Scary Thought - Comments Please

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busman

busman

Senior Member
Location
Northern Virginia
Occupation
Master Electrician / Electrical Engineer
I was installing a Generac Generator the other day with a 12 space automatic transfer panel. While choosing the 12 circuits that would be attached to the transfer panel, I realized that if the existing house wiring has 2 circuits tied together at some point (we've never seen an unintentional joining of 2 circuits before) and one circuit was in the transfer panel and the other not, then this would bypass the effect of the transfer switch and place voltage back on the main bus and the power lines.

I know we cannot be responsible for others mistakes, but this seemed pretty scary. Any thoughts? Has this happened? I thought to test for voltage at the main bus with the generator on, but this would not cover connections thru switches.

Thanks,

Mark
 
Re: Scary Thought - Comments Please

Mark
That is an absolutely excellent point! As a result of this post, I am going push for our company to always test for voltage on all branch circuits that are not in the standby power panel. We need to be sure this scenario does not exist on a new generator system installation in an existing residence/business!!!
Thank-you!
 
Re: Scary Thought - Comments Please

Makes sense that it might happen.

Turn on all the switches in the place, then dump the service disconnect. Turn on the standby and test for voltage on the load side of the service disconnect.

In my experience, the double fed circuit shows up rarely, but when I find it, there is no pattern. Good workmanship, sloppy, and DIY, all have had the problem. . .just rarely.
 
Re: Scary Thought - Comments Please

Thanks for the replies. I originally got to thinking about this in wondering if there was any condition in which a running generator could get connected to live utility lines (still trying to decide what the outcome would be if it did happen). I decided that if the two-pole breaker that feeds the generator panel were to trip then the generator would start and transfer its branch circuits onto generator power while the rest of the circuits in the house would still be on the utility. Then came the idea of circuits crossing the boundary downstream.

I guess the only really reliable method would be to trace every outlet in the house to insure that none connect to more than one breaker. I'm not planning to start doing this, but I will continue to check the mains for voltage with just the generator on.

Thanks again,

Mark
 
Re: Scary Thought - Comments Please

Dont think it would ever happen. First of all the generators' controls sense when power comes back on and safely shuts down generator. Generator kicks on when commercial power is lost. Even if you did a test, you would kill the main and it would never backfeed because the main would be off. Lets say the power goes out, generator kicks in, when power is restored generator would safely shut down. Your statement just wont happen!
 
Re: Scary Thought - Comments Please

Chevy, that only applies to a whole-house transfer switch, one in the main feeder.
 
Re: Scary Thought - Comments Please

On the generac dose it take 1 or both poles on the gen main to kick the A.T.S. siloned ? with the above case only 1 side of the bus would have power so only on leg of the gen main.
 
Re: Scary Thought - Comments Please

Even so, if it was only the breaker to the generator, the generator power wouldnt kick in because it would still sense commercial power. Thats the safety in the controls! His statement just wont happen!
 
Re: Scary Thought - Comments Please

This would involve some luck as well because the two circuits that are somehow tied together would have to be on opposite phases or they would trip under normal usage. If it were a 240v branch circuit then it wouldnt be two circuits but one. Also depending on the xfer panel the power backfeeding back to the panel would then reenter the xfer panel and trip that individule xfer panel OCP for that circuit. In addition to that how much current will it take to excite the secondary windings of the utility xfmr. Remember there is no power in it so reactance is 0 and pure resitance is low. If the given problem circuit is a 20 or 15 A it will mostlikely trip out befor it can develope enough feild to induce a voltage on the primary of the utility.

I think the situation is highly unlikely. Maybe mythbusters can play with this one. ;)
 
Re: Scary Thought - Comments Please

Originally posted by mikeames:
This would involve some luck as well because the two circuits that are somehow tied together would have to be on opposite phases or they would trip under normal usage.
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Nah, that's backwards. For the problem the OP described, the two combined circuits would have to be on the same phase. If they were on opposite phases the breaker would be tripping when he got there. :)
 
Re: Scary Thought - Comments Please

My biggest concern is for a sub-panel arrangement where a large 12KW+ generator is going to be used. That generator could conceivably fire up one of those little 15KW utility transformers. If it did, I know it would only last for a second or less as the generator would become overloaded while trying to power all the neighbors homes off of half its windings (assuming only one 120V circuit was crossed). However, that one second in time could be enough to possibly injure a POCO lineman.
 
Re: Scary Thought - Comments Please

My biggest concern is for a sub-panel arrangement where a large 12KW+ generator is going to be used. That generator could conceivably fire up one of those little 15KW utility transformers. If it did, I know it would only last for a second or less as the generator would become overloaded while trying to power all the neighbors homes off of half its windings (assuming only one 120V circuit was crossed). However, that one second in time could be enough to possibly injure a POCO lineman.
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Ram, read the above posts, the original statement and what you're thinking just what happen. The above posts tell you why!
 
Re: Scary Thought - Comments Please

Nah, that's backwards. For the problem the OP described, the two combined circuits would have to be on the same phase. If they were on opposite phases the breaker would be tripping when he got there. [Smile]
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George if you read what I wrote, and maybe it wasent clear for some, I said the same as you.
 
Re: Scary Thought - Comments Please

Never installed one of these. It is my understanding that those circuits in the transfer panel would not be landed anywhere in the main panel anymore. So regardless of the ATS safety features, if the breaker feeding the x-fer panel were to trip, and the generator energized the emer. circuits, they still would not be connected in any way to the main panel because the 2-pole breaker feeding the x-fer panel would break the wiring path at the breaker when it tripped. That would not help if a emer. circuit was parallel fed by a breaker in the main.
Best solution, sell the homeowner a whole house backup and put the x-fer switch on the mains. Then he can watch tv in his hot tub with a cold one from the frig while his neighbors cuss him out of the darkness.
 
Re: Scary Thought - Comments Please

Originally posted by mikeames:
George if you read what I wrote, and maybe it wasent clear for some, I said the same as you.
Click to expand...
Is this a "go with what I mean, not what I say" deal? :)
 
Re: Scary Thought - Comments Please

That is a good point. How does one approach splitting out circuits that are home run? I have personally never installed one of those "generator" panels where the critical loads are to be moved to it would be a fairly simple process if you were dealing with individual circuits and not one ones that are 3w with shared neutrals. Since both panels would have neutrals from a common point it would be imperative that the line of a 3w circuit being moved to the generator panel be connected to the correct bus on the generator panel in order to remain opposite of the line that has not been moved. But two problems will remain. How does one address the requirement where there is the possibility of the (2) lines landing on the same yoke and future code requirements that most likely will require all 3w circuits to be fed from 2p breakers.
Yes, it's easy for companies like Generac and the others to provide these panels with little or no regard to real world situations and Joe Blow homeowner DYI installs it.
 
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