[ale] Considerations Regarding Dual Power Supplies

Jeff Lightner jlightner at water.com
Mon Oct 6 15:53:22 EDT 2008

You should certainly go to two separate PDUs with separate power
circuits for redundancy.   We do that with all of our servers
specifically to insure loss of one PDU or circuit doesn't take servers

I did see a presentation once on power that talked about the important
thing being that everything had a common ground.  It seems if both
circuits go to the same ground (and it is the shortest path) you don't
have to worry about separate PDUs.

If the PDUs are daisy chained together they're all on the same circuit
anyway so there's no benefit to spreading the load between them.  I've
seen some racks that do things that way (often due to poor planning).

-----Original Message-----
From: ale-bounces at ale.org [mailto:ale-bounces at ale.org] On Behalf Of
hbbs at comcast.net
Sent: Monday, October 06, 2008 3:33 PM
To: ale at ale.org
Subject: [ale] Considerations Regarding Dual Power Supplies

In audio-land, you have to be mindful of not creating ground loops, i.e,
a closed circuit of what should be ground in which EMI (especially 60Hz
hum and harmonics) can be induced.  

Knowing that, does it cause problems to run power cables from a machine
with two power supplies to two different PDUs in a server rack?  Or, is
it preferable to connect the two power cables to as close to the same
point as possible (i.e., two electrically adjacent outlets in the same
PDU)?  Aren't the ground pins in each of the two power supplies going to
be connected together inside the machine?  If so, it seems as though
having the two grounds form a big loop might bring a lot of hash into
the machine through the ground lines.

- Jeff
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