"Gary Schafer" wrote in message
...
You can try for yourself. Take a little 12 volt ac motor or
transformer and connect its 12 volt winding to a 12 volt ac power
source. Hold on to each lead of the motor or transformer bare wires if
you dare. Touch them and remove them from the 12 volt ac source. I bet
you let go rather quickly. :)

Better yet hook your oscilloscope to the winding instead of your
fingers. Look closely as you disconnect the winding from the power
source. Turn the gain down on the scope so you can see the high
voltage spike before it goes off the screen.

That is why equipment is required to have a snubber network. This will
absorb most of the spike down to a safe level for other equipment on the
mains.

Meindert

On Thu, 17 Feb 2005 22:34:26 +0100, "Meindert Sprang"
wrote:

"Gary Schafer" wrote in message
.. .

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.

A few arguments against this: if you use the switch on the A/C unit itself,
you disconnect the motor coil completely. It is virtually unloaded and will
therefore produce a high inductanc voltage.

That is how it is designed to operate. The high voltage not being
connected to anything else will then be harmless.

However, if other equipment is connected while unplugging the shore power,
much of the spike will be dampened by this other equipment. Furthermore,
equipment that is commercially sold, is required by regulations to be able
to withstand spikes up to a certain level, just to prevent this kind of
potential damage caused by other connected (or suddenly disconnected)
equipment.

It may or may not be dampened sufficiently. It all depends on what
type and how many other things happen to be turned on.

Different types of equipment may be able to withstand certian amounts
of overvoltage spikes that are normally seen on the line. But this
situation is far from a normal intended operation.



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.

Well, the voltage spike will at least produce an arc across the switch,
which in turn delivers the spike onto your power system.

It may produce an arc but the kick back energy will not be coupled
into other equipment that is not connected across the source at the
time (the motor coils). Any thing as a result of the arc will be
dampened by the whole mains supply system that is still connected.

You may get away with just unplugging the whole system for a long time
with no problems. But chances are sooner or later it will cause
problems for something.
Always turning things off before disconnecting mains power is the best
bet.

Regards
Gary

Meindert

Gary Schafer wrote:

On Thu, 17 Feb 2005 22:34:26 +0100, "Meindert Sprang"
wrote:



"You may get away with just unplugging the whole system for a long time
with no problems. But chances are sooner or later it will cause
problems for something.
Always turning things off before disconnecting mains power is the best
bet."




"Sooner or later it will cause problems" but 70 VAC will cause problems sooner
THAN later.