CB reset philosophy

[flapsforty]

Reading through the SR 111 Investigation Report I came across a paragraph about the resetting of CBs (http://www.bst.gc.ca/en/reports/air/1998/a98h0003/01report/04safety/rep4_02_04.asp). 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?

[flyblue]

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

[Captain Stable]

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 onceIf 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, flaps.

[mcdhu]

Flaps,

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.

Cheers all,
mcdhu
;)

[Checkboard]

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 (http://www.cadmus.ca/Circuit%20Breakers.htm)

You can have a look at these (http://www.dft.gov.uk/cgi-bin/search.cgi?q=circuit+breakers+reset+fire&cmd=Search&ps=10&o=0&wm=wrd&m=all&wf=101480FF84&t=DFT____) circuit breaker rest fires from the UK Air Accident Investigations Board

[flyblue]

I like it when you talk tech Checkie http://digilander.libero.it/doniuccia/faccine/Love/amore27.gif

[GlueBall]

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

[jetboy]

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.

[flapsforty]

Thanks for all the input folks. :ok:
Checkers, what Flyblue said.................... ;)

Have fired off a mail to the editor of our in-house Safety magazine to propose small article on the subject of CBs in the CC section of said publication.

Huggy, let's keep an eye out for it. :)
Thanks again all!

[UNCTUOUS]

CB Resetting (http://www.iasa.com.au/folders/Safety_Issues/Aircraft_Wire/CBResetFires-1.html)

Dud Wiring and CB's (http://www.iasa.com.au/folders/Safety_Issues/Aircraft_Wire/DowntotheWire.html)

The FAA Guidance on CB Resetting (http://www.iasa.com.au/folders/Telltale_Docs/CBresetter.html)

ALPA's Opinion on CB Resets (http://www.iasa.com.au/folders/Safety_Issues/Aircraft_Wire/CBLetter.html)

New Wiring Rules Soon to Emerge -30Mar01 (http://www.iasa.com.au/folders/Safety_Issues/Aircraft_Wire/CBResetFires-2.html)

(Adobe Acrobat .4mb) So What Happened to the New Wiring Rules? (http://www.iasa-intl.com/pdf/ASW02May03.pdf)

All the above pages are also linked.

[BrianG]

About 6 months ago I rented a c152 to do a bit of sight seeing whilst on holidays. After taking ooff for the checkout flight, the bloke in the RHS told me "we are having a bit of a problem with the flaps, so if the CB pops press it in once - if it pops again don't press it in again". Hmmm.......

[Tinstaafl]

I would suggest the - continuing - problem isn't the flap circuit breaker but the organisation's maintenance controller/management...

[arcniz]

Checkboard - stunner of an explanation! Can we hope to see "Where babies come from" in the near future?

[BrianG]

Tinstaafl,

The flap problem wasn't the only problem, so you are correct. The renting organisation also ran a flight school - imagine if you were a new solo student, CBs popping or, worse, assym flap extension/retraction. Getting off point a bit, but....

I didn't take up the offer to rent the aircraft after the check flight.

[captain_loulou]

In my airline the Book says we can reset one tripped CB only once. If it trips again we let it like this and we call for the Maintenance.

[Captain Stable]

Captain loulou - that's what Swissair did.

[Blacksheep]

I have only one issue with Checkers explanation. Carbon isn't a conductor, nor is it an insulator. Its a semi-conductor - i.e. it conducts electrical current but not very well. Aside from that his description is spot on.

When designing a circuit the total load current determines the size of the wire, which is over-rated by a safety factor. The circuit breaker is then selected to protect that wire from overload. If a circuit breaker trips, this is due to excess current in the circuit such as to exceed the safety rating of the cable, not the equipment that is being supplied. Re-setting a circuit breaker may trigger an already overheated wire sufficiently to break down the insulation and set fire to surrounding material. You should see the dust and fluff that builds up above the ceilings and behind the sidewalls!

I don't believe that ANY circuit breaker should ever be reset even once, without first determining the reason for it tripping. No circuit that has a circuit breaker is THAT important to safe flight. [But an aircraft electrician is bound to say that, aren't I? :hmm: ]

[Did you know that the main generator wiring on large airliners is unprotected against 'hot' short circuits? The cable is allowed to melt until the hot short no longer exists. That doesn't usually take long and the process is startlingly dramatic!]

**************************
Through difficulties to the cinema

[Captain Stable]

Blacksheep - carbon is a bit of an odd element. It comes in several forms. Diamond is a lousy conductor. Graphite is a very good one.

Checkboard is correct - in some forms carbon is a very good conductor indeed, particularly when partially crystallised and including crystallised bits of copper from the wiring it once surrounded.

I like your extreme reluctance to reset CBs at all - however, there inevitably will be circumstances when it is essential to get the aircraft on the deck safely. However, as has been pointed out above, that CB should only be reset when the service is actually needed - not before, and preferably when only a couple of minutes away from the nice firefighters.