Abstract:
A safety system for a building has an earthquake detector circuit for detecting ground accelerations exceeding a threshold acceleration/frequency curve dividing safe and hazardous ground accelerations throughout a predetermined frequency range. The earthquake detector includes an accelerometer for measuring linear acceleration along an axis, and for generating an acceleration signal indicative of a magnitude of the measured acceleration. A filter circuit produces a filtered acceleration signal from the acceleration signal. The gain/frequency characteristic of the filter circuit is a reflection of the threshold acceleration/frequency curve, such that ground accelerations falling on the threshold acceleration/frequency curve yield a filtered acceleration signal having a substantially constant value throughout the frequency range. Finally, a threshold circuit compares the filtered acceleration signal to a predetermined threshold voltage, and produces an earthquake detected signal indicative of the comparison result. The safety system can also include a gas detector circuit capable of detecting a gas in air, and generating a gas detected signal in response to detection of the gas. A smoke detector circuit can also be provided for generating a smoke detected signal. A main unit including a microprocessor is responsive to the earthquake detected signal, the gas detected signal and the smoke detected signal, and generates a first control signal in response to any of the earthquake detected signal, the gas detected signal and the smoke detected signal. A valve controller closes a gas valve to shut off a supply of gas in response to the first control signal.

Description:
BACKGROUND TO THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to safety systems for buildings, and in particular to a building safety system which shuts off the supply of gas to a building in the event of any one or more of a gas leak, a fire or an earthquake, and is also useful for the detection from ships of undersea earthquakes. 
     2. Summary of the Prior Art 
     Recent major earthquakes have demonstrated that the damage sustained by buildings results from two primary sources, namely the ground accelerations induced by the earthquake itself, and subsequent explosions and fires caused by flammable gas leaking from damaged pipe lines. Many older or poorly-built buildings are destroyed by the ground accelerations. On the other hand, modern buildings, which are properly designed and built in accordance with accepted engineering practice for earthquake-prone regions, typically withstand moderate ground accelerations with comparatively little structural damage. In either type of building, gas pipelines within the building can be damaged or ruptured by the ground accelerations, creating an extreme explosion/fire hazard. This hazard is magnified by the likelihood that water-mains may be severed, hampering firefighting efforts. Furthermore, electrical power may also be interrupted, thereby preventing other safety systems from functioning properly, even if these safety systems have not been physically damaged by the earthquake. 
     According to the California earthquake standards, the degree of hazard posed by ground accelerations is dependent on both the magnitude and frequency of the acceleration. This is illustrated in FIG. 7, which shows the earthquake hazard posed by accelerations at frequencies between 0 and 10 Hz. Ground accelerations falling above the threshold acceleration/frequency curve in FIG. 7 are considered to pose an earthquake hazard and thus should trigger an alarm. Conversely, ground accelerations falling below the threshold acceleration/frequency curve in FIG. 7 are considered to be safe and thus should not trigger an alarm. For example, ground accelerations of 0.3 g (i.e. 0.3×9.81=2.94 m.sec −2 ) or greater at 2.5 Hz are considered to be an earthquake hazard. Accelerations of 0.08 g at 1.0 Hz and 2.5 Hz are considered safe, as are accelerations of 0.4 g at 10 Hz. Accordingly, an earthquake detector must be calibrated to be triggered by ground accelerations of greater than 0.3 g at 2.5 Hz. However, in order to avoid false-alarms, such as by passing vehicles, the detector must not be triggered by acceleration levels which are in the safe zone of the graph of FIG. 7, even if the detected acceleration magnitude is higher than 0.3 g. 
     U.S. Pat. No. 5,101,195 (Caillat et al.) discloses a motion detector for detecting an earthquake in order to automatically shut off gas supplies to a building in the event of an earthquake. The detector of Caillat et al. includes a highly damped cantilever beam arranged to generate an electrical signal as the end of the beam moves up and down. The signal generated by the moving beam is passed through a band-pass filter, which passes signal frequencies between 3 Hz and 14 Hz. The filtered signal is then passed to a sensor circuit, which produces an alarm signal. It will be noted that the detector of Caillat et al. attenuates low-frequency vibrations, and as such would be substantially incapable of detecting low-frequency earthquake accelerations i.e. at 2.5 Hz, which, as discussed above, are considered by the California earthquake standards to be hazardous. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a reliable building safety system for shutting off a supply of gas to a building in the event of an earthquake. 
