Fire alarm systems. Aspects of reliability and survivability.
Comparing fire alarm system structures is a tricky business. Each solution has its pros and cons. But let's look at the most common structures using key terms such as «reliability» and «survivability.» What determines the reliability of fire alarm systems? How does reliability differ from survivability? And why must a fire alarm system remain operational even after a fire has started?
The primary task of a fire alarm system is to ensure the timely evacuation of people from a building during a fire. Obviously, the duration of the evacuation depends on the complexity of the facility.
At the beginning of such a complexity scale, for example, you can place a small store with an area of about 500 m2: it is easy to enter, easy to run out in one minute in case of a fire. Much further along the scale, you will have to place kindergartens, hospitals with seriously ill patients, nursing homes: evacuation at such facilities can take hours.
What is necessary for a fire alarm to ensure timely launch of the fire warning system, and most importantly, to allow evacuation even after the fire has started? Probably two things: to be reliable — that's one, and to be durable in an emergency — that's two.
The concept of reliability includes:
reliable detection of a fire at the initial stage of fire development,
the absence of false alarms that reduce confidence in the system.
Survivability is a parameter characterizing the ability of a fire alarm system to function during the development of a fire during the entire period of time required to evacuate people from the building. Previously, the tasks of fire alarm systems included only the detection of the primary source of fire, after which almost instant evacuation was assumed. But what if the fire developed uncontrollably, secondary sources of fire appeared? What if smoke got through the interfloor ceilings into rooms where it was not expected, and cut off the evacuation route? Approaches to the construction of fire alarm systems are changing, the gloomy statistics of recent years force us to.
Let's consider several of the most common structures of fire alarm systems, taking into account, first of all, their reliability and survivability. We will talk about systems:
with a horizontal-vertical structure (Fig. 1);
with a distributed structure (Fig. 2);
with a ring structure (Fig. 3);
wireless with dynamic routing (Fig. 4).
Let's start with the traditional horizontal-vertical structure (Fig. 1). The cable network, as a rule, has one common riser with floor branches. On the horizontal sections of the cable, cable boxes are connected, from which the alarm loops directly branch off. As shown in the figure, if the integrity of the cable between the 1st and 2nd floors is compromised (burns out), the entire alarm system on the 2nd and 3rd floors fails.
Fig. 1 Horizontal-vertical structure of fire alarm systems
When constructing fire alarm systems based on control and monitoring devices with a distributed structure (Fig. 2), in which the units or expanders are connected via the RS-485 interface, as a rule, only one main line is laid. If it is damaged, for example on the 2nd floor, part of the building is again left without an alarm.
Fig. 2 Distributed structure of fire alarm systems
Both the first and second structures have a minimum reserve of survivability in emergency situations and can perform their functions only at the initial stage of a fire. Burnout of wires or cables (indicated by a red cross in the figures) leads to the loss of information from most of the facility and the inability to promptly change the evacuation routes for people.
Fig. 3 Ring structure of fire alarm systems
The ring structure (Fig. 3) with devices for disconnecting short-circuited sections is more resilient — if the integrity of the line is violated, for example, on the 2nd floor, signals from addressable detectors located on the 3rd floor will go through the intact half-ring.
Fig. 4 Wireless structure of fire alarm systems with dynamic routing
To maximize the survivability of fire alarm systems, professional wireless systems implement a dynamic routing algorithm (Fig. 4). The radio channel, as is known, is indestructible, and even if some of the detectors fail as the fire develops, the remaining equipment will continue to function at full capacity, which will allow monitoring the dynamics of the fire and promptly managing the evacuation of people in accordance with the developing situation.
Reliability of fire alarm systems
As has already been said, the reliability of fire alarm systems is determined by several factors. And if, with the current level of technological development, satisfactory reliability of detection can be achieved with any method of constructing systems, then the situation with false alarms is different.
A clear example from everyday practice is fluorescent lamps and fire alarms. Each of us has either personally encountered or heard about the problem of false alarms when detectors and fluorescent lamps are located close to each other.
That is why I would like to draw the readers' attention to false alarms associated with induced electromagnetic interference in communication lines, connecting lines and alarm loops.
