Abstract: A ground fault circuit interrupter (GFCI) interrupts the flow of current through a pair of lines extending between a source of power and a load. The GFCI includes a circuit breaker having a switch located in one of the pair of lines. The switch has a first position in which the source of power in its associated line is not connected to the load and a second position in which the source of power in its associated line is connected to the load. A relay circuit is coupled to the switch selectively positioning the switch in either the first or second position. The relay circuit includes a solenoid which operates in either an energized or a de-energized state. The GFCI also includes a booster circuit for selectively supplying a first voltage through the switch and to the solenoid which is sufficient to cause the solenoid to switch from its de-energized state to its energized state. A power supply circuit supplies a second voltage to the solenoid which is less than the first voltage.

Abstract: A circuit protection apparatus includes a circuit breaker coupled in series with a circuit to be protected. The circuit breaker has a trip input and is responsive to a trip signal at the trip input for changing from a closed circuit condition to an open circuit condition to thereby remove electrical current from the circuit to be protected. A thermal trip element is coupled in series with the circuit to be protected and is responsive to a predetermined current/time characteristic of current flow in the circuit for delivering the trip signal through the thermal trip element to the trip input.

Abstract: A ground fault circuit interrupter (GFCI) with built in intelligence is described that includes circuitry to automatically indicate a device miswiring condition. When a GFCI is wired with its AC and LOAD terminals reversed, downstream devices are still protected in the event of a ground fault but the duplex receptacle on the GFCI itself is not. The invention incorporates miswiring sense circuitry that automatically triggers the generation of visual and audible alarms in the event a miswiring condition is sensed. The invention employs an alarm inhibiting technique that incorporates sense circuitry connected to the AC terminals on one side of the internal GFCI switches or relays and alarm generation circuitry connected to the LOAD terminals on the opposite side.

Abstract: A circuit for the avoidance of electrical shock and burns in an electrical appliance. Designed primarily for a two wire appliance, the device consists of a conditioning circuit at the load and a fault sensor/interrupter at the plug. The conditioning circuit halts the flow of current to the load for a portion of each cycle of an AC excitation. If current flow is detected at the plug during this time interval it is indicative of a fault condition and a circuit breaker at the plug is opened. A fault is detected if one of the following conditions occurs: 1) a resistive path is established between hot to ground; 2) a load other than the conditioned load is placed between hot and neutral; and 3) an overtemperature condition is detected at the load. The circuit is self-resetting so that in the absence of a hot to ground fault, hot to neutral fault, or overtemperature condition, electrical current is allowed to flow.

Abstract: An apparatus and method for the avoidance of electrical shock in an electrical appliance. Preferred for a two wire appliance, the device consists of a current interrupting circuit that impedes current flow for short time intervals in each half cycle. If a fault to ground occurs during these short time intervals, an increased current flow through the plug is detected and this is recognized as a fault, causing a circuit interrupter to open and removing current from the load during the remainder of the half cycle. When the fault is removed, that event is detected within one half cycle and power is restored to the load. The apparatus can provide thermal control in the plug. The apparatus enables the use of a low current, low voltage switch to control high voltages and high currents in the appliance. The apparatus can transmit fault status information or other information to a remotely located controller and can receive control signals from that remotely located controller.

Abstract: A coaxial drop cable safety device is used for cable television, data and telephony applications. The device is used on buried drop cable where 60 Hz powering voltages applied to the drop cable exceed low voltage safety limits and where the drop cable is buried at depths which is less than the minimum required to meet electrical safety codes. Tap end and premise end units are connected to the drop cable. The tap end unit applies a DC voltage to the center conductor of the drop cable. A monitoring circuit in the tap end unit monitors the center conductor DC voltage to a fault from the center conductor to ground along the drop cable. The tap unit will immediately remove the powering voltage In the event of either an open or faulted condition on the center conductor along the entire length of the drop cable.

Abstract: A new and novel system for protecting people and property against electrical shock. It includes a number of preferred and other embodiments which have this as their goal, but which represent a number of distinctive and novel approaches to solving prior art problems. By way of example only, and without limiting the scope of this invention, these approaches include novel immersion detecting circuits, broken wire test circuits, electromechanical circuit breaking including coil/plunger arrangements, and relay circuit breaking mechanisms cooperative with associated circuitry, all of which are able to be incorporated as a system wholly within the load (appliance) and its associated cord set (including a "plug").

