Relay and Contactor based GENSET BACKFEED PREVENTER?

I caught your concept of a "manual transfer device".

Where I live there MUST be mechanical interlocks. That means if either breaker is forced into the other position BOTH breakers change state at the same time. I am talking about internal switching from one device to another. Steps 1&2 would have to happen with one operation.

The utilities used to have a requirement for "visible blade disconnection" I got into a situation where no one made a 4000 amp visible blade disconnect. So they changed the spec to mechanical interlock. I will bet that your contactors are not 'service rated'. Do you know the fault current your generator is capable of? How about the serving utility?

Call the local jurisdictional authorities and talk to them. I do not know of a utility that would allow what you describe. I am familiar with the utilities in 4 western states.

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That would be the case for what I was planning (now, I think, I will just buy an interlock kit).

I believe that the concept of an interlock simply prevents both from being on at the same time. I do not think that it should be impossible to have both devices off at the same time. That situation (both sources off) corresponds to the OFF position on the mechanical transfer switch.

You can see a sample contactor at

Thanks... I think that I will just buy an interlock kit.

I'm not sure what your concern for arcing in the circuit breaker is about. Circuit breakers are made to be turned on and off under load. You could always install wiring and a circuit breaker bigger than the rated capacity of your generator.

John G.

Not a good idea at all!! A transfer switch has to be designed so if it fails it absolutely can not back feed into the utility power. If your contactor fails closed, and believe me it does happen, your generator can back feed per to the utility.

I install and service standby generators, if one of our transfer switches fail there is absolutely no way it will back feed power. The transfer switch may fail in either position, but no way will it cause a back feed! Greg

Contactors can and will fail closed! Not a good idea for a transfer switch! I could not count the number of failed closed contactors I have replaced in the last six years!! Greg

Not a DIY job. Get a UL listed transfer switch, have a licensed electrician install it and get it inspected.

I suppose you COULD use a DPDT contactor - but then why not just use the old standby knife switch. The contactor needs power to activate it.

And if the contactor sticks in the line position you can't use the genset. If it sticks in the genset position, you cannot connect to the grid. - so you are back to manually operating it anyway. Back to the knife switch.

Go get yourself a UL Listed transfer switch of some sort. The cheapest way is a Double-Throw safety switch. Looks just like the fused disconnect for a motor, but the lever goes On-Off-On.

Or do some heavy-duty scrounging and buy a new or good used automatic transfer switch that you can install. DO NOT TRY TO INVENT THIS YOURSELF. It is life-safety critical, and there is absolutely no reason for you to reinvent the wheel. Better you should pay a bit for the right equipment - Use a proven design.

If your homebrewed transfer switch design is flawed, creates a backfeed and kills a lineman or two working on power restoration (or even just surprises the hell out of them when they find out the line is energized) I can guarantee that they will find you, and that the proceedings will not be pleasant.

If you do anything permanent, get it checked out and passed by the local building inspector, and by the local utility. This is critical.

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I've seen plenty of simpler electrical jobs screwed up or just poorly executed by people with more than a bit of sense. In any case, it has to be listed and inspected per the local utility regulations.

In Washington state you are allowed to wire your own house. It then must pass inspection. After that you must live in it for five years before you can sell it. This is done to prevent bad wiring done by people who are building spec homes. Not that everybody who builds a spec home is going to do a bad job. Just that some people will skimp and not use qualified people to do the wiring. ERS

Strange! Here a homeowner can pull a permit to do most anything. Then the job must pass inspection just like anything else and you are done with the process.

Vaughn

That seems like a pretty harsh regulation. Here I believe you can legally wire your own house, but you're supposed to have it inspected afterwards, and certainly before you sell it. But we also have a stupid new law which means that only qualified electricians can buy consumer units. Most people with time and good practical skills, but little money, aren't going to pay an electrician to wire their workshop. They'll find a consumer unit from a friend, or a demolition site, or a dumpster. It just drives work underground. Another sad example of European over-regulation!

Chris

Some of the most screwed up DIY installations I've ever had to rework were done by engineers and (literally) 'Rocket Scientists' who thought they knew better. I think half the parts the former owner used for the additions at this house (before we bought it) "fell off the loading dock" at Rocketdyne in the '60's, though I can't prove it.

In EVERY community I know of, the electrical utility won't tie onto a power panel and provide a meter unless the panel has been inspected and approved as safe by /somebody/ referred to as the Authority Having Jurisdiction or AHJ.

If there isn't a city building inspector, the job falls back to the county or parish level. If it's on state or federally owned property, then their inspector has the authority. The power utility doesn't want the legal liability if someone gets hurt or killed, so they insist on it.

Now after the AHJ has passed the installation, the utility has connected the feed, and both parties are long gone, /then/ you can do additional work without a permit, and cheat as much as you want, with one gigantic caveat: As long as they never see it.

And if the AHJ comes back to check something else and sees you have cheated, now you have a problem. They can (and will) force you to fix it. If you don't, their ultimate recourse is to call the utility and tell them to cut the feed and pull the meter, the installation is no longer safe. Darkness and quiet quickly ensues. (*)

(* At least on things run from utility power, noting the crosspost.)

Except for mobile homes / manufactured housing in California, where inspection of the meter pedestal and outside wiring is a city or county responsibility like normal, but anything electrical inside or on the coach itself is inspected by a State bureaucrat. Our local mobile home inspector drives into Western LA County from Riverside, where the closest office is.

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Christopher, what do you mean by a consumer unit?

Lets see how my grasp of european terms is..

consumer unit = Breaker panel (made of plastic, with DIN rails)

RCD = GFI (or something very similar)

T+E = Twin+Earth = Romex

ring main = circuit fed from both ends (from the same breaker)

If your house is insured it will be void if you install the device. The insurance companies frown on anything that is not UL approved. Michelle

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