165 Halsey recurring power issues

Mon Oct 23 19:16:51 UTC 2023

On Mon, Oct 23, 2023 at 10:38:09AM -0400, Babak Pasdar wrote:
> I wanted to get some feedback as to what is considered standard A/B 
> power setup when data centers sell redundant power.?? It has always been 
> my understanding that A/B power means individually unique and preferably 
> alternate path connections to disparate UPS units.

Generally speaking, the definition of A/B has become muddied in recent decades.  It has almost become an inaccurate marketing term. 

Most sane people have the opinion (myself included) that when "A/B" power is offered, it is at minimum offererd as 2N UPS (different building entrance and MSBs and even physically separate UPS rooms are also desired on a true 2N A/B, but may not always be available).  Some data center operators go even further and architect load switching within their distribution, thereby preventing single-side/one-leg power outages for customers during most of their power maintenance activities

Some data center operators treat "A/B" as convenience for them to undertake maintenance and offload uptime responsibilities to their own customers, and require them to either undertake their own transfer switching and/or dual-cord every equipment, so that they can keep taking one side of the power system down for repeated maintenance.  This does not scale well for retail colo, as not every customer is going to be good at maintaining two PSUs for every single piece of equipment.

Some data centers also view "N+1" system deployment at the UPS as an acceptable form of A/B protection, as long as customer circuits are on different PDUs.

Long story short, whether you're receiving N+1 or 2N or 1N, it's important to inquire about how your power circuits will be architected and delivered by the data center, and either have that codified in the contract or reflected appropriately in SLA offering.  There is nothing wrong with the data center providing N+1 or 1N power, as long as they're transparent about it and that it is what you're willing to accept for the right terms.  However, simply accepting "we are providing you A/B power" or "we've never had primary power failure" are not sufficient to meet proper due diligence during a site selection process, unless you can accept the site outage occurring from time to time, or you're deploying your own power plant (i.e. DC power and batteries) to supplant data center's own power protection scheme.

James