By Mike Sokol
Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM.
I’ve never really had to worry about this because I’m never connected to shore power when I run the generator. But, if that were the case, is there any risk of backfeeding to the pole if the generator starts and I’m already connected to shore power? (It’s a factory-installed Onan generator on a Vengeance Touring Edition toy hauler.) I’m asking because I’m considering installing an Energy Command Auto-Start 30 system to fire up the generator and keep important things powered and running, should I leave the camper for a few hours and the shore power fails. —Brian Z
This had a few answers on the RV Tips Facebook page, two of which are sort of urban myths.
This first answer from Nancy is what a lot of dealers tell their customers, but the dealers are incorrect.
I know very little about this kind of stuff but I know the dealer told us to never have the generator and shore power running at the same time. —Nancy L
While this answer from Dave has a bit of validity, the truth is that transfer switches really can’t fail in that mode.
Before I use one source, I turn off the other. Transfer switch can fail. —Dave S
The answer from Tom, below, is correct.
Generator is fine with shore power. Transfer switch handles it automatically, but if it fails (rarely) then shore power stays connected. All circuits (120 volt or 12 volt circuits) are protected from mix-ups like this… Not to worry about it. (from my decades of experience) —Tom M
And here’s my answer…
The ATS (or Automatic Transfer Switch) consists of one or two mechanical relays that are interlocked so that only one power source at a time can be connected to your RV’s electrical panel, which totally isolates your generator output from the shore power line. This is not like a home generator that’s been improperly installed on a circuit breaker panel without an interlock. The backfeeding of power from your home generator into the electrical grid where it can injure or even kill a power company lineman is a real thing.
However, RVs are built with a generator transfer switch so they can’t backfeed power into the shore power line with your onboard generator. The default condition is line power, and once the generator powers up there’s a 20- (or so) second delay before the transfer switch relay switches the RV over to generator power.
When the generator shuts off, the ATS switches your RV back to shore power, so you can run them both at the same time and nothing bad will happen. If you have an automatic generator starter and the shore power fails, the ATS will automatically switch over to generator power once its running and the 20 second time delay occurs. —Mike Sokol
For more advanced study on RV generator transfer switches and how they work, read my article about it HERE.
OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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