UL Class A ground-fault circuit interrupters
(GFCI) began to be required in kitchens, bathrooms and outdoor outlets in the
early ’70s, and have saved many lives over the years: According to the Consumer
Product Safety Commission, household electrocutions decreased from 270 in 1990
to 180 in 2001. But what about in the workplace? Class A GFCIs cannot be used
where the electrical equipment runs on 480 or 600 V. Yet the danger of electrocution
is real. From 2003 to 2009 there were 801 fatal workplace accidents caused by
worker contact with electrical current (not including the construction
Photo 1. Industrial Shock Block GFCI for circuits 208 V to 600 V (208, 240, 480, and 600 V.)
Traditionally, measures to prevent shock have
included insulated tools, insulated gloves, lockout tagout, etc., but all of
these options are reactive, and none proactively eliminates the hazard. There
are ground fault relays (GFRs), but those react too slowly to provide people
protection, and Class A GFCIs are not practical for industrial settings.
is finally changing, based on the recently introduced special purpose GFCIs in
UL 943C, where new classes are added for 480 and 600 V applications. Although
the new classes of devices trip at higher current levels (20 mA instead of 6
mA), UL calls these devices GFCIs, which UL defines as "a device intended for
the protection of personnel.”
The increase in
personnel protection trip level of the new GFCI classes is allowed by UL
assuming the availability of a reliable ground in parallel with the body.
During a fault, the grounding conductor will shunt the fault current around the
body and cause the device to trip. This provides the let-go protection, while
the 20 mA threshold provides protection against fibrillation. (If there is no
grounding conductor, such as in two-wire household products, then the GFCI must
provide both let-go and fibrillation protection, and a Class A device is
the human body decreases rapidly above 150 volts to ground, and GFCI protection
would require impossibly fast trip times. That’s why UL 943C defines different
classes of GFCI based on the type of grounding (figure 1). If reliable
grounding is provided in a circuit of less than 150 volts to ground, then the
touch voltage will be less than 150 volts to ground and a Class C GFCI may be
Figure 1. How to determine the class of device for a particular application. This figure appears in UL 943C as Figure 1.1 and is used with permission. Copyright © Underwriters Laboratories Inc. UL 943C, Edition 2, 2012.
circuit voltage is between 150 and 300 volts to ground, then the solid reliable
grounding conductor must be the same size as the hot (ungrounded) conductor.
This forms a voltage divider with two equal impedances and acts to reduce the
touch voltage to half (150 volts or less). Then a Class C GFCI may be used.
When the voltage to ground exceeds 300 volts,
UL provides two options. For Class D GFCI protection the circuit requires a low
impedance ground that reduces touch voltage to 150 V or less during a
fault.Class E GFCI protection uses a standard size grounding conductor
but requires a high-speed trip time to limit the body’s exposure and resulting
Class E GFCIs have the same time vs. current
characteristics of Classes A, C, and D up to a fault current of 300 mA (150 V
to ground touch voltage with a 500-ohm body resistance). When the fault current
exceeds 300 mA, the trip time formula changes to a high speed trip requirement.
For the purposes of this article, it will be
convenient to refer to Class A GFCIs as residential GFCIs and refer to the
other classes (Class C, Class D and Class E) as "industrial GFCIs,” although UL
makes no such distinction.
These other classes of UL-listed GFCIs have recently
become available, and they can make worker shock protection proactive,
significantly reducing the risk of injury and death due to shock in the
workplace. Some industry players believe that, as awareness of UL 943C
increases, technology adoption will also increase and, perhaps, the electrical
codes will be updated to require GFCIs in more industrial applications. And
while the NEC® and CEC® do not yet require broad use of industrial
GFCIs, some sections recommend their use for specific applications.
Electrical inspectors and installers may not know
that these devices exist, as until now there was no UL-listed device that could
protect industrial workers from electrocutions. Electrical inspectors will want
to become familiar with the relevant standards and installers will want to
understand how to apply them. What’s more, as industrial GFCIs are gradually
adopted into use, inspectors will start to encounter them; inspectors should
understand how they operate and how they are properly installed.
A little history
In 2000 UL addressed the need for GFCIs
designed for higher power with a draft Outline of Investigation called 943C,
and revised it in 2009, but it was not until 2012 that commercial products
became available. Many users are still unaware that such solutions are
available and not very many workplaces have installed them, still exposing
workers to risk of electrocution.
Where industrial GFCIs are used
Any place where water, people, and significant
voltage equipment meet presents the potential for a shock hazard. Industrial
GFCIs are being installed on equipment subject to washdown, such as food and
beverage facilities; in processing plants handling wet materials using large
pumps or mixers; and in water and wastewater facilities, municipal fountains
and even amusement parks.
Industrial GFCIs are also used for outdoor
applications like welding, mining, or anywhere workers use portable equipment
or equipment with long power cords or cables. It is expected that portable
applications will be one of the first places for adoption of industrial GFCIs.
Classes of industrial GFCIs
residential use are listed under UL 943 as Class A, but they are not suitable
for industrial use for two reasons: they are rated only for 120 and 240 V
installations, and their 6 mA trip level is typically too low for industrial
applications. Many industrial electrical systems have normal leakage current
greater than 6 mA, in which case a residential GFCI would never leave trip mode
and the circuit would never power up.
