Abstract: A subsurface impact protection system for protecting an underground structure is provided. The protection system includes a subsurface polymer layer above the underground structure which is configured to absorb above-ground impact force from reaching the underground structure. A sensor network is embedded in the polymer layer. The sensor network includes a plurality of sensors configured to monitor the polymer layer by generating a corresponding plurality of sensor signals of conditions near the polymer layer and transmitting the generated sensor signals to one or more external receivers. In some embodiments, the polymer layer includes a protective mesh made up of a plurality of high density polyethylene (HDPE) strands in a woven pattern. In one such embodiment, the sensors occupy gaps in the protective mesh. A method of protecting an underground structure is also provided.

Abstract: Apparatuses, methods, systems, and program products are disclosed for selectively accessing content from a content pool. A method includes receiving content for a content-pool from a content contributor. A method includes determining one or more access settings for the content, the one or more access settings determining how the content is displayed. A method includes providing the content to a presenter according to the one or more access settings.

Abstract: A method for monitoring a location performed by one or more processors comprises receiving signals from a smoke sensor; determining one or more minutiae from the received signals; determining a time window based on the at least one determined one or more minutiae; characterizing one or more smoke or fire types in the determined time window based on one or more of the determined one or more minutiae; dynamically determining one or more alarm levels based on the characterized one or more smoke or fire types; evaluating at least one minutiae in the determined time window using the determined one or more alarm levels; and outputting an alarm signal if an alarm condition is determined.

Abstract: A method detects an error in a meter having a current transformer, a burden resistor unit and a resistive path switchably connectable across the burden resistor unit. The method includes obtaining in the processing circuit a first value representative of the voltage magnitude across the burden resistor unit while the resistive path is operably decoupled across the burden resistor unit. The method also includes closing a switching element to operably couple the resistive path across the burden resistor unit, and obtaining in a processing circuit a second value representative of a voltage magnitude across the burden resistor unit. The method further includes determining in the processing circuit whether an error exists based on the first value, the second value, and at least one predetermined stored value.

Abstract: An integrated security arrangement is implemented in a variety of embodiments. In one such embodiment an integrated security arrangement includes a plurality of intrusion sensors that sense an intrusion in a target area, a motion sensor, and an image-capture device that is responsive to the motion sensor. When the image-capture device is enabled it captures images of the target area in response to movement in the target area as indicated by the motion sensor. The integrated security arrangement also includes a mapping database that contains data used to associate one or more of the plurality of intrusion sensors with the image-capture device. A user configures the mapping database to select at least one of the intrusion sensors for enabling the image-capture device. The integrated security arrangement further includes a control unit that enables the image-capture device in response to the selected intrusion sensor as configured in the mapping database.

Abstract: Inventory systems may include one or more sensors capable of detecting spatial positioning of inventory holders and associated inventory items. Data can be received from a sensor, a boundary corresponding to a face of the inventory holder can be determined from the data, and items projecting across the boundary can be determined as items protruding from the inventory holder.

Abstract: A privacy server protects private information by substituting a token or an identifier for the private information. The privacy server recognizes that a communication includes private information and intercepts the communication. The privacy server replaces the private information with a random or pseudo-random token or identifier. The privacy server maintains the private information in a local database and associates the private information for a particular person with the token or identifier for that person.

Abstract: A sensor device detects an object, in particular for optically detecting smoke particles. The sensor device contains a transmitting device for emitting transmit radiation, a receiving device for receiving receive radiation having scattering radiation that is generated by an at least partial scattering of the transmit radiation by the object, and for outputting a measurement signal indicative of the receive radiation, a signal modification device for modifying the measurement signal and for outputting a modified measurement signal, a level of the modified measurement signal increasing after the transmitting device has been switched on, and a calibration device for monitoring the modified measurement signal. The calibration device is embodied such that a reaching of a predefined signal level for the modified measurement signal can be detected and that a time interval between the switching on of the transmitting device and the reaching of the predefined signal level can be determined.

Abstract: There is provided an optical scanning type photoelectric switch capable of preventing interference with another photoelectric switch by use of its own capability, wherein, as for light projection pulse periods of the first and second optical scanning type photoelectric switches, the period is set to 30 ?s in the first optical scanning type photoelectric switch while the period is set to 33 ?s in the second optical scanning type photoelectric switch 1B, the light projection pulses have the same pulse width, and by setting the light projection periods different between the first and second optical scanning type photoelectric switches, even if mutual interference occurs between any optical axes, a phase difference of 36 degrees in rotation period is generated therebetween in a next scan, thereby preventing occurrence of the interference in succession in a plurality of times of scanning.