     Another object of the present invention is to provide a building safety system capable of shutting off a supply of gas to a building in the event of a fire or gas leak. 
     Thus the present invention provides a safety system for a building. The safety system includes an earthquake detector circuit for detecting ground accelerations exceeding a threshold acceleration/frequency curve dividing safe and hazardous ground accelerations throughout a predetermined frequency range. The earthquake detector includes an accelerometer for measuring linear acceleration along an axis, and for generating an acceleration signal indicative of a magnitude of the measured acceleration. A filter circuit produces a filtered acceleration signal from the acceleration signal. The gain/frequency characteristic of the filter circuit is a reflection of the threshold acceleration/frequency curve, such that ground accelerations falling on the threshold acceleration/frequency curve yield a filtered acceleration signal having a substantially constant value throughout the frequency range. Finally, a threshold circuit compares the filtered acceleration signal to a predetermined threshold voltage, and produces an earthquake detected signal indicative of the comparison result. 
     In a preferred embodiment of the present invention, a main unit including a microprocessor is responsive to the earthquake detected signal, and generates a first control signal in response to the earthquake detected signal. A valve controller can suitably be provided to close a gas valve to shut off a supply of gas to the building in response to the first control signal. 
     An embodiment of the present invention also includes a gas detector circuit capable of detecting a gas in air, and generating a gas detected signal in response to detection of the gas. In this case, the main unit is also made responsive to the gas detected signal, so as to generate the first control signal in response to the gas detected signal. 
     Preferably, the gas detector circuit includes both a gas detect portion and a trouble detect portion, so that the gas detector circuit is capable of detecting faulty operation of the gas detector portion, as well as detecting gases in air. 
     An embodiment of the present invention also includes a smoke detector circuit for generating a fire detected signal. In this case, the main unit is also made responsive to the fire detected signal, so as to generate the first control signal in response to the fire detected signal. 
     In another embodiment of the present invention, the main unit includes a gas detector combine circuit, which combines the signals of two or more independent gas detector circuits and produces a single gas detector signal line. By this means, a plurality of gas detector circuits can be used, while retaining a single input line to the microprocessor. The gas detector combine circuit preferably includes respective gas detect and trouble detect portions for combining respective gas detected signals and trouble detected signals generated by the gas detectors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects features and advantages of the present invention will be more readily apparent from the following detailed description thereof given, by way of example, with reference to the appended drawings, in which: 
     FIG. 1 is a schematic diagram illustrating a safety system according to an embodiment of the present invention; 
     FIG. 2 is a schematic illustration of the main unit of FIG. 1; 
     FIG. 3 is a schematic illustration of the gas detector signal combine circuit of FIG. 2; 
     FIG. 4 is a schematic illustration of a gas detector circuit employed in the embodiment of FIG. 1; 
     FIG. 5 is a schematic illustration of an earthquake detector circuit employed in the embodiment of FIG. 1; 
     FIGS. 6 a-d  illustrate the operation of the earthquake detector of FIG. 5; 
     FIG. 7 is a graph showing the threshold acceleration/frequency curve between safe and hazardous ground accelerations; 
     FIG. 8 is a graph showing the gain/frequency characteristic of the band pass filter employed in the earthquake detector of FIG. 5; 
     FIG. 9 is a graph showing the filtered acceleration signal generated by the band pass filter of FIG. 5 in response to ground accelerations falling on the threshold acceleration/frequency curve of FIG. 7; 
     FIG. 10 shows a circuit diagram of a gas valve controller forming part of the main unit shown in FIG. 2 and a gas valve controlled thereby; and 
     FIG. 11 is a flow chart illustrating the operation of the present invention. 
     It will be noted that throughout the drawings, like elements are identified by like reference numerals. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a gas and earthquake detector in accordance with the present invention generally comprises a main unit  1 , an earthquake detector circuit  2 , a gas shutoff valve  3 , one or more gas detector circuits  4  and a smoke detector  5 . In the present embodiment, four gas detector circuits  4  are illustrated, although it will be appreciated that more or fewer such circuits may be used. 
