Thanks fenland! It is just that the reports I have heard of very thin Teflon insulation on aluminium wires make me concerned.

Glad Rag,
My apologies, I jumped ahead of myself in my reply. You asked how come it was not picked up during assembly and test, my point is that as it is thought from the reports that the outer battery cover did the pinching, there would be nothing to see during the assembly process and once it is assembled, inspection and test is all that is left to you. It is possible that inspection would not show anything and if the insulation was not compromised at that time then the unit would pass it's tests too.

Joy Ride,
You are welcome, I think you will find this 'thin' insulation issue goes back a few years and is in the public domain. Quite early on the insulation thickness on the wires carrying 230VAC in the unpressurized areas was found to be on the limit for incipient discharge (corona) when airframe aging was taken into account so it was increased despite the weight penalty that was incurred.

I'm pretty sure the insulation on other wires is no different from other airplanes and most of it is copper, just the really chunky stuff moved to aluminum.

Nope! there have been massive developments in the advancement of "alu" conductors and the termination technology to guarantee they match the service life of the aircraft.

Understood, fenland, but the 787 does seem to be a more risky environment for various proven technologies due to its increased humidity and more easily combustible structure.

Uhh right on humidity- WRONG on more easily combustible structure . .

I wonder how they would find moisture on something that had been through a fire. Unless some idiot had sprayed water on it. :8

I guess corrosion products that are chemically different than oxides from combustion?

DWS, you're, combustible is the wrong word, damageable at a lower temperature than aluminium is probably a better way of putting it.

You are completely WRONG, CFRP has much higher temperature resistance than aluminium, much better temperature-strength curve. It is simply a much superior material. In terms of fire it has self-extinguishing properties.

Albeit after the fuse has punctured at 33000 feet.

Question for the gurus (sort of OT) on temp/humidity etc. As SLF I have sat in many airframes on the tarmac with the temp and humidity well above what is found at cruise (many airport in an Australian summer - particularly FNQ) Surely the fact that the dreamliner has x% increased humidity compared to older aluminium fuselages pales by comparison to the humidity all aircraft are exposed to in say Cairns with the doors open and the A/c packs switched off. obviously in this condition the circuitry is likely to be shut down, but surely when the aircraft is started the humidity inside could be 98% and fully powered up?

Surely aircraft are tested in these conditions too? Does this invalidate the x% higher humidity argument on the dreamliner?

The humidity at 98% on the tarmac is not the problem as the airframe will be at the same temperature and will not cause condensation. At altitude where the outside temperature is well below zero, moisture will condense on colder surfaces forming liquid water around and possibly in components that are not completely sealed and/or insulated from the colder surfaces.

The humidity isn't that high is it??
What IS the humidity in a 787 in cruise??

No, humidity inside 787 is around 15% - by absolute standards still a very DRY air. But everything is relative, comparing to any other jetliner with around 4% you can call it 'humid'.

Blog on the topic:
Dreamliner 787 sweaty passengers may be causing fires | Plane Talking

Note: If you calculate it, you get dew point -3°C (fuselage conditions 80kPa, 26°C, 15%). That means that at FL no liquid water condensation can occur but only icing on cold surfaces. It can melt lately, when a temperature of the part increases. But I am not sure whether 15% is valid for air inlet, average humidity in the cabin or for outgoing air.

BTW I believe that you can hardly feel difference between 5 and 15 percent of humidity.

I suspect you are right although many passengers have sworn there is a noticeable improvement in 'cabin air comfort' on the 787.

Placebo, anyone? ;)

olasek
Can you expand a bit on the bolded part? I am not good at advanced chemistry, nor in some of the exotic materials used nowadays, so I will ask:

once something starts burning, how does it stop itself from burning? :confused:

One way of being 'self extinguishing' is by using/manufacturing a material whose combustion temperature is higher than the temperature of the flame created when it burns.

Certain materials are said to be self-extinguishing when they will not sustain combustion by themselves, once an external source of flame has been removed.

An example would be a material commonly used for printed circuit boards, known as FR-4, which basically is glass reinforced epoxy containing certain additives, usually bromine-based (and environmentally slightly nasty).

See also: Flame Retardant (WP) for information on the exact mechanisms involved.

The speed of evaporation depends much more on a velocity of air flow. If I direct an air outlet to my head, my skin and my eyes could be dried several times more (e.g. 300%) whilst decrease of evaporation between old and new a/c (5 vs. 15%RH) is about 5%.

There can also exist some kind of psychosomatic effects in 787. If you feel calm and relaxing (thanks to advertising) your breathing is light and your respiratory system is dried only little. When you are feeling uncomforted and upset, or you worry about it, both heart action and breathing is much higher and your respiratory system is dried more.