SHOCK PROTECTIVE CIRCUIT WITH ELECTRICAL LATCH FOR SMALL APPLIANCES

Abstract: An immersion detection current interrupter cir­ cuit with an electric latch for use in small electrical appliances. A sensor wire (W) is positioned within an appliance to sense current leakage when in contact with a conductive fluid. A relay or solenoid (R) connected to the sensor is positioned between the load leads (R) in a latch circuit. When the sensor detects a leakage current, it causes a two-pole double-throw relay to latch, breaking the load lines beyond the relay (R). The relay will remain latched untill the power cord is unplugged or reset by a switch which opens and closes the power lines. Once the component has latched, the maximum leakage current will be limited by the voltage drop across a diode (D5).

Description: BACKGROUND OF THE INVENTION Field of the Invention The invention pertains to the field of shock protection circuits in general and in particular to immersion detection current interrupter circuits designed to protect the user of a small hand-held electrical appliance from a serious electrical shock if the appliance is immersed in or otherwise comes into contact with an electrically conductive fluid. Description of the Prior Art Shock protective circuits known in the prior art are of two types, ground fault current interrupter circuits (GFCI) and immersion detection current interrupter circuits. Various devices incorporating such circuits are also known in the prior art. GFCI devices are presently required in new electrical installations in bathrooms and other locations. Typically, such devices are included in an assembly connected to an electrical outlet. Prior art shock protection circuits are designed primarily for electrical devices of substantial size and cost. They tend to be sophisticated, complex devices which, while of substantial economic and safety value, are not appropriate for the volume and variety of inexpensive consumer products in the field of hand-held small electrical appliances. Typical of the prior patent art are the ground fault current interrupting devices disclosed in U.S. Patent Nos. 4,345,289 and 4,358,708 for use in residential applications. A typical industrial hazard protection device is disclosed in U.S. Patent No. 4,103,319. Such devices are inappropriate in terms of size, complexity and cost for small consumer appliances. Nevertheless U.S. Patent No. 4,493,975 discloses a wall-mounted hand-held hair dryer. The principal power switch for the appliance is contained in the wall-mounted housing. When the appliance is removed from the wall mount, this switch is in the "off" position and no power flows to the appliance. Thus, if the appliance should accidentally or intentionally be immersed in a conductive fluid, it is electrically dead and no shock hazard is present. To turn on power to the appliance, the user must activate a switch in the wall mount. Simply returning the hand held dryer to the wall mount turns the switch off. The problem with this prior art device and the UL standard itself is that there is no protection from electrical shock when the power switch is in the "on" position. However, the nearest prior art is European Patent Application EP-A 0 088 390 which discloses a shock protection circuit for use in a small, hand-held electrical appliance, the circuit being capable of detecting a leakage current caused by a conductive fluid and, as a consequence, disconnecting a supply voltage from the electrical appliance. For detecting the leakage current the circuit comprises a sensor means, whose resistance changes in case of a leakage current, while for disconnecting the supply voltage the circuit is provided with a means to interrupt comprising a thyristor and a switch, the thyristor being controlled by the resistance of the sensor means and the switch being controlled by the thyristor, and a means to latch incorporated in the switch. Additionally the circuit comprises a means to limit a leakage current which means is embodied by two resistors. However, the circuit is inappropriate in terms of complexity and use, since the means to interrupt comprises a complex circuit and the means to latch is difficult to reset. Besides, a shock protection circuit for use in a small, hand-held electrical appliance is described in the European patent application EP-A 0 235 859, but this patent application is a prior right published after the filing date of the subject invention in Germany, France and Great Brittain. The shock protection circuit described in the European patent application comprises a first and a second power lead to couple a supply voltage to a load, an electrical connection comprising a diode coupled between the first power lead and a first node, a sensor means coupled to a second node to detect a presence of a leakage current within the appliance, a means coupled between the first node and the second node to interrupt the power leads in case of a leakage current above a threshold limit being