Abstract: A ground fault circuit interrupter (GFCI) interrupts the flow of current through a pair of lines extending between a source of power and a load. The GFCI includes a circuit breaker having a switch located in one of the pair of lines. The switch has a first position in which the source of power in its associated line is not connected to the load and a second position in which the source of power in its associated line is connected to the load. A relay circuit is coupled to the switch for selectively positioning the switch in either the first or second position. The relay circuit includes a solenoid which operates in either an energized or a de-energized state. When energized, the solenoid positions the switch in its second position and when de-energized, the solenoid positions the switch in its first position.

Abstract: A ground fault circuit interrupter (GFCI) with built in intelligence is described that includes circuitry to automatically indicate a device miswiring condition. When a GFCI is wired with its AC and LOAD terminals reversed, downstream devices are still protected in the event of a ground fault but the duplex receptacle on the GFCI itself is not. The invention incorporates miswiring sense circuitry that automatically triggers the generation of visual and audible alarms in the event a miswiring condition is sensed. The invention employs an alarm inhibiting technique that incorporates sense circuitry connected to the AC terminals on one side of the internal GFCI switches or relays and alarm generation circuitry connected to the LOAD terminals on the opposite side.

Abstract: A ground fault circuit interrupter (GFCI) with built in intelligence is described that includes circuitry to automatically indicate a device miswiring condition. When a GFCI is wired with its AC and LOAD terminals reversed, downstream devices are still protected in the event of a ground fault but the duplex receptacle on the GFCI itself is not. The invention incorporates miswiring sense circuitry that automatically triggers the generation of visual and audible alarms in the event a miswiring condition is sensed. The invention employs an alarm inhibiting technique that incorporates sense circuitry connected to the AC terminals on one side of the internal GFCI switches or relays and alarm generation circuitry connected to the LOAD terminals on the opposite side.

Abstract: A circuit for triggering warning lamps including a self-conductive transistor circuit with a triggering circuit connected to the supply voltages. The triggering circuit keeps the transistor circuit blocked if the supply voltages are proper. If voltage fails, the transistors of the self-conductive transistor circuit open a current path from the warning lamp terminal to the control terminal of the power driver which results in switching on of the power driver and, thus, of the warning lamp.

Abstract: Electronic control apparatus for motor vehicles. The apparatus includes a test circuit which causes a test current to flow through a load being operated by the apparatus and which detects the magnitude of the test current. From the magnitude of the test current, an evaluation circuit determines whether the ground potential of the control apparatus and the ground potential of the load are at different levels. In response to a potential difference between the two ground potentials, the evaluation circuit initiates countermeasures for preventing damage to the electronic control apparatus, such as selectively disconnecting the load with a power switch responsive to the evaluation circuit.

Abstract: A leakage protector comprising a lower and upper cover containing a control circuit which includes a bridge rectifier, a pulse absorber, a light-emitting diode, a comparator, a SCR and a relay wherein the output of the comparator causes the relay to interrupt power to a load.

Abstract: A ground fault detection circuit monitors an interlock control circuit for use with a machine or process control system. The interlock control system utilizes electromechanical relays with hard contacts. The interlock control system will disable operation of the machine or process if any ground fault condition exists in the interlock control circuit. A ground condition in the machine or process control circuits is permissible. The system uses a redundant circuit path to verify the integrity of the components in the interlock control system. The control system includes an ungrounded isolation transformer for supplying power to separate monitoring relays interposed between each leg of the transformer and ground. The contacts of these relays are series connected with emergency-stop (e-stop) switches to energize interposing control relays. The outputs of the interposing relays are used to energize the final control relay that controls the operation of the machine or process.

Abstract: A circuit tester comprising a ground fault circuit interrupter having two pair of leads connected to opposite sides of the interrupter in series for connecting the ground fault interrupter between an electrical circuit and a power source to indicate current leakage in an amount greater than about 0.005 amperes, a method for utilizing the circuit tester for detecting amperage leakage in the circuit, and an electrical circuit which will be interrupted whenever there is ampere leakage above that magnitude.

Abstract: A circuit for sensing the loss of a signal ground in an automotive electronic system having a source of electrical power and both a signal ground and a power ground, includes a first sensor coupled to the power ground and between the signal ground and the source of electrical power for generating a first signal responsive to the voltage on the signal ground differing from the voltage on the power ground by more than a first predetermined limit. A latching semiconductor switch is coupled to the first sensor and between the signal ground and the power ground for latching the electrical coupling of the signal ground to the power ground responsive to receiving the first signal.