Abstract: A sensor has a transmitter module for transferring data via a line, the sensor receiving power via the line. At a point in time in which the sensor receives a first power level, it transmits the data for a first time interval. A second sensor which is connected to the line in parallel to the first sensor then transmits its data for a second time interval after the first time interval. A timing sequence control system in the two sensors which is triggered by the point in time of reception of the power ensures the subsequent transmission by the first and second sensor.

Abstract: A normality determination unit determines that a communication function for an outdoor unit of its own is normal when receiving a signal from one of indoor units, and writes a normality flag into an EEPROM. Further, the normality determination unit determines that it is not asserted that the communication function for the outdoor unit of its own is normal when not receiving a signal from any of the indoor units for a predetermined time period, and deletes the normality flag in the EEPROM.

Abstract: A method of and a system for detecting a person occupying a vehicle seat or a corresponding underlay whereby the underlay is provided with a single dynamic sensor and whereby the measurement signal is compared with a reference signal in comparing means, wherein the system comprises first measuring means for measuring a first measurement signal in a first frequency band representing a signal for human presence, and second measuring means for measuring the reference signal as a second measurement signal in a second frequency band representing a signal for background noise, comparing means for indicating an occupancy, wherein the signal strength in the first measurement signal in the first frequency band of human presence signal domination is compared to the signal strength in the reference signal in the second frequency band where essentially only background noise exists.

Abstract: A method, system and circuit arrangement exchange data and supply power to at least one current sink connected via at least one data line pair to a fire alarm control unit. A pulsed voltage is used as the supply voltage for the current sink connected via at least one data line pair. Useful data is transmitted to the fire alarm control unit between two voltage pulses. The voltage required to transmit the useful data to the fire alarm control unit is used by a capacitor of a circuit arrangement in the current sink charged beforehand by the supply voltage.

Abstract: A safety device includes a safety module (11), a data bus (12) and one or several transmitters (15, 16, 17, 18, 19, 20). The safety module transmits a pulse train (one dynamic signal) over the bus (12) constituting a loop, whereby the pulse train is connected to each of the transmitters, wherein the pulse is phase inverted.

Abstract: In a method and a circuit for timing pulse generation, a frame pulse of a corresponding system is masked when an alarm signal of either a working system or a protection system is received, a monitoring window which indicates an absorbable range of delay time difference between the frame pulses is generated upon an arrival of the frame pulse of the system selected by a switching signal, when the alarm signal of a system not selected is generated upon selection of the switching signal. Alternatively, a request signal for regenerating the monitoring window which indicates an absorbable range of delay time difference between the frame pulses is provided upon an arrival of the frame pulse of the system selected by the switching signal, when a slip signal is generated upon selection of the switching signal.

Abstract: A button structure is disposed on a housing and the housing has at least one opening. The button structure includes at least one button portion and a supporting frame. The button portion has a first portion and a second portion. The first portion is fixed on the housing. The second portion covers the opening. The supporting frame is set in the opening. The supporting frame is connected with the housing and faces the second portion. An electronic apparatus, which includes a housing and a button structure, is also provided.

Abstract: A method of accommodating code collisions from multiple SAW identification tag coded response pulses. In one embodiment, the method includes: (1) separating probable candidates by time-domain; (2) classifying the probable candidates by code-division separation; (3) employing known coded identification signals to adjust the probable candidates; and (4) correlating the multiple SAW identification tag coded response pulses to identify the probable candidates for further processing.

Abstract: Gradually lighting/dimming wireless transmission indicator lamp used in a wireless data transmission apparatus. The wireless transmission indicator lamp includes at least one light emitting diode, a wireless transceiver circuit for receiving or transmitting radio signal and converting the received radio signal into digital pulse signal and a controlling circuit having an input terminal and at least one output terminal. The input terminal is connected with the wireless transceiver circuit. The output terminal is connected with the light emitting diode. The controlling circuit is controllable by the digital pulse signal to generate a slowly varying current for the light emitting diode, whereby the light emitting diode can gradually light and dim as human breath to indicate the connection state and transmission state of the wireless data transmission apparatus.

