TEST & MEASUREMENT
PUE category 1 is reported where the IT energy is measured
at the output of the UPS. PUE category 2 is reported where
the IT energy is measured at the output of the power
distribution board. PUE category 3 is reported where the IT
energy is measured at the IT cabinet or rack. Latterly the
definition of PUE has been included in ISO/IEC 30134-
2:2016.
Data centre energy reduction initiatives such as the Green
Grid, EU Code of Conduct for Data Centres and CEEDA
mandate PUE to be reported at category 3 if feasible. It can
be seen that this requires the IT and Facilities department
to share information. Momentary PUE using kW power is
category 0, and while not acceptable for reporting this can
provide an early warning of energy inefficiency.
Managing hidden vulnerabilities
It's common practice for IT departments to provision IT
equipment with duplicate power supply units. Standard
design for a concurrently maintainable data centre is to
connect these PSUs to dual redundant power supplies, fed
from separate supplies as the example in the diagram below
shows.
Causes of Downtime
• A/B circuiting error
• UPS step load failure
In standard out of the box configurations with live power
to both PSUs, the IT equipment normally draws power
equally from the two supplies. If one of these supplies goes
down all of the power draw will transfer to the remaining
active supply. This creates a major risk because it’s
possible for power supply paths to be loaded beyond 50%
capacity when both are live, but the remaining supply can
become overloaded when one of the two fails or is isolated
for (concurrent) maintenance. Any part of the power
infrastructure can be affected by the overload, and the
resultant breaker or fuse failure can cause a complete power
interruption to the critical IT load.
Another hidden vulnerability results from variable power
draw from IT equipment depending on the IT workload,
or even the intake temperature to the equipment. The use
of software defined networking, together with increased
utilisation of IT equipment and variable cold aisle
temperatures tends to cause the IT power draw to vary, and
can exceed the capacity of part of the power infrastructure.
The mitigation of both of these hidden vulnerability risks
can be provided by the power monitoring solution, with
programmed thresholds well below the power infrastructure
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