Abstract: An electrical supply safety plug that prevents power from a mains electrical supply from being accessible to an electrical appliance unless the electrical supply safety plug is properly engaged with the socket outlets and the appliance is turned on. The electrical supply safety plug disconnects the electrical appliance in the event of current imbalance.

Abstract: In an electric power equipment connected to an electrical power source, a defective ground conductor may result in a dangerous situation for a user. To prevent such a situation, the electric power equipment is provided with a safety electrical assembly having a safety switch mounted between the neutral conductor and the ground conductor on the electric power equipment. The safety electrical assembly also has a female plug provided with a ground socket connected to the ground wire of the power source, a neutral socket not connected to the neutral were of the power source, and one or two power sockets connected to one or two corresponding power wires of the power source. The female plug also has a relay having a normally open switch mounted in series with one of the power wires and a control coil mounted between the one power wire and the neutral socket, and a first light indicator mounted between a stationary contact of the relay and the ground wire in such a manner so as to be visible to the user.

Abstract: A ground fault circuit interrupter (GFCI) with built in intelligence is described that includes circuitry to automatically indicate a device miswiring condition. When a GFCI is wired with its AC and LOAD terminals reversed, downstream devices are still protected in the event of a ground fault but the duplex receptacle on the GFCI itself is not. The invention incorporates miswiring sense circuitry that automatically triggers the generation of visual and audible alarms in the event a miswiring condition is sensed. The invention employs an alarm inhibiting technique that incorporates sense circuitry connected to the AC terminals on one side of the internal GFCI switches or relays and alarm generation circuitry connected to the LOAD terminals on the opposite side.

Abstract: Ground fault trip units for circuit breakers are tested by a passive test circuit which includes a test conductor passing through the current transformer sensing coil(s), and a test switch which selectively connects the test conductor in a loop which simulates a neutral-to-ground fault. In a dormant oscillator ground fault test circuit, the test conductor loop passes through both sensing coils. When the neutral-to-ground detector is combined with a sputtering are fault detector sharing a common sensing coil, testing of all the components is accomplished by adding an additional test circuit which injects pulses derived from the line conductor into the sensing coil secondary, or which alternatively, connects a capacitor charged from a DC supply across the sensing coil secondary. Either of these alternative test circuits generates the successive events needed to produce a sputtering arc trip.

Abstract: The present invention teaches a new and novel system for protecting people and property against electrical shock. The invention includes a number of preferred and other embodiments which have this as their goal, but which represent a number of distinctive and novel approaches to solving prior art problems. By way of example only, and without limiting the scope of this invention, these approaches include novel immersion detecting circuits, broken wire test circuits, electromechanical circuit breaking means including coil/plunger arrangements, and relay circuit breaking mechanisms cooperative with associated circuitry, all of which are able to be incorporated as a system wholly within the load (appliance) and its associated cord set (including a "plug").

Abstract: The interrupter includes and apparatus having a pair of oppositely rotatable, spring loaded, sharp edged elements positioned above a pair of wire fuses. Such fuses complete the current path in respective conductors of a two conductor electrical appliance power system. Each edged element is retained by a latching tension of the other fuse, so that opening of either of the two fuses releases the element above the other fuse, causing such element to rotationally descend and apply a cutting force to the other fuse and thus assist in its opening to assure that both conductors of the power system are opened in an immediately successive manner in response to a current overload.

Abstract: A sensing conductor (62) within an appliance housing (56) and a detection circuit (59, 60, 61, 63) preferably within the housing, cause a low resistance shorting of an appliance circuit (57, 58) in response to immersion in water, and an interrupter device (4, 5, 6, 27), preferably located in the plug (65) of the cord set, reacts to the resulting overcurrent by opening two fuses, one in each side of the line (2, 16; 3, 17). An overcurrent opens one fast acting fuse link (27) in the normally ungrounded side of the line (3, 17) and such fuse link (5) in the normally grounded neutral side of the line (2, 16). The fuse link (27) is directly assisted in opening by a spring element (4) so as to have a slight precedence in opening, over fuse link (5) which is indirectly assisted in opening by spring (4), through an insulating coupling block (6), to achieve an initially sequential but substantially overlapping and simultaneous opening, thus disconnecting both sides of the line.

