The decision to create a Tier III Data Centre is a strategic one, usually as as result of the necessity of creating an extremely fault tolerant system, either for customer demand (website hosting for example) or for business continuity reasons (credit card processing, military, financial etc.).
The Tier III data centre was conceived when computer systems were introduced that had dual power supplies. The basis of the Tier III centre is that there is one power path with redundancy, and an alternate power path:
Tier III Data Center Power Flow
Note that essential cooling is now added to the UPS output, whereas with Tier I and II it was expected that the site could cope with short downtime of cooling during which time the generators would power up and restart the system. In Tier III this is not allowed and cooling is continuous. This needs to be borne in mind when selecting the type and size of UPS. It is for this reason that you will often find rotary UPS systems (with their ability to handle mechanical loads better than static systems) used for such applications, although this is by no means a requirement.
The computer systems are dual powered (commonly referred to as A & B inputs). If a computer system is utilised without dual inputs it is expected that a local Static Transfer Switch (STS) is utilised.
In the diagram above we will assume that the primary power path is A, and the secondary power path is B. The Static Transfer Switch (STS) also has primary and secondary inputs, and the primary input is also taken from Path A. The computers are therefore supplied by two power inputs, all of which is provided by the UPS systems. Should the primary power fail anywhere along Path A, then the Static Transfer Switch will revert to its secondary input and continue to supply power along Path B to the load.
NOTE: Static Transfer Switches are capable of switching within μsecs between their A & B inputs provided the two sources are synchronised. If they are not then there will be a switching delay. This is particularly important when you consider the issue of selectivity, i.e. the ability of the source to clear a fault. In order to achieve this selectivity, UPS are synchoronised with their bypass input. Should a fault occur they can switch to bypass instantaneously (quickly anyway) which then will allow a greater fault current allowing the fault to be cleared quickly (ie pop a fuse or trip a breaker) without causing disruption to other equipment on parallel circuits. This means that the primary and secondary sources should be synchronised. Make sure this can be done! Another factor of having unsynchronised inputs is for the potential of having 400V (not 230V) AC within the computer room cabinet.
As you can see, a Tier III is inherently more robust as it will allow failure along the entire path without power being lost. This is what classifies a Tier III system – it is basically a Tier II system with an alternate power path, derived from a seperate source. A Tier III centre has an availability of 99.982% which equates to 1.6 hours of downtime per year.
So how does this help the average computer room user? Well, Tier III is probably way over the top for an SME computer room. I have known small financial companies that require the fault tolerance of the Tier III infrastructure, however Tier III is more strategic and therefore the site is designed from the beginning with Tier III in mind. It is difficult to post fit a Tier III system without severe disruption to the existing business. However, if you required just a little more protection against unplanned outages, it may not be too difficult to install a secondary power path and an STS to feed your computers. Or another alternative may be to look at a halfway house for Tier IV…….
