Note: Descriptions are shown in the official language in which they were submitted.
CA 02503933 2005-04-08
LEAKAGE CURRENT DETECTOR
INTERRUPTER WITH CONTINUOUS DUTY RELAY
This application claims the benefit of the filing date of a provisional
application
having serial number 60/560,460, which was filed on April 8, 2004.
BACKGROUND
Fiel~the Invention:
The present invention generally relates to leakage current detector
interrupters.
Description of the Related Art:
An extension cord can include a plug, having two or more prongs, a cord,
having
two or more insulated conductors and a terminal connector or receptacle for
receiving
one or more electrical plugs to power household devices including lamps,
radios,
televisions and household appliances. A grounded extension cord includes a
plug having
1 S at least three prongs and a three-conductor insulated cord, twa conductors
of which may
be utilized for phase and neutral conductors from a household electrical
circuit and a third
conductor may be utilized as a ground conductor.
Extension cords may be placed underneath rugs where they can be physically
damaged, for example, by being trampled on and pinched by doors and/or
furniture. In
some instances, an electrical extension cord may be used in a wet or damp
surrounding,
such as a garden, a basement or similar places. These, and other, uses can
lead to arcing
or short circuiting in the electrical extension cord.
CA 02503933 2005-04-08
An electrical extension cord can include safety features for the protection of
personnel. One such safety feature is a ground fault circuit interrupter
{GFCI) that can
interrupt electrical power to a device when there is a leakage of current to
ground. The
extension cord can include individually insulated conductors all of which can
be
surrounded by an insulating jacket. When an extension cord is used for an
appliance, a
user of the appliance can be subject to possible injury if a shock hazard
condition should
exist in conjunction with a non-GFCI protected outlet.
Portable devices, such as hair dryers, may not necessarily be plugged into
GFCI
outlets. Thus, a GFCI mounted in the wall can not protect a user from getting
a shock
from a portable device not using the protected outlet.
SUMMARY OF THE INVENTION
An electrical extension cord includes a leakage current detector to interrupt
the
power to a corded appliance. The electrical extension cord includes a
continuous duty
relay having contacts coupled to allow current to flow from an input power
source to a
load when the relay is in an energized state. An electronic switch can switch
the relay
between the energized state and a de-energized state. The electrical extension
cord
includes a shield protected phase conductors and a ground conductor. The
leakage
current detector can detect a leakage current between the shield and the
ground
conductor.
Tn an implementation, an electronic latch is coupled to latch the electronic
switch
in a state which de-energizes the relay upon detection of a leakage current.
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One or more implementations of the disclosure can have some of the following
advantages. The extension cord can protect a user from a shock hazard
associated with
the appliance rather than have a,n electrical outlet into which the appliance
is plugged
provide that protection. The leakage current detector can detect leakage
current between
the shield and the ground conductor of the extension cord. A continuous duty
relay is
used to interrupt power to the appliance. The relay is energized in the normal
state, that
is, with no leakage detected. Hence, a failure of the leakage current detector
itself can
result in de-energizing the relay and intem~pting power to the appliance.
The foregoing has outlined, rather broadly, the preferred feature of the
present
invention so that those skilled in the art may better understand the detailed
description of
the invention that follows. Additional features of the invention will be
described
hereinai~er that form the subject of the claims of the invention. Those
skilled in the art
should appreciate that they can readily use the disclosed conception and
specific
embodiment as a basis for designing or modifying other structures for carrying
out the
same purposes of the present invention and that such other structures do not
depart from
the spirit and scope of the invention in its broadest terms.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an electrical extension cord with leakage
current
protection.
FIG. 2 is a perspective diagram of a receptacle end portion of an electrical
extension cord of the disclosure.
CA 02503933 2005-04-08
FIG. 3 is a block diagram of an extension cord having a leakage current
detector
interrupter (LCDn in accordance with the principles of the disclosure.
FIG. 4 is a schematic of the circuit diagram of a leakage current detector
intemtpter (LCDn in accordance with the principles of the disclosure.
FIG. SA illustrates an electrical safety cord that can be used with the
circuit of
Fig. 1
FIG, SB illustrates a cross-sectional view of the cable of FIG. SA,.
Other aspects, features and advantages of the present invention will become
more
apparent from the following detailed description, the appended claims and the
accompanying drawings in which similar elements are given similar reference
numerals.
DETAILED DESCRIPTION
FIG. 1 illustrates an implementation of a leakage current detector interrupter
102
in an electrical extension and/or power coxd 100 (hereinafter, extension
cord). A leakage
current detector interrupter (L;CDI) is a device that can be used to sense
leakage current
flowing between or from the power conductors in an electrical appliance cord
and can
interrupt the circuit at a predetermined level of sensed leakage current.
