IMT 140

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IMT 140

• Ground Fault Circuit Interrupters

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An inexpensive electrical device, if installed in
household branch circuits, could prevent over
two-thirds of the approximately 310
electrocutions still occurring each year in and
around the home. In addition, injuries to many
thousands of consumers from electric shock and
burns could also be prevented by installation of
this device. The device is a ground-fault
circuit-interrupter (GFCI). The GFCI is designed
to protect people from severe or fatal electric
shocks. Because a GFCI detects ground faults, it
can also prevent some electrical fires and reduce
the severity of others by interrupting the flow
of electric current.
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How the GFCI Works   In the home wiring system,
the GFCI constantly monitors electricity flowing
in a circuit to sense any loss of current. If the
current flowing through the circuit differs by a
small amount from that returning, the GFCI
quickly switches off power to that circuit. The
GFCI interrupts power faster than a wink of an
eye to prevent a lethal dose of electricity. You
may receive a painful shock, but you should not
be electrocuted or receive a serious shock
injury.
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Here is how the GFCI could work in your house
  Suppose a bare wire inside an appliance
touches the metal case. The case is then charged
with electricity. If you touch the appliance with
one hand while the other hand is touching a
grounded metal object, like a water faucet, you
will receive a shock.. If the appliance is
plugged into an outlet protected by a GFCI, the
power will be shut off before a fatal shock would
occur.
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Availability of GFCI's   Three common types of
ground-fault circuit-interrupters are available
for home use wall-receptacle, circuit breaker
and portable plug-in type. Receptacle Type. This
type of GFCI is used in place of standard duplex
receptacle found throughout the house. It fits
into the standard outlet box and protects you
against ground faults whenever an electrical
product is plugged into the outlet. Most
receptacle-type GFCI's can be installed so that
they also protect other electrical outlets
further down stream in the branch circuit.
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Circuit Breaker Type. In homes equipped with
circuit breakers rather than fuses, a circuit
breaker GFCI may be installed in a panel box to
give protection to selected circuits. The circuit
breaker GFCI serves a dual purpose--not only will
it shut off electricity in the event of a
ground-fault, but it will also trip when a short
circuit or an overload occurs. Protection covers
the wiring and each outlet, lighting fixture,
heater, etc., served by the branch circuit
protected by the GFCI in the panel box.
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Portable Type. Where permanent GFCI's are not
practical, portable GFCI's may be used. One type
contains the GFCI circuitry in a plastic
enclosure with plug blades in the back and
receptacle slots in the front. It can be plugged
into a receptacle then, the electrical product
is plugged into the GFCI. Another type of
portable GFCI is an extension cord combined with
a GFCI. It adds flexibility in using receptacles
that are not protected by GFCI's.
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Where GFCI's Should Be Considered   In homes
built to comply with the National Electrical Code
(the Code), GFCI protection is required for
outdoor receptacles, bathroom receptacles, garage
wall outlets, some kitchen receptacles, crawl
spaces and unfinished basements.
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Owners of homes that do not have GFCI's installed
in all those critical areas specified in the
latest version of the Code should consider having
them installed. For broad protection, GFCI
circuit breakers may be added in many panels of
older homes to replace ordinary circuit breakers.
For homes protected by fuses, you are limited to
receptacle or portable-type GFCI's, and these may
be installed in areas of greatest exposure, such
as the bathroom, kitchen, basement, garage, and
outdoor circuits.
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A GFCI should be used whenever operating
electrically-powered garden equipment (mower,
hedge trimmer, edger, etc.). Consumers can obtain
similar protection by using GFCI's with electric
tools (drills, saws, sanders, etc.) for
do-it-yourself work in and around the house.
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Blank face GFCI
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Pilot Light
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Combination-Switch/Receptacle GFCI
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Portable GFCI
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Portable GFCI with auto reset
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TEST and RESET functions work together so that a
tripped GFCI cannot be reset if the GFCI circuit
no longer provides ground fault protection.
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http//www.mindconnection.com/library/handyman/gfc
i.htm
A great web site with detailed instructions on
how to install a GFCI.
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G.F.C.I. by Sam Goldwasser -- excerpts from
Sci.Electronics.Repair FAQhttp//www.repairfaq.or
g/
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What is a GFCI? A Ground Fault Circuit
Interrupter (GFCI) is a device to protect against
electric shock should someone come in contact
with a live (Hot) wire and a path to ground which
would result in a current through his/her body.
The GFCI operates by sensing the difference
between the currents in the Hot and Neutral
conductors.
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Under normal conditions, these should be equal.
However, if someone touches the Hot and a Ground
such as a plumbing fixture or they are standing
in water, these currents will not be equal as the
path is to Ground - a ground fault - and not to
the Neutral. This might occur if a short circuit
developed inside an ungrounded appliance or if
someone was working on a live circuit and
accidentally touched a live wire.
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The GFCI will trip in a fraction of a second at
currents (a few mA) well below those that are
considered dangerous. Note that a GFCI is NOT a
substitute for a fuse or circuit breaker as these
devices are still required to protect equipment
and property from overloads or short circuits
that can result in fire or other damage.
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GFCIs can be installed in place of ordinary
outlets in which case they protect that outlet as
well as any downstream from it. There are also
GFCIs that install in the main service
panel. Note that it may be safe and legal to
install a GFCI rated at 15 A on a 20 A circuit
since it will have a 20 A feed-through. Of
course, the GFCI outlet itself can then only be
used for appliances rated 15 A or less.
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Many (if not most) GFCIs also test for a grounded
neutral condition where a low resistance path
exists downstream between the N and G conductors.
If such a situation exists, the GFCI will trip
immediately when power is applied even with
nothing connected to the protected outlets.
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• How does a GFCI work
• GFCIs typically test for the following condition
• A Hot to Ground (safety/earth) fault. Current
  flows from the Hot wire to Ground bypassing the
  Neutral. This is the test that is most critical
  for safety.
• A Grounded Neutral fault. Due to mis-wiring or a
  short circuit, the N and G wires are connected by
  a low resistance path downstream of the GFCI. In
  this case, the GFCI will trip as soon as power is
  applied even if nothing is connected to its
  protected (load) circuit.

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To detect a Hot to Ground fault, both current
carrying wires pass through the core of a sense
coil (transformer). When the currents are equal
and opposite, there is no output from its
multi-turn sense voltage winding. When an
imbalance occurs, an output signal is produced.
When this exceeds a threshold, a circuit breaker
inside the GFCI is tripped.
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GFCIs and safety ground Despite the fact that a
Ground Fault Circuit Interrupter (GFCI) may be
installed in a 2 wire circuit, the GFCI does not
create a safety ground. In fact, shorting between
the Hot and Ground holes in the GFCI outlet will
do absolutely nothing if the GFCI is not
connected to a grounded circuit (at least for the
typical GFCI made by Leviton sold at hardware
stores and home centers).
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It will trip only if a fault occurs such that
current flows to a true ground. If the original
circuit did not have a safety ground, the third
hole is not connected. What this means is that an
appliance with a 3 prong plug can develop a short
between Hot and the (supposedly) grounded case
but the GFCI will not trip until someone touches
the case and an earth ground (e.g., water pipe,
ground from some other circuit, etc.) at the same
time.
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Note that even though this is acceptable by the
NEC, I do not consider it desirable. Your safety
now depends on the proper functioning of the GFCI
which is considerable more complex and failure
prone than a simple fuse or circuit breaker.
Therefore, if at all possible, provide a proper
Code compliant ground connection to all outlets
feeding appliances with 3 wire plugs.
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The End!!!