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Three phase DB. Turned up and tried to turn the lights on, to find that
all the MCBs controlling circuits with lights (and some without), plus a
submain that also had a lights circuit running from the secondary DB,
we're off.

The only possibly explanation I can think of is that someone had turned
them all off, because they were blindly trying to turn off the lights
after working there and hadn't noticed the timer override master switch.
Problem with that theory is that in order to turn the lights on in the
first place they would have had to know about the override switch!

One obvious explanation is that a different person turned the
lights on and turned them off.

Is there any possible electrical fault which could trip half a dozen
circuits and leave no signs, a maximum of 24 hours since the lights were
circuits were last on. There was no evidence of water leakage.