56
7X24 MAGAZINE FALL 2014
Code-wide Changes
The NEC definition of switchgear,
previously referred to as metal-
enclosed power switchgear, was
revised in Article 100 to be more
comprehensive. It is now a
generic term that encompasses
many different types of metal-
enclosed switchgear, including
1000V and above. Switchgear
was added to the title of Article
408 which governs installation
and construction of distribution
equipment and many other
sections where the term
switchboard was previously used.
The terms switchgear and
switchboard are often
interchanged without
recognizing the fundamental
differences. For example, low-
voltage metal-enclosed
switchgear is constructed to
standard UL 1558 and is
compartmentalized with physical
barriers between circuit breakers
and bussing. Enclosure depth is
typically greater than
switchboard construction and
switchgear requires front and
rear access. Switchgear can be
designed with 30-cycle
interrupting ratings and can also
be designed to be arc-resistant.
Switchboards, on the other hand,
are constructed to standard UL
891 are not typically
compartmentalized, 30 cycle
rated and are not available in an
arc resistant construction.
Switchboards can be designed
front accessible only.
arTiCle 110:
requiremenTs for
eleCTriCal
insTallaTions
Service equipment
is required to
be field-marked with the
maximum available fault current
and the date that the calculation
was performed. The available
fault current is often calculated
using the infinite bus method,
which assumes there is zero
impedance in the upstream utility
system. This calculation method
results in the highest value of
fault current, and is a typical
practice for verifying equipment
withstand and interrupting
ratings. However, this theoretical
value can result in under
calculating the available arc flash
energy due to the apparent
quick operation of overcurrent
devices. These devices would, in
reality, operate slower with the
actual available short circuit
current. The new informational
note in Article 110.24 advises
that these markings should not
be used for determining arc flash
incident energy levels, and
guides users to NFPA 70E,
Standard for Electrical Safety in
the Workplace
for determining
potential exposures, planning
safe work practices and selection
of PPE.
Personnel doors
from working
spaces containing equipment
rated 800A or above (reduced
from 1200A) must have outward
swinging door(s) equipped with
listed panic hardware in
accordance with 110.26(C)(3).
Many data centers have
outdoor
equipment yards
that contain
transformers, generators,
switchgear, and mechanical
equipment. This outdoor
equipment is now subject to the
same dedicated equipment
space requirements as indoor
equipment based on
110.26(E)(2)(b). Outdoor
equipment yards and similar
areas must be carefully
coordinated between all trades
to ensure dedicated equipment
spaces are free of: mechanical,
plumbing, fuel, piping and other
foreign systems.
arTiCle 240:
overCurrenT
ProTeCTion
Article 240.87 Arc Energy
Reduction, now requires all
circuit breakers rated
1200A and
above
, be provided with an
approved
method for reducing
clearing time during a fault
. The
following methods are indicated
for reducing clearing time: Zone-
selective interlocking, differential
relaying, energy-reducing
maintenance switching with local
status indicator, energy-reducing
active arc flash mitigation system
(e.g. high-speed shorting
switches, optical sensors, etc),
and approved equivalent means.
High-speed shorting switches are
most commonly applied to
medium voltage systems. When
an arcing fault is detected, these
devices actively close a switch to
create an intentional three-phase
fault, which increases the fault
current and trips the associated
