Method of and apparatus for signalling an alarm

In the apparatus for signalling an alarm, for example, in a gas or fire detecting installation, a detector is reset after a first response and the time duration until the next-following detector response is determined and classified with respect to three classes of time periods. When the time duration until the further detector response is beyond a predetermined upper or outer time limit, no alarm signal is released and the apparatus returns into its original state. When the further detector response occurs between two time limits, the alarm is immediately signalled. When the further detector response occurs prior to a lower one of the two time limits, there is carried out still one further test during which the detector is once again reset. The alarm signal will only be released if an additional further detector response is still observed prior to the upper time limit. In this manner the reliability of alarm signalling can be enhanced and the frequency of the occurrence of false alarms, particularly due to the occurrence of short-time or brief spurious conditions, can be significantly reduced.

BACKGROUND OF THE INVENTION 
The present invention relates to a new and improved method of, and 
apparatus for, signalling an alarm. 
In its more particular aspects, the present invention relates specifically 
to a new and improved method of, and apparatus for, signalling an alarm in 
which, after a first response of a detector connected to a central signal 
station, the detector is reset for a first time. At a pre-determined 
moment of time after the first detector response there are started in the 
central signal station two test time periods of predetermined different 
time durations and an alarm signal is only transmitted after the 
occurrence of at least one further response of the detector. An alarm 
signal is then transmitted if the detector responds a second time after 
the expiration of the test time period of comparatively shorter duration 
and prior to the expiration of the test time period of comparatively 
longer duration. 
Such methods and apparatus are suited for signalling various dangerous 
states or conditions by means of appropriate detectors which respond to 
the relevant state or condition. An important application is, for example, 
the signalling of a fire break-out by means of fire detectors which 
respond to phenomena due to the existence of combustion or burning 
conditions like, for example, smoke, aerosols formed during a fire, gases 
formed during a fire, radiation of flames, temperature variations, and so 
forth or the signaling of the presence of combustible and/or toxic gases. 
Fire detecting installations, on the one hand, are intended to signal a 
fire at an incipient stage and to release or initiate appropriate fire 
fighting measures. It is required for such purpose to employ highly 
sensitive automatic fire detectors which already react to phenomena 
occurring in the early stage of a fire. Such type of early response 
detectors are, for example, ionization smoke detectors or optical smoke 
detectors. However, when such fire detectors are operated at their highest 
possible sensitivity, there exists the danger that an alarm signal is 
released or triggered by interfering or spurious factors even if no 
dangerous state or condition, i.e. a fire, is present and that, due to 
such faulty or deceptive alarm, complicated fire fighting measures are 
unnecessarily initiated like, for example, the mobilization of the fire 
brigade or the setting into operation of a fire-extinguishing 
installation. 
During use of the method and the apparatus of the initially mentioned type 
as known, for example, from German Pat. No. 2,051,649, certain faulty or 
spurious alarms due to short-time interferences or noise, such as, for 
instance, clouds of cigarette smoke or transient electrical faults can 
already be prevented by automatically resetting a fire detector after its 
response and observing the next following response. In this way spurious 
effects causing unwanted false alarms can be differentiated from more 
persistent actual fire states or conditions. In order to reliably 
eliminate and distinguish such faults, there is required, however, a 
multiple repetition of the resetting and re-activating cycles. During such 
repetitions there is, however, the danger that occasionally an actual fire 
is not or only belatedly detected and signalled and this, of course, is 
extremely undesirable in practice. The suggestion to remedy such situation 
by statistically evaluating the further response signals is not a 
satisfactory proposal because statistical methods can not comply with the 
reliability requirements of fire detecting installations. It is of further 
disadvantage in such known installations that fire detectors of various 
types, for example, fire detectors displaying rapid response, fire 
detectors displaying slow response, fire detectors with or without 
integration circuitry, cannot be connected in common to the same 
evaluation circuit. 
A different approach has been adopted by the known methods of signalling an 
alarm as disclosed, for example, in Swiss Pat. No. 623,154 or in the 
Cerberus brochure FP 198 of the assignee of this application, as authored 
by M. Ruggli and F. Datwyler, entitled "Cerberus Alarm-Konzeption", 
wherein humans are intergrated into the decision-making process. 
Accordingly, an alarm signal triggered or released after one or multiple 
repetitive responses of a fire detector is only transmitted when no 
controlling personnel is present, when the signal is not acknowledged or 
receipted within a predetermined control time period and additionally when 
no message or information concerning the actual state or condition of 
danger is received within a reconnaissance time period of comparatively 
longer duration from a dispatched reconnaissance party. 
