Please reference rotate.aero/forum/ata24-767/343-ata-24-2...ed-power-feeder-wire for some of our previously encountered problems.

Another power feeder wire short. This time the problem was inside the pressure vessel, as we normally find these on the engine or pylon.

The BPCU (Bus Power Control Unit) faults are always the same, DP Trip (Differential Protection) and GEN/FDR Fault (Generator/Feeder).

System isolation is the first step in troubleshooting. The feeder wires from the engine IDG (Integrated Drive Generator) go to the generator contactor. There are some extra wires connected to the three big feed wires. Three (one for each phase) go to the Generator Control Unit or GCU. Removing the box from the rack isolates those wires. Two extra wires, one for a red "Bus Hot" light and a EICAS voltmeter input also need to be separated from the feeds. Removing the bulb and pulling the breaker will accomplish that.
  
     
     
  
Using the megger from the generator end, we found the blue wire with the short.
  
     
  
No registered meter scale and less than 1 Meg-Ohm resistance. (The meggar picked this up as a hard short, but a regular VOM showed an open circuit.)
  
       
  
We went ahead and removed the blue feeder from the contactor just to make sure nothing else was in the system. The reading was the same.
  
     
  
We shook all over the engine and pylon with the megger on..... no joy. Same thing with some wing leading edge panels removed. (Two hours or more)

When we started shaking the wires in the right side forward lower cargo deck, the meter started jumping.

I wonder why....
  
     
     
     
     
  
Fun Stuff!!!

  
 

Not the same day, but still a couple pretty nice shots.....
  
     
     
 

Nice work Mark a lot would forget about the parallel resistor formulas.

Servo valve explanation.....
  

25+ years of aircraft avionics/electrical work..... "0" power feeder wire issues. Last 8 years..... 10+ power feeder wires.

Another one. PW 4000 on a 767. BPCU (Bus Power Control Unit) showing left DP (Differential Protection) Fault and Overlap Fault.

This particular problem reacted differently than most. Instead of just a generator disconnect, it dumped the whole left bus also. "THIS IS NOT NORMAL". The system should dump the generator feed and then the Left Bus Tie Breaker should have closed to feed power from the right to the left.

To isolate the issue we disconnected the three main feeds (and the smaller connections) from the generator breaker (upper left wires).
    
       
  
We disconnected the feed wires from the IDG and fed 1000V from the megger into each lead. As a note: when using the megger, all wires have to be isolated. If we megged forward from the IDG without disconnecting on the other end, we could fry some downstream components. Not so much the breaker itself, but the sense wires going to the BPCU and GCU (Generator Control Unit).

Anyway.....
  
      
  
This was at the pylon disconnect box at the top of the engine. What sucks here is that I was originally shaking the wires and my co-worker on the ground is hearing a buzz. I'm right there and I'm not (it sucks when your hearing is dropping off). After swapping positions, he saw the spark.
  
  
At the forward feed-through of the box is a flange with a short tube. Inside that tube there is supposed to be a thick rubber grommet to center the wires. Where's the grommet???
  
     
  
All items underlined are part of the grommet assembly. The nut and bolt on either end are tightened to squish the assembly inside the tube. With the rubber sealed against the inside diameter of the tube, all the wires are isolated from the edges.
  
     
  
We had one wire shorting on the tube. You can see a pile of molten metal below.
  
     
  
Flange replacement, new wire, and properly positioned grommet.
  
     
  
Removed wire and flange.