Why should we back up the backup?
A short while ago, we told you the story of our upgrade from a single, rather elderly, backup generator to a shiny new pair of N+1 diesel generators - giving us Tier 3 status at the same time. But why is it so vital to have this ‘belt and braces’ approach? Is it just about the standards, or is it really more about the customer?
We think it’s most definitely the latter. The data centre industry is mission-critical, and the consequences of a failure of electrical power to the data centre are severe. Generator systems are often specified and installed to N+1 or 2N standard, meaning there is at least one backup generator available in case a backup generator fails to work. So why doesn’t everyone else in mission-critical environments do that?
Nowadays, office buildings are often specified with backup generators, but rarely to more than N+0 standard. Data centres are mission-critical, but if there is a failure it isn’t a matter of life or death. Whereas in a hospital, such a failure really can be a matter of life or death. People can die on operating tables if the power stops, or they can die in their beds if a dialysis machine stops. Yet hospitals don’t usually have N+1 generators - so why do data centres need them? Even Dealing Rooms don’t have such levels of support - although the trading platforms which are in the data centres do!
Is it just about marketing? Our competitors say they have N+1 so we must have it too? Or to meet the requirement for Tier Levels? Is it really needed for mission criticality, or is it just fashion? Or perhaps it’s designed to guard against poor Facilities Managers who may not do the maintenance properly.
Let’s look at Tier Levels in more detail. These are defined by a commercial organisation in the USA called the Uptime Institute, which has copyright protection on the designation. They are highly US-centric and reflect US conditions. For example, Texas has frequent hurricanes and tornadoes, and much power distribution is on overhead lines because of the distances involved - so long power outages are not uncommon.
Likewise, Florida has frequent lighting strikes - in fact Tampa is often described as ‘the lightning capital of the world’ - which can result in serious prolonged outages. Do specifications that were designed for such locations have any place in a sophisticated and temperate environment like London? A city which is sometimes described as a place where power outages a rare occurrences and only fleeting when they do happen.
As a colocation data centre right in the heart of London, we have to agree that these backup standards are just as vital here as they are in the US. We experience a short outage, just long enough to start the generators, maybe once a year. A few years ago, when the Whiston Road substation in East London blew up, we had a 6 hour outage on one feed and an 8 hour outage on the other.
This was then followed by an 8 hour outage on both feeds exactly two weeks later, and then two years later by a 16-hour outage on both feeds, for reasons which were never explained. The utility providers should provide SLAs, guarantees and compensation, right? But they don’t. They provide power ‘if and when they can’. If they can’t, well that’s a pity - but never mind!
The bottom line is, mains electricity, no matter where in the world, is not reliable. Above all else, data centres must be reliable - so who would design a reliable system that depended on an unreliable supplier? The answer is that data centres do not run on mains electricity. They run on diesel generators - and must be designed so that the backup generators always work. In other words, the backup generators are the primary source of power for a data centre.
Mains electricity is there because it is cheaper than running diesel engines by a factor of about 2:1 - assuming that it would ever be possible to get planning permission to run high power diesel engines continuously in somewhere like central London. Mains electricity is the secondary source of power, sometimes referred to as the ‘economic alternative’. Which leads to an odd, and somewhat counter-intuitive, situation - where the primary source of power only operates 0.1% of the time and the secondary source of power operates 99.9% of the time. But it has to be that way, because only the diesel generator as a source of power can be made to be 100% reliable.
That thinking also leads to an interesting result from the question ‘How many mains feeds are best for a data centre?’ Everyone will be familiar with the salesman who pushes the idea that his data centre has two or three or four mains feeds, and is therefore the best because of this. But if the mains feeds are only the ‘economic alternative’ and exist solely to reduce the data centre operator’s power costs - then why does it matter how many feeds there are? The data centre has to be designed to work continuously at 100% performance level - with any, or all, or none of these feeds working. So perhaps having just one mains feed is just as good as having four, even if that is not what marketing wants to hear!
So in summary, a data centre does need backup generators for its backup generators. The mains cannot be relied on. So if the generators are the primary source of power – the one that can never be allowed to fail – then N+1 or 2N is necessary to guarantee the integrity of the power supply. But having said that, marketing requires it too – if the competitor down the road has N+1 generators, then the data centre which cannot match that will fall behind in its customers’ perception.
