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Relay and Contactor based GENSET BACKFEED PREVENTER?

I

Ignoramus25850

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
 
T

Toller

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.
 
S

SQLit

Ignoramus25850 said:
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.
Click to expand...

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.

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
Click to expand...

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.
 
I

Ignoramus25850

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
Click to expand...

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
Steve Spence
Dir., Green Trust, http://www.green-trust.org
Contributing Editor, http://www.off-grid.net
http://www.rebelwolf.com/essn.html
Click to expand...


--
 
I

Ignoramus25850

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.
Click to expand...

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
 
I

Ignoramus25850

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.
Click to expand...

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.
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.
Click to expand...

What does "service rating" imply?
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.
Click to expand...

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

i
 
C

Chris

SQLit said:
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.
Click to expand...

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.
 
I

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.
Click to expand...

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
 
N

[email protected]

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
Click to expand...
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.
 
N

[email protected]

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.
Click to expand...

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.
 
N

[email protected]

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.
Click to expand...
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.
 
I

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.
Click to expand...


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
 
I

Ignoramus25850

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.
Click to expand...

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

i
 
C

Chris

Ignoramus25850 said:
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.
Click to expand...


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.
 
D

Duane Bozarth

Ignoramus25850 said:
What I am describing is a manual switch. ...
Click to expand...

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.
 
I

Ignoramus25850

Yes, another good choice. Thanks John.
Click to expand...

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
 
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