The operating principles of non-addressable receiving and control devices and the main design options have already been discussed in the industry press.
The analysis was mainly carried out on noise immunity when using various circuit solutions. Let us consider in more detail the electrical characteristics of the loops of two-threshold fire alarm control panels when working with fire detectors of various types.
Requirements for the coordination of non-addressable fire alarm control panels with non-addressable fire detectors are set out in general terms in GOST R 53325-2009 «Fire-fighting equipment. Technical means of fire automation. General technical requirements.
Test methods». Clause 4.2.1.1 states that «fire detectors interacting with the fire control panel must ensure information and electrical compatibility with it.»
Clause 4.2.1.3 contains the requirement: “the electrical characteristics of fire detectors (voltage and currents of the standby mode and the alarm notification mode) must be established in the technical documentation (TD) for fire detectors of specific types and must correspond to the electrical characteristics of the fire alarm loop of the fire control and monitoring device with which the fire detectors are intended to be used.”
In the technical documentation for control and monitoring devices according to clause 7.2.1.5 of GOST R 53325-2009, the “current ranges in the non-addressable alarm loop, including the maximum supply current of the detectors, at which the control and monitoring device registers all the intended types of notifications and the range of supply voltages” must be indicated.
As a rule, the documentation for the fire alarm control panel specifies the maximum permissible current consumption of active detectors, the current limitation level of the loop in the «Fire» mode, and quite often the range of loop resistances corresponding to different modes, but the values of the loop voltages and currents are usually not specified, which makes it difficult to assess the compatibility of a specific type of detector and the fire alarm control panel.
Moreover, at present, for economic reasons, almost exclusively so-called two-threshold fire alarm control panels with identification of the 1st and 2nd detector are used, which determined the emergence of the problem of matching detectors with fire alarm control panels [1].
METHODS OF MONITORING THE STATE OF FIRE LOOPS
Various options for constructing fire alarm control and monitoring devices from the point of view of ensuring reliability are considered in detail in the article by V. Bakanov [2]. In the article by A. Pinaev and M. Nikolsky [3], existing methods for monitoring the state of non-addressable loops are reduced to two types:
— monitoring by loop voltage;
— monitoring by loop current.
A simplified structure of the loop can be represented as a voltage source UXX, about 12-24 V, a current-measuring resistor RППКП (Fig. 1), the value of which for different devices can vary in a wide range, from hundreds of Ohms to several kOhms, and an information processing device with set thresholds corresponding to the boundaries of the loop modes. In this regard, ППКП can be divided into devices with a high-resistance loop output, where the current-measuring resistor simultaneously plays the role of a current-limiting resistor, providing a short-circuit current of the loop at a level of about 20 mA, and with a low-resistance output, about 100 Ohms, where an additional circuit is used to limit the loop current. The voltage value UXX corresponds to the loop voltage without a load, i.e. in idle mode. To control the break in the loop, an end-of-line resistor ROK is installed, usually in the range from 3.3 to 9.1 kOhm, depending on the type
PPCP. The PPCP loop state can be determined by the loop current, by measuring the voltage on the current-measuring resistor. For some reason, the documentation for the PPCP usually only specifies the loop resistance in different modes. In general, the loop resistance RШС is proportional to the ratio of the loop voltage to the voltage on the current-measuring resistor: RШС = RППКПUШС/URППКП. And since a stabilized source is usually used, the sum of the voltages UШС + URППКП is constant and equal to the voltage UХХ, and the loop mode is determined by any of these values.
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| Fig. 1. The fire alarm control panel controls the loop current by the voltage on the resistor |
Let us consider several examples of fire loops with different values of voltage UXX, current-measuring resistor RППКП and end-of-line resistor RОК. Let us determine approximate thresholds for current, voltage and, based on the condition of unambiguous determination of the loop mode in accordance with the requirements of clause 7.2.1.5 of GOST R 53325-2009, let us estimate the permissible current consumption of active detectors in standby mode.
