Jon wrote:
I'm trying to wire a transfer circuit for a generator.

The generator will be located in the barn. The electrical service is at
the house. Two 120vac wires and a ground go to the barn. There is
*one* unused wire from the house to the barn I can use for a signal
circuit.

When the power goes off at the house, I need to disconnect the service
to prevent backfeed and activate for generator power to both the house
and the barn. When the service power goes on I need to disconnect the
generator to avoid damaging it and restore normal service to the house
and the barn.

I have,

*one 240vac 60 amp dpdt mercury relay
*two 120vac 60 amp 3pdt relays.

Any ideas?

Jon Giffen

1: Dig a trench, lay some cable, do it 'right'.

2: Do an enormous science project, and plan on buying more relays.

One way I can think of to do this is to have the generator in the barn
set up to disconnect the _barn_ on power loss, then start itself up.
Power the one wire (against the 120V neutral reference) with what the
generator produces. Then up in the house, use the absence of street
power and the presence of generator power to switch off the street and
onto the generator.

You could complicate this further to automatically switch the generator
off, but you're already looking at a mad scientists dream; were I to do
this (I'd dig the @$%# trench) I'd just plan on _walking_ out to the
barn after the street lights come on and shutting things down manually.

If you're smart you'll check your local codes very carefully before
proceeding -- home generator setups that inadvertently apply power back
to the street can create very hazardous situations for line maintenance
folks, and power companies generally take a dim view of people winging
this sort of design. At the very least you'll need to use some
power-company-approved switchover device that locks out the generator
when you're on street power.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com