Because an LCDI
can detect a current leakage flowing to ground, it may provide graund fault
protection in
addition to protection from arcing between conductors and ground and other
problems
which can arise due to leakage between conductors.
An electrical plug 104 of the extension cord 100 can include the leakage
current
detect interrupter 102 (LCDI). Both the plug 104 and the LCDI 102 can be
provided
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CA 02503933 2005-04-08
within a housing 112 of the plug. A line end of the LCDI can be connected to
three plug
blades 106a, 106b, i05c to access phase, neutral and ground terminals of a
power source
(not shown). A cord portion 116 includes an electrical cable 108, has a phase,
neutral
and ground conductors, surrounded by a conductive shield 110. In an
implementation,
S cord portion 116 can include a first conductor which is a phase conductor, a
second
conductor which is a neutral conductor, a third conductor which is a ground
conductor
and a fourth conductor is a sense conductor. In a three-conductar
implementation, a plug
housing has three blades, one each for the phase, neutral and ground
conductors. The
plug housing further includes cable 108, which is electrically coupled to the
plugJI,CDI
combination within the housing I 12. The conductive shield 110 can be
electrically
connected to a ground conductor (not shown) at receptacle 114. The phase and
neutral
conductors are electrically connected to phase and neufiral terminals (not
shown) of the
LCDI therein. Leakage current may be carried by the conductive shield 1 I 0,
which
extends along the length of the cord portion 116.
1 S In an implementation, conductive shield 110 can be a fine mesh flexible
shield
made of a conductive material (copper, for example) surrounding three-
conductor
extension cord 108. In another implementation, the cable can have four
conductors.
Because the shield is electrically connected to the ground conductor, excess
ground fault
or leakage current is passed to ground while the LCDI detects an imbalance
within the
phase or neutral conductor and trips to interrupt the electrical path through
the cord.
Accordingly, the shield and LCDI combination can protect the electrical
extension cord
itself directly, instead of directly protecting the appliance (i.e., a load )
to which the card
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CA 02503933 2005-04-08
is supplying power. Leakage current can be captured by the shield rather than
by the
ground conductor.
FIG. 2 depicts an electrical a 240 VAC receptacle 204, which can be
electrically
connected to a first end portion of a ground conductor 202a, a first phase
conductor 202b
and a second phase conductor 202c, which may be utilized within an electrical
extension
and/or power cord with built-in safety protection. The receptacle 204 has
cavities 202a',
202b', 202c' that are connected to conductors 202a, 202b, 202c, respectively.
A
connector 204 also receives a portion of conductive shield 206, which may be
electrically
connected to ground conductor 202a at receptacle 204. The configuration of the
cavities
I O of receptacle 204 can be different depending on the rating of the
receptacle. Some
configuration may include a neutral conductor (not shown). In a 120 VAC
implementation, one of the two phase wires is a neutral conductor and the
connector 204
is of a different configuration.
FIG. 3 is a block diagram of an extension cord having a continuous-duty relay
300. The design of the circuit is applicable to both I20 volt and 240 volt
alternating
current {AC) operation. The 240 volt application is shown and will be
described herein.
The 240 volt power has two phases of voltage rather than a single phase as in
a 120 volt
system. AC input power is received from a coupling of a plug 302 to household
power
line phase I and phase 2. The household wiring system also may have a ground
conductor 304. A first line phase, which may be either line phase 1 or line
phase 2, can
be supplied to an electrical circuit 306 that can convert the input AC line
phase voltage to
an output DC voltage, Vdc. The first line phase also is coupled to a relay
coil LI. The
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CA 02503933 2005-04-08
relay contacts RLla, RLIb are normally open and close when relay coil L1 is
energized.
Relay contacts RLIa, RLIb are closed when the relay is energized and
electrical power
fmm line phase 1 and line phase 2 is coupled through the relay contacts to
load phase 1
and load phase 2, respectively. Vdc is coupled to a switching circuit 308,
which, in turn,
S can receive an input from the shield of a cord portion of an extension cord.
The
switching cixcuit 308 wilt energize the relay coil when the level of current
flowing in the
shield is at or below a sp~ified level. The switching circuit will de-energize
the relay
coil when the level of current flowing in the shield exceeds the specified
level.
