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Old 26th Apr 2001, 03:42
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There's more to the humble circuit breaker than meets the eye.

They stare at you from panels at your knees, overhead, behind you, or perhaps on the console between you and your crewmate. What exactly do circuit breakers do and what does it mean when they trip? Should you simply reset a popped circuit breaker? Should a circuit breaker be used as a switch to deactivate unwanted systems?

Generally speaking, most flight training programs do not give circuit breakers the attention they deserve. However, several recent high profile aircraft accidents have reminded us that assumptions, misunderstandings or neglect of critical components, even small ones like circuit breakers, can have tragic consequences.

The problem is even more acute as aircraft become increasingly dependent on highly integrated electronic systems for navigation, stability and control. Fly-by-wire aircraft are obviously totally dependent on electrical systems for safe operations.

Aircraft circuit breakers are designed to interrupt the flow of electrical current when specific conditions of time and current are reached. Those conditions generate heat, and circuit breakers are designed to trip (open the circuit) before this heat damages either wiring or connectors. Such a specification may be for a breaker to trip under a massive short jolt (e.g., 10 times the rated load of the circuit breaker for between .5 to 1.4 seconds) or a longer, less intense overload (e.g., twice the rated amperage for three to 130 seconds, depending on the type of circuit breaker). If the designed overload conditions are not exceeded, the circuit breaker will not trip.
The very tolerances that must be built into a circuit breaker to prevent nuisance tripping, such as the high transient current that flows when a motor or component is started, mean some glitches may not trip the breaker. Ticking faults and arc-tracking are examples.

Ticking faults occur when tiny bolts of electricity intermittently arc from an exposed wire conductor. On wires covered with aromatic polyimide, installed in many aircraft manufactured after 1969, this can burn the thin insulation, converting it into carbon which is an excellent conductor The insulator becomes the conductor! This can in turn lead to very short bursts (microseconds) of violent arcing where localised temperatures can reach temperatures well in excess of 1 000 degrees C, and potentially cause nearby flammable material to ignite.

Nevertheless, short, violent bursts of arc-tracking will not necessarily trip breakers, which are comparatively slow acting devices.

Special arc-fault circuit- interruption devices, still a few years away from widespread use in aviation, are needed to deal with this type of situation. If your aircraft has aromatic polyimide wire, there are very good reasons not to be in a rush to reset any tripped circuit breaker﷓the results could be catastrophic.

Circuit breakers are not intended to protect the electrical equipment, which may have its own built-in protection or mitigation system, but are intended to protect the wiring and connectors, which would otherwise have no defences. Aging, vibration, excessive bending, improper installation, heat, moisture, friction, wind blast, and chemicals, such as de-icing fluid, toilet fluid, hydraulic fluid, oil and fuel, can damage the insulation on the wire, if not the conductor itself. and any connectors. In addition to disabling the circuit and any associated component, this could also create a fire hazard, possibly in an area where it could be impossible to use extinguishers, and that could easily threaten the safety of the flight.

Circuit breakers are thermal-mechanical in nature. Bimetallic elements, with one metal expanding more under heat than the other, pop the breaker open. This also enables them to be reset, albeit only after they have cooled down. However, there are good reasons why it may not be advisable to do so, as we will soon see.

On many light aircraft, the circuit breakers are mounted along the bottom of the instrument panel. Many are flush fit and cannot be manually tripped or pulled. On larger aircraft, they are usually grouped in panels placed around the cockpit in locations where they would not be displacing vital instruments, switches or controls, and most can be manually tripped or pulled.

Having them within sight and reach, although a necessity, is both a blessing and a curse. A blessing because they can be seen and, if need be, reset. A curse, because it is tempting to use them as switches and to reset them when they should not be reset. Circuit breakers are not designed to be used to switch systems on and off and using them for this purpose causes premature wear and increased risk of failure.

When a circuit breaker fails, it may disable a system that is needed for the safe operation of the aircraft. Equally bad, it may leave on-line a circuit that should be de-energised. Both alternatives are unattractive, and both can have catastrophic consequences. It is wise to think twice before resetting any circuit breaker in flight. A popped circuit breaker is telling you that something is wrong - that there has been a serious electrical event. The old rule of thumb to automatically try one reset is not prudent. Safety conscious airlines are now telling their crews not to reset any breakers unless they are essential to safety, and to then do so only once. Wherever possible, this should be done only after consulting the relevant resources (like the quick reference handbook, the minimum equipment list (MEL), the aircraft flight manual and/or the company operations manual).

In most cases it is advisable to delay the reset for as long as possible. For example, there is no need to reset a landing﷓gear circuit breaker that trips after take-off until you are committed to land. If your organisation doesn't have a comprehensive policy on circuit operations it may be time to develop one. Adequate circuit protection and a good knowledge of what circuit breakers can and cannot do is an essential part of flight safety.

CASA Flight Safety Magazine March-April 2001-04-24
Old 26th Apr 2001, 08:34
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When a circuit breaker trips there is a fault. The circuit breaker is there to protect the wiring against overload. Either there is an overload or the circuit breaker is faulty. The former is the most likely cause, circuit breakers usually fail closed rather than trip for no reason. At one time we used to test all the circuit breakers for correct operation with a special tester. This test isn't included in MPDs these days. The CASA (Aussie regulatory authority) advice above is generally good but I'll pick two bones with them. First, carbon is not an excellent conductor. As every electrical/electronic specialist knows, carbon is a semi-conductor. However I'll admit that it conducts quite well in the heat of a fire. Secondly, CASA uses a landing gear circuit as an example, suggesting you leave it for the duration of the flight and close it for the landing. Well, there is always another way to get the gear down or tell if it is locked. If a tripped circuit breaker represents a fault, then closing it again is an even bigger fault. As they say, circuit breakers are there for a reason - to prevent electrical overloads causing a fire. So leave the d*mned things alone.

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Old 26th Apr 2001, 10:42
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Shydrifter and Blacksheep,
Our comp changed the instructions re. the use uf CB's last year. Basically it changed to "keep your bloody hands off them".
Considered a bit of a pain by me and many of my colleagues, since resetting them/using them as on-off switches is in some cases the only way for us to clear up a problem in various cabin systems.

Having read your posts, I now understand the validity of this "annoying" new instruction.
Which makes it a lot easier to comply with, instead of being tempted to "give it a go anyway, we've always done it that way before".
Read Pprune and learn!
Thank you gentlemen.


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