Tier II centres encompass all the features of Tier I centres with the addition of redundant critical power and cooling components.
Tier II Data Centre Power Path
Each component must be capable of operating if the other component fails. This is typically achieved with n+1 redundancy. What this means is that if ‘n’ eg 2, modules are required to support the load then install ‘n+1′, i.e. 3. There is a lot of debate as to what determines true redundancy. For example, some manufacturers have seperate UPS modules, controlled by a single controller. If the controller fails then the system fails, so this is not true redundancy, although they may argue that the controller is designed with redundancy built in.
Some UPS modules don’t have an internal static bypass and instead rely upon a wrap around static bypass. The argument here is that one large wrap around is more robust than several smaller ones, as smaller systems may blow up one by one due to a race condition in the event of a fault. My belief is that the latter is unlikely as static switching can occur in μseconds, probably about a thousand times quicker than the time needed to damage the single static switch. In any case, the static switch is usually rated many times higher than the nominal load current to accommodate fault currents. However, the wrap around is now a single point of failure in the system – although if it fails, this will only cause a problem if the system needs to bypass, so is this a problem?. The debate will continue to rage on.
Historically, UPS were configured in what was known as a “hot standby” configuration to achieve redundancy. In this instance two UPS are fed from the utility, but the output of the standby UPS is fed to the bypass input of the primary UPS. The primary UPS provides power and if it fails (and bypasses), the standby unit will then provide UPS power to the load via the bypass of the primary. Works in principle, and can be used with mixed manufacturers and ratings of different UPS systems, however, the primary UPS output is a single point of failure. In addition, should the primary UPS fail, the secondary UPS will instantaneously be expected to deliver from 0 to 100% load immediately. Shouldn’t be a problem, but sometimes it is!
The modern method of achieving redundancy is to share the load equally amongst the UPS modules (this is how all our Kehua Parallel Systems operate). The UPS talk to each other through redundant communications ports and no one UPS is master over the others. If any UPS should fail, the UPS is isolated from the others automatically.
Enough about redundancy, the Tier II centre is more robust than the Tier I centre however still has one power path and therfore there are times during faults or planned maintenance that the computers have to be powered down. As a result a Tier II centre has availability of 99.741% equating to 22.7 hours of downtime per year. Not much better than Tier I on the face of it (28.8 hours), however Tier II is more robust against unplanned outages.
So how does this impact the normal computer room? As said under Tier I then it depends upon the financial impact of downtime to your business. For SME’s that rely on computer systems, but will all go to the pub if the power is off, then there is perhaps no need to keep the computers running for hours on end, simply shut down gracefully and that’s that. Where you don’t want to have to shut down the system to perform maintenance on the UPS, and don’t want to leave the system vulnerable to power cuts or surges when using an external bypass switch, then you will need redundancy. Like all things in life, it’s a choice based upon your needs and wants.
It’s worth stating that Tier Standards are only a guide as to the robustness of a site against outages, there is no standard or law dictating that this is the way it should be. Use the information as a guide to what is best for your business.
