Making a generator interlock kit

"Ignoramus32538" wrote: (clip) to prevent both breakers from being on at the same time. (clip) ^^^^^^^^^^^ It's neat, conceptually, but a lot of work to do what can be done more simply with a double throw switch.

I've always wondered what it would take to take a slightly rewired reversing motor starter and connect it as an interlock. Don't know how big the rating would have to be (five?) as I forgot all that stuff when I got out of the trade awhile back. One simple switch, wherever you want to put it to make it switch over from the mains to the generator. You don't even have to get out in the cold.

What you are after (I think) is a transfer switch. This is deigned to utterly eliminate the possibility of backfeeding the genset output into the grid. Switching 'breakers' does not meet this requirement! With certainty your local power company will *not* approve any home brew solution. Installing such a non-approved switch may guarantee your being in the dark should they find out about it and will also put service personal at risk. Go with an approved (and admittedly expensive) transfer switch.

Regards. Ken.

$150 !!! Shit, I would want to make my own, too. Bob

Yep, that's what they are.

Yes, it does, so long as the two breakers, which are mounted opposite one another in the panel, has a permanently affixed bar between the two handles so when one is on, the other must be off. There's a commercial panel sold that works like that, for the very purpose of transferring power to a panel from a generator, and disconnecting the panel from the service. I have one mounted in my shop. It's a modified Square D panel, sold by Eylander Electric, in Everett, Washington (no affiliation, just a satisfied customer). The panel is unlimited within reason in that the appropriate breakers can be installed to match the output of the generator at hand.

Harold

A suitably rated double throw, double pole switch costs $329 at least, and would involve serious effort to install due to heavy cables, need to secure a permit, utility disconnecting and reconnecting the meter, etc.

i

Yep. An interlock kit, supposedly, does meet the requirements.

i

The approved transfer switches are often nothing more than the abovementioned breakers which are mechanically interlocked so that only one can be closed at a time.

This does lead to the question, what happens if the breaker that has been locked out, happens to stick closed when it is operated to the "off" position. In that case one

*could* indeed inadvertantly backfeed the service entrance.

This, as you have pointed out, would be "bad."

Jim

Transfer switches are made so that it is impossible, for trivial mechanical reasons, for both inputs to be connected.

i

Not all of them, in fact the Westing house ones come to mind that are nothing but two breakers connected by a motor operated mechanical arm.

The better ones (such as asco) are Much harder to mess up with a stuck contact.

jk

This was the subject of a LONG thread in another newsgroup. We concluded that unless ALL THREE wires (for single phase) L1, L2, and Neutral were transferred (totally disconnected from one source and then connected to the second source) there were failure modes that could send generator power to the pole transformer, thus sending up to 13,000 volts back along supposedly dead lines to where linemen were struggling to restore YOUR power.

For Three phase, I expect that four lines have to be transferred if there is a neutral coming from the transformer. I don't know enough about 3-phase to say for sure about that.

If you only switch the hot leads you are risking killing some poor guy who is trying to get you back on line.

Before replying to this PLEASE PLEASE check the thread from last June (2004) here in rec.crafts.metalworking, and previously in Sept. 1999 in rec.boats. It was hashed to death, but eventually recognized that you need a real transfer switch or at least a three pole double throw switch to make it impossible to send a killing voltage back up the line. SWITCHING TWO BREAKERS JUST ISN'T SAFE.

-- --Pete "Peter W. Meek"

The ones I saw, had an "arm" (one for each electrical leg), that would contact either one input, or another, but there was no way it could contact both, because distance between them was greater than the width of that "arm".

i

Remember that the generator neutral is connected, permanently, inside the main electcical panel, to the ground/neutral buss bar. The connection is very solid and the ground wire from the panel is connected to the copper water pipes. I have seen it.

Thanks, I am looking at that thread right now.

i

If they use stock breakers, one has to consider the issue (as the person who replied above, did) that a breaker could be in the off position, but *still* have the contacts closed. A stuck contact would allow both ciruits to be closed simultaneously.

Jim

This is, indeed, a good thought. Somehow, I hope, breakers are made with a good ability to disconnect in mind, although, as we know, all kinds of things malfunction.

i

I think the issue here is that a accidental backfeed to the line could kill a worker. That's why the utility folks are quite picky about how generators are hooked up.

I could imagine one certain way to do this would be:

1) place main breaker in panel to "off" and lock it in that position. 2) place all other breakers in panel in "off position. 3) use hot circuit tester to check for opens (no continuity) across main breaker on both poles. 4) place all load breakers in panel back to "on" position.

That way one could be certain that there is no chance of backfeeding the service out to the street.

Jim

The usual malfunction for smaller molded case circuit breakers is to fail open, and/or to refuse to close when open. But they could get rusty/dirty inside or weld a contact closed and fail to open, too.

Oh, yeah! If there are linemen working on a line failure, they do go looking for houses with the lights still on and investigate. After the last big earthquake, I had to show the Supervisor that everything was cord connected to the generator.

(Shoemaker's Children go barefoot... The LADWP is rather reliable, save for minor switching transients and dropouts that the UPS's {plural} catch and clean up, so the generator rarely needs to be used. Since there isn't anyone in the house on life support, I'll just run cords when it's needed.)

That works, but for one big problem - How do you tell that the utility power is back on? ;-) If you have neighbors, that's easy - but if you're rural you need binoculars to see if your closest neighbors porch light is back on...

Better to split your house wiring. Run new Romex or conduits to the few lights and appliances that you want on in an outage to a totally separate sub-panel, and use an approved transfer switch to re-route the feeder. When the regular lights come back on, you'll know it.

The other method would be to parallel a smaller breaker with your Main, so you can have indicator lights to see when the utility power is back on. This method is often used to run the fire alarm on commercial power panels, even if they trip that 1200A Main (or turn it off for non-payment) the Fire Alarm has it's own 20A or 30A Main.

The "Kirk Key"

commercial lockout systems could be modified for residential use also - the simplest solution is to have two deadbolt locks with key capture cylinders, lock must be locked to remove the key, and you only have one key on site. And it will satisfy the most picky inspectors...

Put one lock on the Utility Feed breaker, one on the Generator Feed breaker, and you have to turn the one breaker off and lock it off, then you can move the key, unlock and turn on the other.

-->--

I have a little 110V indicator light that I will wire on the utility side of the main breaker.

Do you know what kind of interlock is that?

That's a good thought.

i

You obviously did NOT read the thread I requested you read before replying. The Neutral-Ground bond in the (widely varying by state and jurisdiction) system is NOT reliable. It usually depends on a single screw through the neutral buss into a paint-filled hole in the main panel, as well as bonds of unknown quality through unsupported wires to a ground clamp stuck to a possibly rusty water pipe, through insulating gaskets (which may or may not have been properly bypassed) on the water meter.

And you think this is a good bet to protect the life of a soaking wet lineman who is up a pole doing his best to restore YOUR power. How good is that neutral-ground connection? Say 999/1000? You think that a couple of hundred bucks for a real transfer switch is a bargain in return for a 1/1000 chance to kill the lineman? Think a little before you jury rig something that the life of another person depends on. Risk your own if you think a couple hundred bucks is too much, but you have no right to make that decision for someone else.

-- --Pete "Peter W. Meek"