Abstract: The present invention relates to a safety device comprising a safety module (11), a data bus (12) and on a or several transmitters (15, 16, 17, 18, 19, 20). The safety module transmits a pulse train (one dynamic signal) over said bus (12) constituting a loop, whereby the pulse train is connected to each of said transmitters, wherein the pulse is phase inverted.

Abstract: A system or method for an access code issuer to receive an on-line application including certain personal information from a user of a computer network such as the Internet, to independently operatively connect to a database and obtain or verify demographic and additional personal information regarding the user, and issue an access code to the user. The user enters this access code when accessing various nodes or websites of a plurality of affiliated content providers. The content providers obtain or verify the user's demographics by operatively connecting to the access code issuer, thereby obtaining or verifying the demographics of the visitor to its site without requiring the visitor to enter his or her demographic information or to independently provide proof thereof to the content provider.

Abstract: A method and apparatus for synchronizing a series of interconnected alarm devices. The alarm device detecting an alarm condition produces a signal having a distinct rising edge which is detected at each of the remaining alarm devices. A pulsed alarm signal having a fixed number of pulses is generated at each remaining device in response to the rising edge. A reset signal from the alarm device detecting the alarm condition resets each of the remaining alarm devices after each fixed number of pulses is produced so that each of said devices has a pulsed alarm signal which begins and ends at substantially the same time.

Abstract: A communication protocol facilitates transmission at maximum rates for various types of cables. The protocol can be used to transmit information between a common control unit and a plurality of spaced apart devices coupled thereto. Alternately, the protocol can be used for transmission of information between devices. One control unit or device transmits clock pulses, while in a low impedance state. The devices respond to the clock pulses and transmit data on the communications link, in a low impedance state, when the source of clock pulses assumes a high impedance state. One or more features of transmitted signals can be used to implement one or more synchronized functions.

Abstract: A communication protocol facilitates transmission at maximum rates for various types of cables. Signal to noise characteristics are improved by using voltage waveforms. The protocol can be used to transmit information between a common control unit and a plurality of spaced apart devices coupled thereto. Alternately, the protocol can be used for transmission of information between devices. The common control unit transmits clock pulses, while in a low impedance state. The devices respond to the clock pulses and transmit data on the communications link, in a low impedance state, when the common control unit has assumed a high impedance state. Clock signals and data signals are separated to improve signal to noise characteristics by transmitting same with opposite polarities. A clock detection circuit responds to the polarity of the clock pulses. A data detection circuit responds to the polarity of the data pulses.

Abstract: A communication protocol facilitates transmission at maximum rates for various types of cables. Signal to noise characteristics are improved by using voltage waveforms. The protocol can be used to transmit information between a common control unit and a plurality of spaced apart devices coupled thereto. Alternately, the protocol can be used for transmission of information between devices. The common control unit transmits clock pulses, while in a low impedance state. The devices respond to the clock pulses and transmit data on the communications link, in a low impedance state, when the common control unit has assumed a high impedance state. Clock signals and data signals are separated to improve signal to noise characteristics by transmitting same with opposite polarities. A clock detection circuit responds to the polarity of the clock pulses. A data detection circuit responds to the polarity of the data pulses.

Abstract: A signalling device applies a pulsating signal to supply lines when a head is removed from a base. A monitoring system includes a plurality of such signalling devices and is responsive to the pulsating signal to indicate the absence of a head from its base. When a pulsating signal is already on the supply lines, its waveform is preserved independently of the number of heads removed from respective bases. A timer is triggered either to ensure synchronisation of pulsating signals generated by two or more bases, or to prevent any second base from generating a pulsating signal, if a first base is already generating one. The timer can be an oscillator where a capacitor is prematurely discharged by a pulsating signal already present on the supply lines. The base can include additional means for inhibiting the pulsating signal when a voltage applied to the signalling device either exceeds a predetermined threshold, or changes in polarity, or both.

Abstract: An apparatus and concomitant method of reducing the number of synchronization pulses transmitted to the alarm units for increasing the reliability of the overall alarm system is disclosed. The synchronization signal is implemented as a reference or reset signal from which the alarm units derive a reference time to begin activation of the alarm units. Thus, when an alarm unit receives a reference synchronization pulse, the alarm unit applies the reference synchronization pulse as a reference point in time to trigger a series of flashes or audio tones.

