Negative Phase Sequencing Unit

Last week XL231 decided that she didn't want to accept 200 Volt ground power, this caused a bit of head scratching, and John to get his electrical brain cells into action. At first we assumed that the GPU (ground power unit) to be at fault, however, after carrying out checks we decided that the GPU wasn't the culprit.

The aircraft then suddenly accepted ground power and we were relieved. Unfortunately this was short lived and power soon tripped again, it then re-set for a similar short time before tripping yet again.

We checked the aircraft to ensure no heavy loads were on and tried again, all seemed okay but only for a short time before it re-tripped!

It was then that I noticed an orange warning lamp illuminated on the AEO's panel. The port side NPSU (Negative Phase Sequencing Unit) had tripped.  "No problem I'll reset it" I thought and did so using the push reset button. The Ground power came back on and then re-tripped yet again!

It was then we realised that we had a problem, so out came the books over a coffee in the NAAFI and John decided the NPSU for the port busbars had failed.

The 2 NPSU's were fitted basically as a protection unit during the Tanker conversion programme to protect the aircraft's AC ground and APU electrical systems in case of failure of the powerful AC motor on the Air to Air Refuelling Hose Drum Unit (HDU).  It was deduced that the motor in a fault condition could in a worse case scenario seriously damage the aircraft's electrical system.

Fibre glass distribution board

Port NPSU

Luckily, we had spares-recovered spare NPSU's from Victor K2 XL164 'Saucy Sal' back in 1994 and I was able to remove one of them from the complete distribution board that we had taken out of the aircraft all those years ago.

It was a bit of a task to change it in the aircraft but once fitted proved to be serviceable. Rick will check the removed unit out for repair.