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Safety CountS
What is GFCI
Protection?
By Tom Ruffner
GFCI stands for “ground fault circuit interrupter.” This is an elec-
was grounded, a GFCI would not help. The person’s hand or finger would
tronic device that measures the ac current flowing through the “hot”
essentially act as a bridge between the two conductors and current would
wire and “neutral” wire of the tool or appliance being powered. GFCI’s
flow from the “hot” wire or wires through the body. Since the currents
are usually built directly into receptacles and circuit breakers. They are
in both wires are equal, the GFCI would not disconnect the power and
most commonly used on 120V ac and 240V AC single-phase circuits. They
the person could receive a potentially fatal shock.
are also offered by some manufactures as “in-line” devices that can be
GFCI’s are sophisticated devices that must be wired properly in order
used with just about any power tool or appliance. They are designed
to function as intended. Like any mechanical or electrical device, GFCI’s
with very fast response times that help protect people from fatal electric
can also go bad. Always consult a qualified electrician if there is any
shocks in the event of a ground fault.
doubt about the correct way to install these devices. GFCI devices typi-
What is the difference between GFCI’s, circuit breakers,
and fuses?
GFCI’s are safety devices that protect people from potentially fatal
electric shocks. Circuit breakers and fuses are designed to protect the
wiring to the equipment being powered and to protect the equipment
from being damaged in case of a malfunction. They are designed to
trip or open when excessive currents are detected. Conventional circuit
breakers and fuses do not prevent people from being electrocuted.
How does a GFCI work?
As long as the currents in the power and return conductors are
equal, the GFCI takes no action. If the current differs by more than a
few milliamperes (typically less than 6 milliamperes), the GFCI recognizes
that current is escaping to some alternate path and it quickly disconnects the incoming power to prevent electrocution. The leakage could
be from a short circuit to the chassis of the equipment, to the ground
lead or through a person. Any of these situations are hazardous and
could be fatal if a GFCI in not being used.
Are there any situations when a GFCI would not
protect the user?
It is important to understand that a GFCI does not protect against
every type of electric shock. For example, if a person simultaneously
touched the “hot” and the “neutral” wires of a 120V AC circuit, or
touched two “hot” wires of a 240V AC circuit, and no part of that person
cally contain a “test” and a “reset” button to make sure that the device
is functioning properly. It is recommended that all GFCI’s be tested a least
once a month to ensure they are operating correctly and providing the
required protection.
Can GFCI protection be installed on a three-phase
generator?
The answer to this question is, “not exactly.” A device called an
“equipment leakage current interrupter” (ELCI) can be used on three
phase equipment with line voltages of 230V AC or 460V AC. In order for
a device to be classified as a GFCI by Underwriters Laboratories (UL), it
has to have a trip level setting less than 6 milliamperes of leakage current. For this reason, GFCI’s are considered personal protection devices
while ELCI’s are considered to be equipment protection devices. ELCI’s
have user-adjustable trip level settings, up to 30 milliamps. The reason
for the adjustable trip level is to avoid nuisance tripping. When the line
voltage or line current increases, so does the likelihood of nuisance tripping. GFCI devices are limited to 30 amps while ELCI devices go as high as
100 amps. ELCI’s are “inline” devices that operate on three-phase 230V
AC or 460V ac power sources.
How does an ELCI work?
An ELCI operates in the same manner as the single phase GFCI devices
mentioned earlier. Even though an ELCI allows more leakage current
through in the event of a ground fault, an ELCI still provides an additional
level of user safety because it disconnects the incoming power much
REPRINTED FROM CONCRETE OPENINGS | VOL.16 | NUM.4 | DECEMBER 2007
quicker than a conventional three-pole circuit
motor, but this is what is required to handle
breaker. The disconnecting time of the line
the high surge currents during initial start-up
voltage is 25 milliseconds or less which means
to ensure trouble-free equipment operation.
