Real World Computing
Seize the power!
My recent problems with power interruptions have caused me to think again about the power capabilities in my lab, and also to reconsider the recommendations we make to clients. Answering what might seem to be a simple question of what is an acceptable power outage isn't such a simple question once you dive into the details. There are many conflicting requirements, most notably the cost of any potential solution, and arriving at a compromise with which you're comfortable is a far from simple matter.
First, let me get my prejudices out in the open: as far as I'm concerned, there's one vendor in the uninterruptable power supply space and that's APC (www.apc.com). To be sure, there are other vendors out there, and some of them might provide a longer run time, or more sockets, or a better life expectancy, or even some mixture of all those factors. But, at the end of the day, I've never had any problems with APC equipment, and that's true of my clients' sites, too. Of course, I ought to raise a couple of caveats at this point: the equipment must be appropriately sized, it must be tested, it needs to check its battery state weekly, dead or dying batteries need to be replaced as soon as possible, and so on. But at the end of the day whenever the UPS question arises, I simply reach for the APC catalogue to find my answer.
Let's look at the sort of problems that can arise. First, you need to cover yourself against mains fluctuations. You might believe that the generating companies keep your voltage at a nice steady 230V, but nothing could be farther from the truth. My mains power meters show me that the voltage can stray anywhere from 220V to nearly 240V, and can swing quickly, too, jumping two or more volts in a minute. So the first thing you need to consider is the "brownout", which is when the voltage droops down into the mid-hundreds - most equipment isn't going to be able to run effectively at this level. Then you have the matter of complete power failure, which is a more complicated problem to solve, because although the failure is simple enough - there's no incoming mains voltage - now the runtime performance kicks in with a vengeance.
When it comes to power failures the world divides into the eternal optimists, who believe it will return in just a moment or two, and the eternal pessimists who immediately assume it's gone, let's go down to the pub. The truth normally lies somewhere in the middle, but what is certain is that you can't predict how long any given outage will be: it might be a minute or two; it might be an hour; it might be a day. An outage of a minute or two can be dealt with by almost any decent UPS device. An outage of an hour takes a significant battery reserve and thus tends to be the focus of the extra-long battery runtimes, often using banks of batteries. Getting something with a massively over-specified power generation rating might not yield the most efficient and cost-effective balance between generation and runtime, and it might be better to go with a solution that keeps you at 90% capacity for longer, rather than assuming that a solution that runs at 60% will last longer. Check the specifications carefully.
Independent means
Once you start talking in terms of more than a few minutes, then it's probably best to think in terms of an independent mains supply from a generator, which adds a whole new level of complexity, because it impacts directly on the mains wiring. You need switchover capabilities, and possibly even specially marked wiring that indicates which sockets are connected to the generator. The temptation is to put a large generator in the middle of the main incoming feed, then to run the entire building from this whenever there's no incoming power. This might work well, but bear in mind that you'll need sufficient generating capacity to run that large colour laser printer and the heaters that were turned on in the main office. One good side of this solution is simplicity, since everything keeps running as if nothing had happened, and you can then go round the office switching off the unnecessary devices to reduce generator load and hence improve fuel consumption and runtime. There are plenty of suitably-sized generators out there for reasonable cost: I found sites offering dozens or even hundreds of kilowatts for not too much money, especially in the context of a large business unit. Most business units require three-phase power, which adds more complexity still to the problem, but there are UPS and generator solutions available for this, too, and it doesn't have to be too expensive.