     The main unit  1  can be located at any convenient location within a building, for example in a mechanical equipment room, or near a conventional emergency systems panel typically provided near the main entrance to the building. The earthquake detector  2  can conveniently be located within the main unit  1 . Similarly, the gas detector circuit(s)  4  can be installed within the main unit  1  itself, or can be provided at a suitable remote location. However, in order to provide effective protection, the gas detector circuits  4  should be located near potential sources of gas leaks (such as, for example, near gas valves and meters, and near burner equipment such as furnaces and water heaters), and therefore will generally be located remote from the main unit  1 . The use of multiple gas detector circuits is advantageous, in that it allows simultaneous gas detection and monitoring at various locations throughout a building. 
     As with the gas detector circuit(s)  4 , the smoke detector  5  can be installed within the main unit  1  itself, or can be provided at a suitable remote location. In order to provide effective fire protection, the smoke detector  5  should be located near potential sources of fire (such as, for example, near gas valves and meters, and near burner equipment such as furnaces and water heaters), and therefore will generally be located remote from the main unit  1 . 
     Referring to FIG. 2, the main unit  1  comprises a power supply  6 ; a microprocessor  7  which receives respective inputs from the earthquake detector  2 , the gas detectors  4 , and the smoke detector  5 ; an audible alarm  8 ; a system status display  9 ; and a valve control circuit  10 . A gas detector signal combine circuit  11  is also provided so that multiple gas detectors (in this embodiment four gas detectors) may be used. A system reset line  12  and a system test line  13  are also connected to the microprocessor  7  to allow the gas, fire and earthquake detector system to be reset and tested, as described in further detail below. 
     The power supply  6  produces three regulated direct current outputs at, for example, 5 VDC, 12 VDC and 20 VDC, and can be connected directly to the mains power supply at, for example, 120 VAC. Alternatively, the power supply  6  can be connected to the mains power supply through a conventional wall adapter, which supplies direct current power at, for example, 12 VDC to the power supply  6 . The power supply  6  can conveniently be provided with an internal battery and charger (not shown), so that the system can continue to function in the event of a failure of the mains power supply. 
     The audible alarm  8  is controlled by the microprocessor  6 , and provides an audible alarm indication. A suitable audible alarm can conveniently be provided by a piezo-electric annunciator of the type typically provided in domestic fire alarms. The use of a piezo-electric annunciator is particularly suitable for the audible alarm  8  because it provides a very loud alarm indication, while minimizing power consumption. 
     The system status display  9  can conveniently be provided by one or more LEDs (not shown), to provide a visual indication of the system status. In this case, a multi-coloured LED can be conveniently arranged so that its color indicates the system status. For example, the LED can be controlled by the microprocessor  7  to emit a green light when the system is operating normally. If a problem is detected with the equipment, the LED can be controlled by the microprocessor  7  to emit an amber light. In the event of an gas leak, fire or earthquake, the LED can be controlled by the microprocessor  7  to emit a red light, so that the system status display  9  also provides a visual alarm indication. 
     The gas detector signal combine circuit  11  is designed to receive the respective signals generated by each of the gas detector circuits  4 , and to pass a single gas detected signal to the microprocessor  7 . As shown in FIG. 3, the gas detector signal combine circuit comprises a respective jack  14  for each gas detector. Each jack  14  includes a gas detect signal line  15 , and a trouble signal line  16 . These lines are connected to an input of respective gas detect and trouble detect NOR (i.e. logical NOT OR) logic gates  17  and  18 , respectively. The output of each NOR gate  17  and  18  is adjusted (either upwards or downwards) by respective voltage level translators  19  and  20 , and supplied to a respective input of the microprocessor  7  through combined gas detect line  21  and combined trouble detect line  22 . Additionally, each jack  14  includes a 12 VDC supply contact  23  connected to the power supply  5 , and a ground contact  24 . The 12 VDC supply contact  23  and the ground contact  24  cooperate to provide the respective gas detector circuit  4  with 12 VDC power. 
     During normal operation of the system, the gas detect line  15  and the trouble detect line  16  of each jack  14  will be maintained at a high voltage level by the respective gas detector circuits. In this case, the combined gas detect line  21  and combined trouble detect line  22  will be at a low voltage level, indicating that no gas leaks are detected, and all of the gas detectors  4  are operating normally. 
