On 767's, there are two UFI (Universal Fault Interrupter) units that protect the center tank pumps and wiring.

We recently had a right pump that was only receiving one (out of three) 115VAC inputs. A control relay upstream of the pump had only one phase also.

Traveling further upstream is the UFI. There were some fault lights on the unit indicating a phase problem.

So, you get a replacement and hang it off the plug (thinking it'll work). No lights..... no nothing?

Hang it off the known good side..... same thing?

The only possible issue would be case grounding, which it turned out to be. Most all aircraft electrical components have a wired ground. Not the UFI's.

Not really a big deal, but it sure makes you think for a second.
  
     
     

Let's reevaluate this one.

I still like to know what's happening with the aircraft????

By impedance, we're just talking resistance in a A/C circuit. CSD's (or generators) output 3 Φ alternating current at 115VAC and 400Hz. This output is monitored by both the GCU (Generator Control Unit) and the BPCU (Bus Power Control Unit). Voltage and frequency are two aspects, but current load plays a huge role as to "if" the generator is allowed on the the A/C distribution bus. CT's (Current Transformers) monitor both the output load and user load on all three phases.

If either the Volts/Freq's falls out of tolerance or a differential load is sensed..... the CSD is disconnected from the bus. Usually when this happens, the GCU will kill the field input to the generator. All output from the generator is stopped.

The BPCU should record the actual fault in BITE Memory (Built In Test). When fault history is reviewed, the actual propblem should be known.

Impedance load would be sensed as a load imbalance. This could be caused by a short to ground or open phase (high resistance). Either one will trip the generator off.

Is the generator falling off line? Does it fail internally requiring replacement? Are you sure voltage/freq sensing is good. Is one of the CT's failing?