Abstract: An apparatus and concomitant method of reducing the number of synchronization pulses transmitted to the alarm units for increasing the reliability of the overall alarm system is disclosed. The synchronization signal is implemented as a reference or reset signal from which the alarm units derive a reference time to begin activation of the alarm units. Thus, when an alarm unit receives a reference synchronization pulse, the alarm unit applies the reference synchronization pulse as a reference point in time to trigger a series of flashes or audio tones.

Abstract: The present invention relates to a detection device comprising means (3) able to measure the value of a parameter, said device (1) being able to deliver values lying within a first measurement range which is limited at least by a first limit value. According to the invention, this device (1) furthermore comprises means (5) for modifying before delivery thereof any measured value so as to deliver a corresponding value which is defined in a second measurement range, said second measurement range being limited by at least a first limitation value which is different from said first limit value and which forms part of said first measurement range.

Abstract: A self-monitoring power supply can be incorporated into electrical devices which are part of a multi-device communication system. The power supply is energized by electrical energy received from a medium on an intermittent basis. The self-monitoring circuitry assures that the device has sufficient energy for carrying out its functions. Noise rejection and noise immunity can be enhanced in such devices by circuitry which upon detection of a transmission start signal or a clock pulse locks out data signals for a predetermined period of time and upon detection of a data signal locks out transmission start or clock pulses for a predetermined period of time.

Abstract: In a dangerous condition sensing alarm system which includes a plurality of interconnected dangerous condition sensing units, such as smoke detectors, of the general type in which each unit responds to an apparently sensed dangerous condition by issuing an audible alarm and also by transmitting, from an input/output terminal, a signal to the other units (received as an external alarm signal at corresponding input/output terminals of the other units) so that an alarm condition sensed by any one of the units results in alarms being sounded by all the units, a unit initiating an alarm (which may be false) is marked by an alarm origination circuit. The alarm origination circuit includes a visual indicator (such as an LED) and a latch device (such as an SCR) connected in-circuit such that, when the SCR is turned "on", the LED is activated and remains lit until the SCR is turned "off".

Abstract: In a dangerous condition sensing alarm system which includes a plurality of interconnected dangerous condition sensing units, such as smoke detectors, of the general type in which each unit responds to an apparently sensed dangerous condition by issuing an audible alarm and also by transmitting, from an input/output terminal, a signal to the other units (received as an external alarm signal at corresponding input/output terminals of the other units) so that an alarm condition sensed by any one of the units results in alarms being sounded by all the units, a unit initiating an alarm (which may be false) is marked by an alarm origination circuit. The alarm origination circuit includes a visual indicator (such as an LED) and a latch device (such as an SCR) connected in-circuit such that, when the SCR is turned "on", the LED is activated and remains lit until the SCR is turned "off".

Abstract: A pulse detection circuit to be used in combination with a generator of a train of synchronizing pulses. The detection circuitry remains in a first, inactive state in the presence of an ongoing sequence of synchronization pulses. In the event of a detected absence of synchronizing pulses for a predetermined period of time, the detection circuitry changes state and generates an electrical output signal indicative of one or more missing synchronizing pulses. The detection circuitry can incorporate either digital or analog timing circuitry.

Abstract: An intrusion detector for security systems includes a stationary host unit and a transponder unit secured to a movable door, window or the like. The host unit is secured to a stationary frame in alignment with the transponder unit. A pair of electromagnetic coupling elements are supported upon the host unit and are aligned with a corresponding mirror image pair of electromagnetic coupling units supported on the transponder unit. The host unit provides a burst of power signal which is magnetically coupled to the transponder unit and rectified therein to provide operative power for the transponder unit. The host unit further provides a data signal magnetically coupled to the transponder unit which communicates a random number to the transponder unit. The transponder unit calculates a pseudo-random number using a predetermined algorithm and communicates the random number back to the host unit. The host unit performs an identical calculation upon the random number to provide a reference pseudo-random number.

