I'm being unnecessarily flippant.
I've not run into electro-mechanicals before. I'm genuinely interested.
( Kroney, Mon 5 Sep 2011, 13:03, 2 replies, latest was 14 years ago)

I'm no expert but I think the point is that if you lose power, a relay-based system will fall into a known state.
Pesky digital components won't necessarily do the same, being governed less by mechanical physics. That's probably a gross over-simplification.

/used to do a bit of 40XX and 74LS electronics as a kid ...
(BrianHequator was stretching owls, on, or around, Mon 5 Sep 2011, 13:05, Reply)

The known state presumably being one of the switches hasn't (or has) switched
and it's Bartleby's job to figure out which one.
( Kroney, Mon 5 Sep 2011, 13:06, Reply)

I guess so.
That and the fact that a relay-based system has proper bits of wire as conduction paths, so even if the logic power fails (i.e., the relays won't switch) the established circuits are still good, in contrast to semiconductor-based circuitry.
(BrianHequator was stretching owls, on, or around, Mon 5 Sep 2011, 13:08, Reply)

All this is true.

( Bartleby A dead man on vacation, Mon 5 Sep 2011, 13:13, Reply)

Think of fly by wire aircraft.
There are anynumbers of computers each coming up with an identical solution, which then moves a control surface to the required position. If one of the computers fails, the overall supervisor, and its backup, spot the different solution and reject that one. To make sure these sytems are safe, different software engineers write the software, so a systematic fault cannot occur. All this means big money. My soultion to fly by wire, would be add a bit of good quality string between the control surface and the pilot.
( Bartleby A dead man on vacation, Mon 5 Sep 2011, 13:10, Reply)

Yeah, I've got a basic understanding of flight control.
Unfortunately your bit of string would immediately be the cause of a huge crash, due to modern planes being so aerodynamically unstable the pilot wouldn't be capable of flying it.
( Kroney, Mon 5 Sep 2011, 13:12, Reply)

That is so, it was a crap analogy.
Apply the reasoning to shutting down an oil storage depot, say Buncefield.
( Bartleby A dead man on vacation, Mon 5 Sep 2011, 13:15, Reply)