Relay and Contactor based GENSET BACKFEED PREVENTER?

I have been playing with using relays of various kinds, as well as contactors. I already built 2 phase converters, and a remote switch, for instance. I have a few 90A and 75A contactors and solid state relays and mag starter buttons and whatnot.

Here's what I have been thinking about. I have a 7 kW Onan DJE generator that I have for emergencies. I want it to power my entire house in emergencies (I know that I cannot run AC and some other devices while under generator power).

It is expensive and painful to install a transfer switch. I want to make something easier and cheaper to power the house in case of emergencies, while at the same time preventing backfeeding.

I can, instead of a transfer switch, build a system that includes a contactor, a mag starter, and a relay that only closes the main generator contactor when the main circuit breaker is in the OFF position.

It would work something like this. There will be a mechanical device and a switch such that the switch could be closed only when the main breaker is off. When the main breaker is on, the switch could not be closed.

There will be a DC circuit, powered by a 9V battery, that would be a signal input to a solid state crydom relay. The power contacts of the relay would be in series with the power from generator. When the relay is closed (only when the mains breaker is open), and a START button is pressed on the start/stop switch (like ones used for mag starters), the main contactor would close. The STOP button interrupts input to the contactor, causing it to open. Turning the little switch near the main breaker off would also interrupt the circuit, opening the contactor.

This seems to be a very fool proof system. I can build it in 30 minutes, except that I would need more time to fabricate a mechanical switch opener/closer.

I would like to ask that those who can visualize what I am describing, to comment on this plan. Thanks

i
Reply to
Ignoramus25850
Loading thread data ...

Our 50amp transfer unit only switches power from grid to generator after

30 sec> I have been playing with using relays of various kinds, as well as
Reply to
Steve Spence

I don't claim to have really followed it, but do know that the contacts on my TS relay fused closed. Unless your device accomodates that (on the line contacts) safely, it is no good.

Reply to
Toller

Where I live the utilities require physcial and electrical interlocks. Most residential panels do not lend themselves to a physcial interlock. Square D is the only one that I know of that has a physical interlock between the main and another breaker. Then comes to the issue of the equipment being service rated. Then comes UL listings. A contactor/motor starter off the shelf I do not believe is service rated.

Check with your local authorities and utility before embarking on the project. If you can build this in 30 minutes, your a lot better than I am.

Reply to
SQLit

What I am describing is a manual switch. The user would have to perform the following steps manually:

1) Turn off the main breaker. 2) Turn on the mechanical switch for DC input circuit to the relay, which becomes possible only if the main breaker is off. 3) Turn on the generator (could be done at any other time in this sequence) 4) Press the START button on the start/stop switch.

At any time, if the interlocked mechanical switch is turned off, the contactor would open and electrical power would no longer be supplied to the house from the generator. The interlocked switch must be in the off position for the main breaker to be turned on.

i

Reply to
Ignoramus25850

What is a TS relay?

I want to use a large contactor, rated for way more than what my generator can supply. My genset generates about 28A, whereas the contactor that I would use is rated for 30A.

i
Reply to
Ignoramus25850

Well, in my case there will be a physical interlock between the DC signal switch and the main breaker. Should not be hard to fabricate.

What does "service rating" imply?

It will take me a lot longer to build an interlocking switch, but the electricals themselves are not hard to put together.

i
Reply to
Ignoramus25850

One piece of advice. I am not seeing it in your description, but I might be missing it. Do you have a plan for seeing that the incoming power from the street is back on?

I only mention this as I once had a friend do something similar, and failed that part. Worse yet, he had to get into his car to drive by a few neighbors to see if power had been restored. Even worse yet he had to drive by two of them as they had generators. Remote area it was.

Reply to
Chris

Chris, you raised a great question. As of now, I do not have any provisions for seeing incoming power. I see it as a fully separate issue from what I am asking, but it is a very good one.

My concern with doing anything to see incoming power, when the mains breaker is off, is that I would connect an electrical load (a light bulb perhaps) before the main breaker. That seems a little bit scary.

