Switch Ratings, What's it all Mean?

1 Copyright 1998 Robert L. Nuckolls, III, Wichita, Kansas. All rights reserved. This document may be reproduced electronically or mechanicallyfor distribution in a not-for profit, educational endeavor if it is published in its entirety and without 1 Bob Nuckolls6936 Bainbridge RoadWichita, Kansas 67226-1008 Voice/Fax: 316-685-8617E-mail: ratings , What's it all Mean? Just catching up on piles of snail-mail and e-mail that tendsto build up while we are flitting from fly-in to up a copy of the Oct 97 issue of Van's Air Force andread an article on Switch selection that makes some goodpoints but arrives at the wrong conclusion.

2 The author wasprivileged to observe some work done at UL Laboratorieson switches and expressed some concern for builder naivetywith respect to AC versus DC correctly cites an increased difficulty for breaking a DCcircuit versus an AC circuit .. particularly when inductiveloads are involved. Quoting from the article:"Typical of this is the roller and bar micro switches madeby MICRO( Switch ) Corporation. Rated at 10 amps for125/250 volt AC, the same Switch can only carry ampsat 250 volts DC! The voltage stayed the same!"The statement is true but not relevant to our task.

3 We're notbuilding 250-volt airplanes, we build 14 and a few 28-voltairplanes. Check out this data table plagiarized from thesame Microswitch catalog . As one picks from the variousswitch products in the catalog, an "electrical code rating" isquoted for each device .. the chart above states the ratingsfor each code. When one buys a toggle Switch fromMicroswitch .. the choices above are all chart cites a variety of conditions for applyingswitches. Various combinations of AC or DC voltage alongwith loads can have a profound effect on Switch life.

4 Induc-tive loads do call for some derating .. about 25%. However,look at the column for lamp loads .. it calls for the greatestderating .. on the order of 75%!I'll call your attention to the 250 VDC column for ALLswitches. Note that none are rated at more than amps inspite of the fact that the same switches are good for 6 ampsat 250 VAC and MANY more amps at lower again from the article:"Those of you who can still remember the old Ketteringcoil ignition systems will recall that when the condenser inthe distributor went bad, the points generally turned blueand melted down in a few minutes.

5 "The cited capacitor was to slow down a the rate-of-rise forvoltage across relatively slow moving, cam driven switchcontacts. If an arc were allowed to form between theopening points, energy intended to spark combustiblemixtures in a cylinder would be used up at the points instead.. the most notable result of bad "condenser" was the carran very badly if at all .. the points were indeed subject tomore electrical stress but seldom for very long .. thissituation demanded timely repairs. Switches of choice forairplane panels are not cam driven.

6 Toggles use springloaded, over-center mechanisms that provide higher contactspreading velocities. Going on with the article .."Cockpit switches don't have benefit of the condensers toabsorb the electrical inertia present in a DC circuit and asa result, the gap temperatures get hot enough to weldcontacts. That includes AC rated switches, even those madewith exotic high temperature alloys."The Kettering ignition example is an excellent way toillustrate"inductive" circuits. However, there are few suchcircuits in an airplane. Most notable of these are battery andstarter contactor coils.

7 We don't put "condensers" on thesesystems but we do install "catch diodes" or MOVs (metaloxide varistor) to protect Switch contacts. This has beenstandard practice in airplanes for 30+ years (just worked onmy kid's '72 Chevy truck today and saw a 1N4001 diodecrimped into the connector for the air-conditioning compres-sor clutch). The article also overlooks the differences inphysics between burning contacts and welding 1998 Robert L. Nuckolls, III, Wichita, Kansas. All rights reserved. This document may be reproduced electronically or mechanicallyfor distribution in a not-for profit, educational endeavor if it is published in its entirety and without 2 Toggle Switch ratings Table from Microswitch CatalogElectCodeRating28 VDC115 VDC250 VDC115 damage to switches is done during the breaking of acircuit where an arc forms in the widening gap.

8 Dependingon contact spreading velocity -AND- thermal mass of thecontacts, this can be the most stressful task for , this is when the contacts are getting farther apart.. an improbable scenario for physics for closing a circuit are different. Here, poten-tially high inrush currents are impressed across contacts thatmay have small hills and valleys eroded in them fromprevious Switch openings. These little hills become potentialwelding material when the large inrush current is forced toflow through a small cross section of material.

9 This canhappen to any Switch with either DC or , switched circuits in airplanes are resistive butlet's look again at the purloined data from Microswitch. Innearly all cases (except Code 5) DC ratings at 28 VDC forthe switches cited are better than the ratings for 115 VAC!(????) As a general rule of thumb, I've told builders that the115 VAC rating is directly translatable to 14 VDC applica-tions. I'll offer the chart above in support of this advice. Thearticle continues .." what often happened during UL testing was that thepoints welded shut, making it impossible to open thecircuit.

10 "Sure, let me pick the test parameters and I can probablyweld about any Switch shut. Keep in mind that UL has to testfor EVERY possible safety contingency in product designand utilization. We design and build failure tolerant designsand select electrical devices that perform in narrowlydefined settings. In years of fiddling with airplanes, I'venever seen a manual Switch weld shut. I've seen themcorrode open, I've seen over-center springs rust out, I've seewires break off the back, etc. But never a welding.