Line interruption supervisory device for fire alarm systems

Abstract: A fire alarm system is equipped with a terminating capacitor (CE) connected across the terminal (EL) of a pair of lines (L) to which fire detectors (DE) are connected and with cut-off means (Tr1, Tr2) for cutting off the power supply to the lines in the case of a line interruption test. Furtheron, a discharge means (Tr3) causes the terminating capacitor (CE) to discharge when the power supply to the lines (L) is interrupted, and a discrimination means (MPU) compares the voltage available on the lines before interrupting the power supply with the line voltage after a predetermined time and judges, by the difference of these two voltages, whether there is an open in the lines, or not. Thereby, false operation of the line interruption supervision is prevented, due to a fluctuation of the power supply voltage, and a line interruption is detected reliably, even in fire alarm system having very long lines and a great number of fire detectors connected thereto, by which the discharge voltage is decreased.

Description: The present invention relates to a line interruption supervisory device of a fire alarm system according to the introduction of claim 1. (Prior art) GB - A - 2 115 966 describes an example of a line interruption supervisory device of this kind. In this example, a terminating capacitor is connected across the terminal of a pair of power/signal lines to which fire detectors are connected. For supervising the lines and for stating any interruption, the power supply to the lines is cut off periodically ba a pulse oscillator. During the cut-off phases, the terminating capacitor is discharged through a resistor. In case of a breakage of a line, the voltage across the resistor is decreasing rapidly within the cut-off pulses. When the voltage drop falls below the voltage of a comparision voltage source, within a supervision pulse, a line interruption indicator is switched on. (Questions that the present invention intends to solve) Voltage on the lines having fire detectors connected thereto varies with length of the lines and kind and number of fire detectors connected. In the above example, the longer the line length extends and/or the more the number of fire detectors connected increases, the lower the discharge. voltage becomes. Therefore, the discharge voltage could fall below the divided voltage, causing the line interruption indicator to light up even if there is no line interruption. The present invention aims at offering a line interruption supervisory device of a fire alarm system which is capable of surely excercising the line supervision even if the line length extends longer and/or the number of fire detectors increases. (Means to solve the questions) The above-indicated questions are solved by the characteristic elements of claim 1. Therein the terminating capacitor of the power/signal lines is caused to discharge when the power supply to the lines is cut-off through discharge means comprising a constant-current circuit, and the voltage on the lines immediately before the power supply cut-off and after the lapse of a predetermined time after cut-off is compared and a line interruption is stated if the voltage difference at these two times exceeds a predetermined threshold voltage. (Effect) Since the line interruption supervisory device of the fire alarm system according to the present invention causes the terminating capacitor to discharge gradually with time by using a constant-current circuit, and to judge whether there is an open in the power/signal lines by forming the difference between the line voltage immediately before cutting-off the supply voltage and at a predetermined time thereafter, an by checking if this voltage difference exceeds a predetermined threshold, it is possible to exercise the line interruption supervision without fail even in systems with very long lines and a great number of fire detectors connected thereto. (Embodiments) Fig.1 is a circuit diagram showing an embodiment according to the present invention. Shown in Fig.1 are repeaters T1, T2, fire detectors DE and a terminal unit EL. The repeater T2 is identical to the repeater T1, and additional repeaters identical to the repeater T1 and provided. These repeaters T1, T2-------- are connected to the control panel RE. The repeater T1 is equipped with a power supply PS, a receiving circuit 11, a transmission circuit 12, a microcomputer MPU, inverters INV1, INV2, transistors Tr1, Tr2, Tr3, a constant-current circuit for charging CC1, a constant-current circuit for discharging CC2 and an A/D converter 10. The fire detectors are connected with the repeater TI through the power/signal lines L. The terminal unit EL is equipped with a terminating capacitor CE, a resistor for rush current prevention R1, a zener diode ZD, a diode D and a resistor R2. The microcomputer MPU executes the flowchart program shown in Fig.2 and is equipped with data input ports D0 ∼ D7, output ports for controls OUT1, OUT2, a built-in comparator and a reference voltage input port Vrf of the comparator. The constant-current circuit for charging CC1 is a circuit which supplies a predetermined constant current to the fire detectors DE and the terminal unit EL through the power/signal lines L. The constant current circuit for discharging CC2 constituting a part of the discharge loop of the terminating capacitor CE is a circuit which keeps the inclination of the characteristic showing changes of the discharge current constant to avoid rapid drop of the discharge voltage (voltage on the power/signal lines L). The A/D convertor 10 is a circuit which converts the analog voltage on the power/signal lines L to a digital value. The transistors Tr1, Tr2 switch off when the power supply to the power/signal lines L is interrupted. Combination of the microcomputer MPU with the transistors Tr1, Tr2 is an example of means to cut off the power supply to the power/signal lines. The transistor Tr3 causes the terminating capacitor CE to discharge. Combination of the microcomputer