Abstract: A protective device for an externally powered electrical apparatus is disclosed. The protective device is designed to prevent accidental electrocution of the user if the power supply is subject to a short circuit. The protective device is an electromechanical device incorporating a device for detecting current differentials between non-ground electrical conductors, a normally closed switch in association with a fuse device, and a trigger circuit for terminating the current flow through the electrical apparatus when a short circuit is detected.

Abstract: What is described is an electrical safety socket arrangement, comprising live and neutral socket inlets, switches connecting the live and neutral socket inlets to respective live and neutral socket outlets, and a circuit control which operates the switches when live and neutral supply feeders are connected to the socket inlets and a circuit is completed such that the circuit is completed by the connection of an appliance across the live and neutral socket outlets.

Abstract: A transformer ground fault protector is provided and includes sensing means in the form of at least one conductive plate adjacent but spaced from the output of each secondary winding in a transformer having its secondary windings connected in series and at the midpoint of the series circuit to ground. The at least one conductive plate forms a capacitive circuit with the windings and provides an output voltage when the secondary windings are energized and unbalanced. The output voltage generated by the at least one conductive plate in normal operation of the transformer, is substantially equal to zero volts. However, when a fault occurs in the transformer, the output voltage induced in the at least one conductive plate has a magnitude greater than zero. The resultant voltage in turn is conveyed to a comparator which compares the magnitude of the resultant voltage with a predetermined threshold voltage.

Abstract: A low-frequency power amplifier with an integrated BTL output circuit drives a load. The amplifier incorporates a bias circuit for supplying a bias voltage to each circuit composing the amplifier, a control circuit for switching between the normal operation mode and the mode for detecting the load being in an open-circuit condition, a detector circuit for detecting the load being in an open-circuit mode by supplying current to the load and sensing the current flowing through the load, and an output circuit for telling an external circuit of the integrated circuit that the load is in an open-circuit mode. All these circuits are squeezed into a single chip. The control circuit controls the bias circuit and detector circuit. During normal operation, the BTL output circuit is in an operative state, while the detector circuit is in a nonoperative state.

Abstract: A circuit interrupter is disclosed which provides for tripping of a line lead on occurrence of a loss of neutral or an earth leakage fault without the need for an earth connection. The circuit interrupter includes a circuit breaker which is activated by discharge of a capacitor in a closed loop, the capacitor being connected across the output of a D.C. power supply. A loss of neutral fault is detected as the potential of a neutral lead rises through a pull-up resistor and tripping will occur before the neutral rises to a potential at which the power supply becomes inoperative.

Abstract: A device and method for interrupting current flow to an electrically-powered apparatus is disclosed. The device includes a means for detecting a selected value of a physical parameter associated with the operation of the externally powered electrical apparatus, a trigger circuit incorporating an integrated circuit, a firing device which responds to the trigger circuit by generating a conductive plasma, an electronic measurement circuit to prevent unwanted firing of the firing device and thin conductors which are destroyed by the firing device when the conductive plasma forms an electric arc between them. Current flow to the electrically-powered apparatus is thereby terminated.

Abstract: A new and novel system for protecting people and property against electrical shock. It includes a number of preferred and other embodiments which have this as their goal, but which represent a number of distinctive and novel approaches to solving prior art problems. By way of example only, and without limiting the scope of this invention, these approaches include novel immersion detecting circuits, broken wire test circuits, electromechanical circuit breaking means including coil/plunger arrangements, and relay circuit breaking mechanisms cooperative with associated circuitry, all of which are able to be incorporated as a system wholly within the load (appliance) and its associated cord set (including a "plug").

Abstract: A sensing conductor and a detection circuit cause a low resistance shorting of an appliance circuit in response to immersion in water or other electrically conductive liquid, and an interrupter device, preferably located in the plug of the cord set, reacts to overcurrent, by opening both sides of the line. The sensing conductor must be strategically placed, within the appliance housing, to pass in proximity to liquid access points and the current carrying parts of the appliance. The detection circuit may be within the appliance or in the plug, with the sensing conductor extended to the appliance as a third conductor in the line cord. When a small current passes between the sensing conductor and the ungrounded or grounded neutral sides of the line or earth ground, current is shunted through the detection circuit from the ungrounded to the grounded neutral conductor, and away from the appliance circuit.

Abstract: A leakage current interrupter device for use with appliances. The device of the present invention includes a switching device having an electromagnet which is energized by a sensing circuit upon the detection of leakage current or a ground fault condition. The magnetic field generated by the electromagnet moves a lever arm to instantaneously interrupt the flow of current through the circuit to disconnect the appliance from the power supply.