It is, however, a disadvantage of this state-of-the-art technique that such 
method is dependent on human shortcomings, that is due to negligence and 
inattentiveness of the control personnel or due to faulty assessment of a 
dangerous situation, the alarm signal may fail to be triggered or released 
or may be belatedly initiated with partially catastrophic consequences. 
In the prior art method of differentiating between a persistent false alarm 
due to a defective detector and an actual false alarm, for example, as 
disclosed in German Patent Publication No. 2,816,192, there have been 
suggested means for use with a fire detecting installation. Such means 
interrupt the detector voltage after the first alarm signal at least once 
for an adjustable period of time and indicate, as a spurious signal, an 
alarm signal which is immediately present when the detector operating 
voltage is again turned on. Such means transmit, as actual alarm signals, 
alarm signals which arrive at a certain delay. The therein described 
central fire detecting station thus is designed such that a permanent 
response of a detector due to a defect is not further transmitted as an 
alarm and only secondarily so that there can be suppressed deceptive 
alarms due to brief spurious effects. In such central fire detecting 
station, signals originating from all detectors, particularly from those 
with and without time-delayed response, are processed in the same manner. 
SUMMARY OF THE INVENTION 
Therefore, with the foregoing in mind, it is a primary object of the 
present invention to provide a new and improved method of, and apparatus 
for, reliably signalling an alarm in a manner which is not afflicted with 
the aforementioned drawbacks and limitations of the prior art heretofore 
discussed. 
Another and more specific object of the present invention is directed to 
the provision of a new and improved method of, and apparatus for, 
signalling an alarm in which the reliability of alarm signalling is 
improved and faulty or deceptive alarms are avoided to the greatest 
possible extent. 
Still a further significant object of the present invention is directed to 
a new and improved method of, and apparatus for, reliably signalling an 
alarm in a manner such that deceptive alarms are positively and 
effectively suppressed. 
Another, still important object of the present invention is directed to a 
new and improved method of, and apparatus for, signalling an alarm in 
which deceptive or spurious alarms are reliably suppressed and in which, 
during signal processing at the central signal station, the differences in 
the characteristic response of detectors with and without time delay are 
taken into account and compensated, so that such method and apparatus can 
be readily utilized in already existant installations working with various 
types of detectors which have different response characteristics. 
Still another important object of the present invention is directed to a 
new and improved method of, and apparatus for, signalling an alarm in 
which human error is precluded as far as possible when the installation is 
staffed with control personnel. 
Now in order to implement these and still further objects of the invention, 
which will become more readily apparent as the description proceeds, the 
method of the present development is manifested by the features that, the 
central signal station resets the detector a second time when the same 
responds a second time prior to the expiration of the test time period of 
comparatively shorter duration and an alarm signal is triggered or 
released in such case when the detector responds a third time after a 
delay time period which is shorter than the difference between the 
durations of the two test time periods and prior to the expiration of the 
test time period of comparatively longer duration, and the central signal 
station returns into the original state if the detector does not again 
respond prior to the expiration of the test time period of comparatively 
longer duration. 
As alluded to above, the invention is not only concerned with the 
aforementioned method aspects, but also relates to a novel construction of 
apparatus for the performance thereof. Generally speaking, the inventive 
apparatus comprises a central alarm signal station and at least one 
detector which is connected therewith and responds to ambient conditions, 
a reset circuit located in the central alarm signal station for resetting 
the at least one detector which has responded at least one time, and an 
evaluating circuit for transmitting alarm signals. This evaluating circuit 
comprises a timing circuit for generating two predetermined test time 
periods of different time durations and which transmits an alarm signal 
when, after a first reset of the detector, such detector responds a second 
time after expiration of the test time period of comparatively shorter 
duration and prior to the expiration of the test time period of 
comparatively longer duration. 
To achieve the aforementioned measures, the inventive apparatus for 
signalling an alarm, in its more specific aspects, comprises: 
switching elements in the reset circuit reset a detector, which has 
responded, a second time, when such detector has responded a second time 
prior to the expiration of the test time period of comparatively shorter 
duration. The evaluating circuit transmits an alarm signal when the 
detector, which has been reset a second time, responds a third time after 
a predetermined delay time period which is shorter than the difference 
between the two test time periods and prior to the expiration of the test 
time period of comparatively longer duration; and 
a gate circuit in the evaluating circuit blocks the transmission of an 
alarm signal when no detector response again occurs prior to the 
expiration of the test time period of comparatively longer duration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Describing now the drawings, it is to be understood that only enough of the 
construction of the apparatus has been shown as needed for those skilled 
in the art to readily understand the underlying principles and concepts of 
the present development, while simplifying the showing of the drawings. 
