CA2503933A1 - Leakage current detector interrupter with continuous duty relay - Google Patents

Abstract

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 protecting phase conductors and a ground conductor. The leakage current detector can detect a leakage current between the shield and the ground conductor.

Description

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.

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.

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.

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 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 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 S

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 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 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 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, 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, 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 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 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 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.

Claims (16)

1. An electrical extension cord including a leakage current detector interrupter comprising:
a continuous duty relay having contacts coupled to allow currant to flow from an input power source to a load when the relay is in an energized state;
an electronic switch to switch the relay between the energized state and a de-energized state; and an electronic latch coupled to the electronic switch, wherein in response to detection of a leakage current, the electronic switch is latched in a state to de-energize the relay.

2. The electrical extension cord of claim 1, wherein the latch comprises a silicon controlled rectifier.

3. The electrical extension cord of claim 2, wherein the electronic switch comprises a transistor.

4. The electrical extension cord of claim 3, comprising:
a test switch to trigger the latch, wherein the electronic switch is latched in a state to de-energize the relay; and a reset switch to un-trigger the latch, wherein the electronic switch is in a state to energize the relay.

5. The electrical extension cord of claim 1, comprising:
a cable comprising:
a conductive wrap surrounding an insulated first load conductor and an insulated second load conductor, and a ground conductor located outside of the conductive wrap but within a non-conductive flexible outer wrap, wherein the insulated first load conductor and the insulated second load conductor are coupled to respective contacts of the relay.

6. The electrical extension cord of claim 5 wherein the detected leakage current flows between the ground conductor and the conductive wrap.

7. The electrical extension cord of claim 5 wherein the conductive wrap comprises a braided shield.

8. The electrical extension cord of claim 6, comprising:
an electrical plug having a first line conductor, a second line conductor and ground plug blades wherein the shield is electrically connected to the ground conductor;
and a receptacle having a first load conductor, second load conductor and ground receptacles to receive external plug blades.

9. The electrical extension cord of claim 6, comprising:
a plug to receive an alternating current (AC) power supply input having a first line voltage, a second line voltage and an AC ground; and a direct current (DC) power supply to receive the first line voltage and provide a DC voltage output and a DC ground, wherein the DC power supply floats between the first line voltage and the second line voltage and is off center from the midpoint between the first and second line voltages, wherein the first line voltage and the second line voltage are inputs coupled to respective relay contacts and the AC ground is coupled to the extension cord shield, and wherein the DC power supply output is coupled to the electronic latch.

10. The electrical extension cord of claim 9, wherein a resistor divider is arranged to offset the DC power supply from the midpoint between the first and second line voltages.

11. A method for detecting leakage current in an electrical extension cord::
energizing a continuous duty relay to allow current to flow from an input power source to a load; and switching the relay between the energized state and a de-energized state.

12. The method of claim 11, comprising:
detecting a leakage current in the extension cord; and latching the relay in the de-energized state in response to detection of a leakage current that exceeds a predetermined value.

13. The method of claim 12 wherein the extension cord comprises a phase conductor, a neutral conductor, a ground conductor and a shield.

14. The method of claim 13 wherein the leakage detected is from the ground conductor to the shield.

15. The method of claim 12, comprising:
latching the relay in the de-energized state in response to actuation of a test switch; and resetting the relay to the energized state in response to actuation of a reset switch.

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