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Thursday, February 23, 2012

RV Inverter AC Ground Problem

We sure enjoyed your seminars at the FMCA Rally! We have a Magnum inverter in our Monaco. On shore power and generator power, all the receptacles check “good” using a three-light, plug-in tester. However on inverter power, the receptacles show an open ground on both circuits. I have checked connections in the inverter block and it is wired correctly plus all connections are tight. The same in the main breaker panel. I have owned this coach since new and have used the inverter only twice for short periods of time, so it may have come out of the factory like that. Since everything worked, I never gave it a thought to test the circuits. I was checking which circuits were inverter powered with the idea of installing a residential refrigerator at some point. Bill W., (Gilbert, AZ)

Bill, I’m not a huge fan of those plug-in testers. I, too, used one for many years until one of my electrical engineering buddies found some inconsistencies while comparing and using them. Now I always rely on a digital volt, ohm multimeter (VOM), for all electrical measurements just to be safe. But you state your circuits work fine and show the correct polarity using the generator or the shoreline, so chances are your plug-in analyzer is working. But to be sure you indeed have an open ground situation on the inverter output, it’s better to use a VOM while troubleshooting. There should be an integral ground connection from the main panelboard distribution box through the inverter and any sub-panels, and ultimately to chassis ground. Here’s a paragraph from the Magnum manual for your inverter:

Wiring the Inverter AC Ground 1.  Combine the ground (green) wire from the main panel’s ground buss bar and the ground (green) wire from the sub-panel’s ground buss bar (or the ground connection on the outlets, if using the output breaker versions). After these grounds are combined, use field wiring leads to connect them to the inverter’s AC GROUND (green) wire.

So be sure you can read continuity between the ground buss bar in the main panel and the ground terminal on the Magnum. The ground path should remain constant throughout the entire AC circuitry. The hot and neutral conductors will be switched through the automatic transfer switch, but the ground conductor should not be switched. It could be a ground wire came loose some where in the system between the inverter and the breaker box(es). But measuring with a VOM should determine if and where the open exists. Since an open ground circuit is potentially dangerous, it may behoove you to contact a certified RV technician to perform further testing and measuring just to be safe. 

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