     If a gas leak is detected by any one (or more) of the gas detectors  4 , the respective gas detect line  15  is switched to a low voltage level. In response, the combined gas detect line  21  switches to a high level, which indicates to the microprocessor  7  that a gas leak has been detected. 
     Similarly, if a fault is detected by any one (or more) of the gas detectors  4 , the respective trouble detect line  16  is switched to a low voltage level. In response, the combined trouble detect line  22  switches to a high level, which indicates to the microprocessor  7  that a fault has occurred with one of the gas detectors  4 . 
     Referring to FIG. 4, each gas detector circuit  4  generally comprises a voltage regulator  25 , a detection circuit  26  and a detector condition display  27 . The voltage regulator  25  receives the 12 VDC power from the power supply  6  of the main unit  1 , and outputs a regulated voltage at, for example 5.2 VDC. This regulated voltage is then suitably divided to produce reference voltages REF  1 - 4 , by means of, for example, a voltage divider (not shown). The reference voltages REF  1 - 4  are used to calibrate the gas detection circuit  26 , and to set the threshold limits for gas detection. 
     The detection circuit  26  comprises a detector element  28  (such as, for example, a FIGARO (trade name), model TGS813, manufactured by ) having first and second conduction plate contacts  29  and  30 , and first and second heater element contacts  31  and  32 . The first conduction plate and heater element contacts  29  and  31  are connected to the regulated output of the voltage regulator  25 . The second conduction plate contact  30  is connected to the inverting input of a first comparator  33 , and to the positive input of a second comparator  34 . Similarly, the second heater element contact  32  is connected to the inverting input of a third comparator  35 , and to the positive input of a fourth comparator  36 . The output of the first comparator  33  is connected to the gas detect line  37  of the gas detector circuit  4 , and the outputs of the second, third and fourth comparators  34 ,  35 , and  36  are connected to the trouble detect line  38  of the gas detector circuit  4 . 
     The first and second comparators  33  and  34 , and the third and fourth comparators  35  and  36  each cooperate to define two window comparators  39  and  40  which compare the voltage levels of the second conduction plate contact  30  and second heater element contact  32  against the reference voltages REF  1  and  2 , and REF  3  and  4 , respectively. The output voltages of the window comparators  39  and  40  indicate the operational condition of the detector element  28 . 
     In particular, during normal operation, a low current will flow between the conduction plates of the detector element  28 , so that the second conduction plate contact  30  will be at a voltage between predetermined high and low levels. If a gas enters the detector element  28 , the conductance between the two conduction plates will increase so that the voltage of the second conduction plate contact  30  will rise to a high level. When the voltage of the second conduction plate contact  30  reaches a threshold defined by voltage REF  1 , the output of first comparator  33  will go to a low level indicating that gas has been detected. On the other hand, if the voltage of the second conduction plate contact  30  drops to a low level threshold defined by voltage REF  2 , then the output of the second comparator  34  will go to a low level indicating that a fault has occurred with the detector element  28 . 
     Similarly, during normal operation a low current will flow through the heater element of the detector element  28 , so that the second heater element contact  32  will be at a voltage between predetermined high and low levels. If the voltage of the second heater element contact  32  rises to a high level threshold defined by voltage REF  3 , then the output of the third comparator  35  will go to a low level indicating that a fault has occurred with the detector element  28 . On the other hand, if the voltage of the second heater element contact  32  drops to a low level threshold defined by voltage REF  4 , then the output of the fourth comparator  36  will go to a low level indicating that a fault has occurred with the detector element  28 . 
     The gas detect line  37  and the trouble detect line  38  of the gas detector circuit are connected to respective inputs of the detector condition display  27 . The detector condition display  27  is suitably comprised of an LED  41  and an associated driver circuit  42  to provide a convenient visual indication of the operational status of the gas detector circuit  4 . For example, in the embodiment illustrated in FIG. 5, when the outputs of all of the first through fourth comparators  33 - 36  are at a high level, the LED  41  is controlled to emit a green light. When the output of the first comparator  33  switches to a low level indicating gas detected, the LED  41  is controlled to emit a red light. Finally, when the output of any of the second, third or fourth comparators  34 ,  35  or  36  switches to a low level indicating a fault with the gas detector element  28 , the LED  41  is controlled to emit an amber light. 
