R. Measures wrote:
> On Jan 5, 2006, at 2:08 PM, John Popelish wrote:
>> R.Measures wrote:
>>> Without a ground/bond wire wouldn't connecting the Neutral wire to the
>>> metal enclosure hold the enclosure to a safe potential until the
>>> breaker trips?
>> The neutral carries load current,
>
> • In a 240v appliance, why would it?
If there is actually no load of any kind connected to the neutral and
that line is dedicated to that appliance (not shared with any other
loads) then it is effectively an equipment grounding conductor, not a
neutral. But many 240 volt ranges, for instance, also contain clocks
and timers and oven lights that use the neutral as a load carrying
conductor, so this precludes that line from being a legitimate
equipment grounding conductor.
>> so it may have some voltage drop at
>> the appliance end, compared to the end at the power panel. And if it
>> ever goes high resistance, then the moment any load is connected to
>> it, it has full line voltage on it. So a single failure (open
>> neutral) would produce a shock hazard if the case were attached to it,
>> the moment the appliance was turned on (a moment someone is likely to
>> be touching the case, also).
>
> • For a shock hazard to exist, the person would also need to be
> grounded. The Neutral wire on the 240v outlet in my radio room is
> #6-ga. What would make the resistance of the Neutral go high and
> simultaneously connect one Line wire to the enclosure?
A lose connection at either end would be the most common cause of a
high resistance neutral. If that neutral had also been used as an
equipment grounding conductor (safety ground) than the case would be
hot the moment the equipment was turned on.
Even if the neutral is well connected, in the event of a line to
neutral fault in the equipment, the two sides of the line circuit
would each drop about half of the line voltage till the over current
device opened, and you could receive a shock during that moment. This
case of shock hazard also applies to safety ground conductors, and in
many cases is worse, because safety ground conductors are allowed to
be smaller than line conductors, in many cases.
Even when the neutral is intact, there must be some voltage drop
(resistive and inductive) across it because of load current. You
would get some very nasty ground loop currents when different pieces
of grounded equipment were connected together (with signal wiring or
just incidental contact), if each used its own neutral as case ground.
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