redundant ps

Hi,

I have a question about redundant power supplies on servers.  I think I had a dell, which if the primary ps died, the other took over.  I think on my DL 380 G5 the two ps are both live, so if one looses power, the other if it is getting power keeps the server on.

When buying equipment that talks about redundant ps, is there a way to know which is being referred to - if the primary ps has to die for a switch over vs if both are looking for power all the time, and if either one gets power you are good.

As for the later description, it would work if both ps were plugged into different ups and one ups died, whereas in the ps fail over only method, if the primary ps is plugged into the ups that dies, the server goes down, because it senses that ps is fine, it just doesn't have power so no switch to the second ps ever happens.

Thanks,
gsgi
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gsgiAsked:
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Ian MeredithCommented:
As you can read from the attached link, a servers redundant power supply is one that has two separate power supply units.  You connect mains or UPS to one or both.  When either one fails the other takes over.

There will be a primary psu, and when that fails the secondary takes over.  You can operate off the secondary psu regardless of the state of the primary.  An example is plugging mains power into just the secondary (primary empty), the server will operate just fine.

They are designed to switch either way, depending on where the power is coming from.  

http://www.computerhope.com/jargon/r/redunpow.htm

hope this helps
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gsgiAuthor Commented:
So this Dell I had was weird?   The primary power supply into a UPS and the backup into the wall.  I turned off the Dell.  I turned off the UPS.  I turned on the Dell.  It would not start.  I rewired the Dell, so the primary was plugged into the wall.  It started.  The reason, as I understood it, was because the primary power supply was not actually degraded or broken, so it didn't look to the secondary power supply.  This was on a PE 2600 I think.  Maybe I am dreaming.  I remember thinking it was odd at the time.

Thanks,
gsgi
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Ian MeredithCommented:
Interesting gsgi, I see the Dell PE 2600 was built around the 2004 mark.  So, nearly 10 years old.  I would think that your observation was of a variation of redundant power supply that has improved over time....

Between manufacturers and even models from the same manufacturer you would find differences in their implmentation of redundant power supplies.

This link explains the redundancy in Dell PE blade servers. Talk about choices!!
http://www.dell.com/downloads/global/power/ps2q06-20060273-Devireddy.pdf

I found an un-answered post (2009) on the Dell support forums that asks a similar question to yours. Interesting that it was never answered...  http://en.community.dell.com/support-forums/servers/f/956/t/19287963.aspx
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andyalderCommented:
There isn't normally a primary or secondary PSUs but rather both PSUs are actively supplying power to ensure there is no dip in current to the components when one PSU is unplugged. On some HP ProLiants you can set a preferred PSU but both are still active with one providing about 80% of required power and the other 20%, this is slightly more efficient than 50:50.

Not starting with a failed PSU is strange but not impossible but that's completely different than not continuing once powered on should a PSU or power feed fail. Blades are very different since they have multiple PSUs rather than just two.
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gsgiAuthor Commented:
Thank you.
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