Are you talking about testing a loom installed on an aircraft with fuel in the tank?
Do you not have an AMM to refer to for this? What is the a/c type?
Remember that fuel vapour explosion risks relate to values of a number of factors, which if known and controlled can enable you to carry out such tests safely.
GTF-6, PSD-60/90 fuel qty test sets, like the barfields posted above. They will tell you if you have a bad probe or wiring problem.
You wouldn't want to meg into a probe anyway.
FQIS harnesses are wet, but pump/valve wiring is usually in conduit. A DMM will tell you if there is a problem in the conduit.
Thank you all for your reply. Here is the situation: what's the best way to check a wire connection to a fuel probe, in a drained/ vented fuel tank (Bombardier CRJ 100/200/705)? Fuel probe was tested OK, but apparently the connecting wires were at fault. FIM recommended wire check by meggering method (wire insulation to ground). What would be the alternatives to meggering in special conditions (fuel tank)?
One Mike
If you find that you cannot find an alternative to using a Megger, then purge the tank with dry nitrogen and ensure that all of the oxygen is out before you do.
B.E.
Fuel probe system can be tested while electrically connected to the aircraft provided there is no fuel or vapor and that the tank is drained and purged to 90PPM (1.5%LEL)or less. We air purge our tanks until this level is met after sumping and de-puddling. (ref USAF TO 1-1-3)
The PSD/GTF type FQIS test sets MEG the wires and probes and can be accomplished with the systems installed. The reason I would not use a Megger is that I do not know at what level the PSD test equipment operate at. A megger can supply 250, 500 or 1000 VDC (low amp). I do not know what output the qualified FQIS test equipment operates at and therefore would not recommend using one unless there were procedures stating what settings to use. Damage to probes or even electrical arc could occur.
Another thing to think about is that if there is a wiring problem in the tank, chances are your only option is to replace the wire/harness. All aircraft I ever worked with do not allow splice repairs to wire harness inside the tank and harnesses are potted.
If the FIM says to Megger, then there should be a corresponding AMM or other reference on preparing the tank for such an operation (maybe even referenced in the FIM?) such that the safety issues are addressed.
I agree that you are unlikely to be able to do a worthwhile repair, even by returning to the manufacturer, so as you've obviously got the tanks open already to vent them (or will shortly do so,) you might want to do a visual inspection.
You might be lucky and find it's just a loose connection. If you rely on the Megger (or alternative) you might end up ordering a replacement harness unnecessarily in this case. Fuel harnesses aren't cheap of course.
If you end up Meggering, don't forget to be sure to isolate the harness from the FQIS. You don't want more problems than you have already.
The "devil is in the details", and I'm pretty sure the probes are wired in parallel,( and I bet there's half-a-dozen in the tank) so even if using one of Mr Barfield's excellent testers, ( which, by the way are configured to individual aircraft by special connectors, and have the various aircraft types & their tolerances listed, ie total capacadence,individual probe capacadence, conductor resistance to shielding, shielding to ground resistance, ect ) you are still going to have to isolate, by dirty old troubleshooting techniques, the particular harness in the system at fault.
Thank you all for your valuable input. Issued was finally fixed consulting AMM (Fuel Tank - no tools/testing/measuring equipment batterry/electric powered that can initiate a spark), also acquired a TDR as per WDM Standard Practices.
Dumb as this may seem - I would like to see NO wires in a fuel tank. Isn't that what doomed TWA flight 80 (???) off the east coast about 10 years ago??
A TDR is great if you have one. Not all MROs do. But great that you solved the problem.
To MiketheEngineer. Without doing a very long post that goes way off thread, there are finally a lot of long overdue issues with wiring in tanks and aircraft wiring in general now being addressed. It will though be a while yet before we see the disappearance of in-tank fuel harnesses on new medium and large aircraft, and a lot longer before the last in-tank harness leaves the sky.
Last update from Bombardier TechSupport and Safety Team - fuel tank wire meggering allowed and safe if fuel tanks are drained and vented. I'm srill having my doubts here, so I'll stay with the TDR just to be on the safe side.
When I was younger, and even stupider, I decided to fix a pinhole in the bottom of a terneplate 14 gallon gasoline tank by soldering a penny over it.
I chose a windy day, drained the tank, removed the filler cap and the sender unit (which left a decent size handhole, swabbed the remaining liquid out of the bottom with a rag on a stick, and wiped down all the accessible surfaces, until there was no liquid fuel remaining, and no detectable odor of fuel. Then I let it sit for an hour so the wind could further ventilate it.
You can see where this is going.
You know how you learned in college that a little lighter fluid dribbled on the mouth of an empty beer bottle makes an amusing display as flame shoots out of the mouth while the flame front proceeds down inside the bottle in search of more oxygen? Same thing, on a much larger scale.
First application of the torch to the pinhole set off the vapor and air in the tank. Got a really pretty blue flame out of the sender hole, and a smaller one from the filler cap, and a loud acoustic noise, sort of a cross between a moan and whoosh. Not some gentle lick of flame either; pretty decent velocity, extending out maybe five feet. It burned out in about a minute, while I was wondering about the tank seams.
It cooled off enough to touch in half an hour, by which time the air supply in the tank had been replenished. The second flame was just a little less violent than the first, and didn't last for quite as long.
Recurse eight more times until the fuel supply was actually exhausted, and I was able to sweat the penny on nicely.
Next time I'll just use a sheet metal screw and a little Permatex.
“If the FIM says to Megger, then there should be a corresponding AMM or other reference on preparing the tank for such an operation (maybe even referenced in the FIM?) such that the safety issues are addressed.[eng300]”
The above says it all. Nitrogen purge during the test should be mandatory. I think you need to make sure that the insulation is not cracking anywhere in the wire run. If the wire is in a bundle it may have short path through to another wire, not necessarily airframe ground. Take a look at whatever gaseous residuals or fuel is in the tank, obviously these things will have different dielectric constants, which is something to think about.
USAF T.O. 1-1-655, Restricted Use of High Potential Voltage Testing Apparatus on Aircraft Containing Fuel, addresses your question.
As a last resort, high potential testers may be used in fuel areas provided they are drained, purged and maintained at less than 20% LEL.
Funny thing about the military....they have tech data for everything!
onemike,
Sorry, the company I work for gets these books for me through a contract that does not allow redistribution of any data.
I would like to, but can not.
Field Team: 20% LEL. Seems a bit high even for the military.
Eng3000: almost nobody has a TDR. They are soooo expensive - but sweet. You are right about the tank wiring. People have a bit of a cavalier attitude towards fuel tank - we burned out two aircraft in a hangar because we didn't bother with bonding the (plastic) bucket or earthing the aeroplane that we were draining the AvGas from.
Onemike: You are right to question. Your situation may have seemed obvious - consult the AMM and do that BUT you must question everything. Good Man!!
