Hi Roy
On (08 May 96) Roy J. Tellason wrote to Robert Sayre...
 RS>  I've never tried this; but, it seems possible....
 RS>  I would probably use a small series resistance between the 
 RS> main AC feed and the house.
 RS>  I would apply these two samples to a comparitor. If the power 
 RS> co. voltage were higher, I'd assume that the power was 
 RS> incoming. If the generator voltage were higher, I'd assume that 
 RS> the power was outgoing. 
Technically you are quite wrong. BUT, it just might work OK at your
home, Take care!
In an AC power circuit, voltage measurements won't define power direction or
magnitude. There is a nasty bit of maths involved concerning power factor and
reactive power. This Conference is not the place for fancy maths.
Example, a 138 KV AC power line can run at 140 kV at the loaded end and at 
he
same time 130kV at the generating station end yet the power is flowing out
from the 130kV end. The actual voltages that occur, are affected by lots of
features including load power factor, line capacitance, line inductance. But
as long as the throttle is open at the generator, the power will flow away
from the generator quite happily toward a load running at a higher voltage.
The arithmetic is dead easy with DC. Power flows ONLY from a higher voltage
bus, toward a lower voltage bus.
To safely start a standby generator you need a change over contactor or
breaker and a time delayed no- volts relay. On voltage failure the utility
side breaker is tripped, proved tripped, then the generator run up. With
generator volts established, the generator AC breaker/or contactor is closed.
A changeover contactor is best ie one that is mechanically unable to close to
both utility and generator at the same time. Unfortunately! these are 
ncommon
and costly. Most designers use two separate contactors that are electrically
interlocked so only one may be closed, and having closed an auxiliary contact
of that contactor, blocks any closing command to the other one.
But we all know about MURPHY don't we!   Cheers..........ALEC
.........MURPHY not welcome at HOMEPOWER sites!
... ........The wheel is environmentally unviable
--- PPoint 1.92
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