MPU with the transistor Tr3 is an example of discharge means which causes the terminating capacitor to discharge when the power supply to the power/signal lines is interrupted. The power supply PS comprises a constant-voltage circuit which converts the power supplied from the control panel RE through the power/signal lines L to voltage required for the internal circuits and the fire detectors. The microcomputer MPU contains a memory which is an example of memory means to memorize the voltage available on the power/signal lines immediately before the power supply is interrupted. Furthermore, the microcomputer MPU is also an example of line interruption discriminating means to judge whether there is an open in the power/signal lines by difference between the voltages available after a lapse of the predetermined time from power supply interruption and immediately before the power supply interrupion. Next, operation of the above embodiment is described hereinafter. Fig.2 is a flowchart showing the operation of the repeater T1 in the above embodiment. Firstly, the variables j and k representing the number of calls from the control panel RE and the number of detections of line interruption respectively are initialized to zeros. Also the outputs OUT1 and OUT2 of the microcomputer MPU are initialized to L and H respectively (S1). With the output OUT1 of the microcomputer MPU set to L the transistors Tr1, Tr2 switch on, and with the output OUT2 set to H the transistor Tr3 switches off. If a signal is received from the control panel RE and it is a call signal (S2, S3), the number of call j from the control panel RE is incremented by one (S4). If the number of call j does not reach a predetermined number J (e.g. 10 calls) (S5), the output VA1 of the A/D converter is read in (S6) and compared with the fire signal discriminating voltage VF (S7). If the fire detector DE is in operating state at this time, the output VA1 of the A/D converter falls below the fire signal discriminating voltage VF, then the repeater transmits a fire signal together with, for example, its self-address to the control panel RE from the transmission circuit 12 (S8), and the program returns to the step S2. If the output VA1 of the A/D converter is above the fire discriminating voltage VF, it does not mean 'fire', and the program returns to the step S2. Provision may be made so that a response signal will be transmitted to the control panel RE in case of no fire. On the other hand, when the number of call j to the repeater T1 from the control panel RE has reached the predetermined number J, the output voltage VA2 of the A/D converter 10 (voltage available immediately before power supply interruption) is read in (S11), and then the output OUT1 is set to H (S12). As the output OUT1 is set to H, the transistors Tr1, Tr2 switch off and the constant-current circuit for charging CC1 goes into the OFF state. Consequently, power supply to the power/signal lines L is interrupted. Next, the output OUT2 is set to L (S13), and this causes the transistor Tr3 to switch on and the constant-current circuit for discharging CC2 and the discharge circuit to go into the ON state. The charge on the terminating capacitor CE is now gradually released through the power/signal lines L, the constant-current circuit for discharge CC2 and the transistor Tr3. After a predetermined time (e.g. 1 ms), the output voltage VA3 (voltage available after power supply interruption) of the A/D converter 10 is read in (S15), and this read-in voltage VA3 is written in the memory in the microcomputer MPU. Then, the discharge circuit is set to the OFF state, and the charge circuit is set to the ON state. In other words, the output OUT2 is set to H (S16) to set the constant-current circuit for discharging CC2 to the OFF state, and the output OUT1 is set to L (S17) to actuate the constant-current circuit for charging CC1. A calculation of a voltage difference ΔV (=VA2 - VA3) is then made by subtracting the voltage available after power supply interruption VA3 from the one available immediately before power supply interruption VA2 (S18). If the voltage difference ΔV is less than a threshold voltage V TH (e.g. 6V) as a reference for line interruption judgement (S19), it is judged that there is no line interruption. And the line interruption flag is checked for its state at this time (S20). If the line interruption flag is set (ON), the line interruption restoration signal is transmitted (S21) to clear (OFF) the line interruption flag (S22). On the contrary, if the voltage difference ΔV is greater than the threshold voltage V TH (S19), the interruption flag is checked for its state (S31). If the line interruption flag is OFF, the number of detection of line interruption k is incremented by one (S32). When the number of detection of line interruption has reached a predetermined number of detection K (e.g. 5) (S33), the line interruption signal is transmitted to the control panel RE (S34) to set (ON) the line interruption flag (S35) and to initialize the number of detection of line interruption k and the number of call j from the control panel RE to zeros (S36), (S37). Then, the program returns to the step S2. The line interruption flag is set (ON) when the line interruption signal is transmitted to the control panel RE, and is cleared (OFF) when the line interrupted state has been restored to the normal state. Since the above embodiment forms a judgement on line interruption by looking at whether the difference ( ΔV) between the line voltages available immediately before the start of discharge and after a lapse of the predetermined time from the start of discharge has reached a predetermined voltage, it is capable of surely discriminating line interruption even if the line length, the number and kind of detectors connected are varied. Fig.3 is a drawing illustrative of the above embodiment. In this Fig.3 no line interrupted state is shown on the left hand side and the case with a line interruption is shown on the right hand side. Although the voltage VA2 available at