Doug Miller wrote:
In article , Jeff Wisnia wrote:
What I'd like to learn is just what kind of faults and accidents drove
the code to require a separate ground conductor for those kind of
dedicated 230 volt appliance circuits, mainly clothes dryers and stoves
I suppose. By "dedicated" I mean that the circuit only serves one
receptical/appliance.
First off, note that clothes dryers and stoves are *not* purely 240V
appliances: the heating elements are 240V, but the control circuits (and, in
the case of a dryer, the motor) are 120V and thus need a neutral.
The neutral in a 120V circuit carries current. When the equipment chassis is
bonded to the neutral conductor, any person touching the chassis becomes a
secondary, parallel path to ground for the return current. Under certain
adverse circumstances (e.g. barefoot on a wet concrete floor) the possibility
exists for hazardous levels of current to flow through that person - even when
there is no fault in the wiring or in the appliance.
You're correct of course, I considered that parallel path condition and
asumed a maximum of 10 amps of motor current flowing on the neutral
lead. With 75 feet of No. 10 wire, 10 amps will create only about 0.9
volt of drop in that neutral lead, and I figured that if you could drive
enought current through a human body with that low a voltage, even with
wet skin, I would have heard about people electrocuting themselves with
a single D cell battery by now. G
Wiring faults (e.g. a high-resistance connection in the neutral) greatly
increase this hazard, again even when there is no fault in the appliance.
I can't argue with that either.
These risks are essentially eliminated when the neutral is isolated from the
equipment chassis, and the chassis is bonded to ground.
Yes, as long as the chassis ground doesn't develop a high resistance or
open too.
All that said, I have to agree that the fourth conductor does add
another level of protection.
Jeff
--
Regards,
Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt.
And who wants to leave buttprints in the sands of time?
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
"Truth exists; only falsehood has to be invented."
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