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Thursday, October 22, 2009

AC Electrical Enigmas

I have a 31-foot Southwind motorhome. Since I purchased the rig one of the 30-amp power cord prongs shows evidence of arcing. One prong is burnt looking with the rubber around it melted slightly. I replaced the male end but now the new plug is doing the same thing. Any idea on what is going on or what tests to run to find the problem? Also, I recently installed a PROsine 2.0 inverter and all seemed well with one large problem that I cannot run the air conditioner on the generator power. Shore power will run the air conditioner with no problems. When the air conditioner is turned on all AC power in the motorhome shuts off with a time delay of 1 to 2 seconds before coming back on. There was no problem running the air conditioner on shore or generator power prior to the installation of the inverter. I do not think there is a shore power/generator power transfer switch involved here because I have a separate female receptacle in the rear compartment that the power cord must be plugged into in order to get generator power.
Daniel, (Las Vegas, NV)

Daniel, I’m thinking you have two separate issues; the first being the burned shoreline plug and the second being the inverter/air conditioning problem.

The arcing shoreline plug could become a potentially dangerous situation so begin your troubleshooting with that one. Start by checking all the receptacles inside the coach with a simple plug-in circuit tester like the one shown here. Inexpensive testers are available at building and/or hardware stores as well as RV accessory stores. With the coach plugged into shore power, plug the circuit tester into every wall outlet in the coach and verify the polarity is correct and that a good ground exists.

Next, using a multi-tester or voltmeter, perform what RV technicians call a “hot skin” test. With the coach plugged into shore power, set the meter to AC Volts and attach the black probe to a good earth ground such as a water pipe, building ground rod, or other buried metal object. Touch the red probe to several different metallic surfaces on the RV. If you get a voltage reading at any location you have a hazardous situation; a leakage of alternating current to ground.

It may be necessary to have a shop perform an insulation breakdown test, also called a hi-pot test to determine if a short truly exists. Too much current is likely not the problem since the coach is protected by a breaker at the power pedestal as well as inside the coach. Even if one of the breakers is faulty, which is unlikely, it is highly unlikely that they both would be flawed.

The other possibility is a direct short. Although under normal circumstances, a circuit breaker will protect against a short circuit, there are cases where they simply might not. With the shore power cord disconnected, use an ohmmeter and make sure there is no continuity between either of the line prongs and the ground prong. I suspect that one of these tests will reveal a specific wiring problem in the coach.

In terms of the air conditioner problem, this may be a relatively simple issue. The inverter is capable of delivering 2,000-watts which is actually just under 17-amps at 120-volts AC. Obviously this is not enough to power the air conditioner which typically requires a dedicated 20-amp circuit. If we assume that the inverter was correctly installed, then it has been properly isolated from the converter and the converter has been modified, if necessary, to accommodate the inclusion of the inverter, thereby bypassing its own charge module. The PROsine inverter includes a charge module and automatic transfer switch. The fact that the air conditioner causes the coach to lose power for a few seconds is indicative of the inverter trying to supply the power to the A/C instead of the generator being the voltage source. Simply what is happening is the inverter is going into over-current mode, shutting down and then resetting. This is likely caused by either a problem with the way the inverter is wired or it could be a generator output issue. It is possible that the inverter transfer switch is not sensing the generator output, although it is sensing the shore power. If this is the case, then the generator output is flawed, but since the A/C worked fine on generator power prior to the inverter installation, check that the installer didn’t inadvertently trip the genset breakers.

It’s possible that what you believe to be the air conditioner not working off generator power is actually the generator not providing any output at all, where everything is trying to run off the inverter. That is until you turn on the air conditioner. Then, of course, the lack of available current shuts down the inverter. Check the circuit breakers located on the generator itself and see if they are tripped. It would be a good idea to have the generator output voltage and frequency checked by a professional as it is possible that the inverter is more sensitive to voltage and/or frequency than the air conditioner.

So it appears you may have two completely different, yet similar, electrical issues going on. It’s imperative that the above tests be performed properly and expertly to gain a clearer understanding.

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