Primary and Secondary Power Options
Posted by Victor Goodman on 30 July 2012 04:46 AM


Each circuit you purchase comes attached to one UPS- that first connection is called “Primary Power.”

The fact is most customers opt for only “Primary Power,” it is backed by Utility Power and failing that Generators.

If the Utility fails, the generator kicks on in about 8 seconds, but during those 8 seconds the UPS has to run the data center on its batteries.

It could probably hold the data center for fifteen minutes or longer if the generator took longer to start, but usually its just a few seconds.

Some deployments that need high availabity utilize two Separate UPS Systems, a Primary and a Secondary.

The truth is that the UPS is the weakest link in the data center.

It is a complex electronic machine; filled with batteries and lots of power and heat, further it is required to run 24/7/365.

At Corporate Colocation, we follow regular maintence schedules, as do most commercial data centers, but even with that, the possibility of failure always exists.

To create a higher level of availabity (increase the 9’s) companies opt for “A and B” Power or Primary and Secondary as it is sometimes called.

The most optimal method would be to purchase servers with dual power supplies ( added redundancy there) and plug each server into two PDU’s  ( Primary and Secondary)  using  not more than 50% of the 80% of allocated power per PDU.

For example:  If you had one 20 Amp 120V Circuit in a cab- per the 80% allocation rule you would have a total of 16 amps of power, to be divided between the two PDU’s.

So, you would plug 8 amps in one PDU and 8 amps in the other.

Therefore if one UPS failed, the other could hold the entire 16 Amps-


Secondary Power makes it less likely that your servers will suffer reboots due to UPS Failure. Deployments in which downtime, or the cost of unintended reboots exceed the cost of a Secondary Circuit are ideal candidates for Secondary Circuit. 

 As UPS failure is unpredictable by nature, it is hard to determine if such an event will occur during your deployment, going with "B" Power is something you have to decide, but if you can afford it, it's a super idea!


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