Relay and Contactor based GENSET BACKFEED PREVENTER?

Page 1 of 3  
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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
--
Chris

If you can read this, thank a teacher. If it is in English, thank a
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
--
Chris

If you can read this, thank a teacher. If it is in English, thank a
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Our 50amp transfer unit only switches power from grid to generator after 30 seconds from when it sees power from the gen.
http://www.green-trust.org/wiki/index.php?title=Automatic_Transfer_Switch
Steve Spence Dir., Green Trust, http://www.green-trust.org Contributing Editor, http://www.off-grid.net http://www.rebelwolf.com/essn.html
Ignoramus25850 wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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

--



Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ignoramus25850 wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

http://www.green-trust.org/wiki/index.php?title=Automatic_Transfer_Switch
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? 10000 amps is pretty common I have seen places where special circuit breakers were needed because the fault current was over 10k.
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.
--------snipped----------------
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
http://yabe.algebra.com/~ichudov/misc/ebay/Furnas75AContactor/dscf0036.jpg

Thanks... I think that I will just buy an interlock kit.
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I'm happy with my automatic switch.
Steve Spence Dir., Green Trust, http://www.green-trust.org Contributing Editor, http://www.off-grid.net http://www.rebelwolf.com/essn.html
Ignoramus25850 wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 06 Sep 2005 15:21:20 GMT, 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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 06 Sep 2005 12:04:31 -0400, snipped-for-privacy@sny.der.on.ca

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Since you are going for a manual changeover, you might want to consider one of these simple kits: http://www.interlockkit.com/intro.html

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Yes, another good choice. Thanks John.
i

--



Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 06 Sep 2005 16:31:40 GMT, Ignoramus25850

As of now, my favorite plan is as follows.
1. Install a premade interlock kit rather than dick around with relays. Mostly for legal reasons.
2. Also have a mag starter style switch in the line from generator to the power panel, to help prevent arcing in the circuit breaker that is used to feed the panel.
i
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Site Timeline

Related Threads

    HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It 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.