the time t1 immediately before power supply interruption in the case with no open in the lines is equal to the voltage VA2 available at the time t3 immediately before power supply interruption in the case with an open in the lines, the voltage VA3 available at the time t2 after power supply interruption in the former case is higher than the voltage VA3 available at the time t1 after power supply interruption in the latter case. Accordingly, the voltage difference ΔV in the case with no open in the lines is smaller than the voltage difference ΔV in the case with an open in the lines. Based on this difference a judgement is made as to whether there is an open in the power/signal lines. The voltage difference ΔV is little influenced by line length or the number of the fire detectors connected. Further, it is possible to discriminate between line interruption and no line interruption more easily if the discharge current of the terminating capacitor CE is restricted by providing the constant-current circuit for discharging CC2 because this lessens the voltage drop due to the line resistance at the time of discharge and consequently the voltage drop at the time of discharge in normal condition regardless of the line length. The above description relates to the operation of the repeater T1 but is also applicable to the other repeaters T2 and so on. Fig.4 is a circuit diagram showing another embodiment according to the present invention. The repeater T1a shown in Fig.4 is basically identical to the repeater T1 shown in Fig.1 but differs in the way of voltage input to the A/D converter 10a from that to the A/D converter 10. Describing it more precisely, the A/D converter 10 has direct input from the power/signal lines L while in the case of the A/D converter 10a the voltage on the power/signal lines L is divided by the resistors R5, R6, R7 and fed to the A/D converter 10a. While the A/D converter 10 in the embodiment shown in Fig.1 uses the maximum value of the voltage on the power/signal lines L in case it can be inputted as it is, the A/D converter 10a shown in Fig.4 can not have the voltage on the power/signal lines L inputted as it is and uses the lowered voltage because of the small withstand voltage. The A/D converters 10, 10a of the repeater T1 may be built in the microcomputer MPU. In the above embodiments, the constant-current circuit for charging CC1 and the constant-current circuit for discharging CC2 may be omitted, and the microcomputer built in the A/D converter is used as such. While each of the above embodiments shows the line supervision by repeater, same applies to the case where the line supervision is exercised by control panel. In this case, however, the steps S2, S3 in Fig.2 are omitted and the number of call j is replaced, for example, by count of the timer output. (Effect of invention) The present invention has such an effect that the line supervision can surely be exercised even if the line length extends longer and/or the number of the fire detectors connected increases.

Claims: A line interruption supervisory device of a fire alarm system which is equipped with: a terminating capacitor (CE) connected across the terminal of a pair of power/signal lines (L) to which fire detectors (DE) are connected, a cut-off means (Tr1, Tr2) to cut-off the power supply (PS) to the power/signal lines (L), a discharge means to cause the terminating capacitor (CE) to discharge upon interruption of the power supply to the power/signal lines (L), and a discriminating means to state a line interruption on the basis of the voltage drop on the power/signal lines (L) after interruption of the power supply to the power/signal lines (L), characterised in that: the discharge means for discharging the terminating capacitor (CE) upon interruption of the power supply (PS) comprises a constant-current circuit (CC2) for discharging the terminating capacitor (CE) gradually with time, and the discriminating means (MPU) comprises a memory means to memorize the voltage (VA2) available on the power/signal lines (L) immediately before interruption of the power supply, and further comprises means to form the difference (ΔV) of the voltage (VA2) on the power/signal lines (L) immediately before interruption of the power supply (t3) and the voltage (VA3) on the power/signal lines (L) after the lapse of a predetermined time (t4) from interruption of the power supply and to state a line interruption when the said voltage difference (ΔV = VA2 - VA3) exceeds a predetermined threshold voltage (V TH ). A device according to claim 1, characterised in that the discriminating means (MPU) is designed to judge that there is an open in the power/signal lines (L) when a line interruption state has been detected during a predetermined number (K) of detections at plural times. A device according to claim 2, characterised in that the discriminating means (MPU) is equipped with a restoration discriminating means to judge whether the line interruption state has been restored to the normal state, and initiates a line interruption restoration signal upon the restoration of the line interruption. A device according to one of claims 1 to 3, characterised in that the terminating capacitor (CE) is charged through a constant-current circuit (CC1), and the discharge means causes the terminal capacitor (CE) to discharge through the constant-current circuit (CC2). A device according to one of claims 1 to 4, characterised in that the cut-off means (Tr1, Tr2), discharge means (CC2, Tr3, MPU), memory means and line interruption discriminating means (MPU) are provided in a control panel (RE). A device according to one of claims 1 to 4, characterised in that the cut-off means (Tr1, Tr2), discharge means (CC2, Tr3, MPU), memory means and line interruption discriminating means (MPU) are provided in a repeater (T1).

Inventors: SHUTOKU RYUJI, TAKAHASHI KAORU, TSUMUJI AKIO, SHUTOKU, RYUJI, TAKAHASHI, KAORU, TSUMUJI, AKIO
Applicants: NOHMI BOSAI LTD, NOHMI BOSAI LTD.
Application year: 1990
Main classification: G08B29
IPC codes: G08B29, G08B17