58
protective relays faster. Optical
relaying may be alternative
method for consideration, where
deemed an “approved
equivalent means” by the local
Authority Having Jurisdiction
(AHJ). These systems detect an
arcing fault by looking for
specific light and current
characteristics and trip only when
both are present. The changes to
240.87 have the potential to limit
the number of equipment
manufacturers who offer these
systems and may increase
equipment costs. Engineers must
weigh the various pros and cons
for each method and determine
the most appropriate strategy on
a case-by-case basis.
The two main factors that affect
arc flash incident energy are
current and time. The longer a
fault exists, the higher the
resultant arc flash severity. Unlike
short circuit current where the
device closest to the source has
the highest value, a higher arc
flash hazard may exist at
locations further from the source
due to the time it takes the lower
fault current to activate the
overcurrent device.
Unfortunately, maximum
selectivity and maximum service
continuity can be in direct
conflict with one another. Ideally,
any overcurrent protective device
that detects a fault should open
instantaneously to reduce arc
energy. However, at the same
time, selectivity is often necessary
to ensure that upstream devices
are restrained long enough for
the device nearest to the fault to
open first to minimize the impact
of the fault on the remainder of
the system.
arTiCle 250:
grounding and
Bonding
Many data centers are supplied
by outdoor electrical substations
that are often enclosed by metal
fences to restrict access to
qualified personnel. Based on
the new provisions in 250.194,
metal fences and other metallic
structures
must be grounded and
bonded
to limit the rise of
hazardous step, touch and
transfer voltages.
IEEE 80 Guide
for Safety in AC Substation
Grounding
is the industry
standard for fence grounding.
arTiCle 310:
ConduCTors for
general wiring
Conductors and cables installed
on rooftops exposed to direct
sunlight are subjected to higher
ambient temperatures than the
surrounding environment. These
conditions require the ampacity
of the conductors to be
temperature-adjusted, which
typically results in larger size
conductors. An exception added
to 310.15(3)(c) allows Type
XHHW-2 conductors to be used
without temperature-adjustment
.
Type XHHW-2 conductors are
comprised of a thermoset
insulation suitable for high
ambient temperatures, which
may be simpler and more cost
effective than increasing
conductor and raceway sizes.
7X24 MAGAZINE FALL 2014
Basic Shock Hazards in
Substations
(Copyright © IEEE 80-
2000, Figure 12)
Example of a Metallic
Fence Around a Substation