     Referring now to FIG. 5, the earthquake detector  2  is conveniently formed as a “plug-in” board designed to be removably installed within the main unit  1 , and is conveniently provided with power, for example at 5 VDC, from the main unit power supply  5 . The earthquake detector  2  is formed essentially as an acceleration detection circuit based upon conventional integrated circuit accelerometers  43 , such as, for example, model AD-XL05 accelerometers manufactured by Analog Devices. Each accelerometer  43  measures accelerations along one axis. Accordingly, the earthquake detector  2  of the present invention employs three accelerometers  43 ( a ),  43 ( b ) and  43 ( c ) oriented so as to detect accelerations in respective x, y and z orthogonal directions. Preferably, the earthquake detector  2  is installed such that both of the accelerometers  43 ( a ) and ( b ) are oriented in a substantially horizontal plane, so that horizontal accelerations in any direction can be detected, and the accelerometer  43 ( c ) is oriented in a substantially vertical plane. 
     Each accelerometer  43 ( a ) and ( b ) generates a respective accelerometer output signal, as shown in FIGS.  7 (A)- 7 (B), and the accelerometer  43 ( c ) generates a corresponding accelerometer output signal (not shown), indicative of lateral ground accelerations in the respective directions of the accelerometers  43 ( a )-( c ). The accelerometer output signals are supplied to respective band pass filters  44 ( a ),  44 ( b ) and  44 ( c ) through accelerometer output signal lines  45 ( a ),  45 ( b )and  45 ( c ). The band pass filters  44 ( a )- 44 ( c ) produce respective filter output signals on filter output lines  46 ( a )- 46 ( c ). As illustrated in FIG.  6 (D), the filter output signals vary about a mean voltage V mean . Each filter output signal is supplied to respective threshold comparator  47 ( a )-( c ) through filter output lines  46 ( a )-( c ), and compared to threshold voltage V 1 . If the voltage of any of the filter output signals exceeds the threshold voltage V 1 , the respective threshold comparator  47  ( a )-( c ) generates an earthquake detected signal, illustrated in FIG.  6 (E), which is supplied to a respective input of a logical OR circuit  48 . The output of the logical OR circuit  48  is supplied as a combined earthquake detected signal to the microprocessor  7  through an earthquake detector line  49 . The use of the logical OR circuit  48  allows the use of independent accelerometer/band-pass filter/threshold comparator networks to monitor ground accelerations in orthogonal directions, while requiring only a single earthquake detector line  49  to supply an earthquake detected signal (generated for any one of the accelerometers) to the microprocessor. 
     As shown in FIGS.  6 (D) and  6 (E), if the peak value of the filter output signals remains below the threshold voltage V 1 , the threshold comparators  47  maintain the inputs to the logical OR circuit  48 , and consequently also the earthquake detector line  49  at a constant high voltage level. However, if at any instant the level of one of the filter output signals rises above V 1 , the respective window comparator switch  47 ( a )-( c ) switches the earthquake detector line  49  to a low level, thereby signaling to the microprocessor  7  that a hazardous earthquake has occurred. 
     Accelerations considered to be hazardous are a function of frequency and magnitude. In particular, accelerations of 0.3 g (i.e. 0.3×9.81=2.94 m.sec  −2 ) at 2.5 Hz are considered to be an earthquake hazard, whereas accelerations of 0.4 g at 10 Hz and 0.08 g at 2.5 Hz are considered safe. This is illustrated in FIG. 7, which shows the earthquake hazard posed by accelerations at frequencies between 0 and 10 Hz. Ground accelerations falling above the threshold acceleration/frequency curve in FIG. 7 are considered to pose an earthquake hazard and thus should trigger an alarm. Conversely, ground accelerations falling below the threshold acceleration/frequency curve in FIG. 7 are considered to be safe and thus should not trigger an alarm. 