Abstract: An arcing fault detector is set up to identify arcing faults and to distinguish them from other disturbances in a circuit. A plurality of electrical conditions are monitored and the conditions are tested against predetermined parameters for establishing a series of logical conditions relating to the presence of an arcing fault. A logic circuit combines the logical conditions for deciding on the presence of an arcing fault. An output signal of the logic circuit is used to activate a relay for interrupting the supply of electrical power to the circuit.

Abstract: A protective device for portable apparatus supplied from a high-voltage line is disclosed, which includes a connecting line from an electrical supply. The device also includes a protective switch with contact sets located in the supply lines and pre-stressed contact springs which are maintained in a closed position by a hoop-like fusible wire. An electronic firing circuit is disposed on a flat plate, together with the protective switch and the connections to the supply line and to the apparatus. The two-pole outlet of the firing circuit terminates in contact lugs with which the ends of the fusible wire can be connected mechanically and electrically. The circuit plate is, preferably, seated in the housing of a right angle plug, the contact pins of which are cast or pressed into the bottom of the housing and support the circuit plate while, at the same time, their heads constitute the opposite contact for the contact springs of the protective switch.

Abstract: A ground-safety controller circuit containing a thyristor that senses grounding of electrical equipment and enables interruption of power to the equipment.

Abstract: A circuit for controlling the orientation of magnetic fields is provided. The circuit has particular application to a flux shunt trip device in a circuit breaker. The flux shunt trip device has a spring-loaded plunger which when released to its extended position causes the circuit breaker to trip. Under normal operating conditions, the plunger is maintained in a retracted position by the force of a permanent magnet. A trip coil is mounted adjacent to the plunger. When the trip coil is energized a magnetic flux is generated opposing the magnetic flux of the permanent magnet. When the opposing flux neutralizes the flux of the permanent magnet the spring-loaded plunger is released to its trip position. A control circuit is provided to aid in maintaining the plunger in its retracted position under high current conditions close to, but below, the trip value.

Abstract: The voltage (U.sub.DS) on a power MOSFET (1) is compared with a voltage (U.sub.V) derived from the sum of the voltages of a Zener diode (3) and the threshold voltage (U.sub.T) of a second MOSFET (5) to detect a short circuit in a load (2) in series with the power MOSFET (1). When this total voltage is exceeded, the second MOSFET conducts. Its load current is then evaluated as the short circuit signal.

Abstract: An improved mains outlet and method of operating the same that automatically and safely distinguishes between the conditions of human or animal contact with the outlet terminals and contact with appliances such as light bulbs and consumer products, to prevent any substantial voltage or power from being drawn in the former case and automatically applies substantially full mains voltage when the appliance is connected to the outlet, including appliances presenting inductive loads. Use with ground fault detector systems is also feasible.

Abstract: A protection device for an electric appliance comprising a differential transformer comprising a magnetic core disposed around a pair of wires leading to the appliance from the source and a sensing wire thereon; a solenoid driven switch for cutting off the source in response to a leakage current in the wires; an SCR which is normally non-conducting for connecting the solenoid to the source in the conducting state so as to cause the switch to be in a cut-off state; a voltage regulator for supplying suitable regulated voltage to an amplifier and a latched-trigger circuit; an amplifier connected to the sensing wires for providing a signal to the latched-trigger circuit; and a latched-trigger circuit for taking the signal from the amplifier and in response thereto provide a trigger signal to cause the SCR to be in a conducting state so as to operate the solenoid and cut-off switch.

Abstract: An electrical safety device for use in power tools that is designed to automatically discontinue operation of the power tool upon physical contact of the tool with a concealed conductive material. A step down transformer is used to supply the operating power for a disconnect relay and a reset relay. When physical contact is made between the power tool and the conductive material, an electrical circuit through the disconnect relay is completed and the operation of the power tool is automatically interrupted. Once the contact between the tool and conductive material is broken, the power tool can be quickly and easily reactivated by a reset push button activating the reset relay. A remote reset is provided for convenience and efficiency of operation.

