SWITCHES AC VS DC
by Art Bianconi (EAA 92330)Technical Counselsor 121699 Dover
GreenStaten Island, NY 10312
Some years ago I w.s fortunate to beable to work alongside
engineers fromUnderwriters Laboratories (UL) duringdestructive
testing of electrical devices.It was part of my apprenticeship as
adesigner for a major electrical manufac-turer and it was during
this period thatI acquired an appreciation for the vitaldifferences
between AC and DC ratingsfor switches.
I share this with you because I amgrowing increasingly concerned
at thewidespread lack of appropriatenessmost aircraft builders
demonstratewhen selecting switches for the cockpitenvironment. Each
time a builder asksme to perform a pre-FAA inspection ofthe
aircraft, I carefully inspect theswitches and to date over
three-fourthsof the projects inspected have turnedup AC rated or
non-rated switches inDC circuits.
There is a large scale misconceptionthat any switch can be used
so long asits current rating exceeds the maximumload in the
circuit. "Current is current;what difference does it make
whether
it's DC or AC? Besides, I'm using a 125volt AC switch in a
circuit with only 12volts!" The differences in load
carryingcapability are dramatically non-linearand are best
appreciated by carefullyinspecting a high-quality switch
carryingboth AC and DC ratings. Typical of thisis the roller and
bar micro switch madeby MICRO Corporation (Part No. DT-2RV23-A7).
Rated at 10 amps at 125or 250 volts AC, the same switch canonly
carry .3 (that's three-tenths!) of anamp at 125 volts DC. If DC
voltage isincreased to 250 volts, the current rat-ing drops even
further to .15 amps! Inreal terms, this represents less than 1/60
of the original load carrying abilityand all we did was go from 250
voltsAC to 250 volts DC!
Those of you who can still rememberthe old Kettering coil
ignition systemswill recall that when the condenser inthe
distributor went bad, the points gen-erally turned blue and melted
down injust a few minutes. Cockpit switchesdon't have the benefit
of condensers toabsorb the electrical inertia present in aDC
circuit and, as a result, the gap tem-peratures get hot enough to
weld con-tacts, even those made with exotic hightemperature
alloys.
The reason for this is simple enoughto appreciate: because AC
currentchanges directions 120 times a secondin a 60 cycle circuit;
there are 120 timeswhen there is no current flowing at all.The
current actually helps turn itself offthe moment it sees a gap and
switchdesrgners use this phenomenon to helpreduce the cost of
manufacturing AC
switches. In DC circuits, however, the"push" is constant even
when the pointsbegin to open and the resulting arc isDC current's
way of demonstrating itsresistance to termination.
"But won't my circuit breakers protectme?" No, they won't. Fuses
and CB'sprovide overload protection and awelded set of contacts
will not, by them-selves, cause an increase in circuitload.
Furthermore, what often hap-pened during UL testing was that
thepoints welded shut making it impossibleto open the circuit.
Cycling the switchto the open position was often mislead-ing yes,
the lever moved but insidethe switch, the cam had separated fromthe
welded points and while it appearedto have interrupted the circuit,
the circuitwas, in fact, still hot. If the circuit in-volved was
your fuel transfer pump orfuel boost pump and you thought itturned
off when in fact it was still run-ning, what would the
consequencesbe? If it were a flap or elevator trimmotor or a gear
retraction device, howwould a tripped circuit breaker save youif
the activating switch was weldedclosed and in a mode other than
whatis required for a safe landing?
A DC rated switch will cost you about3 times more than an AC
rated switchof identical current capacity. If yourpanel sports 10
switches (which is notlikely) the difference will be less than$35
(in 1986 dollars). You've gotten thisfar. Is it worth jeopardizing
your invest-ment or your safety by cutting cornerswith cheap or
improperly ratedswitches?
EAA Membership Honor RollThis month we continue our recognition
of persons who have qualified for the EAA Membership Honor Roll.
When you receive your new or renewal EAA
Membership Card, the reverse side of the attached form will
contain an application with which you can sign up a new member.
Fill in your new member's name,enclose a check or money order and
return to EAA Headquarters and you will be recognized on this page
in SPORT AVIATION - and there is no limit to howmany times you may
be so honored here.
Introduce your friends to the wonderful world of EAA . . and be
recognized for your effort. (The following list contains names
received through April 10.)THEODORE M. WRIGHTCutten. CAJULE D.
LAMMLos Angeles, CAKARL G. BUTTSTADTBarrington, ILS. CHARLES
MARINGELLINorthridge, CAMARION CHARLES PYLESEdgewood, KYMIGUEL
MENDEZ-OJEDAEstepone Malaga, SpainROBERT D. LAUGHLINNewcastle,
CALARRY GONZALESFremont, CALOUIE G. FERREROCocord, CA
GEORGE RAINSFt. Worth, TX
JAMES W. STARKEYAshland, ORJOHN B. MCMASTERKansas City, MOMARY
ANN DUNCANLexington, KYCHUCK VIBBARDConneaut. OHLARRY JAMES
DRISKILLArvada, COPAUL BURDETTEClifton, COMAX A KRUEGER, JR.San
Antonio, TXTIM LOWEEuless, TX
MAURICE CLAVELWauchula, FLDARROLL J. PETERSONCameron, WlRON
DENIGHTNorth Glenn, COBILL SCHELTEMAMuskegon, MlGIL J.
DEGUIREWindsor, Ont., CanadaKEN COUILLARDAppleton, WlWAYNE P.
BOYCEOrlando, FLJACK D DIXONDes Moines, IAWILLIAM BOBBTucson,
AZ
BILL RODGERSMcAlpin, FLJEROME EICHHORNPortage, INJACK
ORRLakewood, CADENNIS L GEIGERAppleton, WlW. R. BILYEUSalem, ORKEN
D. GAVER, JR.Ridgecrest. CAXEN MOTSINGERCayce, SCJESSE V
SMITHPrairieville, LADON HEDEMANDubuque, IA
BARBARA PHILLIPSRed Bluff, CAJOYCE ENRENBERGMenasha, WlROBERT C.
JOHNSONJackson, MNJOHN A. SINKLombard, ILRAY CALLAHANLake City,
FLROY GARDNERSomersworth, NHJOHN A. GOWINSUhrichsville, OHRAY
OLCOTTNokomis, FL
SPORT AVIATION 33