     The band pass filters  44  are designed to pass accelerometer output signal frequencies within a range of approximately 0.1 Hz to 2.5 Hz, and to increasingly attenuate signals below approximately 0.1 Hz, and above 2.5 Hz, as shown in FIG.  8 . Thus the gain/frequency characteristic of the band-pass filters  44  is designed to closely match the acceleration/frequency relationship of the threshold acceleration/frequency curve separating hazardous and safe ground accelerations, shown in FIG.  7 . Consequently, acceleration/frequency values falling on the threshold acceleration/frequency curve (shown in FIG. 7) will yield a filter output signal Vout having a value which initially increases and, above approximate 0.1 Hz, becomes substantially constant and slightly less than the threshold value, as shown in FIG.  9 . By this means, a single threshold voltage (V 1 ) can be used to reliably discriminate between hazardous and safe ground accelerations, without regard to the frequency of those accelerations. 
     The threshold comparators  47  define the threshold acceleration magnitudes, indicated by the amplitudes of the filter output signals, corresponding with hazardous earthquakes. The threshold voltage V 1  is selected so that accelerations which are too low in magnitude to pose a significant earthquake hazard do not generate an “earthquake detected” signal. 
     For example, the above-mentioned Analog Devices AD-XL05 accelerometers are supplied with 5 VDC power from the power supply of the main unit  1 . The respective accelerometer output signals are supplied to the band-pass filters  44 , which produce respective time-varying filter output signals having a mean voltage of V mean =1.8 VDC. The filter output signals Vout are then supplied to the window comparator and compared with threshold voltage V 1 =2.2 VDC. 
     A circuit diagram of the gas valve controller  10  of FIG. 2 is shown in FIG.  10 . An output line  50  from the microprocessor  7  is connected to an amplifier circuit comprising transistors Tr 1  and Tr 2  and resistors r 1-3 r , which amplifies pulses from the microprocessor  7  to a level sufficient to operate the gas shut-off valve  3 , which is implemented as a solenoid valve. Actuator of the shut-off valve  3  interrupts a supply of gas through a gas line  52 . 
     FIG. 11 is a flow chart illustrating the operation of the microprocessor  7  employed in the present invention. Upon initial start-up of the earthquake/gas detector system, the microprocessor  7  performs an initialization sequence at step S 1 , and turns on the system status display  9  and turns off the audible alarm  8 . At step S 2 , the gas detectors  4  (via the combined gas detect line  21  of the gas detector signal combine circuit  11 ) are checked for the presence of a gas detected signal. If the result of step S 2  is “NO”, the system proceeds to step S 3 , and the earthquake detector signal line  49  is checked for the presence of an earthquake detected signal. If the result of step S 3  is “NO”, the system proceeds to step S 4 , and the smoke detector  5  is checked for the presence of a fire detected signal. If the result of step S 4  is “NO”, the system proceeds to step S 5 , and the gas detectors  4  (via the combined trouble detect line  22  of the gas detector signal combine circuit  11 ) are checked for the presence of a trouble signal. If the result of step S 5  is “NO”, the system proceeds to step S 6 , and the system test line  13  is checked to determine whether a system test button (not shown) has been pressed. If the result of step S 6  is “NO”, the system proceeds back to step S 2 , and the sequence of checks at steps S 2 -S 6  is repeated. Conversely, when the result of any one of the checks performed in steps S 2  through S 6  are “YES”, the system proceeds as described below. 
     Earthquake or Fire Detected 
     When an earthquake detected signal is found at step S 3 , or when a fire detected signal is found at step S 4 , the microprocessor proceeds immediately to its earthquake/fire alarm sequence at step ES 1 . In the example embodiment, the system status display  9  is controlled to emit a red light; the audible alarm  8  is activated; the dial output is activated, and the gas valve controller  10  is triggered to close the gas shut-off valve  3 . The microprocessor  7  then proceeds to step ES 2  where the earthquake and combined gas detector signal lines  49  and  21 , respectively, and the smoke detector  5 , are repeatedly checked for the presence of either an earthquake detected signal, a gas detected signal, or a fire detected signal. If any one of an earthquake detected signal, a gas detected signal or a fire detected signal are detected at step ES 2 , the system repeats the check. When none of an earthquake detected signal, a gas detected signal or a fire detected signal are found at step ES 2 , the microprocessor  7  proceeds to step ES 3 , where the system reset line  12  is repeatedly checked to determine whether the system reset button (not shown) has been pressed. When the result of the check at step ES 3  is “YES”, the microprocessor  7  returns to step S 1 , and re-initializes the system. 
