> To reiterate: The danger comes when neutral and ground are bonded together
> at
> the appliance AND both neutral and ground wires back to the service
> entrance
> become open. Under that circumstance, at least some of the "hot" AC will
> appear
> on the chassis of the appliance.
For that to occur with both the neutral and ground opening, the grounded
equipment chassis needs to find an alternate neutral path through other
connections (e.g., grounded PL-259, RCA jacks, BNC, a grounded operator,
etc.). This probably presents the most serious potential fault condition if
both ground and neutral become open.
However, it doesn't necessarily take the opening of both the neutral and
ground to present a dangerous condition. A danger still exists even when
neutral and ground are bonded at the equipment and there is no wire-open
fault of any kind because an equal portion of neutral current can divide at
the equipment chassis (say 50% neutral conductor; 50% ground conductor if
they're sized evenly) and result in two separate current paths back to the
service entrance panel. In this case, the danger is in the flow of current
through the ground wire -- when it shouldn't.
Witness the many older Alpha and Henry amps in service in North America
where 3-wire service is incorrectly used and although the ground lead is
correctly tied to the amplifier chassis, that same chassis is tied to the
neutral lead for the 120 VAC blower. For these older amps fed with 3-wire
service, ground and neutral become one. Of course, equipment with an
exposed chassis where the neutral and ground are tied would present a much
more dangerous condition if located outside near water or perhaps a damp
basement.
Another serious condition can still occur without both neutral and ground
opening if: (1) the neutral and ground are tied at the equipment; and (2)
only the neutral opens between the equipment and service entrance panel
while ground remains intact. In that case, neutral current divides and
flows through the equipment chassis, the ground conductor, grounded PL-259s,
BNCs, and a grounded operator.
Paul, W9AC
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