Mark

The whole point of a Constant Speed Drive is to do just that.... hold the frequency within a set tolerance. Without looking, 400Hz +/- 10 Hz.

If the frequency drifts higher or lower that 390/410Hz. The Generator Control Unit will take it off line.

These units a pretty damn reliable. 397-403 is usually seen as a frequency.

If you're reading low freq's, the CSD is NOT doing its job.

If the frequency is showing low, but generator is "not" dropping off line, you need to be verifying the indication system.

I have never seen an indication that was wrong when the generator is staying online.

You would be looking at a bunch of areas of possible faults, not just grounds.