White Paper

Switching
Switching within rack PDUs is vital to being able to remotely
turn on or off the connected equipment, and is enabled by
the use of relays at each outlet. The relays used within
rack PDUs can be one of three types: normally-open;
normally-closed or bistable relays.
• Normally-open relays require power to be supplied to
them in order for the outlets to be able to deliver power to
the connected loads. In the event there is an issue with the
power supply feeding the relays, the relays remain open
and there is no power provided to the connected outlets.
• Normally-closed relays only require power to open the
outlets. Under normal operation, they remain closed.
This means if there are any issues with the power supply
feeding the relays, the outlets continue to provide basic
power distribution to the connected loads.
• Bistable relays (or latching relays) act as normally-closed
relays during normal operation, in that they also resort to
providing basic power distribution in the event of a power
supply failure. They do provide an additional advantage in
that they allow a choice of state when power is restored
after an outage. Outlets can either be turned on or off,
or returned to the same state they were in prior to the
outage. Bistable relays only require power if they change a
state. They will keep outlets up and running with no power.
In addition, their power consumption during normal
operation is low, which helps minimize the overall energy
footprint of Switched rack PDUs. (Figure 4)
External Connectivity
In the event that the primary network to the rack PDU goes
down, some rack PDUs provide redundant communications
through integration with out of band management devices,
such as serial consoles or KVM switches. However, if the
external communication to the rack PDU is just not available,
the design of the rack PDU should ensure that basic power
distribution, as well as the operation of local management
modes such as onboard display, is not affected. It is important
that rack PDUs have an automated management path
that will maintain basic power distribution. An automated
management path also will ensure that when input power to
one of the phases of a three-phase intelligent rack PDU is lost,
the outlets connected to the unaffected phases continue to
be powered.
Maintainability
As computing demands and complexity in the data center
continue to rise, unplanned data center outages remain
a signicant threat to organizations in terms of business
disruption, lost revenue and damaged reputation. A 2013
survey of U.S.-based data center professionals by the
Ponemon Institute and sponsored by Emerson Network Power
showed that an overwhelming majority of respondents had
experienced an unplanned data center outage in the past 24
months (91 percent).
Regarding the frequency of outages, respondents experienced
an average of two complete data center outages during the
past two years. Partial outages, or those limited to certain
racks, occurred six times in the same timeframe. According to
survey responses, complete outages lasted an average of 107
minutes and partial outages lasted an average of 152 minutes.
(Figure 5) The second part of the study quantied the cost of
an unplanned data center outage at slightly more than $7,900
per minute.
While the survey provides good data for a more broad
discussion on data center downtime and the steps that can be
taken to increase availability, it does underline the importance
of minimizing the Mean Time To Repair (MTTR) if an intelligent
rack PDU goes down.
Figure 4. Bistable (or latching) relays resort to
providing basic power distribution in the event of a
power supply failure.
Normally-open relay:
Normally-closed relay:
Bistable (or Latching)
relay:
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