In a message dated 97-05-02 00:20:32 EDT, you write:
<< For standard operation, you are 100% correct. However,
in order to maximize safety, and minimize the likelihood
of undesired shorts, it is always best to switch the
hot lead, and keep the section of unswitched hot wire
as short as possible. >>
Ken:
Your description of the safe way to wire a house is right on, but I must
disagree with transferring that criteria to an automoble. 110V can be fatal,
not only to a technician doing repairs, but to a home resident as well. For
this reason, safety is the over-riding concern, pushing aside all other
concerns. In a car, however, we do not have a shock hazard (12 volts is
harmless from a shock standpoint) so other concerns become more important.
To illustrate, consider the O/D relay which started this topic. In a TR6, it
is located about 2-3" from the fuse box. A very short piece of wire can be
used to run from the fuse box to the relay, and can be routed and protected
from most hazards with very little difficulty. The permissive switches on the
transmission, on the other hand, are located a fair distance from the relay,
and are in a harsh environment. They are exposed to all the road hazards -
mud, oil, dirt, rocks, etc. Which wire should we switch? If we want to switch
the hot wire, we must run a long wire to the transmission, and another long
wire back to the dash switch, and then to the relay. We have significantly
increased the amount of hot wire exposed to the world, plus we now have it in
the worst place possible. If, on the other hand, we run the ground side of
the relay to the switches, we not only reduce the amount of wire, but if a
short should occur, the worst thing that can happen is the O/D is switched
on. In this case, we are far better off to switch the ground lead.
Next, consider the reverse and stop lamps. They are located in a fairly harsh
environment, as people often carry car parts and other "junk?" in the trunk.
This material can slide around and damage the wiring. Which wire do we switch
in this example? First of all, we have to run power to the lamps, no matter
which wire we switch, so either way, we will have a length of hot wire
exposed to damage. The best we can do here is switch the hot lead. That way,
the amount of time the wire is hot is limited to the time we are in reverse,
or we are pressing the brakes. We cannot avoid having a powered wire in this
area, but we can reduce it's exposure.
The same logic applies to the power lead to the O/D solenoid itself. Unlike
the O/D relay, here it is better to switch the power lead.
Another example to consider is the overhead, or dome, lights used on modern
cars, and some of ours. The hot wire is safely tucked away, out of sight,
behind the headliner. The switching is done in the ground lead, as this lead
runs to the door jam switches, as well as the headlight switch. On a four
door car, this can be a lot of wire.
In general, when adding devices to our cars using a relay, I recommend
putting the relay as close to the fuse box as possible, and switch the ground
side. That way, if the wires are routed into the car in such a manner as to
be abraded (which is very easy to do), the worst thing that can happen is the
device is turned on - much better than having a melt down.
The next question is why not add a fuse, and prevent the problem? In the case
of driving lights, for example, it may be more convenient to have the lights
come on, rather than having to replace a fuse, especially if other devices
are hanging off the same fuse. In some cases, it is not possible to use a
fuse. If the O/D solenoid works the way it was explained by a poster over on
the MG list, and it sounds reasonable that it does, a fuse would provide no
protection at all (this gets into another area all together, worthy of a
whole article by itself. Are your headlights fused? Ever wonder why not?
Given a choice, which would you prefer - a, sitting beside the road watching
your car burn to the ground, or, b, having your headlights go out on a dark,
cloudy night, while you are doing 65 on a winding, twisty, country road?).
There are no cut-and-dry rules. Each situation has to be considered, and
evaluated on it's own merit.
As for hazards while working on the car, always follow the recommendation of
disconnecting the ground side of the battery (whether positive or negative).
Dan Masters,
Alcoa, TN
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