Re: O/T Electric wireing

Posted by KEB on March 29, 2006 at 20:14:06 from (207.94.26.1):

In Reply to: Re: O/T Electric wireing posted by VADAVE on March 29, 2006 at 11:35:44:

VADAVE: Not all faults blow a breaker. It is entirely possible to have a fault that will supply enough current to give a fatal shock without the breaker ever knowing it ever happened.

An example here would be somebody using a drill under the kitchen sink. A fault between the motor windings and the drill case (assuming its not a double insulated one) would make the case hot with respect to the plumbing, which is connected to earth. You're doing really well to get the resistance of a single ground rod down to less that a few 10's of ohms, so there will be some resistance in the path back from the pipe to the ground rod to the neutral buss in the breaker panel. 120 volts across 20 ohms is 6 amps, well below what will pop a breaker but many time higher than the current necessary for a fatal shock.

A safety ground wire will have a lot less resistance, and would therefore carry the fault current back to the breaker instead of the current flowing through a hazardous path. Without the safety ground wire, the drill would operate normally even though the case is at 120 volts above ground.

Gound fault interrupters in high risk places such as kitchens & bathrooms add another layer of protection against fault conditions that don't blow breakers. A ground fault interruptor monitors the current in the hot & neutral leads, and opens the circuit if there's ever a difference. Any difference in current means that some of the current is flowing in a fault path somewhere instead of returning on the neutral.

Codes are in place for a reason, and even though that reason may not be intuitively obvious, I can gurantee you that the codes are based on well thought out engineering practices.

Keith

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