My Two Cents -
Either method of switching is valid, and each has its
advantages. Putting the switch on the power side of the load (High side
switching) means that the hot lead is "hot" only when the switch is ON,
so that a short circuit to ground on this wire can lead to smoke only
when the switch is ON. In autos, this can also reduce the amount of wire
needed (weight and cost savings in manufacture) because the chassis can
be used as the return circuit.High side switching is required for
building electrical codes because it means that when the switch is OFF,
no voltage (with respect to ground) is present at the load (helps to
prevent fried electricians). If the switch were on the ground wire,
there would be full voltage present at the load (with respect to ground)
even with the switch OFF. This is more of a danger with voltages found
in buildings than it is in autos because the voltages involved are much
higher (120 volts or more). Yes, I know that you are supposed to remove
the fuse before working on the circuit, but it doesn't always happen).
Putting the switch in the ground wire (Low side switching) can
have advantages in modern (negative ground) automobiles, where the
"switch" may actually be a Power MOSFET (Metal Oxide Field Effect
Transistor) that is controlled by some sort of sensor. For reasons of
efficiency and cost, the MOSFETs are usually N-Channel (as opposed to
P-Channel) because it is cheaper to manufacture N-Channel MOSFETs, and
they are more efficient.
With negative ground, N-channel MOSFETs can be used with less
(electronic) circuitry in Low side switching configuration (They do not
require a "High Side Driver" chip, such as an LTC1155).
My conclusion is that either switching mode is acceptable in a
Healey, just do a careful job and fuse the circuit.
- Bernie Johnsen 1967 BJ8
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