UPS - single points of failure

[Jeff Kinz]

On Fri, Dec 01, 2006 at 12:41:52PM -0500, Rich Braun wrote:
> > If you want to eliminate that single point of failure purchase 2 UPS's
> > and buy a server has has 2 power supplies.  One goes into each UPS.
> 
> At a consumer-price level???!?  Where do I get a dual power supply with
> auto-failover without breaking the bank?

You have a good point.  

I have seen regular power supplies wired up to do this, using standard
off the shelf components, but its tricky. At one of my client companies
they had done a competitive upgrade to gain a new customer and as a
result, inherited all that customers old "boxen".  A LOT of Boxen!

So they were basically awash in unused PC's and UPS's.  Eventually these were
"wholesaled" to a reseller but before that happened all the extra equipment
sparked a few imaginations.  That summer they were having a lot of power
problems killing their servers and affecting critical operations.
Because of internal politics, (New dept perceived as threat by rest of
company :-) ), they couldn't get funding to buy the right equipment so
they built failover circuits for both UPS's and power supplies.

The basic idea was :


UPS1 --------------|
                   |
UPS2------------FAIL BOX-===>power out>===PC

The magic FAIL BOX monitored the power from UPS1 and if it dropped it
activated a relay (three relays actually) that pulled contacts over from
UPS to UPS2 lines.

Then the built a similar things for the Power Supplies, but it was more
complicated because each PS could fail on three different Voltage
supplies. :-)


The sticky point (iirc) was making sure that the relays worked well
because if the contacts weren't switched over cleanly from one power
infeed to another, and simultaneously... "Bad things would happen..."

They eventually had 4 servers set up this way.

Jeff Kinz
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