By Mike Sokol
I recently bought an RV with a built-in generator and the sales guy told me I had to be careful not to run the generator while plugged into a pedestal or I would “backfeed” electricity into the power lines and do something bad, like kill a line worker. So I have to unplug power from the pedestal before I start up my generator or be responsible for killing someone? Is that correct? I feel like I’m going to have a panic attack worrying about accidentally starting the generator while the power is on. —Fast Eddie
Dear Fast Eddie,
Well, like most urban myths, there’s usually a little truth buried in all the misconceptions and obfuscation, and this myth falls into that category. Yes, there is such a thing as a generator backfeeding electricity into the power grid and, yes, it does some pretty awful things. But NO, it can’t happen to your RV generator due to one special little box you probably have installed from the factory, the ATS or Automatic Transfer Switch.
The job of the generator transfer switch is to route current from the shore power connection into your RV’s power center whenever it’s available. But if the power fails for some reason and you need to turn on your generator, then the transfer switch changes over the connection so the RV’s power center is connected to only your on-board generator and is kept isolated from the shore power cord and the pedestal. When the shore power comes back on, you can take your time switching off the generator and the procedure is reversed.
You can see in the diagram that this 30-amp ATS has two separate relays, one of which is connected to shore power and the other is connected to generator power. What you can’t see in the diagram is many larger transfer switches have a mechanical interlock that won’t allow both relays to make contact at the same time. So a voltage detector circuit looks for generator power and if there is none (and the pedestal voltage meets it’s requirements), then the shore power relay makes contact.
However, if the generator is then started up the controller starts a counter running for 20 seconds or so, and if the generator power looks to be stable, then it first disconnects the shore power relay and then connects the generator relay. The mechanical lockout is an extra safety on large switches which prevents a controller failure from accidentally energizing both relays at the same time that could actually create a backfeed situation. But don’t worry, your ATS makes sure that can’t happen.
But what is an actual generator backfeed the sales guys was describing? Well, that’s when someone in a house during a power outage connects a generator into a wall outlet and “backfeeds” it to the rest of the house. To do that they use a male-to-male suicide cord. Now, unless they remember to also turn off the service panel’s main circuit breaker, the generator power will indeed backfeed into the power company’s line transformers and step up your generator’s 120 volts to around 14,000 volts. And if a power company worker is trying to reconnect downed power lines that were supposed to be unenergized, a line person can indeed get killed from the generator in your house backfeeding power into the electrical grid.
If you do want to use a backup generator at your house you’ll need to install some sort of generator isolation switch, just like your RV has. It can also limit the generator power to appliances that won’t draw too much current, so you don’t accidentally turn on your oven and kill the genny power.
So, in short, your RV salesman doesn’t know what he’s talking about. You already have an Automatic Generator Transfer Switch in your RV, so you can start up your generator while connected to shore power anytime you like without any worries at all.
Hope this helps relieve your panic attack. All’s well….
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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