On Tue, 2006-08-22 at 19:01 -1000, Ken Brown wrote:
>
>
> > I am changing out some 2 prong (non grounded) plugs with three prong
> > (grounded) plugs on some equipment to make the equipment more safe.
> >
> >
> Be careful about this. In equipment that uses a transformer power supply
> it is probably okay to do this. Just make sure that the hot lead of the
> power cord is the lead that goes to the fuse and the power switch. There
> may be some transformerless equipment that does not have one of the two
> wires connected to the chassis, and it may be okay to connect the
> chassis on this gear to the ground lead. I AM NOT ADVOCATING ANY
> MODIFICATIONS!
>
> Most transformerless radios have one of the leads connected to the
> chassis. Some have polarized plugs that are intended to insure that it
> is plugged in such that the neutral lead is the one going to the
> chassis. They are also built so that all of the parts you can touch,
> knobs, switches, case are either insulating material (bakelite or other
> plastic) if conductive they are insulated from the chassis and both the
> hot and neutral power leads. They are built to be safe even if you do
> get the plug in backwards. With this kind of radio there is usually no
> place to connect the ground wire, because the ground and the neutral are
> never supposed to be connected except at your building's load center
> (circuit breaker panel or watt hour meter box). The three prong plug may
> help prevent plugging the cord in backwards, but you cannot really use
> the ground wire without violating the rule of keeping ground and neutral
> separate.
>
> If you have GFCI outlets or breakers in your building, connecting the
> ground and neutral together should result in tripping the GFCI. A GFCI
> protected outlet should trip whenever the hot and neutral currents are
> not equal and opposite. With the ground connected to the neutral, the
> return current will be divided between the ground wire and the neutral
> wire. So the current in the neutral wire is no longer equal to the
> current in the hot wire.
>
> If you don't have GFCI protecting you, connecting the neutral to the
> ground can create a hazardous situation. The voltage drop from the
> current in the finite resistance of the ground wiring can raise the
> potential of the ground prong. Another device, with a three prong plug
> and grounded chassis, plugged into another outlet sharing the same
> ground wire back to the load center, will now have a small AC voltage on
> it's chassis, instead of the zero volts it would have if there were no
> current flowing in the ground lead.
>
> DE N6KB
>
And if the radio used a 0.05 paper to chassis from each wire to sort of
ground the chassis, the current in that capacitor will sometime trip
the GFCI, even if the capacitor is good. More often that paper capacitor
is quite leaky, why I said put in new ones rated for line voltage
applications (with lots of head room for transients).
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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