Turning attention now specifically to FIG. 1, there has been schematically 
illustrated a block circuit diagram of an apparatus for signalling an 
alarm and which is constructed as a fire detecting installation. A 
multiple number of fire alarms or fire detectors D are connected to a 
central signal station S via a common connecting line system L. The fire 
detectors D all may be of the same known type, for example, ionization 
fire detectors or optical smoke detectors, or may be constituted by known 
detectors which react to other phenomena related to burning or combustion 
conditions. Preferably the fire detectors D are equipped with a suitable 
self-holding circuit so that the fire detectors D, following their 
response and after exceeding a threshold value, for example, of smoke 
concentration, remain in their alarm or response state until they have 
possibly been reset. However, there can also be simultaneously connected 
to the central signal station S fire detectors D of different types, for 
example, detectors having differently delayed responses, since the 
different characteristics of such detectors are accounted for by the 
special design of the central signal station S so that no specific 
adaptation or matching is required. 
The signals originating at the fire detectors D are supplied to a reset 
circuit R containing switching elements RS and arranged at the input side 
of the central signal station S. After their first response 1.AL, the fire 
detectors D are immediately reset by means of the central signal station 
S, i.e. by the reset circuit R therein, as shown in the functional diagram 
with respect to time depicted in FIG. 2. An evaluating or evaluation 
circuit A is operatively connected to the reset circuit R and is capable 
of transmitting or initiating transmission of an alarm signal. The 
detector reset operation is effected, for example, by a short-time 
lowering of the supply voltage of the fire detectors D as is illustrated 
by R.sub.1 and by techniques well known in this technology. 
Simultaneously, a timing circuit T in the evaluating circuit A of the 
central signal station S is started by the reset circuit R. This timing 
circuit T comprises a time window, that is, it generates, at a 
predetermined moment of time after the first detector response 1.AL, two 
test time periods t.sub.1 and t.sub.2 of predetermined different time 
durations which, when conventional ionization fire detectors are used, 
preferably can be selected in the order of magnitude of 20 and 90 seconds, 
respectively. In the illustrated example, the predetermined moment of time 
at which the test time periods are started, is immediately after the first 
detector response 1.AL. There is now tested by means of such timing 
circuit T at which time after reset of such detector the fire detector D 
again responds. This second response is indicated in FIG. 2 by the 
reference character 2.AL. Three cases can be differentiated for such 
second response 2.AL: 
1. The second response 2.AL occurs after expiration of the second test time 
period t.sub.2 of comparatively longer duration, i.e. more than 90 seconds 
after the first response 1.AL. This is interpreted to mean that no 
persistent fire phenomenon is present, but only a short-lived or brief 
irregularly occurring spurious effect like, for example, electric noise 
pulses or clouds of cigarette smoke. Consequently, the release of the 
alarm signal is blocked and no alarm is signalled. 
2. The second response 2.AL occurs prior to the expiration of the longer 
second test time period t.sub.2, however, after expiration of the first 
test time period t.sub.1 of comparatively shorter duration, i.e. between 
20 and 90 seconds after reset. This is interpreted to mean that there is 
present a more enduring fire or combustion phenomenon, that is a 
persisting smoke concentration, and that the signal has originated from a 
fire detector D of longer response time, for example, from an ionization 
fire detector containing an integration circuit which, anyhow, responds 
only when the threshold value thereof is exceeded for a more prolonged 
time and thus, in any case, has enhanced reliability with respect to 
triggering faulty alarms. It is for this reason that there is therefore 
immediately activated an alarm signal transmitter AS connected with the 
evaluating circuit A and an alarm signal AS' transmitted. 
3. The second response 2.AL already occurs prior to the expiration of the 
shorter first test time period t.sub.1, i.e. prior to 20 seconds after 
reset. This is only possible in the case of rapidly responding detectors 
D. In such case it may be doubtful whether the state or condition of 
danger is persistent or whether there is only present a more persistent 
spurious effect. In this case, therefore, the reset circuit R is caused to 
again reset the fire detector D a second time after a predetermined delay 
time period t.sub.3. There is then tested whether a further response 3.AL 
occurs within the still running test time period t.sub.2 of comparatively 
longer duration. If such is the case, the alarm signal transmitter AS is 
instantly activated. When this is not the case, the alarm signal 
transmission is blocked by means of the gating circuit or gate means G of 
the evaluating circuit A and the central signal station S is returned into 
the original state. In order to achieve the effects hereinbefore 
described, the delay time period t.sub.3 must be smaller than the 
difference t.sub.2 -t.sub.1, i.e. the difference between the test time 
period of the predetermined comparatively longer duration t.sub.2 and the 
test time period of the predetermined comparatively shorter duration 
t.sub.1. The delay time period t.sub.3 may have a duration of, for 
example, in the range from 0 to preferably 30 seconds. 
