Protection of fire alarm systems from lightning and switching surges.
This article will consider the issues of protection from lightning and switching surges of fire alarm equipment.
The article is intended for technical specialists involved in the installation and operation of fire alarm equipment.
Sharing experiences with other manufacturers, as well as representatives of installation and operating organizations, confirms that equipment failure due to lightning discharges is a fairly common occurrence.
And it's not just about the quality of the equipment, installation errors, or violation of operating rules. The same equipment can work properly for years at some sites and regularly fail at others. The unpleasant consequences of surge voltages are not only manifested in equipment failure.
Failures in operation are no less dangerous.
For example, there are known cases of false starts of automatic fire extinguishing systems: in this case, the equipment is in good working order, the protection of the starting circuits is triggered, protecting the output electronic keys from destruction.
However, the pyropatron is activated, since the current induced by the interference is sufficient to detonate it.
Attempts to use any additional protection devices increase the cost of equipment and installation, but also do not give a noticeable result.
So, why does reliable equipment fail and how to deal with it?
Sources of dangerous impulse overvoltages.
1. Lightning discharge is the most powerful source of impulse overvoltages.
During a lightning discharge, huge currents arise in its trunk, during the flow of which dangerous voltage potentials arise.
Lightning protection systems, which include lightning rods and grounding, are designed to protect buildings and people from electric shock, but not to protect electronic equipment and communication lines.
Therefore, a direct lightning strike on a building almost always results in failure of electronic equipment.
Real protection from lightning discharge can be discussed if the distance to it is at least hundreds of meters.
Fortunately, a direct lightning strike is a fairly rare occurrence.
Therefore, the most probable impact on the OPS equipment should be considered to be an electromagnetic pulse arising between clouds and a remote lightning strike to the ground.
For the central regions of Russia, the intensity of thunderstorm impact is approximately 50 hours per year, with lightning affecting 1 km2 of terrain on average twice a year.
For the northern regions of Russia, lightning affects 1 km2 of terrain once a year, for the southern regions – up to five times a year.
Therefore, for the middle zone on communication lines or power lines, dangerous interference in the form of 10 kV voltage pulses should be expected once a year and up to 50 times a year – pulses of about 1 kV.
For southern regions with increased thunderstorm activity, the frequency of occurrence of dangerous voltages, accordingly, increases fivefold.
It should be noted that thunderstorms are not the only source of overvoltages that can disable electronic equipment; there are other causes that can create sufficiently powerful impulses. These include three more large groups.
2. Switching impulse interference.
The main source of switching impulse interference is transient processes during the following operations in the electrical network:
Switching on and off electrical consumers (electric motors, incandescent and fluorescent lamps, computers and other equipment).
Switching on and off circuits with high inductance (transformers, starters, etc.).
Emergency short circuits in low-voltage networks and their subsequent switching off by protective devices.
Emergency short circuits in high-voltage networks and their subsequent switching off by protective devices.
Switching on and off electric welding units.
The source of impulse interference is urban electrified transport, including the metro, as well as electrified railways. This group of interference, as a rule, is single impulses with an amplitude of up to several kilovolts.
In accordance with GOST, the presence of switching interference impulses with an amplitude of up to 4.5 kV and a duration of up to 5 ms in a 220 V network is considered acceptable.
In reality, the frequency of occurrence of single impulse interference with an amplitude of up to 300 V is on average 20 interferences per hour for industrial enterprises, and 0.5 interferences per hour for residential buildings.
The most dangerous interferences with an amplitude of 1 to 10 kV make up to 0.1% of the total number of impulse interferences.
Thus, in an office located on the territory of an industrial enterprise, electronic equipment is exposed to powerful interference on average three times a week, and in a residential building — up to four times a year.
In addition to single impulse interference, periodic impulse interference occurs in power supply circuits due to the operation of fluorescent lamps, power supply converters, etc.
This type of interference reaches an amplitude of up to 1 kV, has a wider spectrum and leads to both failures and damage to equipment.
Switching impulse noise of various durations in the 220 V power supply circuits of most fire alarm system equipment under normal operating conditions can disable it only if the amplitude of the interference exceeds 1 kV.
The probability of damage to equipment in the power supply circuits increases many times in conditions of high humidity or in conditions of high dustiness, which is typical for industrial facilities.
Damage to power supplies of fire alarm system equipment is a consequence of the impact of impulse noise in the electrical network.
It should be noted that pulse power supplies are damaged much more often and linear ones are damaged less often.
3. Overvoltage and voltage dips in the power supply network
The reasons for overvoltage in power supply networks are primarily due to the low quality of power grids and low energy consumption culture. Therefore, we will highlight only the most typical problems of power supply.
Maximum voltages in the power grid are usually associated with minimum loads on the power system and are observed at night. The greatest fluctuations in voltage in the power grid occur at the beginning and end of the working day.
In reality, at industrial facilities, periodic («day-night») fluctuations in the 220 V power grid from 160 V to 260 V with short-term increases to 300 V are possible.
