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Old 04-20-2015, 06:18 PM   #11
N3LYT
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Join Date: Jun 2012
Location: Maine
Posts: 709
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Quote:
Originally Posted by NN5I View Post
A bit simplistic, and not true on a three-wire or four-wire 120/240 supply.

Consider the first of two schematics. It represents 120/240 house (or RV) wiring with two appliances. One has 100 ohms resistance (maybe a 150 watt lamp) and the other has 1000 ohms (maybe a 15 watt lamp; the resistances are approximate). One lamp is on one leg of the 120/240 supply, and the other is on the opposite leg. Each lamp has 120v across it and all is well.

Now, in the second schematic, the neutral has opened, perhaps because of a dirty connector, or a broken wire, or a loose screw. The 240v is across the two resistors in series, 1100 ohms total. The current in both lamps is about 0.218 amperes. The voltage across the 100 ohm lamp is 21.8 volts, and the voltage across the 1000 ohm lamp is 218 volts. The 100 ohm (150 watt) lamp is dissipating 4.76 watts, not enough to light it up; but the 1000 ohm (15 watt) lamp is dissipating 47.6 watts and won't last very long.

Substitute your toaster and your soldering iron for the two lamps, and you'll soon need a new soldering iron.

So I guess your statement is almost true; a two-wire appliance won't work very long with an open neutral.

As for the isolated neutrals, they're isolated only from the service panel onward. There's only one neutral in the pedestal, and there's only one neutral in your shore-power cable, and most likely there's only one neutral all the way to that service panel (breaker panel) in your RV. Every neutral in the RV is connected to the one-and-only neutral in the shore-power cable and pedestal.
Yes that could happen in a twin 120 volt 50 amp set up but not in a 30 amp. Some devices with a 3 pin plug could still work with out a neutral if they had a grounded frame common to the neutral but a two prong plug with an open neutral won't. The service panel is the source panel with the neutral and ground bonded together the camper is the sub panel with an isolated neutral.
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