one more last word on starter switch current

["Brian Vogt" <bvogt@domain.elided>]

Generally, I'm a read-only kind of digester. This stems from the fact that all
my automotive knowledge would easily and securely fit in the smallest of
thimbles. But occasionally, when the subject turns electrical, I can actually
understand the problem. This might have something remotely to do with the
degree in electrical engineering that I struggled for multiple years in
obtaining.  So with the strong possibility that some other digester will
correct my description, here goes-

The current through the starter switch discussion has missed a large and very
important point about electrical circuits and switches. The problem overlooked
is contact erosion, which is one of the main failure modes for our electrical
switches.

Unlike pure resistive loads, motor loads present a brutal environment for
switch contacts. Closing the switch contact on a motor start-up load causes
very large current surges of about 3 to 8 times the running current. When the
switch is opened and the current decreases, the magnetic field of the inductor
collapses and an electromotive force is induced. The polarity of the induced
voltage is such as to oppose any change in current flow. This induced voltage
aids the source voltage in striking an arc and maintaining it as the contacts
separate.

Non-motor inductive loads, such as those seen in solenoids, have inrush
currents that greatly exceed the normal operating currents of the device. This
inrush current can easily reach 8 to 10 times the steady state current. As a
switch on an inductive load is opened, the inductor, or transformer, induces
counter option "voltage" in the circuit. This voltage opposes any change in
the circuit current and can reach hundreds of volts. This extremely high
voltage can restrike the arc as the switch contacts open resulting in severely
eroded or welded contacts.

In AC circuits, the voltage and current are varying in a sinusoidal pattern.
Both the voltage and current cross the zero reference 120 times per second for
60Hz. Therefore, the chances of closing or opening a switch when the voltage
and current are at their maximum in AC circuits is remote. But with DC
circuits, the voltage and current do not vary and are always at their rated
levels. Compared to AC circuits with the same voltage and current, DC circuits
handle 1.414 (rms) times the power. Therefore, when opening or closing a
switch on a DC load, the arc developed is more severe, more energetic, and
lasts longer causing more contact erosion and a shorter switch life. Any
switch intended for a DC circuit should have its AC capacity rerated for DC.

Now I'm not sure how much this helps you pick out your "under the dash
switch". But it does serve to point out that like all things in life, it's
never so simple as just Ohm's Law. (V=IR, voltage equal current times
resistance).

Hey, then we could also talk about switching Capacitive loads - Oh, and then I
could go on about contact bounce, and switch materials, and switch contacts,
and switch designs, and dielectric strength, and .

Brian Vogt
Memphis
69 roundtail

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