detected, a means to latch the power leads in an open state, and a means to limit a current through the sensor means. In addition, the shock protection circuit comprises a further diode coupled between the second power lead and the first node, while the means to interrupt comprises a solenoid to set the power leads into the open state and the means to latch comprises a mechanical latch. When the appliance is immersed in or comes into contact with an electrically conductive fluid, the sensor means draws a leakage current, which current energizes the solenoid. Consequently the solenoid sets the power leads into the open state, while the means to latch continuates the open state, the means to latch being not easily resettable. In conclusion, the general prior art fails to disclose a simple, inexpensive circuit embodied in a small component device which serves to protect the user of a small electrical appliance from electrical shock if the appliance comes into contact with an electrically conductive fluid such as water. The most frequent hazard of this type is the use of hair dryers in a bathroom. When a personal care appliance such as a hair dryer or electric razor come into contact with water, the user may receive an electrical shock which can be fatal. The object of the present invention is to protect the user from a serious shock (above .005 amps) with a simple and inexpensive device. SUMMARY OF THE INVENTION The present invention is a simple, reliable, inexpensive immersion detection current interrupter circuit to provide shock protection when a hand-held small appliance comes into electrical contact with an electrically conductive fluid such as water, said circuit designed to be packaged as a small component for inclusion within such an appliance or the plug of its power cord. When the appliance is plugged into an electrical power source such as a wall outlet and is immersed in or otherwise comes into electrical contact with a conductive fluid, there will be a current leakage. The object of the invention is to provide a simple, reliable, inexpensive shock prevention circuit which will limit the maximum current leakage to .005 amps after 0.25 seconds. A bridge and relay (or solenoid) circuit in conjunction with a simple sensor achieves this object. The circuit utilizes a sensor to detect this leakage current. The sensor then activates a relay or solenoid within a bridge circuit which disconnects the load leads and causes the relay contacts to latch. After the relay contacts latch, no current will flow to the appliance. In the preferred embodiment of the invention using an electric latch, the sensor is a wire positioned in various parts of the appliance and in close proximity to the two power leads which senses current leakage when in contact with a conductive fluid. A relay or solenoid connected to the sensor is positioned between the load leads in a latch circuit and connected thereto by diodes which rectify the current. When the appliance is connected to (plugged into) a power source and the sensor comes into contact with a conductive fluid, the sensor provides a short path to ground with low resistance for the relay (or solenoid) current. When the sensor conductor detects a leakage current, it causes the two-pole double-throw relay to latch, disconnecting the load lines beyond the relay. If the relay (or solenoid) requires .010 amps to activate, the leakage current will be interrupted in less than .025 seconds, meeting the standard of less than 5 mA maximum after .025 seconds. The bridge latches the relay contacts in whether the ac line is high or low. When the relay contacts latch, the bridge keeps a low voltage on the sensor no matter how the plug is put in an outlet (normal or reversed polarity). The relay will remain latched until the power cord is unplugged or reset by a switch which opens the power lines. Once the component has latched, the maximum leakage current to ground will be limited by the voltage drop across the diode. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a circuit diagram for a shock protection circuit for small appliances using an electrical latch. Figure 2 is a cut-away plan view showing the circuit of Figure 1 installed in the plug of a hand-held hair dryer. Figure 3 is a cut-away plan view showing the circuit of Figure 2 installed in the housing of a hand-held hair dryer. DESCRIPTION OF THE PREFERRED EMBODIMENT The shock protection device of the present invention is a simple, reliable, inexpensive immersion detection current interruptor circuit which addresses the hazard of the possible severe, even fatal, electrical shock to a person if a hand-supported appliance is dropped into a conductive fluid such as water. Since non-distilled water is a conductive fluid and the hand-held hair dryer is the small appliance most frequently used in close proximity to water, resulting in statistically significant injuries especially among children, the present invention will be described in an embodiment useful