EXAMPLE No. 1
Combined loop, i.e. detectors with normally open contacts and normally closed contacts are included, while the response of the 1st and 2nd detectors to closing and opening is determined (Fig. 2). This type of loop has a maximum number of modes 7:
— break in the loop;
— two detectors triggered to open — «Fire 2»;
— one detector triggered by opening – “Fire 1”;
— standby mode;
— one detector triggered by closing – “Fire 1”;
— two detectors triggered by closing – “Fire 2”;
— short circuit of the loop and, accordingly, 6 thresholds.
As initial characteristics, we will set typical parameters: open loop voltage UXX equal to 20 V, current-limiting resistor of the loop RППКП we will take 1 kOhm to ensure limitation of short-circuit current at the level of 20 mA, end-of-line resistor RОК 7.5 kOhm ± 5%, maximum loop cable resistance RКАБ 220 Ohm and minimum leakage resistance RУТ between loop wires 50 kOhm. Then the nominal loop current in standby mode will be Iдеж = UXX/(RPКП + RОК) = 20 V/(1 + 7.5) kOhm = 2.35 mA. We will determine the maximum spread of loop parameters, i.e. with the minimum value of the terminating resistor ROK – 5% we will take into account the leakage resistance of the loop of 50 kOhm, and with the maximum value of ROK + 5% we will take into account the cable resistance of 220 Ohm. Taking into account these assumptions, the loop resistance can vary within 6.24 kOhm38.1 kOhm, respectively, the standby current can be in the range from 2.2 to 2.76 mA. Thus, the spread of the standby current exceeds 0.5 mA! Accordingly, the loop voltage in standby mode at the output of the fire alarm control panel can be within 17.24 V317.8 V.
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Fig. 2. Combined loop with double triggering for closing and opening |
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Table. 1 |
We include detectors with normally open contacts in a loop with additional resistors RДОП = 1.6 kOhm ±5%, detectors with normally closed contacts – with ballast resistors RБАЛЬ = 4.7 kOhm ±5% (Fig. 2). The loop parameters for the minimum, nominal and maximum loop resistance for different modes are given in Table 1.
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Fig. 3.Combined loop modes |
Usually, the documentation for the fire alarm control panel provides the loop resistance limits corresponding to different modes, but consideration of the corresponding currents and voltages provides additional information, allows you to assess noise immunity and determine the maximum permissible current consumption of detectors in standby mode. The data in Table 1 show that the areas of operation of one and two detectors for opening intersect, with a leakage resistance between the loop wires of 50 kOhm and when two detectors are triggered, the loop current will correspond to the rated current when one detector is triggered. That is, the device will not be able to identify the triggering of the second detector! In addition, it should be noted that even the rated currents and voltages of the loop, without taking into account the cable, differ insignificantly when the detectors are triggered for opening. When the first detector is triggered, the loop current decreases by 0.83 mA, and when the second detector is triggered, only by 0.4 mA.
Now let's define the permissible current consumption of detectors in standby mode. Alexander Zaitsev suggested introducing a term that clearly defines the problem at hand: «loop break current». Indeed, in accordance with the requirements of GOST R 53325-2009, paragraph 7.2.1.1, «Fire alarm control panels must ensure… automatic integrity control of communication lines with external devices (IP and other technical means), light and sound signaling of a malfunction». In general, a loop break is identified by a decrease in the loop current when the end-of-line resistor is disconnected. In this case, it is necessary to take into account the current consumption of fire detectors and the leakage resistance between the loop wires. What current consumption of detectors is desirable to ensure? If one loop protects up to 10 rooms, with 3 detectors per room, with a standby current of the detector of about 0.1 mA, it is necessary to ensure a current of 3 mA. However, according to the data in Table 1, if the loop break occurs at the end of the loop and the current value is 2-3 mA, the fire alarm control panel will remain in the standby mode and will not detect the malfunction. If, when the loop breaks, approximately half of the detectors are switched off, and the remaining detectors consume approximately 1.5 mA, the device will generate the «Fire 1» signal, since this loop current value corresponds to the operation of one detector to open (Fig. 3). Accordingly, if the loop break detects a detector current of about 1.2 mA, the device will generate the «Fire 2» signal! What is the «break current» in this case? Naturally, it should be less than the loop current corresponding to the formation of the «Fire 2» signal when two detectors with normally closed contacts are activated. Based on the data provided in Table 1, we can determine the “loop break current” at which the “Fault” signal will be generated to be less than 1 mA, and taking into account the loop leakage current, which can reach 0.4 mA, the maximum permissible current consumption of the detectors should be reduced to approximately 0.5 mA.
