RVelectricity: Will my RV generator backfeed power and kill someone?

14

By Mike Sokol

Dear Mike,
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.

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.

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Jim
4 days ago

So my question is how do I know if I have the transfer switch and where would I look? 2017 forest river toy hauler 4kw Onan gen

Brian Moore
4 days ago

I thank you for this info. Very helpful. I enjoy reading your column and learning more about basic electricity. You make it easy to understand with your writing skills. I just wish all the other electricians didn’t always try to “school” you.

Rory R
3 months ago

You may be surprised at some of the ol’ wives’ tales, told by sales personnel on an RV lot to potential buyers.

GaryM
3 months ago

I had a class A motorhome in the late 90’s that had an Onan generator. It had an electrical outlet in the basement storage box where the shore power cord was stored. In order to use the generator you just plugged the shore power cord into the receptacle box. No way to use both at once.

Phil Atterbery
3 months ago

I wish there was an RV salesman certification. It would help wipe out some of the sources of bad information. Thank you for your continued battle for truth, facts, and the safety of RVers everywhere.

Wolfe
3 months ago

Mike: I would be careful being over-confident. Every coach with a built-in generator absolutely SHOULD have an auto transfer switch, but I don’t trust manufacturers not to cut corners even when risky, and I don’t trust “helpful” dealers and DIYers not to get it wrong with double connections to the bus. So, I’d say confirm that you DO have an auto transfer switch. If not, I know where to get them for $30-40.

Nanci: Precedence in an RV should be for the generator, while in a house it’s for the line input. That’s not guaranteed, but “typical” setup because the assumption is that if you’re running the genny while there is line power, something is wrong with the line and you intended to transfer. At home, the power was probably out and you prefer the line once it’s good again.

Jim: You are correct that the backfeed is likely to only last as long as whatever the circuit breaker takes to overload. But, that CAN be long enough for the backfeed to hurt someone. I’d stay with “be safEST rather than pretty safe.”

Nanci
3 months ago

We exercise our generator every month and I usually shut off shore power in order to put a full load on the generator. Which takes precedence with the transfer switch if the 50 amp power pedestal would stay on and the generator is on too? Can I exercise the generator without turning off shore power?

Jim Thomas
3 months ago

Mike, I agree with everything you’ve said here, with one caveat. While it is possible for a generator to backfeed the utility when improperly connected, the chances of it doing that for more than a second is very unlikely. Remember all the connected loads on that line. Assuming the RV generator could successfully provide the inrush current to energize the pole mounted transformer, it would immediately try to provide power to every load in every house on that sub-distribution line. Not gonna happen. It might be possible for a lineman to get hurt working next door to your house for the moment the line could get energized, but in reality, it’s an extremely remote possibility. That said, the use of suicide cords and the like are never a good idea. (I both worked in the power industry and was an Onan Level III RV generator tech).

Kevin
3 months ago
Reply to  Jim Thomas

Mike I agree with what your saying and it is correct. But I need to clarify just one point. If a transformer is serving several homes then yes a generator would trip out as there is no way it could carry the backfeed load. But in the more rural areas it’s not uncommon for a single transformer to feed just a single house. Under these conditions if a Lineman were holding the hi side leads or touching the hi side bushing and provided a path to ground or to the primary wire then it might be lights out permanently for him/her. That’s why we always pulled the house meter just incase. Seen allot of strange things over the years.