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bgflyguy

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I'm trying to trace down some of the 120v side of my "camper" and there is this box that I dont know what does. I have a couple of guesses, but I'm probably wrong. Hopefully it's easy to identify for those in the know, because I'm not in the know.
 

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The picture itself only shows a 30-40 amp 2 pole double throw relay. Cannot tell from picture if it is part of an automatic transfer switch or just activated by some other way like a generator or shore power source energying relay coil.

That could be called automatic but a true automatic transfer switch has two relays and additional electronics that primarily puts some delay gap on switch over to prevent a sudden AC source phase shift between two independent AC sources like a simple inverter and shore power or generator.

DIYer may not know that appliances like refrig and air cond can be damaged by a sudden AC source switch between two independent AC sources (not phase synchronous sources). It can be a little like throwing your car in reverse while driving forward.

Try to find a label on relay indicating what the coil voltage is. Likely either 120vac or 12v dc relay coil. If 120vac coil it is likely energized by gen or shore power. If 12v there is likely a manual toggle switch to power relay coil.

The coil draws about 2 watts of power so you usually want to wire relay so the generator or shore power runs the relay coil and when running on battery powered inverter the relay coil is not energized.
 
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To add to what RC said about ac source phase shift, you can find inverters with built-in transfer switches that will synchronize with an a/c source before switching (thereby avoiding any phase shift).
 
Okay, so just a relay, and in this case I'm guessing its 120v activated by shore power. The only wires are all 120. I dont see any other electronics.

So dont run the AC while plugging in shore power? This isnt a big deal for me right now, but it's good to know. If I did want this a little more robust could I just swap it out with a proper ATS?
 
Have you tried to contact the manufacturer to see if you can get a wiring diagram?
 
Okay, so just a relay, and in this case I'm guessing its 120v activated by shore power. The only wires are all 120. I dont see any other electronics.

So dont run the AC while plugging in shore power? This isnt a big deal for me right now, but it's good to know. If I did want this a little more robust could I just swap it out with a proper ATS?
A discrete ATS can't synchronize the inverter to the generator/mains and switch on the zero crossing.
That feature requires an inverter/charger.
The do make some inverters with ats but without charger, those would also work.
 
Do you have to have the synchronization? Is the delay enough to protect most loads? What is most common for mobile systems?

I'll probably just leave it as is. I dont need the auto transfer, as much as just need to insure the inverter and shore power are switched and protected.
 
Unless you have a specialized inverter to sync power virtually all RVs do not sync the power. A typical RV transfer switch will be normally closed to connect the utility cord. The switch delay is to give the on board generator time to get up to speed and begin regulating voltage before the switch activates. Delay is maybe 30 seconds. Then bam the switch is made and everything continues running. Best to reduce loads to lessen the wear on appliances and the switch itself. Although rare to start the generator if you actually have utility power.

That 2w coil will not add any significant power loss from the battery powering an inverter.
 
I'm not sure where I read it, either in my Onan manual or in the owner's manual for the trailer, but it is very specific that all loads should be turned off before firing up the generator as well as before turning off the generator. I'm pretty careful about that, but sometimes I slip up. I had my two stage air compressor connected when I fired up the generator. There is a delay for the interior AC circuits and then there seems to be a longer delay for the exterior circuits before they get power. I'm not sure how that happens. Either way, the air compressor didn't care but it's kind of industrial grade. I would be a lot more careful with sensitive electronics.
 
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