62
Traditionally, selective
coordination can be
accomplished by overlaying the
Time-Current Characteristic
(TCC) curves for each
overcurrent protective device
and adjusting their trip settings
to ensure the TCC curves did not
overlap. Selective coordination in
the instantaneous region (below
0.1 seconds) poses significant
challenges when modeling
similarly rated circuit breakers,
while simultaneously trying to
minimize arc flash incident
energy. Previous editions of the
NEC did not define specific time
or current parameters where
selective coordination must exist;
this resulted in many systems that
were coordinated only in the
short and long time regions
(beyond 0.1 seconds) or under
“bolted-fault” conditions in an
effort to avoid installing larger,
more expensive equipment.
For example, the typical 20A
circuit breaker is not selectively
coordinated with the typical
225A main panel board circuit
breaker. This is also true of many
molded case circuit breakers with
instantaneous trips. It will be
necessary to specify and select
different equipment and devices
to comply with this requirement
for selective coordination.
The
revised definition in Article
100 now states that selective
coordination
must include: “the
full range of available
overcurrents
, from overload to
the maximum available short
circuit current, and for the full
range of overcurrent protective
device opening times”.
Conventional TCC analysis may
no longer be sufficient.
Manufacturer selectivity tables,
which specify the relative
operating times of various circuit
breaker combinations, and other
techniques such as instantaneous
Zone Selective Interlocking (ZSI),
may be necessary to achieve
selective coordination in the
instantaneous region. According
to the NEC, ‘selective
coordination’ should not be
confused or interchanged with
the term ‘coordination’
referenced in other code articles,
such as 517.30(G). Coordination
refers only to “the period of time
that a fault’s duration extends
beyond 0.1 second”.
arTiCle 646: modular
daTa CenTers
Modular Data Centers (MDCs), or
offsite constructed data centers,
represent a growing trend in
data center architecture. MDCs
are prefabricated units rated 600
volts or less, which are
preassembled at a factory and
shipped to the construction site.
An MDC may consist of one
module that contains ITE and all
supporting power and cooling
equipment, or it may consist of
multiple interconnected modules
with the ITE separated from the
supporting power and cooling
equipment.
MDCs offer several advantages
over traditional “stick-built” data
centers: they are constructed off-
site in controlled environments
and can be shipped nearly
anywhere in the world, enabling
accelerated project deliveries
and reducing project risk due to
weather or other unforeseen site
conditions. MDCs are inherently
scalable and provide greater
flexibility, with nearly unlimited
load-on-demand opportunities.
7X24 MAGAZINE FALL 2014
Typical TCC Curve used for Selective Coordination
