GB2101784A - Fire alarms - Google Patents

SPECIFICATION Fire alarms This invention relates to fire alarms, and is concerned with arrangements for checking the continuity of a fire alarm system.

For example, the invention is concerned with systems in which each of a number of detectors is connected between a pair of lines, so that they are in cascade, the lines being connected to a panel including a d.c. supply in series with a device responsive to current arranged to give an alarm if the current exceeds a threshold value. An important feature of such a system is that a check of the continuity of such lines should be possible from the panel.

The check has customarily been done by connecting a resistor across the lines at their ends remote from the panel. This causes a standing current to flow down the line, and any break in the lines will cause this current to cease, and indicate a lack of continuity.

According to the present invention, a fire alarm system including a number of detectors connected along a pair of lines so as to be in cascade, the lines being connected to a panel arranged to give an alarm if a detector is operated, characterised by a capacitor connected between the lines at their end remote from the panel, and means for periodically modifying the supply to the lines from the panel and monitoring the change in the system resulting from such modification. For example, there may be, for alarm operation, a current responsive device in series with a d.c.

supply, and the voltage of the supply may have its normal value for intervals of 50 milliseconds, and a modified value for periods of 10 milliseconds.

In practice, when a circuit with detectors is being checked, the supply cannot be turned off completely for two reasons. First, if there is a break in the lines, the part of the line still connected to the fire alarm panel would experience large supply voltage changes equal to the full supply voltage every time the line continuity was checked. This would provide adverse supply conditions to the fire detectors.

Secondly, in the event of an alarm, a supply voltage is required to maintain a thyristor in the detector in the latched state. When a fire detector assumes an alarm condition, it causes current in the lines to increase by means of its internal thyristor.

Accordingly, in a preferred form of the present invention, the modification referred to above comprises a periodic reduction of the voltage from a normal value to a lower value.

In the case of a 24 volt system, for example, the normal voltage may be 24 volts, but assuming a worst-case-drop-in-line voltage of 2 volts in 10 milliseconds with the supply switched off, a back-up supply may be employed of 24-3 volts, that is to say 21 volts.

The invention may be put into practice in various ways, but one specific embodiment will be briefly described by way of example, with reference to the accompanying diagram, which is a schematic circuit diagram of part of a fire alarm system.

A system includes a number of fire detectors 10 connected in cascade between positive and negative lines 11 and 1 2 respectively. The lines 11 and 1 2 are connected to a fire alarm panel shown in chain lines. The lines 11 and 1 2 are connected in series with a transistor 1 4 across a suply, shown as a battery 1 5. The transistor 14 is controlled by a timing circuit to switch off the supply at intervals of 50 milliseconds for periods of 10 milliseconds, as indicated by the waveform 16.

At their ends remote from the panel, the lines 11 and 1 2 are connected through a capacitor 20, for example of nominally 22 micro-farads.

A voltage-responsive device 1 7 is connected across the lines at their ends adjacent the panel.

Accordingly, when the transistor 14 is periodically switched off, the voltage between the lines is maintained solely by the capacitor 20 and falls gradually. If however, there is a discontinuity in the lines, the voltage will be maintained only by the inherent capacitance of the lines, or a portion of them, and the voltage across the device 1 7 will drop rapidly and indicate a lack of continuity.

As indicated above, it is undesirable to switch off the supply completely, and accordingly, the circuit includes in parallel with the transistor 14, a second transistor 25 in series with a Zener diode 26 which transistor is switched on as the transistor 1 4 is switched off. The diode 26 drops 3 volts, thereby reducing the voltage applied when the transistor 14 is cut off from 24 volts to 21 volts.

The remaining parts of the system may be in accordance with known practice.

The device 1 7 measures the voltage across the lines while the supply is still on and then measures the voltage again at the end of the 10 milliseconds period. By knowing the maximum current, which would be drawn from the line by fire detectors and the minimum value of end of line capacitance, the largest drop in line voltage in 10 milliseconds can be calculated. If there is any line discontinuity the line voltage will fall much more rapidly because it will only be held up by line capacitance. Thus, by measuring the difference between the line voltages prior to, and at the end of, the 10 milliseconds off period, line continuity can be checked.

The principal advantage of using an end of line capacitor is that there is not standing current passing down lines for monitoring purposes. This means that the fire panel current consumption is greatly reduced while it is in a standby state and the battery back up capacity may be reduced. An end of line capacitor can have a much larger tolerance (e.g. + 100%-50%) than can an end of line resistor component (typically i 10%).

The invention includes a method of checking the continuity of a fire alarm system including a number of detectors connected along a pair of lines so as to be in cascade, the lines being connected to a panel arranged to give an alarm if a detector is operated and including a capacitor connected between the lines at their end remote from the panel. in which method the supply to the lines from the panel is periodically modified and the change in the system resulting from such modification is monitored.