Generator Overload, L-R
High load indicated on affected IDG.
[G-450 QRH MB-35] The Caution Messages section simply says "Indicated generator load exceeds 100%," and "Reduce Load on indicated generator." It does not refer to the Electrics/APU section where a checklist exist, but that checklist doesn't say much more:
[G-450 QRH EA-10]
Each AC generator normally runs at less than 30% of capacity so something has gone wrong. The high draw AC items, we are often told, are "heaters, motors, chargers, and pumps." Looking at the Master Table of Electrical Component Availability, EA-17, we see the following items from those categories under L MN AC, R MN AC, and ESS AC:
Missing from this list are the four TRUs, which each get their power from a main AC bus. And of course that means all on-side electrical loads are suspect.
Getting rid of a high draw item also has the salutary effect of identifying the root cause of the problem, if you are methodical about it. You should pull up the synoptic and watch the load on the generator as you switch off your high draw suspects.
The easiest ones to get rid and probably one of the higher draws are fuel boost pumps. How much should a fuel pump reduce your load? I watched the right generator and right main TRU while shutting off the right alternate fuel pump. The generator dropped from 24% to 21% load, the TRU from 21% to 12%.
I would kill the on-side alternate pump, look at the load, and if it goes down appreciably — much more than the expected 3% AC — suspect it. If, for example, the load goes from over 100% to under 30%, you have just identified a pump that is drawing way too much and could be a fire hazard; leave that breaker pulled and consider yourself lucky. If it only goes down a few percent, open the crossflow valve and kill the on-side main boost pump. If you still only end up with a negligible reduction, bring one of the pumps back and keep looking.
How do you know what the high draw items are? Look at the number on the circuit breakers. But doing that you see mostly small numbers. There are just a few over 20, such as the windshield heaters, PSU fans, and even the vacuum toilet. One big draw item that should be an easy kill are the HF R/T units, 25 each. The source of really big numbers, however, is the PDBs. Here's the left:
And the right:
Most of these are feeders to other circuit breaker panels but there are a few worth noting: fuel pumps and the galley.