Abstract: A temperature sensing system for over-heat detection includes a temperature sensing cable, interface unit and termination unit. The interface unit is configured to issue interrogation, communication and power signals to the termination unit through the temperature sensing cable. During normal operation issuance of the interrogation signal results in no reflection pulses, as the signal is dissipated through the termination resistance in the termination unit. When an over-heat, short circuit or open circuit condition exists, a reflected pulse is detected by the interface unit and this information is used to determine the type and location of the condition. The interface unit is also configured to supply power to the termination unit which in turn is stored in the termination unit to power its operations, including setting of the resistor values. The termination unit is used as an instrument of calibration in the subject system.

Abstract: Self-contained smoke detector systems each have internal self-diagnostic capabilities and function as components of an automatic fire alarm communication system implemented with a conventional two-wire alarm initiating circuit. Each system includes a microprocessor-based self-diagnostic circuit that periodically checks sensitivity of radiation sensor electronics to smoke obscuration level. By setting tolerance limits on the amount of change in voltage measured in clean air, the system can provide an indication of when it has become either under-sensitive or over-sensitive to the ambient smoke obscuration level. An algorithm implemented in software stored in system memory determines whether and provides a sensitivity fault condition signal indicating that for a time (such as 27 hours) the clean air voltage has strayed outside established sensitivity tolerance limits.

Abstract: In a system where the same alarm is held and displayed in different apparatuses, alarm information held in either an alarm detection apparatus or an alarm processing apparatus for a predetermined time is subjected to synchronization with respect to transmission (holding), the alarm information being displayed in a display/transfer part.

Abstract: A switch module for connection between a power source and a controlled load, and a method for its operation, the switch module being testable without regard to the nature of the load, the nature of the power source, or whether the switch is actually connected to the power supply and its controlled load. The switch module has two pairs of MOS FET solid state switches, each pair connected back-to-back to allow switching of alternating current (ac) as well as direct current (dc) in either direction. Each pair of switches in the module has an associated independent power source, which is used in the preferred embodiment of the invention to supply a control signal to each switch and to supply a test current through the switch when it is closed. Test pulses invert the state of each switch momentarily and corresponding test current pulses are observed if a switch is operating properly.

Abstract: A communication system includes a control element, a bidirectional communications link, and one or more devices coupled to the link. The control element, via driver circuits, reversibly applies a predetermined potential to the link for the purpose of energizing the one or more devices as well as communicating therewith. The devices can respond at appropriate time intervals. During the responding time intervals, the control element can continue to supply power to the devices.

Abstract: A danger alarm system includes a plurality of detectors, each of which having a microprocessor, a current drain controllable by the microprocessor for data exchange with a central station, an address register, and a nonvolatile memory for containing an individual binary serial number.

Abstract: An alarm system suitable for use in a telecommunications network in which a central switching system is capable of selectively providing a signal of a predetermined polarity to a plurality of remotely located stations without being dependent upon or impeding the ability of the central switching system to process relatively high volume communications traffic. Each of the remote stations is connected to the telecommunications network by an electrical conduction path. The stations are substantially unresponsive to the predetermined polarity signal. In response to an alarm indication signal, the system effects a reversal in the polarity of the signal provided to a selected subset of the remote stations. Each station in the selected subset responds to the polarity reversal by indicating an alarm condition.

Abstract: An output metering pulse signal is developed by measuring impedances at input terminals which are connected through lead wires to a metering unit shifted between low impedance and high impedance states while output malfunction signals are developed in response to open and shorted conditions of the lead wires, each open or shorted condition malfunction signal being developed by sensing the magnitude of the voltage at an input terminal during periodic brief applications of a strobe voltage to the input terminal through an impedance. Such output metering pulse and malfunction signals are developed in each of a plurality of automatic meter readers and are stored in the automatic meter for subsequent transmission over telephone lines to a utility control center.

Abstract: A signal control system for transmission circuits which includes two ring counter type transmission circuits respectively formed of a plurality of transmitter-receivers located opposite to each other, a control circuit, and a couple of signal lines respectively formed of two alternate signal lines, a control signal line and a transmission line. The first pair formed of the first transmitter-receivers of the respective two transmission circuits are simultaneously operated, and subsequent pairs of the transmitter-receivers of the transmission circuits are synchronously operated in turn by a shift pulse. The system can transmit signals and control the transmission circuits without complicated circuits such as an address circuit and a verification circuit so that the construction is simplified and its power consumption is largely reduced.