     Gas Detected 
     When a gas detected signal is found at step S 2 , the microprocessor  7  proceeds to its initial gas alarm sequence at step GS 1 . In the example embodiment, the system status display  9  is controlled to emit a red light; the audible alarm  8  is activated; and a ten second delay is initiated. The microprocessor  7  then proceeds to step GS 2  where the earthquake detector signal line  49  is checked for the presence of an earthquake detected signal, and the smoke detector  5  is checked for the presence of a fire detected signal. If either an earthquake detected signal or a fire detected signal is found at step GS 2 , the microprocessor  7  proceeds immediately to the earthquake alarm sequence at step ES 1 , and continues through the successive steps ES 2  and ES 3  as described above. If the result of the check at step GS 2  is “NO”, the microprocessor  7  proceeds to step GS 3 , where the 10 delay is checked. If the 10 second delay has expired, the microprocessor  7  proceeds immediately to the earthquake alarm sequence at step ES 1 , and continues through the successive steps ES 2  and ES 3  as described above. Alternatively, if the 10 second delay has not expired, the microprocessor  7  proceeds to step GS 4 , where the combined gas detect line  21  is again checked for the presence of a gas detected signal. If a gas detected signal is found at step GS 4 , the microprocessor  7  returns to step GS 2 , and the sequence of checks at GS 2 , GS 3  and GS 4  are repeated. If a gas detected signal is not found at step GS 4 , the microprocessor  7  returns to step S 1 , and re-initializes the system. 
     Trouble Detected 
     When a trouble detected signal is found at step S 5 , the microprocessor  7  proceeds to its initial trouble alarm sequence at step PS 1 . In the example embodiment, the system status display  9  is controlled to emit an amber light, and the audible alarm  8  is activated. The microprocessor  7  then proceeds to step PS 2  where the earthquake detector signal line  49  is checked for the presence of an earthquake detected signal and the smoke detector  5  is checked for the presence of a fire detected signal. If either an earthquake detected signal or a fire detected signal is found at step PS 2 , the microprocessor  7  proceeds immediately to the earthquake/fire alarm sequence at step ES 1 , and continues through the successive steps ES 2  and ES 3  as described above. If the result of the check at step PS 2  is “NO”, the microprocessor  7  proceeds to step PS 3 , where the combined trouble detect line  22  is again checked for the presence of a trouble detected signal. If a trouble detected signal is found at step PS 3 , the microprocessor  7  returns to step PS 2 , and the sequence of checks at PS 2  and PS 3  are repeated. If a trouble detected signal is not found at step PS 4 , the microprocessor  7  proceeds to step PS 4  where the system reset line  12  is checked to determine whether the system reset button (not shown) has been pressed. If the system reset button has not been pressed at step PS 4 , the microprocessor returns to step PS 2 , and the sequence of checks at PS 2 , PS 3  and PS 4  are repeated. When the result of the check at step PS 4  is “YES”, the microprocessor  7  returns to step S 1 , and re-initializes the system. 
     System Test Activated 
     When the system test button is pressed (step S 6 ), the microprocessor  7  proceeds to its test alarm sequence at step TS 1 . In the example embodiment, the system status display  9  is controlled to emit a red light; the audible alarm  8  is activated; and the gas valve controller  10  is triggered to close the gas shut-off valve  3 . The microprocessor  7  then proceeds to step TS 2  where the system test line  13  is repeatedly checked do determine whether the system test button (not shown) is still being pressed. When the system test button is released, the microprocessor  7  returns to step S 1 , and re-initializes the system. 
     It will be appreciated that the present invention is not limited to the example embodiment described above, but may be varied without departing from the scope of the appended claims. 
     For example, in the above-described embodiment, the gas detector signal combine circuit  11  is provided with jacks for connection with four gas detector circuits. It will be appreciated that more or fewer such jacks may be provided. Further, it will be apparent that one or more jacks may be unused in a particular installation. Finally, it will be understood that different types of gas detectors (such as, for example, natural gas, propane, or CO detectors) may be suitably connected to the jacks, and that gas detectors of more than one type may be used in combination to allow simultaneous detection of more than one type of gas.