But if there are detectors in the loop for opening, in our case, connecting detectors with a current consumption of 0.5 mA is also unacceptable. The nominal current of the loop in the «Fire 2» mode, corresponding to the operation of two detectors with normally closed contacts, equal to 1.12 mA, will increase to 1.62 mA, which corresponds to the «Fire 1» mode. That is, the device in principle does not allow the simultaneous inclusion of normally closed detectors and current-consuming detectors in the loop.
EXAMPLE No. 2
To eliminate the obvious shortcomings of the loop shown in example 1, in practice, two or three types of loops are used in the fire alarm control panel: a loop with only normally closed detectors (Fig. 4) and a loop with only normally open detectors (Fig. 5) with detection of the response of two detectors; sometimes, a combined loop with different types of detectors is also allowed, but with detection of the response of only one detector and with a minimum current of the detectors in the standby mode. In this case, for a loop with active detectors, with the same initial parameters of the fire alarm control panel, the «break current» should not fall into the area allocated for the standby current, and taking into account the leakage current, the maximum current consumption of active detectors could be increased to approximately 1.5 mA. However, the boundary between the Fire 1 and Fire 2 modes is only 1 mA, and in order for the Fire 1 signal to be generated when one detector is triggered, and not Fire 2, the detector current must be, accordingly, less than 1 mA.
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Fig. 4. Loop with normally closed detectors
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Fig. 5.Loop with normally open detectors |
In a combined circuit, a ballast resistance value approximately twice as large is usually selected, for example, Rbal = 10 kOhm, and an additional resistance that is half as large. Accordingly, the delta between the standby mode current and the “Fire” mode current increases when a normally closed detector is triggered (Table 2). However, the “break current” remains the same as in example 1, therefore, the current of active detectors should also be less than 0.5 mA.
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Table 2 |
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Table. 3 |
Sometimes there is a recommendation to compensate for the current consumption of active detectors by increasing the end-of-line resistor.
Obviously, in a combined circuit with a circuit current in the «Fire» mode from the detector for opening of about 1 mA, there can be no talk of any compensation.
In a circuit with smoke detectors, compensation for the increase in current of a large number of detectors by reducing the current of the terminating resistor allows you to «fit» into the thresholds of the standby mode and the «Fire 1», «Fire 2» modes, but if the «break current» is exceeded, the device will not detect a break in the circuit.
EXAMPLE No. 3
Let's change the parameters of the loop, to increase the standby current we will increase the maximum loop voltage UXX to 26 V, set the end-of-line resistor to 3.9 kOhm ± 5%, and take the current-limiting resistor of the loop RППКП to be 1.2 kOhm.
In this case, the nominal current of the loop in standby mode will increase to 5.1 mA. The short-circuit current of the loop will be less than 22 mA, which makes it possible to connect detectors without current-limiting resistors.
To generate the «Fire 1, 2» signals, we connect detectors with normally open contacts to the loop with additional resistors of 2.7 kOhm ± 5%, detectors with normally closed contacts with ballast resistors of 2.2 kOhm ± 5%.
We will leave the maximum resistance of the loop cable and the minimum leakage resistance the same as in the first two examples RКАБ = 220 Ohm, RУТ = 50 kOhm. The calculation results are given in Table 3.