Abstract: An independent transmitter/receiver, or a transponder in a communication system, receives signals distorted by outside, unwanted conditions such as noise, line capacitance, and so forth. The status of a switch is checked at periodic times, by pulsing, rather than by continuous current flow, to conserve power. The invention includes a debounce circuit for monitoring the switch status; the debounce circuit is adjusted by a debounce select signal received from outside the transmitter/receiver. In addition a sensing circuit is coupled between the switch being monitored and the debounce circuit. The sensing circuit also has its sensing operation adjusted as a function of the debounce select signal. This allows for accurate monitoring of the switch condition, notwithstanding noise, line capacitance and other factors.

Abstract: A self-diagnostic circuit for use in alarm systems and the like, of the kind having a plurality of alarm detection switches connected in a loop, indicators associated with each of the switches for indicating locally the detection of an alarm condition and a central controller for reversing voltage polarity in the loop whenever an alarm condition is detected by one of the switches. The self-diagnostic circuit comprises: a first switch for detecing an alarm condition connected in an alarm loop; a second switch automatically activatable in response to operation of the first switch; an alarm indicator automatically operable in response to activation of the second switch; and, an automatically operable circuit for preventing deactivation of the second switch until after diagnosis of alarm system operation, even when the first switch no longer detects the alarm condition. Any such first switch exhibiting erratic switching behavior can be easily identified by continued operation of the associated alarm indicator.

Abstract: An individual source identification of several signal providers or pick-ups with closable contacts disposed in a twin-wire signal line, in particular for fire detectors in fire alarm systems. The signal pick-up system allows to distinguish actuation of individual sensors separately and apart from a short circuit formation. The fire alarm system also allows connection to automatic sprinkler devices. The fire sensor system is easy to install in that the individual settings of the individual detectors at a construction site are minimized.

Abstract: A detector system having multiple sensors, each sensor detecting a different danger condition, has an address comparator for determining whether an access address coincident with a selected address is present in an access signal received from a central monitoring unit. The detector exchanges an information signal with the central monitoring unit using the coincidence detection signal of the address comparator. Success coincidence detection signals, corresponding to successively selected addresses being present in the access signal, are counted by a counter. If the detector has two sensors, the counter may be a D flip-flop circuit, and if the detector has more than two sensors, the counter may be a ring counter. The counter generates a signal for operating respective analog switches connected to the outputs of each sensor so as to successively through-connect the output of one sensor to the output of the detector system for transmittal to the central monitoring unit.

Abstract: This invention relates to a system for monitoring faults in various functions of rail road cars from the lead car or from the locomotive of a train. This system scans the train once for one identical function of every car, and the scanning is repeated for subsequent identical functions. During scanning if this system detects fault in any car or cars, it illuminates corresponding lights on the display panel in the cab, indicating the types of faults and the car numbers. This system is capable of scanning a maximum of fifteen cars for nine different functions of each car.

Abstract: An electrical apparatus for monitoring electrical continuity includes a number of input circuits each associated with a respective arm of a multi-phase rectifier circuit the continuity of which is to be monitored. Each input circuit includes a comparator which generates a train of pulses P while an input current persists in the arm. If a discontinuity arises in an arm for a first preset interval of time a monostable in that arm generates a first trigger signal. If the discontinuity persists for a further preset interval of time another monostable, common to all the arms, triggers an alarm via a bistable circuit.

Abstract: A fault tolerant control system employing input sensing network and in association therewith a signal processing system which eliminates the effect of radiofrequency interference, electromagnetic interference, transients, and random false alarm signals. The sensing network comprises a linear circuit or a resistor disposed in parallel with one or more semiconductors which are preferably diodes or the like. The network senses current flow so as to provide a signal having a maximum voltage established by the semiconductors with the linear circuit or network providing a linear change in voltage with current until the voltage of the semiconductor is reached when the voltage then becomes non-linear.

Abstract: A diagnostic status indicating apparatus is used with an alarm system having a plurality of normally-closed switch contacts which are opened upon occurrence of an alarm condition. Light-emitting diodes are connected across the normally-closed switch contacts at an opposite polarity to that of the current normally passing through the contacts in the operating mode. In order to diagnose the alarm-generating condition of the electrical contacts, a switch removes the operating voltage and connects a test voltage at a reverse polarity, whereupon the LED for any open circuited electrical contact is activated. The device is preferably based upon a double-pole-double-throw switch and includes indicators for a test condition and for a ground fault.