The most commonly used plugs and recep-
that the person operating the equipment may
If a single-phase motor is to be powered, fol-
tacles on portable equipment are “twistlock”
only receive a mild shock instead of a fatal one.
low the same procedure but only multiply the
devices. These devices plug together and have
Because ELCI’s are “in- line” devices, they can
horsepower by 2½ times for selecting the gen-
a locking feature to keep them from pulling
be added to practically any type of three-phase
erator size. Single-phase motors are even more
apart. They are available in current ratings
equipment.
difficult to start than three-phase motors and,
from 20 amps to 50 amps and cover voltages
therefore, require a larger generator.
up to 600V AC three-phase depending on the
An ELCI should be considered as an
additional safety device and in no way should
be a substitute for the equipment grounding
conductor. A secure ground should be
attached to the metal frame or housing of the
equipment at all times. Always select an ELCI
with the proper line voltage and load current
What will happen if a generator is
too small?
Some of the most common components
that fail from selecting a generator that is
too small are circuit breakers, fuses, motor
rating for the application.
starters, motor windings, motor start wind-
What size generator is right to power
various pieces of equipment?
generator windings, and generator voltage
This is probably one of the most asked and
the current demanded, the magnetic field in
least understood questions for anyone using
the generator winding starts to collapse and
power equipment. In order to avoid costly
this causes a rapid drop in line voltage and
downtime and damage to the generator and
line frequency. When this happens, generator
the equipment being powered, it is import to
and motor winding temperatures increase and
properly size the generator for the job. On
damage to the winding insulation can occur. In
the nameplate of the generator there will be
this type of situation, the supporting electrical
two ratings kVA (Kilovolt Amperes) and kW
components are also being overworked which
(Kilowatts). The kVA number will always be
leads to premature failure.
higher than the kW number due to a variable called “power factor”. The ideal power
factor number is 1. This means that all of the
power drawn is used to perform work and
kVA equals kW. In real world conditions, the
power factor number is more often about .80
due to the energy losses within the equipment
being powered.
Manufacturers of generators will usually
designate a continuous output rating for a
generator at a given temperature rise of the
winding, usually 125 degrees centigrade. Use
the kW output rating at this temperature when
choosing the generator size. When operating
three-phase induction motors, a good rule of
thumb is to select the size of the generator by
doubling the horsepower of the motor to find
ings and capacitors (single phase motors only),
regulators. When the generator can’t supply
What type of plugs & receptacles
should be used?
model chosen. Each device is numbered with a
National Electrical Manufacturers Association
(NEMA) code for standardization purposes.
The NEMA number on the plug and receptacle have to be identical, otherwise, they will
not connect together. As long as the number
matches, different brands with the same ratings are interchangeable. Devices called “pin
and sleeve” are available for currents above 50
amps three-phase. These devices are physically
bigger in size and usually carry a much higher
price tag than “twistlock” type devices. They
are also available up to 600V AC and currents
up to 100 amps.
When choosing any plug and receptacle
configuration, always choose devices that are
rated for the maximum current and supply
voltage at which the equipment will be
operated. It is important that the devices be
installed properly to avoid loose connections or
“hot spots. ” Always perform a visual inspection
of plugs and receptacles before
each use and repair or replace
any device that shows signs of
damage or excessive wear. To
prolong lifetime and reduce
the likelihood of failures,
always keep plugs and
receptacles away from
excessive moisture or
water.
Tom Ruffner is the Electrical Engineer at Diamond
Products, Inc. in Elyria,
Ohio. He can be reached
at 800-321-5335.
how many kW is needed.
For example, if the equipment has a 20
horesepower three-phase motor, you will need
a total of 40 kilowatts of useable power from
the generator to insure enough surge capacity
just for starting the motor. At 40kW, this same
generator would have a total of 50kVA assuming a .80 power factor. This may sound like
a large safety cushion for a 20 horesepower
REPRINTED FROM CONCRETE OPENINGS | VOL.16 | NUM.4 | DECEMBER 2007