On Thu, 17 Feb 2005 11:02:05 -0400, Terry Spragg
wrote:

No, Gary, we don't. Do you?

There is no difference between shutting off your A/C either at it's
control panel, breaker box, or by shooting up the distribution
transformer down the street. Non. Except for the legalities, of course.


By not turning off your air conditioner systems before disconnecting
your shore power cable or by only shutting down your main breaker this
is what happens:

The fields in the air conditioner motors suddenly collapse when power
is removed. The collapsing fields will generate a high voltage spike
throughout your boats mains system. Any other equipment that is turned
on at that time will receive those high spikes of voltage. Everything
that is turned on is connected to the air conditioner motors. You can
guess what can happen with voltage spikes fed to some types of
equipment.

If you shut down the AC systems first you avoid any kick back voltage
spikes being fed to other equipment. The fields just collapse and no
voltage spikes go anywhere.

And yes an ac motor is no different than a DC motor when it comes to
generating emf. A collapsing field is a collapsing field. If it was
originally powered by dc or ac you still get the same effect. Both
have voltage disconnected and the field collapses in the same manor.
Other than if you happen to switch the ac motor off at zero crossing.
Any transformer connected to the line will also do the same. Those old
battery chargers with the large transformer in them can also cause the
same problems. Although motors are the worst culprits as they usually
operate at higher currents and generate more kick back.

When the power company shuts down large areas it does not cause as
many problems because there are usually many loads still on line that
tend to absorb most of the kick backs from motors on line. But still
there can be some damage done by this.

Regards
Gary