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. I've know that to kill appliance users
on the spot. By running separate neutral and ground if the neutral opens
the user is still protected by the ground though the 120 volt
application fails to operate. If the ground fails the situation is safer
than in the 3 wire case, but its difficult to predict the possibility of
shock because it depends on the quality of the insulation of the appliance.
I've had to explain at least one such death in my working career, not
caused by me but as an expert witness hired by the attorney for the
widow and the estate.
73, Jerry, K0CQ
On 4/4/2011 12:59 AM, Ken Brown wrote:
> Why the requirement for separate ground and neutral wires is reasonable
> and not just the "result of nanny state busybodies."
>
> In a device that runs on 240 VAC the currents in the two hot wires are
> equal magnitude and opposite phase. No current flows in the ground wire,
> except in the case of a fault. The I * R voltage drop in the ground wire
> is zero, since I = 0. This keeps the chassis of the device at ground
> potential, zero volts relative to ground. This is a three wire system,
> which is safe and conforms to present day codes.
>
> If there is a portion of the equipment (such as a motor, clock or
> control circuitry) which is run on 120 VAC, not using a 240 / 120 step
> down transformer inside the device, powered by one hot lead and using
> the ground wire as the return, the current in the ground wire is not
> zero. The I * R voltage drop in the ground wire is some finite value
> greater than zero, and the chassis of the device is at some potential
> greater than zero volts relative to ground. This is an unsafe system
> which does not conform to present day codes.
>
> If there is a separate neutral wire for the 120 VAC load(s) in the
> device AND a ground wire, the ground wire carries no current, except in
> the case of a fault. The I * R voltage drop in the ground wire is zero,
> since I = 0. This keeps the chassis of the device at ground potential,
> zero volts relative to ground. This is a four wire system which conforms
> to present day codes.
>
> DE N6KB
>
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