Brian Gaff wrote

There have been lots of storms, maybe a spike did it.

Nope, spikes don’t do that.

"The Natural Philosopher" wrote in message
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On 27/09/2019 10:49, NY wrote:
"harry" wrote in message
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On Friday, 27 September 2019 09:54:45 UTC+1, wrote:
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!

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.

If they have tripped, the toggles would be in the mid position. To
reset/turn back on, they have to be turned to the fully "off" position
and then "on".
So it should be obvious if they have tripped or been turned off.

That assumes the MCBs have that design. Ours (MCBs and RCDs) have only
two positions: fully on or fully off - there's no half-way state to say
"tripped" as opposed to "turned off manually".

Maybe it's different for 3-phase distribution boards rather than
domestic 1-phase ones.
Overvoltage on one or more of the phases can cause breakers to trp