Abstract: A leakage current preventing means for a drier includes a power circuit connected to an AC source, a touch control circuit connected to the power circuit and having a touch control IC and a touch control terminal capable of actuating the touch control IC, an output circuit connected to the touch control circuit and having a control medium which can be triggered into conduction by a pulse output from the IC and controls two junctions of the fire and ground wires from the AC source for being power supplied, and a leakage current detecting circuit connected to the touch control circuit and including a sensor for detecting an impedance change in the drier and an actuating medium capable of being energized into conduction in response to the impedance change to enable that the IC has no voltage output which results in non-conduction of the control medium which in turn results in no possible leakage current between the two junctions.

Abstract: In an electrostatic paint spraying installation or the like, a high-tension voltage generator comprises a voltage rectifier-multiplier in a mobile subsystem and a grounding connection in a fixed subsystem. A conductive wire connects an input terminal of the voltage rectifier-multiplier to the grounding connection. When the high-tension voltage generator operates, a high-frequency alternating current residual signal injected into this conductive wire is sensed and measured. The measured value is compared against a reference value that may be variable. The high-tension voltage generator is disabled if the difference between the measured value and the reference value reaches or crosses a predetermined threshold.

Abstract: Apparatus for protecting electrically operated devices from damage as a result of penetration of moisture has an electric switch or a normally intact conductor, and an explosive or ignitable actuator which initiates an opening or closing of the switch or at least partial destruction of the conductor when operated by one or more sensors which detect the presence of moisture in or close to the electrically operated device. The explosive actuator can be used to clear the path for movement of a movable switch contact from open to closed position or vice versa, or to break up a conductor when ignited or otherwise operated in response to a signal from the moisture sensor or sensors. A combustible actuator can employ a flash light whose envelope confines a portion of a conductor and further contains a combustible material serving to generate heat on ignition in response to a signal from the sensor or sensors whereby the heat destroys the integrity of the portion of the conductor in the envelope.

Abstract: An improved mains outlet and method of operating the same that automatically and safely distinguishes between the conditions of human or animal contact with the outlet terminals and contact with appliances such as light bulbs and consumer products, to prevent any substantial voltage or power from being drawn in the former case and automatically applies substantially full mains voltage when the appliance is connected to the outlet.

Abstract: A ground fault detector employs a current feedback loop which makes use of a differential current sensing configuration to in essence increase the gain of the current feedback signal, thereby lowering the detector trip level when ground fault currents are present. In a preferred embodiment, ground faults are detected in a circuit having an AC-to-AC converter and a useful load by positioning a differential sensing configuration in the DC link of the AC-to-AC converter.

Abstract: Positive assurance of correct connection, in supplying electrical utilization equipment from a current source, is provided when it is critical to safety and particularly when receptacle outlets are used or a fixed wiring installation required which may be subject to wiring error. An electromagnetic relay is employed to prevent equipment use and to provide audible indication in the event of hazardous connective error such as reversal of ungrounded and grounded neutral conductors and the omission of a grounding connection. The relay may be one that is also otherwise controlled for on-off power switching.

Abstract: A safety device for disconnecting an electrical appliance from a power supply, and thereby preventing electrical shock hazards. A normally closed switch connects the appliance to the power supply. A frangible device, such as a flash bulb, maintains the switch in the closed position. A sensor detects a dangerous condition and actuates the flash bulb, which action fragments the envelope of the bulb, whereupon the switch assumes the open position, thereby disconnecting the appliance from the power supply.

Abstract: An adapter unit comprising a moisture resistant housing within which is carried an improved, self testing ground line fault interrupter device. The improved device is electrically interconnected with a connector carried externally of the adapter housing so that the unit can be plugged directly into a standard duplex outlet of an existing circuit. The apparatus includes circuitry that automatically tests the operability of the device when it is plugged into a duplex outlet without the need for manual manipulation of test buttons or other overt action by the user.

Abstract: A power MOS switching device is provided wherein certain protective circuits in a control portion thereof remain functional when a ground connection to said control portion is lost.

Abstract: To detect if a tool bit, such as a drill, a saw blade, or the like, upon being worked into a wall, meets a metallic obstruction, such as a conduit, reinforcement rod, hydraulic pipe or the like, an alternating voltage, preferably between 2 and 20 kHz, is coupled through a coupling capacitor (9) to the tool bit, and current flow to the tool bit is detected by a current measurement stage (10; 31, 32) to disconnect energy to the drive motor (5), for example by disabling firing of a thyristor (15), or closing a valve of a compressed-air tool. The coupling capacitor should have a capacity small enough to provide high impedance to network power frequency, and to insure a substantial change in current being supplied to the tool bit holder, typically a chuck, if the tool bit meets a metallic object in the wall.