Circuit Breakers - The Deliberately Weak Link

Document Sample
Circuit Breakers - The Deliberately Weak Link Powered By Docstoc
					                                   The Deliberately
                                   Weak Link
                                   Mike Murphy

                                  CirCuit breakers probably don’t            breaker for between .5 to 1.4 seconds)
                                  get the attention they deserve. How-       or a longer, less intense overload (e.g.
                                  ever, several recent high-profile air-     twice the rated amperage for 3-130
                                  craft disasters have reminded us that      seconds, depending on the type of cir-
                                  assumptions, misunderstandings or          cuit breaker). if the designed overload
                                                                             conditions are not exceeded, the circuit
                                                                             breaker will not trip. some breakers
                                                                             are temperature sensitive and will trip
               Circuit breakers! They stare at you from                      earlier when warm than cold (opposite
    panels at your knees, overhead, behind you or                                this highlights one of the limita-
                                                                             tions of circuit breaker design. the
    perhaps on the console between you and your                              very tolerances that must be built into
                                                                             a circuit breaker to prevent nuisance
       crewmate. Occasionally, they trip. Just what                          tripping, such the high transient cur-
                                                                             rent that flows when a motor or com-
      do these humble yet hardworking devices do,                            ponent is started, means some glitches
                                                                             may not trip the breaker. ticking
      what does it mean when they pop and, just as                           faults and arc-tracking are examples.
                                                                             ticking faults occur when tiny bolts
                        importantly, what do they not do?                    of electricity intermittently arc from
                                                                             exposed wire conductor. On wires
                                                                             covered with aromatic polyimide
                                  neglect of critical components, even       wrap, installed in many aircraft built
                                  small ones, like circuit breakers, can     since 1970, this can burn the thin insu-
                                  have tragic consequences. the prob-        lation, converting it into carbon, which
                                  lem is even more acute as aircraft         is an excellent conductor - a nasty case
                                  become increasingly dependent on           of the insulator turning into the con-
                                  highly integrated electronic systems       ductor! this can in turn lead to very
                                  for navigation, stability and control.
                                  Fly-by-wire aircraft are obviously
                                  totally dependent on electricity for
                                  safe operations.
                                      aircraft circuit breakers are
                                  designed to interrupt the flow of elec-
                                  trical current when specific conditions
                                  are reached. those conditions of time
                                  and current, generate heat. Circuit
                                  breakers are designed to trip (open
                                  the circuit) before this heat dam-
                                  ages either wiring or connectors. a
                                  specification might be for a breaker
                                  to trip under a massive short jolt (e.g.         An Aviation Circuit Breaker
                                  10 times the rated load of the circuit-     Picture courtesy of Texas Instruments

28 Recreational Flyer                                                                              July - August 2010
                                Typical Trip Curve
                           Courtesy of Texas Instruments

short bursts (micro-seconds) of violent
arcing where localized temperatures
                                            such an option may not always be
                                            readily available (e.g. in mountain-
                                                                                       It is wise to think
                                            ous, arctic or oceanic areas) adequate
can reach extremely hot temperatures
(well in excess of 1,000°C) capable of      circuit protection and a good knowl-
                                                                                       twice before
igniting nearby flammable material.         edge of what it can and cannot do, is
Nevertheless, short, violent bursts of      essential.                                 resetting any
arc tracking will not necessarily trip          Circuit breakers, are thermal-
breakers, which are comparatively           mechanical in nature. bimetallic ele-      circuit breaker in
slow-acting devices. special arc fault      ments, with one metal expanding
circuit interruption devices, still a few   more under heat than the other, pop        flight. It is telling
years away from widespread use in           the breaker open. this also enables
aviation, are needed to deal this type      them to be reset, albeit only after they   you something is
of situation. if your aircraft has aro-     have cooled down. However, there
matic polyimide wire, there are very
good reasons not to be in a rush to
                                            are good reasons why it MaY NOt be
                                            aDVisabLe to do so, as we will soon
reset any tripped circuit breaker. the      see.
results could be catastrophic.                  On many light aircraft, the cir-
     Circuit breakers are not intended      cuit breakers are mounted along the
to protect the electrical equipment,        bottom of the instrument panel. Many
which may have its own built-in pro-        are flush fit and cannot be manually
tection or mitigation system, but the       tripped or pulled. On larger aircraft,
wiring and connectors, which would          they are usually grouped in panels
otherwise have no such protection.          placed around the cockpit in locations
aging, vibration, excessive bending,        were they would not be displacing
improper installation, heat, moisture,      vital instruments, switches or controls,
friction, wind blast, chemicals such        and most can be manually tripped
as de-icing fluid, toilet fluid, hydrau-    or pulled. Having them within sight
lic fluid, oil and fuel can damage the      and reach, although a necessity is both
insulation on the wire, if not the con-     a blessing and a curse. a blessing
ductor itself and any connectors. in        because they can be seen and, iF NeeD
addition to disabling the circuit and       be reset. a curse, because it is tempt-
any associated component, this could        ing to use them for a purpose they
also create a fire hazard, possibly in an   were never intended (i.e. as a switch)
area where it could be impossible to        and to reset them when they should
use extinguishers and that could easily     not be reset.
threaten the safety of the flight. With         the electro-mechanical construc-
any in-flight fire, especially one in an    tion of a circuit breaker was not
inaccessible location or close to criti-    designed for use as a switch, and using
cal components, an immediate landing        it for this purpose causes premature
becomes a very high priority. because                           continued on page 36

July - August 2010                                                                               Recreational Flyer 29
Circuit Breakers / continued from page 29   this should be done only after con-         is no time to develop a good policy.
                                            sulting the relevant resources (e.g. the    in the meantime, logging any circuit
wear and the risk of failure. When a        Quick reference Handbook, the MeL,          breaker anomalies gives maintenance
circuit breaker fails, it will take down    aircraft Flight Manual, Company             a much more accurate picture of the
a system, which may be needed for           Operations Manual, and/or mainte-           nature of the problem.
the safe operation of the aircraft; or it   nance.) this approach might suggest             Circuit breakers: a willing friend,
will leave on line a circuit that should    that the reset be delayed until the ser-    ready to save you from harm’s way,
be de-energized. both alternatives are      vice is needed. there is no need to         provided you understand and respect
unattractive, and both are capable of       reset a landing gear circuit breaker that   their limitations.
inflicting catastrophic consequences.       trips after take off until one is commit-
    it is wise to think twice before        ted to landing.                             Mike Murphy, former ATPL pilot and
resetting any circuit breaker in flight.        unless your organization already        ex-TC executive, now chair of the Air Pas-
it is telling you something is wrong        has a comprehensive policy on circuit       senger Safety Group, thanks Mark Van
- that there has been a serious electri-    breakers, it is time that Flight Ops and    Berkel at Transport Canada Aircraft Ser-
cal event. this danger signal must be       engineering/Maintenance           develop   vices for his insights into this important
interpreted with extreme caution. the       one. even if you have one, don’t            topic, Texas Instruments (Klixon Circuit
old rule of thumb to automatically          assume that everyone is aware of it,        Breakers) for permission to use the above
allow one reset is not prudent. safety-     understands it and is using it. better to   graphics, and a group of his former col-
conscious airlines are now telling their    be surprised by finding out now that        leagues for vigorous peer review of this
crews not to reset any breakers unless      they are not than to learn about it after   article.
they are essential to safety and then to    a tragic event. being at altitude with a    This article is reprinted from the July-
do so only once. Wherever possible,         deteriorating situation on your hands       August, 2001 Recreational Flyer.

36 Recreational Flyer                                                                                          July - August 2010

Shared By: