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Old 06-23-2018, 05:50 AM   #1
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Generator vs Shore power

Is there any problem running the generator while plugged in to shore power? We will be staying at a friends house in his driveway and will be using an adapter to plug into a 110 outlet. Maybe enough to run one AC but not two during 95 degrees heat. So we will use our generator during the day and just use the 110 at night for one AC. Just wondering if I have to unplug the 110 outlet each time I turn on the generator. Thanks
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Old 06-23-2018, 06:02 AM   #2
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You should have no problem doing that. The generator will have priority.


We just went through 3 days of campground power going on and off and the generator would run everything fine even with park power back on (we left it on due to the fact power from the campground would drop too low and couldn't handle the load)
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Old 06-23-2018, 06:11 AM   #3
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The auto transfer switch disconnects the shore power, and connects to generator, when it senses power from the generator, so doing what you propose is fine. There is no chance of crossed lines.

You will want to switch off the ACs for 5 to 10 minutes between transfers, to let the compressors equalize their pressures.
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Old 06-23-2018, 06:55 AM   #4
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You have two yea’s, and now a nay.
I agree with what has been said above, that the generator will have priority and the transfer switch will take care of business.
But, parts can, and do, fail. It’s so simple to go unplug from the shore power and then fire up the generator, why not just do it? If the transfer switch should fail and you get power feeding from both sources there’s a good chance it will ruin your day.
Most likely the transfer switch will not fail to that degree, but why chance it?
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Old 06-23-2018, 07:22 AM   #5
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You have two yea’s, and now a nay.
I agree with what has been said above, that the generator will have priority and the transfer switch will take care of business.
But, parts can, and do, fail. It’s so simple to go unplug from the shore power and then fire up the generator, why not just do it? If the transfer switch should fail and you get power feeding from both sources there’s a good chance it will ruin your day.
Most likely the transfer switch will not fail to that degree, but why chance it?
The contacts in the auto transfer switch are normally closed. That carries the shore current thru the contacts to the RV.

When the generator power energizes the coil, an electromagnet in the relay pulls the shore power contacts open before the generator contacts can close. If they stick closed, there is no transfer, so no danger of backfeeding or cross connecting.

If somehow, in the unlikely event that the relay came completely apart and did cross connect or short out, there are circuit breakers in the RV panel, generator and shore power post, waiting to do their job.

There is just no reason to go out and pull the plug.
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Old 06-23-2018, 07:24 AM   #6
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I'm sorta in the camp the I really prefer to disconnect shore power then fire up the generator and visa versa. But...I also tend to be to sympathetic to old wive's tales too. LOL

OTOH, and I say this with all due respect to twinboat (a really smart dude whom I wouldn't normally cross swords with. ) I don't think you need to switch off the AC units to protect the compressors. They have their own timing mechanisms to do that. (Ya, I seem to pick wives tales I like/agree with too. LOL)

OTOH, when you are going from generator to 120V you need to shut off the ACs and any other high draw devices. This is to allow the generator to run at idle for a few minutes to cool down before you shut it down. This would also seem to minimize the chance for a jolt of the ATS switching under a high load so you might consider that if you decide to stay plugged into shore power.

Not to twist this off topic but...

There could be something you haven't thought about. I would suspect that the 120V outlet you are going to plug into is on a GFI circuit. This might not work in some cases. It seems that I have seen (and others report) that if you have an RV refer they seem to cause GFIs to trip. This mostly seems to occur with those that have ice makers which in my case was the cause. I unplugged it and no GFI tripping. I don't know if that applies but the info is yours for consideration.
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Old 06-23-2018, 07:27 AM   #7
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The contacts in the auto transfer switch are normally closed. That carries the shore current thru the contacts to the RV.

When the generator power energizes the coil, an electromagnet in the relay pulls the shore power contacts open before the generator contacts can close. If they stick closed, there is no transfer, so no danger of backfeeding or cross connecting.

If somehow, in the unlikely event that the relay came completely apart and did cross connect or short out, there are circuit breakers in the RV panel, generator and shore power post, waiting to do their job.

