On Sun, 31 Aug 2008 15:31:35 -0700 (PDT) someone who may be Jonathan
wrote this:-

I'm also unsure what extra complexity an "automatic" transfer switch
has, beyond perhaps a short time delay circuit that will activate the
contactor once the secondary supply starts up.

To add to what others have said, some things to consider in your
musings. There are probably more things to consider, these are off
the top of my head:


What do you want to do?

Automatic changeover on failure of external supply?

Automatic change back when external supply healthy?

What equipment are you dealing with?

What happens while the generator is starting up?


How are you going to do it?

What constitutes a failure, how do you detect it?

How do you design and install the equipment?

How do you test it?


Some thoughts on these:

Automatic changeover on failure of external supply? This is "easy"
from the operational point of view. You don't have to worry your
pretty little head about it, a box of tricks does the work. However,
such complication may not be necessary.

Automatic change back when external supply healthy? This is a little
more problematic. Does the box of tricks switch back to the external
supply immediately, shutting down the generator just as the external
supply fails again, or does it wait? For how long? How does the box
of tricks monitor the health of the external supply? Is automatic
change back necessary, even if automatic changeover is considered
necessary?

What equipment are you dealing with? If this is in support of a
large computer installation then that is a different matter to a
house or office.

What happens while the generator is starting up? Is it acceptable to
have a total failure of electricity in the building while the
generator starts up? Does the building have a few battery powered
lights so there is not total darkness? Are you going to use this
time to shed loads (few generators are rated for the full load of a
building)? Is there any equipment which must keep going, via a UPS.
How many attempts at starting up is the generator to be allowed and
can the UPS run for this long?

What constitutes a failure, how do you detect it? Not as obvious an
answer as it seems. While in many cases a voltage operated device
will do the job in some circumstances a frequency operated device
may be a better one to use.

How do you design and install the equipment? The equipment must
detect the failure of the external supply, initiate the starting
sequence of the generator, shed loads as necessary, disconnect the
external supply, change over the earthing arrangements without the
possibility of leaving the building un-earthed, detect that the
generator has run up and is ready to take the load and connect the
generator to the building. It must prove these steps have been taken
before it goes to the next step. That is just automatic changeover.

How do you test it? Not just the box of tricks. The rule of
generators is that no matter how often they are tested by simulating
the failure of the external supply, at least some of the dammed
things will fail to start properly when needed for real.

In my opinion for a generator at a house there is no need for all
this rigmarole. Have a torch by the bed, with charged cells. Have a
few emergency luminaires on the stairs. Have a few candles and
matches in the house. Excavate the generator from the garage and
connect it up.




--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54