On Aug 29, 12:16*am, "Jon" wrote:
Hi,

I just completed wiring and testing my circuit.

* no backfeed from the generator into the grid
* generator is disconnected from the load when service is on
* generator is disconnected from the load when service disconnect is on

http://jons-math.bravehost.com/transfer.html

Jon

Jon
since there is still a connection to the service disconnect you have a
single point of failure. If anyone were to close the service
disconnect a short pulse of current would be imposed on the service
entry conductors. It takes a few milliamperes to kill. I'm sure you
believe that this can never happen but your belief does not make your
circuit good practice. The addition of an auxiliary contact, of the
break before make type, to the Service Disconnecting Means switch or
breaker would add the human factor safeguard that is missing. With
that auxiliary contact any closing of the Service Disconnecting Means
would cause your contactor to drop out and thus disconnect the
generator from the load wiring before that wiring could be physically
connected to the Service Entry Conductors. Even better would be to
add a shunt trip to the Service Disconnecting Means. A shunt trip
would open the Service Disconnecting Means breaker. If the Service
Disconnecting Means is a fused switch You would need to add an
enclosed breaker to serve as the shunt trip device. Everything should
be wired so that any failure of the Generator provided control current
to go to the house and back will prevent the contactor at the
generator from closing. The only vulnerability in that scheme is the
possibility of a short circuit in the control circuit that would close
the contactor without the auxiliary contacts at the Service
Disconnecting Means actually being closed to pass the current. Since
you only need two control wires to make the contactor work and you
have three you could use one to pull in a control relay located at the
contactor to complete its coil circuit. That control current would
only be present if the auxiliary contact on the Service Disconnecting
Means has changed position in response to the Service Disconnecting
Means having been opened or the shunt trip having operated. This
would eliminate the need for a control relay at the Service
Disconnecting Means although continuing to use it would do no harm.
--
Tom Horne