Using the method as described hereinbefore for measuring the further 
response times after detector reset and for classifying the further 
response time into three time classes, there could be advantageously 
achieved, according to a practical embodiment of the inventive apparatus 
using commercially available ionization fire detectors as the fire 
detectors D, a surprising reduction in the frequency of false alarms 
during practical operation up to a factor of 5 in comparison to known 
evaluating methods using reset and response repetition and two-stage 
classification (YES/NO). Additionally, the inventive apparatus 
automatically adjusts to the response characteristics of the fire 
detectors D connected therewith, so that no adaption or matching measures 
are required. 
It will be readily apparent that the aforedescribed principles are 
applicable to individual fire detectors or fire detectors connected 
together into detector groups, as for instance shown in FIG. 1. 
Modifications and further developments are possible within the scope and 
teachings of the inventive concepts. Thus, it can be advantageous to 
connect a repetition stage in such a manner as to precede the steps of the 
inventive method, so that the start of the test time periods t.sub.1, 
t.sub.2 is only released when a fire detector D has responded a second 
time after a first response and the consecutive detector reset. It may be 
preferable to employ a number of such repetitions. 
The inventive method has been described hereinbefore with reference to a 
fully automatically operating apparatus. 
However, it may also be preferable and advantageous to use the inventive 
concepts with respect to an apparatus for signalling an alarm, for 
example, a fire detecting installation which at least partially is 
monitored by control personnel, for example, during the daytime by 
employing a suitable daytime-circuit. FIG. 3 shows a time diagram of such 
apparatus. After a first response 1.AL of a fire detector D the related 
detector group i.e. the responding detector of the group is reset and the 
time period .DELTA.t is determined until the next-following response 2.AL. 
When this time period .DELTA.t is greater than a predetermined time limit 
t', the second response 2.AL is assessed as a new event and the entire 
cycle is restarted. In the other case, i.e. with a shorter further 
response time .DELTA.t, as shown in FIG. 3, there are started at the 
second response 2.AL a control time period V.sub.1 which, for example, may 
last two minutes, and simultaneously therewith a reconnaissance time 
V.sub.2 of, for example, ten minutes. The control time period V.sub.1 can 
be interrupted by an acknowledgement or receipt signal Q signalling the 
attention of the control personnel and the reconnaissance time V.sub.2 can 
be interrupted by a reset signal E which is released or triggered by the 
dispatched reconnaissance party or by a manually released or triggered 
alarm signal. When these manual interruptions fail to appear, the group of 
detectors D is once again automatically reset at a moment of time .DELTA.t 
prior to the expiration of the control time period V.sub.1 or, 
respectively, of the reconnaissance time V.sub.2 and the test time periods 
t.sub.1 and t.sub.2 begin to start at such moments of time t.sub.0 or, 
respectively, t.sub.00. The moment of time at which the test time periods 
are started, is shortly prior to the expiration of the control time period 
V.sub.1 and of the reconnaissance time period V.sub.2 by a differential 
time .DELTA.t which is defined by the time difference between the second 
response 2.AL and the first response 1.AL or by the time difference 
between the first response 1.AL and a further response preceding the same. 
At the occurrence of a further response, that is a third response within 
the test time period t.sub.2 there is then immediately released or 
triggered an alarm signal AS'. In such arrangement the test time period 
t.sub. 1 can be set equal to 0. 
It is also possible that the aforementioned steps are only initiated after 
one or more response-reset cycles of the fire detector D. The arrangement 
may also be such that the acknowledgement signal or the reconnaissance 
signal blocks the start of the first and second test time periods. 
In this manner there is achieved the result that even in the case of human 
failure there is ensured a reliable release or triggering of an alarm 
signal AS' with the least possible frequency of false alarms. 
While there are shown and described present preferred embodiments of the 
invention, it is to be distinctly understood that the invention is not 
limited thereto, but may be otherwise variously embodied and practiced 
within the scope of the following claims. ACCORDINGLY,