[Beowulf] Re: Emergency Power Off

David Mathog mathog at caltech.edu
Fri Mar 23 10:20:58 PDT 2007

Jim Lux wrote:
> Relays are your friend.
> EPO switch has Normally Closed contacts (i.e. they open when you hit 
> the switch)
> Power flows through contacts to small relay with NO and NC contacts
> (you can use DC or AC supplies)
> relay contacts go to UPSes
> Multiple relays can be paralleled.
> It's "fail safe" because if the AC power to the relay system fails, 
> the relays drop out, and turn off the UPSes.

That pretty much defeats the purpose of the UPS in the first place!
If the AC power fails, but there is no emergency, the UPS units must
stay on.

My understanding of the code is that the EPO switch must work no matter
what the conditions in the room.  So if the main power has already
failed, for instance 2 minutes before, the EPO still has to be able to
kill the UPS units when the fireman hits the button.  That means
there needs to be some sort of battery backup to be able to drive a
relay.  I'm assuming that most of the pro circuits use a small
lead acid battery that is kept trickle charged.
The thing is, I know from personal experience that those systems are
failure prone.  (One of our alarm systems died  without indicating
that it was down since the AC was still on.)   The lead acid
battery eventually craps out, and it can do it fairly suddenly by
developing an internal short or a crack in the case.   So it all
becomes quite complex, with a requirement for an ongoing test of
the health of the internal battery, without actually triggering
the EPO circuit.

If I can get away with a switch block, I'll use a switch block.


David Mathog
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech

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