaker or blow the fuse. In the USA for residences, the code says that this ground resistance must measure 25 ohms or less.

If you have a high resistance or an open ground, you have a highly dangerous situation in that the case of your appliance will now be elevated to near or full line voltage (and possibly use you as a ground if indeed you are standing barefoot on a damp floor or touching a water pipe or faucet).

A good ground system is best of all and much preferred to a system without a ground.

A faulty or open ground is worse than no ground at all.

Beachcomber