>The big hazard in the 3 wire situation with some load current in the
>common ground/neutral conductor is that when the ground wire fails, 120
>volts WILL be found on the case.
Probably the larger issue in the 3-wire 120/240V configuration (e.g.,
unmodified Alpha 70/77 series) is if neutral and ground are tied at the
equipment -- and neutral accidentally lifts above ground potential, then the
chassis becomes hot with 120v-circuit current and that current will find its
way back to its source via BNC, PL-259, RCA and any other connector that is
grounded to the chassis.
In fact, left unmodified without going to 240V/120V 4-wire service and power
cord, that current is still dividing between the equipment's neutral/ground
point and the grounded chassis connectors. It's a good thing that 99+% of
the time, the neutral/ground point on the equipment is of much lower
impedance back to the service panel than the chassis connectors!
So, left unmodified, older amps with 120V blowers are still a safety concern
if they are serviced by only 3-wires. There's only one right way: Pull
4-wire, 240/120v service into the shack with proper conductor and breaker
size, and replace the 3-wire power cord with a 4-wire power cord. The
latter part is easy with Alpha 70/77 series amps.
Paul, W9AC
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