On Tue, 13 Dec 1994, Rick Colombo wrote:
> Electrically speaking, to disable an electrical circuit, you want to remove
> the ground cable first. So, on a positive ground circuit, you want to
> remove the positive cable. Since battery posts are different sizes and this
> item only fits the (smaller) negative post, its use would not be effect on a
> positive ground circuit.
One generally does remove the ground connection to a car battery first,
but I don't think the reason is because that is the only way to disable
the circuit. When you remove the cables from a car battery, the wrenches
used may touch the chassis (ground). If you remove the "hot" connection
first and the wrench touches ground, there will be a large current through
the wrench and at the very least a shower of sparks. Not good. So it is
good practice to remove the ground connection first. If the wrench
touches ground while removing the ground connection, nothing happens.
After the battery ground is disconnected, the "hot" terminal can be
removed without worrying about whether the wrench will touch ground.
So the ground connection is removed first for practical reasons; removing
either battery cable will disable the circuit. In fact, in wiring
appliances in your home, it is wise to put the switch in the "hot" side,
rather than the neutral (ground) side of the circuit for safety reasons.
Or better still, use a double pole switch and break both sides of the
circuit.
My guess is that the cutoff switch cannot be used on the hot side of a
positive ground system because of something in the design of the cutoff.
The post size isn't the only issue; one does not want large expanses of
uninsulated metal that can short to ground with spectacular results. I
would have to see one of the cutoffs, but the design of a cutoff for the
ground side might well be different from that of a switch designed for the
hot side. As just one example, the switch may make and break a connection
between the battery post and the uninsulated case of the switch, which in
turn could be attached to ground by an uninsulated cable. This would be
fine on the ground side of a negatively grounded battery, but unsuitable for
use on the hot side of a positively grounded battery.
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8910
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