Misuse of switches can be bonehead stunt. Gain a little real application information. Here is an old article, by an old gentleman, note the info source,
as there is an incredible wealth of "real" & proven information available in this aviation community.
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[PDF] SWITCHESAC VS DC
... in load carrying capability are dramatically non-linear and are best appreciated
by carefully inspecting ahigh-quality switch carrying both AC and DC ratings. ...
SWITCHES AC VS DC
by Art Bianconi (EAA 92330)
Technical Counselsor 1216
99 Dover Green
Staten Island, NY 10312
Some years ago I w».s fortunate to be able to work alongside engineers from
Underwriters Laboratories (UL) during destructive testing of electrical devices.
It was part of my apprenticeship as a designer for a major electrical manufacturer and it was during this period that I acquired an appreciation for the vital
differences between AC and DC ratings for switches.
I share this with you because I am growing increasingly concerned at the
widespread lack of appropriateness most aircraft builders demonstrate
when selecting switches for the cockpit environment. Each time a builder asks
me to perform a pre-FAA inspection of the aircraft, I carefully inspect the
switches and to date over three-fourths of the projects inspected have turned
up AC rated or non-rated switches in DC circuits.
There is a large scale misconception that any switch can be used so long as
its current rating exceeds the maximum load in the circuit. "Current is current;
what difference does it make whether it's DC or AC? Besides, I'm using a 125
volt AC switch in a circuit with only 12 volts!" The differences in load carrying
capability are dramatically non-linear and are best appreciated by carefully
inspecting a high-quality switch carrying both AC and DC ratings. Typical of this
is the roller and bar micro switch made by MICRO Corporation (Part No. DT-
2RV23-A7). Rated at 10 amps at 125 or 250 volts AC, the same switch can
only carry .3 (that's three-tenths!) of an amp at 125 volts DC. If DC voltage is
increased to 250 volts, the current rating drops even further to .15 amps! In
real terms, this represents less than 1/60 of the original load carrying ability
and all we did was go from 250 volts AC to 250 volts DC!
Those of you who can still remember the old Kettering coil ignition systems
will recall that when the condenser in the distributor went bad, the points generally turned blue and melted down in just a few minutes. Cockpit switches
don't have the benefit of condensers to absorb the electrical inertia present in a
DC circuit and, as a result, the gap temperatures get hot enough to weld contacts,
even those made with exotic high temperature alloys.
The reason for this is simple enough to appreciate: because AC current
changes directions 120 times a second in a 60 cycle circuit; there are 120 times
when there is no current flowing at all. The current actually helps turn itself off
the moment it sees a gap and switch desrgners use this phenomenon to help
reduce the cost of manufacturing AC switches. In DC circuits, however, the
"push" is constant even when the points begin to open and the resulting arc is
DC current's way of demonstrating its resistance to termination.
"But won't my circuit breakers protect me?" No, they won't. Fuses and CB's
provide overload protection and a welded set of contacts will not, by themselves,
cause an increase in circuit load. Furthermore, what often happened during UL testing
was that the points welded shut making it impossibleto open the circuit. Cycling the switch
to the open position was often misleading — yes, the lever moved but inside
the switch, the cam had separated from the welded points and while it appeared
to have interrupted the circuit, the circuit was, in fact, still hot. If the circuit involved was your fuel transfer pump or fuel boost pump and you thought it
turned off when in fact it was still running, what would the consequences
be? If it were a flap or elevator trim motor or a gear retraction device, how
would a tripped circuit breaker save you if the activating switch was welded
closed and in a mode other than what is required for a safe landing?
A DC rated switch will cost you about 3 times more than an AC rated switch
of identical current capacity. If your panel sports 10 switches (which is not
likely) the difference will be less than $35 (in 1986 dollars). You've gotten this
far. Is it worth jeopardizing your investment or your safety by cutting corners
with cheap or improperly rated switches.