Electric arcs generate significant
amounts of heat. At phase-to-ground
voltages above 150 volts (V), such as
277V, an arcing fault is readily sustained.
A ground fault is typically not a solid or
“bolted fault” condition, so dynamic arcing impedance is introduced in the circuit,
reducing the fault current seen by a standard overcurrent device and increasing
the time the fault can exist. This enables
arcing faults to manifest into destructive
events. During an arc event, ionized gas
is dispersed, creating a conductive gas or
plasma in the atmosphere surrounding
the bus bars within the equipment. This
condition often escalates from a phase-to-ground fault event to a phase-to-phase
This is why the NEC requires GFPE.
Protection is generally required for solidly grounded wye services and feeders
of more than 150V to ground but not
exceeding 1,000V phase-to-phase for
each disconnect rated at or above 1,000
amperes (A). GFPE is required for nominal 480Y/277V, three-phase, 4-wire,
wye-connected systems. The maximum
settings are 1,200A and not longer than 1
second for fault currents 3,000A or more.
GFPE is not permitted for fire pumps or
in systems where a nonorderly shutdown
or interruption would introduce additional hazards.
Types of GFP equipment
Two types of GFP equipment are
ground-strap type and zero-sequence.
The main bonding jumper (MBJ) is
what is meant by the terms “ground
strap” or “neutral ground strap.” The
ground-strap type of GFP equipment
monitors the MBJ for high inrush current triggered by a ground-fault event.
Both types provide protection from
load-side ground faults. GFPE installed
in a service does not provide protection
on the line side of the GFP equipment.
A line-side ground fault is not detected
by the GFP sensors, and equipment can
be severely damaged or destroyed by the
A zero-sequence GFP equipment
is often referred to as residual because
this type of equipment sums up all of
the current in the phases and grounded
conductor, and any excess current is
residual or left over. The GFP equipment
operates when the amount of residual
current exceeds predetermined set values. Like the ground-strap type, this
type of system is equipped with a control relay, one or more sensing current
transformers, and a shunt-trip breaker
or disconnecting means.
GFPE system coordination
Where overcurrent and GFP devices are
selectively coordinated, they provide
the benefits of restricting outages to
the circuit or equipment closest to the
ground-fault or short-circuit event by
operating the local overcurrent or GFP
device, rather than causing the entire
system to suffer a failure. The point of
a ground fault in any system is never
known, so system coordination is all
about anticipating a ground-fault event
and strategically locating GFP equipment at each level desired in the system.
The NEC defines selective coordi-
nation, and applying GFP equipment in
multiple levels of feeders on the load side
of the service GFP device can effectively
localize ground faults and simultaneously
provide power continuity. A coordinated
system of ground-fault protection in cas-
cading feeder levels affords the ability to
isolate an offending fault to one location
or feeder level. The coordination often
incorporates zone-selective interlocks
or differential relay settings.
Zone-selective interlocking involves
installing signaling circuit wiring between
each level of GFP equipment. Where
necessary or desired, the tripping function of the service GFP device can be
delayed while the feeder GFP closest
to the offending fault can be activated.
GFP equipment coordination involves
analyzing the system characteristics and
applying appropriately selected GFP
equipment in a way that effectively
provides a deliberate separation of the
tripping bands from the lowest-rated to
the highest-rated GFP system device.
Equipment ground fault protection systems must be performance-tested by
qualified people when installed to ensure
proper operation. The test must involve
a primary current injection process. The
requirements for qualified people and
primary current injection are new in the
2017 NEC. Testing verifies the system will
interrupt a ground-fault event at selected
current pickup and time settings.
The performance testing must be
in accordance with the manufacturer’s
instructions. Section 230.75 requires a
neutral disconnecting means to be provided within the service equipment for
testing purposes. Although the term
“neutral disconnect link” is not used
in the NEC, it is used by manufacturers of equipment that is suitable for use
as service equipment. Once the link is
removed, a test can verify that the neutral
is isolated from grounding connections
on the load side of the service disconnect.
Complete test records must be available
to the authority having jurisdiction.
Protecting equipment from arcing
GROUND-FAULT PROTECTION OF EQUIPMENT (GFPE) provides protection from devastating arcing events and destructive arcing burn-downs. National
Electrical Code (NEC) sections 215.10, 230.95, 240.13 and 517.17 provide GFPE
requirements, and Article 100 defines it.
JO HNS TON is NECA’s executive director of standards and safety. He is chair of the NEC
Correlating Committee; chair of the NFPA Electrical Section; and a member of the IBEW,
NFPA Education Section and the UL Electrical Council. Reach him at email@example.com. IST