Networks Europe May/June 2019 | Page 42

42 POWER DISTRIBUTION UNITS "The constraints of the electrical grid and the need to lower our power consumption are two issues that aren’t going away, and we must all respond accordingly" meters have the same accuracy – it may range from 1% to 5% – so it’s worth investigating the technical specifications. PDU electronics also consume power at levels which can be significant, especially in a large scale data centre; given that it can range from as little as 6 watts to 60 watts. If intelligent PDUs are installed in a 200 rack data centre with dual PDUs in the rack, then their power consumption could be as little as 2.4kW or as much as 24kW. The switching function allows users to remotely shutdown individual sockets and to hard boot any device that’s connected to it. It’s worth checking which type of switch device (or relay) is within the PDU construction – either monostable or bi-stable. The monostable needs a permanent supply of power to change state and stay there. Bringing it back means removing the power. The bi-stable needs to be pulsed with electricity to change state and pulsed again to come back. Bi-stable, therefore, uses less power than monostable, which can again be significant in terms of the aggregate consumption from a lot of sockets. 3. How reliable is it? Typically, bi-stable relays have a higher mean time between failure (MTBF) than the lower-cost monostable option, which is in excess of 80,000 hours. This is significant given that most PDUs operate at the back of the rack where ambient temperatures can exceed 40°C, which, unsurprisingly, can shorten the lifespan of the conventional relay. Metering also an impact on the overall resilience of the power system. It allows users to determine their power consumption and how close the system may be getting to the tripping points of the circuit breakers, particularly if warning alarms are also enabled. 4. Is it easy to integrate? Larger PDUs must fit into the back of the rack. However, if possible, they shouldn’t extend into the 19in space because of the potential this has for conflict with 19in mountable equipment. So, mounting PDU positions that sit in parallel with the 19in angles can be beneficial and should be possible in 600 wide racks if the PDU profile is slim enough. Most intelligent PDUs use SNMP protocol to communicate so integration into a DCIM or NMS is easily achieved. 5. Will it last the lifetime of the data centre? This is an almost impossible question to answer because it’s difficult to predict the development of future data centre technologies. It would be sensible to allow for future capacity by providing additional sockets within the PDU, and also to ensure that you have spare capacity from the PDU incoming feed. If you work in a fast-changing environment, then it’s worth considering modular PDUs that allow you to add additional sockets if required. They’re slightly more expensive, but they offer greater flexibility enabling you to change the socket configuration without taking the IT system down. Profiling demands The constraints of the electrical grid and the need to lower our power consumption are two issues that aren’t going away, and we must all respond accordingly. PDUs allow you to profile your IT power demands, and if you can’t measure it you can’t improve it. n www.networkseuropemagazine.com