in said hair dryers. It is to be understood that the invention is not limited thereto. Figure 1 is a circuit diagram of the simple immersion detection current interrupter circuit of the present invention. A special sensor conductor in the form of at least one uninsulated (exposed) wire W is positioned in various parts of the small appliance to sense leakage currents to ground when the sensor wire comes into contact with a conductive fluid such as water. The sensor wire is positioned so that it will conduct a leakage current over the shortest path to ground, either the external ground or the internal ground of the appliance. The sensor leads are positioned at locations in the appliance where the water path to the ground would normally be shorter than the path from the hot lead to ground. It may be in close proximity to one or both of the power leads or an additional ground wire. Thus, in certain cases conduction may be between the sensor wire and one of the power leads. In most cases the sensor lead would be placed as close as possible to the ground lead, thus, making a shorter sensor current path when the appliance is placed under water. The precise positioning of the sensor lead is critical to the effectiveness of the interrupter, and will depend on the configuration of the appliance in which it is installed. In any case it should be close to the power leads also. A solenoid or two-pole double-throw relay R is connected within a modified bridge circuit between the two power (load) lines and through the sensor lead to ground. When an electrical path is provided between the sensor lead and ground sufficient to cause the solenoid or relay to activate, the relay contacts latch into the normally open (NO) position. When power is applied and the solenoid or relay is connected to the normally closed contacts as shown in Figure 1, and the leakage current rises above a threshold, the sensor will conduct to ground or to either wire, hot or neutral. Within .025 seconds the relay will close to the normally open contacts and latch, thus removing power from the appliance. Once latched, the power lines to the appliance are disconnected and the relay will remain latched until it is unplugged from the power source. Once the relay or solenoid has latched, the maximum current drawn by the sensor to ground will be limited by the voltage drop across diode D5. Typically the current will always be less than .005 amps. The diodes D1 and D2 in the modified bridge rectify the current to the relay while D3 and D4 maintain a latching current on the relay until the unit is unplugged. If the distance between the sensor lead and the power leads is small, the current required to pull in the relay could be .010 amps, thus making the relay less expensive. The relay coil can be operated higher than .005 amps, to ground, and still maintain its protective function since the unit will open in 0.25 seconds. The diode D5 provides the limiting factor within this time. In the embodiment illustrated in Figure 1, the diodes are drawn for a particular current direction. It should be obvious that the diodes could be reversed so that current will flow in the opposite direction. The circuit of Figure 1 operates independently of the polarity of the power lines. The ac line plug could be reversed without affecting its operation. Also, if one of the two power leads is broken, the circuit will still operate if the sensor lead is in tact. In an alternate embodiment, the minimum required components for the circuit would be one-half the bridge shown in Figure 1 using only diodes D1, D4 and D5. The device of the present invention is a simple, reliable and inexpensive immersion detection current interrupter circuit which can be incorporated within the housing of a hand-held appliance as shown in Figure 3, said circuit being used to disconnect power to the appliance via a relay or solenoid. In order to prevent bodily injury, the triggering current required to energize the relay must be kept about 5 mA. This threshold level allows for leakage currents that are inherent in most electrical devices and minimizes false triggers. This current level may be achieved by incorporating a resistor in series with the relay coil or by setting the resistance of the relay coil to the limiting value. One or two relays or solenoids may be used. The circuit of the present invention is user resettable. The unit may be reset by disconnecting the power leads from their power source and reconnecting them again. For the customary appliance this is usually done by unplugging the unit and plugging it in again. However any means to break and reconnect a power line between the power source and the bridge circuit will suffice. Of course, the unit must be dry to reset. When a conductive fluid comes into contact with any part of the immersion detector or sensor within the housing, current will flow through the fluid to ground, triggering the control circuit to disconnect the appliance from the ac line. An additional