Reducing the nominal value of the terminating resistor by approximately 2 times determined a significantly smaller effect on the value of the loop resistance of the parallel connection of the cable leakage resistance, but, accordingly, the effect of the series-connected cable resistance increased.
Let's determine the «loop break current» for this case.
The minimum standby current is 4.71 mA, which seems to suggest a higher current consumption of the detectors, compared to the previously considered examples, but another limitation appears here. The maximum standby current without taking into account the current consumption of active detectors can reach 5.59 mA, and the minimum loop current when the first detector is triggered is 6.91 mA.
Therefore, to avoid false «Fire 1» signals in the standby mode, the maximum current of the detectors must be less than 1 mA. On the other hand, it should be noted that the maximum loop current in the «Fire 1» mode is 9.73 mA, and the minimum loop current in the «Fire 2» mode is 8.8 mA (Table 3), i.e. in this example, it is possible to generate a false «Fire 2» signal when one detector is triggered, or when the second detector is triggered, the device may remain in the «Fire 1» mode.
The areas of the «Fire 1» and «Fire 2» modes intersect, which does not allow the thresholds to be selected correctly even in the absence of current-consuming detectors.
For a loop with normally closed detectors, the areas of the «Fire 1» and «Fire 2» modes, although they do not intersect, their boundaries practically coincide.
In addition, when assessing the stability of the device, one should also take into account the instability of the device parameters, temperature threshold drifts, drift during aging, etc.
Obviously, the complexity of constructing two-threshold devices determined the development of the PPCP with adaptive thresholds, which allows to some extent to take into account the initial parameters of each loop.
However, the capabilities of auto-compensation are limited and not everything can be compensated, for example, the spread of the nominal values of resistors RДОП and RБАЛЬ for each detector, cable resistance and cable leakage resistance are of a distributed nature and their influence depends on the location of the detector in the loop.
In the best case, the nominal parameters of the loop can be ensured.
In conclusion, it should be noted once again that the documentation for the fire alarm control panel usually provides only the ranges of loop resistance for various modes, despite the fact that clause 7.2.1.5 of GOST R 53325-2009 states that “the fire alarm control panel must have the following designation indicators, the numerical values of which are provided in the technical documentation (TD) for the fire alarm control panel of a specific type:
– current ranges in a non-addressable alarm loop, including the maximum current supplying the detectors, at which the fire alarm control panel registers all the provided types of notifications.”
The lack of information in the documentation on the fire alarm control panel modes depending on the loop current does not allow us to correctly determine the permissible current of the detectors in standby mode and assess the compatibility of the device with fire alarms of various types, especially with smoke fire alarms with a non-linear volt-ampere characteristic, but this is the topic of a separate article.
In practice, a fairly simple method can be recommended to check the provision of a «loop break current»: turn off the last fire alarm, in the base of which an end-of-line resistor is installed, and monitor the formation of the «Fault» signal on the fire alarm control panel.
If the «Fault» signal is missing or the «Fire» signal is generated, then the standby current of the detectors exceeds the «loop break current».
In this case, it is necessary to switch off the detectors one by one until the «Fault» signal appears.
After this, switch off several more detectors to ensure a technological reserve, and connect the bases of the removed detectors to an additional loop or to additional loops if the number of removed detectors is greater than the number of remaining detectors.
Links on the topic:
Analysis of the loop parameter of a two-threshold fire alarm control panel. Part 2
LITERATURE
1. Neplokhov I. Classification of non-addressable loops, or Why there are no two-threshold devices abroad //»Algorithm of safety», No. 3, 2008.
2. Bakanov V. Key to high-reliability fire alarm systems //SECURITY.UA, No. 2, 2010.
3. Pinaev A., Nikolsky M. Assessment of the quality and reliability of non-addressable fire alarm devices //»Algorithm of safety», No. 6, 2007.
Source: magazine «Algorithm of safety» No. 5, 2010