FIG. 4 illustrates a schematic of an implementation of a LCDI having a
continuous duty relay to control the flow of current through a power cord. The
invention
is applicable to both 120 volt and 240 volt applications. The 240 volt
application is
described herein. The LCDI disclosed may be contained in a plug that can be
plugged
into a receptacle, which provides 240VAC represented by Line Phase T and Line
Phase 2.
The LCDI provides electricity to flow to a downstream cable, which is
indicated by the
connections Load Phase I and Load Phase 2. In a 120V embodiment the
connections
would be Line and Load Phase and Line and Load Neutral.
The LCDI can power a cable having conductors for two line phases, an optional
ground conductor, a neutral conductor and a shield. The shield may be present
along the
entire length of the cable and is electrically connected to ground at the
LCDI. The LCDI
can include a double-poled relay RL1, capable of interrupting power to Load
Phase I and
Load Phase 2 upon the detection of leakage current that causes switch Q 1 to
turn OFF
(i.e., not conducting). The relay RL1 is normally open and is held closed by
relay coil L1
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CA 02503933 2005-04-08
when the electronic switch Ql is ON (i.e., conducting). When the electronic
switch Ql is
OFF, a spring (not shown), that can be part of the relay ltLl, pulls the relay
back to its
normally open state. The solenoid must be energized or driven continuously for
current
to be supplied to the Ioad. This type of relay may be referred to as a
continuous duty
relay. A ground conductor also may be present with a hardwired connection from
Line to
Load side. The ground may not be disconnected upon detection of leakage
current.
The protective circuitry includes a continuous duty, normally open relay in
conjunction with an LCDI. Current through the relay coil will close the
normally open
relay contacts. The electronics for the LCDI circuitry may be powered by a DC
power
supply 402 which, in the embodiment illustrated, can be achieved through a
full-wave
diode bridge diode D1-D4, a capacitor Cl (for smoothing the fully rectified
wave), and a
zener diode Z1 to regulate the voltage. In a 240 Volt application, the relay
coil may be
powered from one phase, line phase 1 in this example, of the 240 Voit AC
supply line.
The selected phase may pass through a diode D6 for half wave rectification.
Capacitor
1 S C3 can smooth the voltage output of the half wave supply at the cathode of
D6, so that
the current flowing through the relay coil is sufficient to keep the relay
contacts closed
(without chatter) at AC voltages close to 50% of nominal voltage. A second
capacitor C4
helps to limit back electromotive force (EMF), when the relay is de-energzed.
The half
wave rectified phase is used to energize the relay coil L1.. The transistor
switch Q1 is
used to enableldisable the half wave rectified current flow through the relay
coil and thus,
control coupling of the input line phases to the respective load phases. An
indicator LD f,
such as a light-emitting diode (LED), is connected in series with the relay
coil L 1 to
CA 02503933 2005-04-08
indicate when the relay coil is energized and power is connected to the load.
The half
wave rectified current returns to a ground output 404 of the full wave bridge.
When no leakage is present from the sense shield, a resistor divider R4, R7,
R8
can supply voltage to the base B of the transistor Ql. Tlie values of the
resistors may be
chosen such that transistor Ql is normally in the ON state, which, in_turn,
can allow
current to flow through the relay coil.. A current leakage to the sense shield
of the cord
can enable current to flow to the gate of a Silicon Controlled Rectifier
{SCR). The SCR
cathode is connected to the DC ground of circuit 402. If su~cient leakage
current is
detected from any one of the load conductors to the shield, the Silicon
Controlled
R~tifier (SCR) fires and voltage divider R6 and RS limits the current flowing
to the gate
of the SCR from the shield, also sets the sensitivity of the SCR. A capacitor
C2 may be
included to filter electrical noise from the shield to reduce false firing of
the SCR.
Capacitor C4 also may help prevent false firing of the SCR due to voltage
spikes on the
power line. A diode DS protects the SCR from a voltage surge on the DC, ground
line by
clamping the gate of the SCR to the SCR's cathode. Additional protection can
be
provided by a varistor MV1, which can protect the LCDI from voltage spikes on
the line
phase conductors.
When the current flow to the gate of the SCR exceeds a predetermined value,
the
SCR will fire and current can flow from the anode to the cathode of the SCR.