First of all, these are false alarms that occur as a result of the control and monitoring device's response to interference induced in the alarm loop (see Fig. 5). Just imagine such an explosive mixture: a long loop, high input resistance of the device itself, high resistance of the loop's terminating resistor, and a loop status control mode that monitors the input voltage rather than the current. The result: instead of a fire alarm, we get a very good detector receiver with a sensitive antenna. You click the light switch — an alarm sounds. You turn off the pump — an alarm sounds. You turn on the welding machine — another alarm sounds…
Fig. 5 Impact of electromagnetic interference on wired control and monitoring devices
Two other types of interference are directly related to wired smoke threshold detectors (see Fig. 6). Induced interference along the power supply circuits can affect both the input and output circuits of the detector. The output circuits of these detectors are always accessible to pulse interference, especially since good filtering in the detector is rarely done. The effect of interference from the loop on the input power supply circuit may occur less frequently and only at the moment of measuring the smoke in the detector chamber, but the sensitivity of the input circuit is greater than that of the output circuit. Accordingly, such an alarm system will rather constantly disturb than ensure fire safety for the customer.
Fig. 6 Impact of electromagnetic interference on wired fire detectors
It is possible, of course, to lay alarm loops of minimum length and, as required, at a distance of at least 0.5 m from power cables. It is possible to use shielding of cable networks… But then the cost of work will increase with an unknown final result. The way out of such a vicious circle is to use communication channels that have greater (compared to traditional wired) immunity to electromagnetic interference. We are talking about professional wireless alarm systems of the latest generation. Anticipating skeptical assessments of readers, I suggest paying attention to such a parameter as the degree of rigidity for resistance to electromagnetic oscillations. Where wired systems barely provide the II degree, modern wireless ones easily provide the IV degree. Antennas are much shorter, and accordingly, resistance to electromagnetic interference is much higher (see Fig. 7).
Fig. 7. Insensitivity of wireless fire detectors to electromagnetic interference
Thus, if you take a detector with a high-quality smoke chamber and ensure the transmission of signals via communication channels without wires (accordingly, without induced interference), the number of false alarms can be reduced to almost zero. If it is possible to transmit not just generated notifications about the detection of a fire, but also transmit in digital format the current level of smoke in the room or dustiness of the smoke chamber, which can be analyzed and assessed, then we are talking about a significant increase in the quality of service of these detectors during operation.
Conclusions: reliability of fire alarm systems: the higher, the shorter the total length of wire lines: shorter antennas; the higher, the fewer analog loops are used and the more connections via digital channels are used: error correction during signal transmission; wireless systems are higher than wired systems: the impact of electromagnetic interference is significantly lower.
Survivability of fire alarm systems
Let's analyze the survivability of fire alarm systems over two time intervals:
during the development of a fire at the facility,
day after day in the process of current operation.
Over the past few years, the simplified view of the survivability of a fire alarm system has been revised. An understanding has emerged of the need to ensure the operability of a fire alarm system (and not just a warning system!) for the entire time required to evacuate people from buildings and premises. These changes are reflected in the «Technical Regulations on Fire Safety Requirements» — the new fundamental federal law in the field of fire safety.
As has already been discussed, people will run out of a store after the salespeople in a couple of minutes at most during a fire – try to linger there! The situation is much more complicated with socially important facilities: how long will it take to evacuate a four-story nursing home? At least an hour, or even two or three, it is quite difficult to calculate. During this time, most of the fire alarm wires will naturally fail, and secondary sources of fire in the adjacent wing of the building will not be detected. Smoke gets onto evacuation routes through the stairs and ventilation ducts, making it impossible to leave the building (Fig. 8). It is necessary to urgently change the order and sequence of evacuation, and possibly also the operating mode of the smoke removal system on the evacuation routes. It is for this reason that in 2008, a standard for the non-flammability of connecting lines in warning systems was introduced into NPB 104. However, non-flammability alone is not enough – to save people from emerging smoke traps, it is necessary to monitor the spread of smoke throughout the building and, accordingly, change the access routes of fire brigades, which is only possible with wireless alarm systems (Fig. 9).
Fig. 8 Inability to control evacuation after the start of a fire
So, how can we increase the survivability of a fire alarm system in accordance with modern requirements? Let's consider the experience of telecommunication networks.
