Power Play
Is your power supply redundant?
Proper loading of cabinet power-distribution units can affect server operation.
by Calvin Nicholson
Over the past decade,
enterprise reliance on networks and data
centers to provide information, run
businesses, access and store data, and
facilitate global commerce has grown
considerably. As businesses and
organizations plan for the next decade and
build out their data centers, power
consumption and distribution to the devices
in the cabinet should be taken into
consideration early in the planning stages.
Increased densities, proper installation,
power and cord choices, and redundancy all
affect the efficiency and reliability of the
devices in the cabinet.
To provide redundancy,
most IT equipment is designed with two or
more power supplies. Ideally, each power
supply draws power from a different cabinet
power-distribution unit (PDU). Each cabinet
PDU is fed by a separate power source. Under
normal operating conditions, each power
supply delivers roughly 50 percent of the
load to the server, so if one fails or power
is lost, the other will deliver 100 percent
of the load.
If proper installation and cabling is not implemented, data center managers may be operating under a false sense of security.
One critical aspect of
redundancy that should not be overlooked is
how each cabinet PDU is loaded. Each circuit
should be designed so that it is capable of
handling the entire load of the rack;
therefore, in a tier 3 or tier 4
system-plus-system setup, each PDU under
normal operating conditions should not be
loaded to more than 40 percent of the
circuit capacity. This is due to the
National Electric Code (NEC), which requires
that a circuit be loaded to no more than 80
percent of capacity.
Since two cabinet PDUs
typically power each rack, each PDU should
be loaded no more than 40 percent, because
if one circuit is lost, the other PDU will
reach 80 percent of the circuit capacity.
A typical example
illustrates how two PDUs, properly loaded
under normal operating conditions, might
still fail when one circuit is lost. Each
cabinet PDU is rated and capable of
supplying 20 amps; however, the circuit can
be loaded no more than 80 percent of its
capacity, or 16 amps (derated).
Each circuit feeds one
input of a rack of dual power supply
servers. The total load is 12 amps, which is
less than the derated value. When circuit A
is lost, however, the full load is drawn
from circuit B, resulting in a total current
draw of 24 amps. This will cause circuit B
to blow a fuse or trip a circuit breaker,
leading to the loss of power for the whole
rack of servers.
In another example, each
20-amp circuit is loaded for a total of 8
amps. When circuit A is lost, the total load
on circuit B is 16 amps, which will not
cause a secondary failure. To provide power
redundancy for critical systems, the power
capacity at the rack level will need to be
at least two times the amount of power
consumed by these devices.
If proper installation
and cabling is not implemented, tier 3 and
tier 4 data center managers who believe they
have implemented redundant systems may be
operating under a false sense of security.
Not only should load requirements be taken
into account, but installation of new
devices should be monitored by either
facilities personnel or the IT group.
As in many installations,
the power requirements to operate a
redundant system, when one of the in-feeds
is down, is not taken into account. Also, in
many cases, devices that are dual power
corded are plugged into a single in-feed,
breaking one of the critical rules of
providing redundant power. Often, proper
supervision, inspection and review are not
implemented until there is a problem and the
system that was believed to be redundant
goes down.
Calvin Nicholson is director of product marketing for Server Technology, Reno, Nev., which designs and manufactures intelligent power-distribution products for enterprise data centers.
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