feature of this immersion detection current interrupter is that the protection is available to the user whether the hand-supported appliance is switched off or on, that is, protection is available whenever the appliance is energized in the presence of a hazardous condition. The circuit of Figure 1 can be incorporated within the housing of a small appliance as shown in Figure 3, in which case only a two-wire power cord is required. Alternatively, the circuit could be incorporated within the power plug as shown in Figure 2, in which case a three-wire cord is required, the third wire being the sensor lead. This sensor lead outside the appliance housing could be a wire mesh around the power leads, or a bare wire with a loose porous sleeve around one or both of the power leads. Within the appliance, the sensor could be a conductive strip, a conductive coating or a ring. The sensor could be one or more wires, at least one of which is in close proximity to the power leads or an additional ground wire. In certain cases, conduction may be between a sensor lead and one of the power leads, depending on polarity of the leads. In any event, the sensor should be close to the power lines to provide a good path for conduction in water. The circuit of Figure 1 is thus a very simple circuit with few components, thereby yielding a very reliable circuit which is very inexpensive to manufacture and hence appropriate for low cost small consumer appliances. Most importantly, the circuit of Figure 1 is immune to electrical noise and will not false trigger or nuisance trigger. The circuit of the present invention works with the power switch in both the on- and the off-position of the appliance power switch. The shock protection is available any time the unit is energized. The advantage of the present invention is that it provides the lowest possible cost for a user resettable shock protection circuit which is fast and reliable. In addition, after latching, the current in the sensor is limited to safe levels until the appliance is unplugged. Furthermore, if the shock protection circuit is placed in the appliance plug, no components of the appliance are required to be sealed. A fault indicator signal could be added to the latch mode to warn of an existing fault. This could be a signal light connected to the NO relay contacts.

Claims: A shock protection circuit for use in a small, hand-held electrical appliance, the circuit comprising: a first (upper "POWER") and a second (lower "POWER") power lead for coupling a supply voltage to a load, an electrical connection comprising a first diode (D1) coupled between the first power lead and a first node (D1/R), sensor means (W) coupled to a second node (D4/R) to detect the presence of a leakage current within the appliance, means coupled between the first and second nodes to interrupt the power leads in case of the leakage current being detected above a threshold limit, means to latch the power leads in an open state after the leakage current being detected above the threshold limit and means to limit the leakage current through the sensor means, characterized in that the means to interrupt comprises a solenoid or two-pole double-throw relay (R) to set the power leads into the open state, the means to latch comprises a second diode (D4) which is connected to the second power lead through a contact closed by the solenoid or two-pole double-throw relay (R) after the leakage current being detected above the threshold limit and the means to limit comprises a limiting diode (D5) inserted between the second node and the the sensor means. A circuit as claimed in claim 1, characterized in that the electrical connection comprises a third diode (D2) coupled between the second power lead and the first node and in that the means to latch comprises a fourth diode (D3) for coupling the first power lead to the second node. A circuit as claimed in claim 1 or claim 2, characterized in that the solenoid is part of a relay. A circuit as claimed in claim 1, characterized in that the means to interrupt comprises a contact controlled by the solenoid, the contact being adapted to couple the second power lead to the second node via the second diode (D4) in case of the leakage current. A circuit as claimed in claim 2, characterized in that the means to interrupt comprises a further contact controlled by the solenoid, the further contact being adapted to couple the first power lead to the second node via the fourth diode (D3) in case of the leakage current. A circuit as claimed in claim 1, 2, 3, 4 or 5, characterized in that the sensor means comprises a wire positioned throughout the appliance, the wire being in close proximity to the power leads. A circuit as claimed in claim 1, 2, 3, 4, 5 or 6, characterized in that the means to latch comprises means to indicate the presence of the leakage current.

Inventors: MADSEN, ELMER WILBUR
Applicants: NORTH AMERICAN PHILIPS CORPORATION
Application year: 1987
Main classification: H02H5
IPC codes: H02H5