The fundamental quality of telecommunication systems is their multi-connectivity, in other words, a signal from point A to point B can come via more than one route. Special devices are used — routers — with a built-in automatic path search system, allowing networks to perform their functions even if several sections fail. For this purpose, routers have a special algorithm for searching for a signal transmission path from a device with one address to another. When there are several connected levels with ring lines, a sufficient number of reserve bypass routes appear, therefore, the survivability of networks increases significantly.
In security and fire alarm systems, both wired and wireless, this problem has not yet been solved. For wired fire alarm systems, this is a relatively expensive pleasure and things have not gone beyond devices for disconnecting short-circuited sections or using ring lines (Fig. 3). In wireless systems, such tasks were not set at all due to the fact that the overwhelming majority of these were small amateur systems.
With the advent of professional wireless systems on the market, the issue of dynamic routing has become relevant. Already at the initial stage of development, the task was to maximally increase the survivability of wireless fire alarm systems in the event of any emergency situations at the facility (Fig. 4). Of course, some of the equipment may fail as the fire develops, but the part of the premises where there is no open fire yet (and the speed of its spread is two orders of magnitude lower than the speed of smoke spread) will be under control, which will allow monitoring the dynamics of the fire and making adequate decisions.
In other words, one of the main conditions of dynamic routing in professional wireless systems is the automatic search for the shortest signal delivery route when the conditions for radio signal propagation change or when certain nodes fail.
This allows us to talk about a qualitatively new level of survivability of fire alarm systems in emergency situations. A unique opportunity has appeared to receive signals about fires in premises as long as there is at least one fire alarm, and to promptly manage evacuation as long as there is at least one voice alarm.
Fig. 9. Wireless systems: managing evacuation after a fire has started
Briefly about the life of a fire alarm system «day after day»: how long can a facility exist without redevelopment or ordinary cosmetic repairs of individual rooms? A year or two. And often even a couple of months after the facility is put into operation, it is necessary to change the configuration of the alarm system, for example, it was decided to rebuild some of the rooms on the second floor. At the same time, the wired fire alarm system is constantly in a disabled state — either there the loop needs to be temporarily dismantled, and this means several rooms at once without control, or here the equipment needs to be moved — another whole group of rooms is left without protection. Often the reality is such that the customer does not call specialists who will carry out restoration work in a timely manner. So an almost new, but non-working alarm system stands for several years. As signalmen say: «There is a connection, but it does not work.» Using wireless fire alarm systems reduces the risk of such situations. The repair began — the detector was removed from the ceiling and put in a desk drawer. The repair is finished — the detector was hung back. The system as a whole does not suffer from this, no one cuts any connecting lines or cables.
The above concerns finishing works, but there are also electrical networks, computer networks, and, finally, industrial automation. Each of the specialists in their field, laying their communication lines, one way or another comes into contact with the fire alarm communication lines. Not by malicious intent, but due to their insufficient qualifications, they leave an indelible mark on the performance of the fire alarm system. And this is day after day, all year round.
If we take into account that a turnkey installation of a wired and wireless fire alarm system at the same facility can be done at the same cost, then we can confidently say that the transition from wired fire alarm systems to wireless ones is as inevitable as the mass transition to mobile communications was inevitable.
For large distributed facilities, it is advisable to use hybrid (wired and radio) fire alarm systems. When using such a structure, a maximally protected ring signal line-trunk is laid with radio extensions covering individual buildings, floors and rooms. Thus, maximum reliability and survivability of all levels of the alarm system are ensured at minimum costs.
Conclusions: survivability of fire alarm systems: higher for systems whose devices can send an alarm signal along more than one route. For example, using ring lines (Fig. 3) or dynamic routing (Fig. 4); significantly higher for professional systems using wireless technologies, which are truly fire-proof (Fig. 4).
This article is the first attempt to analyze the most common structures and classes of fire alarm systems in terms of their survivability and reliability:
the choice of an alarm system should be made taking into account the configuration and purpose of the protected building in order to avoid the risk of unstable operation in life «day after day» and complete failure of equipment in the event of an emergency. For example, for a small store, a traditional wired system will be enough, and in a regional hospital it is necessary to install the most reliable and survivable system that allows monitoring the dynamics of a fire and promptly managing evacuation even after the start of a fire;
it is in the area of fire safety that the most striking advantages of modern professional wireless alarm and notification systems are manifested.