Fire and overhear detectors (firewires) out of resistance.
 

At my airline we had a 747 engine fire detector element (firewire) that waa out of resistance. The resistance was 10.2 M ohms and it should be no more than 80 ohms.

We Megger'ed the firewire from the core to the outside with 500 volts and it brought down the resistance to below 80 ohms.

Does anyone know why this process works? Some people call it whetting but I struggle to understand why it works.

Thanks

found this after a quick google search, seams to answer your query.

wiki says "In electrical engineering, wetting current (sometimes also spelt as whetting current in archaic sources) is the minimum electric current needing to flow through a contact to break through the surface film resistance.[1] The film of oxidation occurs often in areas with high humidity. Providing a sufficient amount of wetting current is a crucial step in designing systems that use delicate switches with small contact pressure as sensor inputs. Failing to do this might result in switches remaining electrically "open" when pressed, due to contact oxidation.".

And meggering a firewire, is an approved maintenance technique then is it? Must have missed that in the AMM somewhere ;) Seriously though why did you think that is a good idea?

This is a well known way of bringing a firewire back into tolerance. Are you one if those people who follows the AMM?

John are you a wind up?
Yes I do know why I follow the AMM for your other question.

agree
 

I agree with John pal. No its not an approved AMM procedure, but a push button megger works wonders for sorting firewires. has done for 30+ years. I would stay away from the ole wind up ones tho! :)

also a couple of D cells in series also works well for "electrical cleaning"

:)

So what would be your defence before a judge? I am concerned the "ex" may disappear from your name excon.

trouble is the manual can be wrong!

of course thats why there are procedures to get it corrected. The only wrong course of action is that which these cowboys are doing.

BCAR Section L,
I have to agree with you.
Stick with the manual and have the procedures changed by the correct manual revisionl if the manual is wrong.

A380 AMM ref:

TASK 36-22-00-600-801-A Wetting current procedure

Yes, this thread has been dormant for a good number of years but your original post was about firewire on a Boeing 747, so rather pointless to highlight a manual reference from a completely different aircraft type that has a different firewire system. There was never anything like that in the Boeing 747 manual and believe me given the number of firewires I had apart on Boeing 747s in the past and the number of times I had to climb into the engine core to find and rectify faults, I would have loved it if that were a legitimate way to work. By meggering the system, you never fixed the fault [it would temporarily dry out any moisture in the connections] it would always reappear at a later time, so although you might have got the aircraft away, it just meant someone else would eventually have to fix it properly when it failed again [which it always would] as you could not be bothered to fix it properly. Quite a lot of the time on a Boeing 747 it was a case of moisture in the connections as the crush washer hadn't been properly tightened down or had been overtightened [so not making a proper seal] Sometimes it could be even stranger reasons. I once noticed a connection on a B747 firewire that only seemed to be on one thread, only to find that although tight, whoever had it apart before either didn't realise the old crush washer was in there or couldn't get it out, so just fitted a second one over the top.