On 11/10/2011 1:00 PM, rfi-request@contesting.com wrote:
> What difference does it make that it should ONLY be done
> at service panel?
The difference is what happens if a combined neutral/ground conductor
opens. With a combined conductor the phase-to-neutral parts of the
appliance will put a hazardous voltage on the chassis if that conductor
opens. That requires only a single failure to be hazardous. That's
particularly important for laundry dryers, which usually are near
plumbing fixtures, on cement (conductive) basement floors, next to the
(grounded) washing machine, etc. If this failure occurs, there is a good
chance of a serious or fatal electric shock.
With separate neutral and ground conductors, the operating current
returns through one wire (neutral, white) and there is no significant
current in the other wire (ground, green or bare). If the neutral opens,
the appliance stops working--but there is no shock hazard on the
chassis. If the ground wire opens the voltage and current on the chassis
are so low that they are not hazardous. It requires two failures to have
a shock hazard: an open ground conductor and an insulation failure
between the chassis and a hazardous circuit. This is much less likely to
happen than the single failure, which is why the N.E.C. now requires
separate conductors.
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