UL 943C has three classes for industrial GFCIs: Class
C, for use in circuits with maximum line-to-ground voltage of 300 V where
reliable equipment grounding or double insulation is provided; Class D, for use
in circuits with line-to-ground voltage higher than 300 volts with oversized
grounding to prevent the voltage across the body during a fault from exceeding
150 volts; and Class E, which covers systems similar to Class D, but with
high-speed tripping required, therefore the oversized ground of Class D is not
required. Figure 1 shows how the class of device for a particular application
There was also a
Class B, which applied only to swimming pool underwater lighting fixtures that
were installed prior to local adoption of the 1965 edition of the NEC.
These are considered obsolete and have been removed from UL 943, the UL
Standard for Ground-Fault Circuit-Interrupters. They will seldom be encountered
in practice and will not be further discussed here.
differ from Class A units in another important way: UL requires that they
monitor the equipment ground wire, to detect whether the monitored equipment is
properly grounded, and to cause a trip if the grounding connection is lost.
According to UL, Class C, Class D, and Class E
GFCIs follow the same inverse-time protection curve as Class A GFCIs, which is
defined by this equation.
This curve is
shown in figure 2, where the current (I) is in mA and the time (T) is in
seconds. The unit should trip in about 1 s at a current of 20 mA (quickly
enough to prevent injury at that current level), and within 20 ms at currents
of 300 mA or higher. The higher the current, the faster the GFCI must trip. The
advantage of the inverse trip curve is that it minimizes nuisance tripping for
transient low-current faults while providing full people protection from
Figure 2. UL 943C requires industrial GFCIs to follow this trip curve.
In practice, a
Class C, Class D and Class E GFCI will trip at current between 15 and 20 mA (in
order to allow for a range of operating conditions). UL 943C Figure 1.1 states
that Class C GFCIs must "provide protection against ventricular fibrillation
(let-go protection optional).” Clause 15.2 of UL 943C allows the lower limit of
the trip threshold of a Class C device to be reduced to any value between 6 and
15 mA to provide some limited let-go protection as an option.
UL 943 says that Class A GFCIs should have the
inverse-time trip curve shown in figure 2 over a leakage current range of 6 to
264 mA, but some manufacturers of lower-priced Class A devices do not implement
the curve because it makes the design more complicated, and instead use an
instantaneous response that is lower than the quickest response required by the
code just to pass testing. The UL curve is the absolute highest time response
accepted but it is not restrictive. A device will fail UL testing if it responds
to a fault slower than the curve suggests but will pass as long as the response
time is less than the curve time. Too-quick response to transient ground-fault
currents of low magnitudes will cause nuisance tripping; this is the main
reason for not using residential type Class A GFCIs in industrial applications
even where the line-to-line voltage is 240 V or lower and the system leakage is
less than 6 mA.
How to install industrial GFCIs
An industrial GFCI can be built into a panel or
a cabinet, as shown in figure 3; there are units with their own NEMA 4X
enclosures that can be added to existing equipment. A separate load ground
monitoring wire must be connected to the chassis of the equipment to be
protected through a termination device, as shown in figure 4.
Figure 3. An industrial GFCI can be built into a panel or a cabinet, as shown, or added to existing equipment (using units with their own enclosures).
Figure 4. Circuit diagram of an industrial ground-fault circuit interrupter
As shown in figure 4, some GFCIs have internal fuses
to increase their interrupting capacity, protect the internal contactor and in
some cases eliminate the need for an upstream current-limiting device. In
addition, some models can detect undervoltage, brownout, and contactor chatter.
EGFPDs and how they differ from GFCIs
circumstances the 20 mA trip level of an industrial GFCI can make its use
impractical. In those cases, an equipment ground-fault protection device
(EGFPD) can be used. EGFPDs offer protection similar to GFCIs but are allowed
by UL to have an adjustable trip level (GFCIs have a fixed trip level) and
monitoring the equipment ground wire is not required (a mandate for industrial
GFCIs). EGFPDs can be adjusted to trip in the range of 6 to 50 mA. EGFPDs are
rated by UL for equipment protection only.
Don’t confuse a GFCI or EGFPD with a ground fault
relay (GFR). Unlike a GFCI or EGFPD, both of which contain their own circuit
interrupters, a GFR works by detecting a ground fault and sending a signal to
the trip input of an upstream circuit breaker or contactor. Because GFRs have
adjustable trip times, and power breakers often take up to 50 ms to trip, they
do not protect people against cardiac arrest.
Industrial GFCIs provide workers
with vital protection against shock injury and electrocution at a cost that is
trivial compared to the multiple costs involved with a serious injury or a
fatality. Inspectors can play a critical role in informing users of this
technology to help them increase safety.
P.Eng. is the technical product specialist for the protective relay products
line for Littelfuse. He received his B.Sc. and M.Sc. in Electrical Engineering
from Ain-Shams University, Cairo, Egypt, in 2001 and 2005. He serves on several
professional societies, committees and working groups including UL STP 943, CSA
C22.2 No.144 standard committee, IEEE IAS and the IEEE Working Group on
Industrial and Commercial Power Systems Communication Based Protection and
SCADA. He is a registered professional engineer in Saskatchewan and has a
patent pending. He is also an IAEI associate member, Canadian Section. Nehad
can be reached by email at firstname.lastname@example.org.