There is just no reason to go out and pull the plug.
Aren't both contactor's in a transfer switch normally open till a electrical input closes them Twinboat??? and with loss of electrical source they open.......
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Old 06-23-2018, 07:30 AM   #8
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Aren't both contactor's in a transfer switch normally open till a electrical input closes them Twinboat??? and with loss of electrical source they open.......
Answer at 16S https://youtu.be/snNMsPWr-7o?t=16s

Making sure I'm not arguing with TB on this one. LOL
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Old 06-23-2018, 09:51 AM   #9
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The transfer switchs that have surge protectors, and most 50 amp switchs, as in the video will be normally open, as pointed out. I believe they have mechanical lockouts that will not allow both relays to be closed at the same time.

A single relay 30 amp transfer switch will be normally closed. There are 50 amp transfer switchs that are NC also. See below.

On my AC, the delay is in the thermostat. Since that is powerd by 12 volts or battery, and it stays powered without 120 volts, the delay doesn't kick in.

So, like any RV, it depends on the system you have.
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Old 06-23-2018, 10:10 AM   #10
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Switching with high current loads active (such as air conditioning) causes arcing of the relay contacts in the switch. This usually does not cause immediate catastrophic failure - rather there is a cumulative degradation of the relay contacts over time.

Disable high current draw items before switching. Or better to disconnect shore connection before switching.
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Old 06-23-2018, 10:11 AM   #11
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I used to leave mine plugged into shore power at home and occasionally turn on the generator to exercise it or needed additional power for AC's ect. I noticed a louder clunk(more than the normal) when I did this. My Auto Transfer Switch went out when it was less than two years old. Discussing it with the mechanic he said all wire connections were tight so it could have been one of two things, one the switching back/forth without unplugging shore power damaged the ATS, or two luck of the draw and I just got one that was bad to begin with. Since that time I now unplug before turning on the generator, and have not had any further problems. It only takes me a few seconds to unplug and start up the generator so I just don't take the chance anymore.
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Old 06-23-2018, 10:28 AM   #12
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If it was my RV I would disconnect shore power before starting generator. Then going the other way I would shut generator down before plugging in to shore power with both A/C shut down.
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Old 06-23-2018, 10:41 AM   #13
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I used to leave mine plugged into shore power at home and occasionally turn on the generator to exercise it or needed additional power for AC's ect. I noticed a louder clunk(more than the normal) when I did this. My Auto Transfer Switch went out when it was less than two years old. Discussing it with the mechanic he said all wire connections were tight so it could have been one of two things, one the switching back/forth without unplugging shore power damaged the ATS, or two luck of the draw and I just got one that was bad to begin with. Since that time I now unplug before turning on the generator, and have not had any further problems. It only takes me a few seconds to unplug and start up the generator so I just don't take the chance anymore.

What is the purpose of an Automatic Transfer Switch if not to make the switch when plugged in? Not trying to argue, it's just the logic of the mechanic doesn't make sense to me. I can see taking some precaution by turning off high draw AC devices before making the switch, but to unplug beforehand seems to defeat the purpose of the ATS. If switching to gen power while still plugged in to shore was detrimental to the ATS, wouldn't the operator's manual state not to do that?


I have looked at both 30 amp and 50 amp instruction manuals (of course, not all possible makes/models), and they both state, in the testing of function section, that shore power should be ON when testing for proper switching to gen power. Both state to turn off high draw devices beforehand, but neither state, as an operating procedure, to disconnect shore power before starting the generator.


If one wants to unplug first because it gives them peace of mind, that's certainly worth doing. I'm just questioning the logic of the mechanic in stating that the operation of the switch as it was designed, contributed to the failure. That's like saying your bicycle derailleur broke because your were pedaling while changing gears.
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Old 06-23-2018, 11:18 AM   #14
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Well out of all the systems on my MH, or house for that matter, the electrical part/knowledge is my weakest point. I do all my own maintenance and am able to maintain/repair just about anything on the MH. Replacing electrical parts is easy, but trying to find what caused the damage or failure to begin with is a whole different thing. So I just followed his advice.

The ATS is only there to figure out what source of power you are using, not necessarily just to keep switching back and forth, at least that was the way it was explained to me. To me it really isn't that big of deal to unplug when I'm going to use the generator. It may be an urban legend that keeps me doing that, but the louder clunking when making the change over when still plugged in didn't sound like it was doing it any good.
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