I could, however, install a little button style light or some such.

i
Reply to
Ignoramus25850

Sounds like a good idea - BUT. Can you ensure the generator contactor can NOT stay closed when the switch is shut off to turn the main disconnect on? You need a failsafe system that makes it IMPOSSIBLE for the generator to "lock on" - whether by killing the generator before being able to reconnect to mains or whatever. That is why virtually all transfer switches employ what boils down to a mechanical DPDT knife switch.

Reply to
nospam.clare.nce

Since you are going for a manual changeover, you might want to consider one of these simple kits:

formatting link

Reply to
John Grabowski

You want AT LEAST a 50 amp rated contactor, at 500 volts, for a safety factor. 30 amps is cutting it a bit fine, since you do not have zero crossing switching.

Reply to
nospam.clare.nce

On my brothers recreational property the meter is at the road (with a disconnect and main breaker) and there is a red pilot lite on the back of the service box that tells him power is on. His transfer switch is back at the buildings. When the light is on, he knows the grid is up.

Reply to
nospam.clare.nce

I am open to suggestions here. I would think that that's what contactors are made for, to open and close reliably, when operated at or below the rated power. Otherwise they would be very unsafe for all kinds of machines that they control.

i
Reply to
Ignoramus25850

Thanks. I have a 3 pole 75A contactor that I am planning on using. Should be plenty for a 28A generator.

i
Reply to
Ignoramus25850

Crossposts deleted

I'm all for building stuff that does what you want it to. But when you start talking 7 kw of power plus the potential to have life endangering consequences I start to drag out my "safety first" speech.

There must be a POSITIVE interlock to avoid backfeeding to the grid. This interlock should satisfy BOTH the power company and the electrical inspector. Period. No discussion. Positive in this case means you, your neighbor, an electican, or your power company guy can all work it correctly without fear of screwing up. It should also be physically positive in the sense that it won't bend short of using a vice grip or prybar. Your description says something about some vague "mechanical device" to accomplish this. I don't buy it.

Other than that, why are you going though all the hassle of relays, 9 volt batteries, etc when all you need to do is switch a couple of circuits?

I also think you will have some issues with trying to power the whole house with the 7 kw generator. I see a few neighbors trying to run their houses with 5 kw (granted these are a bit smaller), modern houses take a LOT of power, if several things come on at once, you kill the generator. But with proper power management, you should have enough power to run a small window AC unit as well as the rest of the required items like refrigerator, freezer, well, and a few lights.

Most folks just use the 60 amp (??) Square D transfer switch to have 4 circuits that can be cut over to the generator. $100 or so, UL listed, NEC compliant, looks like a sub panel, takes an afternoon to install. The even simpler route is to have the critical items (eg furnace)on plugs, just move the plug from grid to generator as required.

Cheers.

Ignoramus25850 wrote:

Reply to
RoyJ

Ok just wanted to make sure you had it in mind. I know how we can get. Focus on one thing and forget the obvious.

Reply to
Chris

Well, I agree about the need for positive interlock. And that's what I want to do.

I thought about it. In reality, it is much better to switch the whole house (and turn off some power hog devices like A/C and hot tub), that switch circuits piecemeal.

You know, you have a good point, but I already tried to run the house from a generator. It all went just fine.

Yes, all I will do is turn off A/C and possibly hot tub, and instruct everyone to use the electric stove sparingly.

My genset can easily run a window AC, which we have at home.

A furnace cannot have a plug, actually...

In any case, any non-whole-house transfer switch would very severely limit usability of the house. I have very numerous small circuits that power small lights and such. It's a 5,000 sq ft house if I count the basement, with about 30 circuits. If I used a small transfer switch,

3/4 of the house would be dark and unusable, when there is no power related reason for it. Many lights in the house are fluorescents. I have plenty of fluorescent bulbs that I could swap in remaining lights (which see very little use).

It's either a whole house transfer switch, or an add on contraption like the one I described.

i

Reply to
Ignoramus25850

Yes, another good choice. Thanks John.

i

Reply to
Ignoramus25850

Yes. And?

You will definitely need to check w/ your local utility. They may have requirements that any transfer switch you put in place be a listed device.

Reply to
Duane Bozarth

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.