The node,
between R4 and R7 is pulled to a voltage near the ground of the full wave
bridge. In
turn, transistor switch Q 1 turns OFF (open circuit). The current flowing
through the SCR
is sufficient to hold it latched on, thus ensuring that the relay contacts
remain open,
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CA 02503933 2005-04-08
interrupting the current flow from the line to the load conductors. Hence,
firing of the
SCR can drive the base of the transistor low enough to turn transistor Q 1
OFF. The SCR
will continue to conduct DC current until the SCR is reset. Thus, the SCR is
latched into
a conducting state which, in turn, drives the base of transistor Q1 low and
latches the
transistor Ql into the OFF state. When transistor Ql is OFF, relay coil L1 is
de-
energized and contacts SLla , SLIb open to remove electrical power from the
load (and
the leakage fault). That is, switching transistor Q1 remains OFF even when the
voltage at
the gate of the SCR is removed.. This ensures that the LCDI will go to the
tripped, or
current-intemtpting, state once a fault is detected and remain tripped even
after the fault
is removed.
Conduction of electricity to the load can be reset once a fault is removed.
The
SCR can be reset to a non-conducting state. A switch SW1 in parallel with the
SCR is
provided to reset the SCR to the a non-conducting condition by shorting
current around
the SCR. When SWl is actuated, the SCR is shorted, which brings the current
level
flowing through the SCR to zero which, in turn, resets the SCR to a non-
conducting state.
That is, when the reset switch SWI is closed, the SCR is commutated (starved
of the level
of current required to hold it on). When the reset button is released, because
the SCR is
not conducting, the voltage is allowed to rise at the base of the transistor
QI and the
transistor again can conduct, energizing the relay coil L1 and closing the
relay contacts
RLIa, RLIb. Thus, the circuit can be reset after a fault is detected. Note
that if leakage
current is still present after resetting, the LCDI will trip again. Other
reset methods may
CA 02503933 2005-04-08
be available including disconnecting either an anode or a cathode of the SCR
to unlatch
the SCR.
For a 240 volt system, the LCpI is able to detect leakage from the ground
coinductor to the shield because the DC power supply voltage 402 floats
between the two
phases, and between phase and neutral, for the 120 Volt implementation. In the
240 Volt
implementation, the DC power supply floats oil center from the voltage
midpoint of line
phase I and line phase 2. Thus, there is a voltage potential difference
between the ground
of the DC supply and the ground conductor of the AC power supply. The off-
center float
may be achieved by selection of a resistor divider Rl, R2, R3. The potential
of the
IO ground conductor (in 240VAC systems} may be halfway between Phase I and
Phase 2
voltages when the resistance of R1 plus R2 is approximately equal to that of
R3. The
ground of the full wave bridge will be at a potential halfway between Phase I
and Phase
2. Because the cathode of the SCR and the ground conductor will have the same
potential, it may not be possible to fire the SCR with current from the ground
conductor.
~ Thus, the circuit is designed so that the resistance of Rl plus R2 does not
equal that o~ R3
to allow a potential difference between the cathode of SCR and the ground
conductor to
exist. The potential difference between the grounds of the AC and DC supplies
can
enable a leakage current to flow between the two;grounds during a leakage
current event,
which can trigger the LCDI.
A test circuit can be pxovided to test the LCDI's operability, A test button
SW2
can be actuated to cause current to flow from one of the load phases to the
gate of the
SCR and fire the LCDI as described above. A capacitor CS may be coupled in
series
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with the test button as a DC blocking capacitor to prevem any DC voltage held
in the
shielded cable from discharging back into the LCDI circuitry. The AC current
can pass
through CS to trigger the SCR. The LCDI may be reset as described above.
FIGS 5A and SB illustrate an implementation of a 3 conductor 120 VAC cable
500 having a conductive wrap 502 surrounding the line 506 and neutral 508
conductors.
A ground conductor 504 can be outside of the conductive wrap 502. All three
conductors
can be within a non-conductive outer jacket 510, which can be flexible. The
Line side
includes a line conductor 512 surrounded by a conductor insulation 514.
Similarly, the
neutral conductor 5I6 and the ground conductor 516 may be surrounded by
conductor
IO insulation 520, 522, respectively. In another embodiment, a braided
conductive shielding
can be substituted for the conductive wrap 502. The cable can be used to
provide for arc,
leakage and ground fault protection. A fault to ground (energizing the ground
conductor)
may not occur before the fault energizing the conductive W rap. That is, the
conductive
wrap can be energized prior to a fault to ground. The LCDI powers a cable
consisting of
the conductors for the two phases, an optional ground conductor and a shield
incorporated into the insulation surrounding the conductors. The shield is
gresent along
the entire length of the cable and is electrically connected to ground at the
LCDI,
While there have been shown. and described and pointed out the fundamental
novel features of the invention as applied to the preferred embodiments, it
will be
understood that various omissions and substitutions and changes of the form
and details
of the apparatus illustrated and in the operation may be done by those skilled
in the art,
without departing from the spirit of the invention.
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