Circuit Breaker Reset Philosophies for aircraft

posted 30th April 2003 10:29

CB reset philosophy

Reading through the SR 111 Investigation Report I came across a paragraph about the resetting of CBs. It states the need for "clear and unambiguous message stipulating the acceptable CB reset philosophy, and the consequences of an inappropriate CB reset".

We (FA's) used to be allowed to reset the CB in galleys and the Purser work station. It changed to only reset after contact with the pilots and last year to "no reset".
Point is, not everybody knows about it, the rule change wasn't emphasised and as a result, I still see FAs resetting CBs without giving it a thought.

Anyone who can tell me what the norm is at other airlines, how much emphasis is put on it and how well CB procedures get followed by the (cabin) crew?

Never reset it alone, ask first the F/D.
Never saw anyone (C/A) resetting a CB in my career.

At my previous airline, the philosophy was that, should a CB pop in the air, a reset could be attempted, on several conditions:-

The service was essential to the flight
It is only reset when needed - i.e. if it was for the gear, only reset just before wanting to lower the dangly bits
It is reset only once

If non-related CB's pop as well, before reset, on reset or subsequently, then you've got a possible major situation on your hands.

IMHO, under no circumstances whatsoever should your FAs be resetting CBs on their own

From the A321 QRH re 'Tripped C/B Re-engagement'

''In flight, do not re-engage a c/b that has tripped by itself, unless the captain (using his/her emergency authority) judges it necessary for the safe continuation of the flight. This procedure should be adopted only as a last resort, and only one re-engagement should be attempted.''

I believe that says it all really, whether the c/b in question is on the flight deck or elsewhere in the aircraft.

When an electrical system short-circuits, instead of current flowing from the power source through a service (like a ridio, or oven) and back to the power source, it can take a "short ct" and avoid the resistance of the service. It's a bit like a hose in the garden, with your thumb over the end providing a resistance to the flow, take away your thumb (the resistance) and the water can shoot out of the hose. Electricity does the same, take away the resistance, and electricity can shoot along the wire. Now electricity moving along a wire creates heat, and the wire can only handle the amount of heat due to normal operations - given a short circuit, the wire will quickly overheat, and eventually cause a fire.

Circuit breakers are designed designed with bimetallic strips, which bend under heat, popping open the circuit before this heat damages either wiring or connectors. A specification might 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 3-130 seconds, depending on the type of circuit breaker). If the designed overload conditions are not exceeded, the circuit breaker will not trip. Some breakers are temperature sensitive and will trip earlier when warm than cold.

The very tolerances that must be built into a circuit breaker to prevent nuisance tripping also mean that when it does trip, it has a fairly major problem with the circuit, which is now heated up from that problem. Reset the breaker, and the wire, with it's insulation may catch fire before the extra energy can re-trip the breaker. Some glitches may not trip the breaker at all. Ticking faults and arc-tracking are examples. Ticking faults occur when tiny bolts of electricity intermittently arc from exposed wire conductor. On wires covered with aromatic polyimide wrap, installed in many aircraft built since 1970, this can burn the thin insulation, converting it into carbon, which is an excellent conductor – a nasty case of the insulator turning into the conductor! This can in turn lead to very short bursts (micro-seconds) of violent arcing where localized temperatures can reach extremely hot temperatures (well in excess of 1,000°C) capable of igniting nearby flammable material. Nevertheless, short, violent bursts of arc tracking will not necessarily trip breakers, which are comparatively slow-acting devices.

It is wise to think twice before resetting any circuit breaker in flight. It is telling you something is wrong – that there has been a serious electrical event. This danger signal must be interpreted with extreme caution. The old rule of thumb to automatically allow 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 then to do so only once.

>paraphrased from this article

You can have a look at these circuit breaker reset fires (below) from the UK Air Accident Investigations Board

We have a ban on resetting any fuel pump CBs; but it's the crew's option to reset any other CB once.

Leave it Out!

It's quite alright to pull and reset a circuit breaker to reset a faulty unit (much like turning your computer OFF and ON when it locks up). But if it pops on its own, leave it alone! It did exactly what it was designed to do - that is - save your day! Some years ago, a DC-9 crew reset a toilet pump circuit breaker that popped. (One reset). Instead of popping the second time, the pump motor caught on fire, which spread to the cabin. Diversion to Cleveland (or was it Cinci?) and half the pax perished before the EVAC. Circuit Breakers are not hi tech, as earlier described, and they may only work the one time. Did you know that all system breakers on the A330/340 are now located in the Avionics Bay, where the crew can't get to them? There are still some commercial breakers in the cabin, but the FAs are specifically told not to reset them.

See Also the Official FAA and ALPA Policy: http://www.iasa.com.au/folders/Telltale_Docs/CBresetter.html

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