[vox-tech] [ot] Power rating on switches?

[Jeff Newmiller]

On Thu, 13 Jun 2002, Mark K. Kim wrote:

> Another EE question on a LUG mailing list -- after all, this list is the
> "swiss army list" of anything technical. :)

Hey, on the internet, no one knows I am a dog impersonating an engineer.

> If I have a switch (SPDT, not that it matters) rated at 0.5A 112VDC, is it
> *guaranteed* to *not* work at higher current of any voltage?  For example,
> 1.5A 5VDC?

No.  It is not a fuse.  The actual conditions under which it will fail
will in general vary, as there is little incentive to apply accurate
quality control to the breakdown point as long as the manufacturer is
comfortable with the safety margin for normal operation.

The voltage rating is primarily related to the isolation between the
contacts and approach conductors in the switch.  High voltages may allow
current to jump from one side of the switch to the other, with consequent
risk of fire due to the arc, damage to the contacts, or just lack of
control where control was desired. (Clearly a reduced concern in your
application.)

Current rating is primarily related to the resistance in the switch,
particularly in the contacts, which may overheat and weld closed or just
degrade the plastic switch body.  (This is probably the best reason to
think twice about your proposed alternate operating conditions.) Also, it
turns out that there is often at least a microscopic arc upon opening the
switch, and high current corresponds to more energy in the arc, which can
then damage or weld the contacts. (Probably a reduced concern in your low
voltage application.) In high-power applications, switches are often
designated "non-load-break" if it is not normally going to be operated
while current is flowing through it... yielding a cost and space savings
over having load-break-capable switches at all points.

If you want to, you can evaluate the characteristics of the switch
yourself, and decide whether it makes a difference... but without further
characterization you probably shouldn't consider the substitution.

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