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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
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