Overvoltage in the power grid disables standard simple protection circuits against impulse interference (varistors, etc.), impulse power supplies. Separately, two common installation errors can be singled out that lead to overvoltage:
phase imbalance of the power grid due to overload of one phase by electricity consumers;
overload of the neutral of the power grid due to a smaller cross-section of the conductor at the neutral than at the phase.
4. The electrostatic charge that accumulates during the operation of process equipment is interesting in that, although it has little energy, it is discharged in an unpredictable place.
Paths of penetration of impulse overvoltages into the fire alarm system equipment.
Regardless of the source of the surge voltage, the penetration paths of surge voltages are similar. The main condition for penetration, in addition to the source of surge voltage, is the presence of a long line in which the interference occurs.
Such lines are:
1. RS-232 computer connection cables.
2. Analog threshold signaling loops.
3. Low-voltage (12 V) power supply wires for the units.
4. High-voltage (220 V) power supply wires for the units.
5. Connecting wires for electronic keys with load.
6. Digital addressable fire alarm loops.
7. Connecting wires for optoelectronic relays with load.
8. Connecting wires for electromechanical relays with load.
9. Automatic fire extinguishing starting circuits.
10. Analog video cables.
11. Interblock LAN cables.
12. Interblock CAN network cables;
This list is ranked by the degree of resistance to overvoltage. The list shows the advantage of distributed systems based on the CAN interface.
Let's take a closer look at the mechanisms of the impact of high-voltage impulse interference on connecting lines.
When lightning strikes objects located in the immediate vicinity of the network installation sites, the potential of the building and the PC may increase to a significant value due to the spread of lightning currents.
The distribution of potentials over the earth's surface will depend on the distance to the epicenter of the lightning strike and the power of the lightning discharge.
The example (Fig. 1) shows the potential distribution in clay soil (specific resistance p = 60 Ohm*m) depending on the distance to the lightning strike.
The lightning current is 20 kA. It follows from the figure that when such lightning strikes, a potential difference of 6.4 kV is formed between the buildings, which will lead to the failure of equipment connected by a UTP cable.
The use of arresters in this case helps to significantly reduce the dangerous potential.
Fig. 1. Distribution of potentials when lightning strikes the ground. Power supply is provided by two different substations
Fig. 2. Main routes of overvoltage penetration into buildings and structures of security facilities.
Thus, external electromagnetic pulses, regardless of the source that generated them, lead to the formation of a potential difference on an extended communication line. The value of the potential difference depends on the intensity of the external electromagnetic field, the rate of its change, the length of the communication line, and can reach tens of kilovolts under certain unfavorable conditions.
Protection from impulse overvoltages.
To protect an object from the effects of any type of overvoltage, it is first necessary to create an effective grounding and potential equalization system.
In this case, it is desirable to switch to TN-S TN-C power supply systems with separated neutral and protective conductors. The figure on the right shows a diagram of such a 3-wire connection.
The third wire PE is used to ground the equipment and is connected to the physical ground at one point with the neutral wire N.
The neutral and phase wires are protected. Excess potential «flows» into the ground through the PE wire.
Combining PE and N wires worsens protection. The basic principles of using surge protection devices are discussed in [3].
An effective method of protection is zonal division of the object.
In an object divided into zones, when moving from one zone to another, the peak values of overvoltages are limited to levels permissible in a given zone. The higher the zone number, the lower the values of permissible levels of impulse interference.
Arresters are used as first-stage protection diverting devices — gas-discharge devices that have a certain breakdown voltage, at which their resistance decreases sharply.
After passing through the 1st stage (arrester), the potential in the line is limited at the breakdown potential level, which is usually ~ 350–500 V for short pulses; for long-term breakdown processes, it is about 90 V (cases of dangerous voltages from other sources, for example, when a power cable falls on the line).
Note that the use of fuses will not give results, since their response time to the pulse significantly exceeds the time of the overvoltage itself.
To further limit dangerous voltage, a second stage of protection is performed. It is separated from the first stage of protection by current-limiting elements (chokes, resistors).
The second stage is usually built on zener diodes or suppressors.
They further limit the voltage from 350–500 V to 6–7 V; the transmitted power is up to 1.5 kW. In many cases, this is enough to prevent equipment failures.
The practice of operating the equipment shows that not all facilities where the fire alarm system is installed have zonal protection against impulse overvoltages. In some cases, the facilities do not even have grounding.
And their creation is not always within the competence of the design and installation organizations of the fire alarm system because these are expensive engineering and technical structures and in many cases the customer is not ready to bear such costs.
In the absence or insufficiency of protective measures in difficult thunderstorm conditions (or other sources of overvoltage), additional electronic products can be used: «surge protection devices» (SPD) in the form of separate devices that allow you to increase the degree of protection.
The general principle of operation of all SPDs is to reduce the hazardous potential and its timely discharge to grounding.
In this case, SPDs use multi-stage combinations of protective elements with different characteristics:
Products of this type are widely used to protect household and professional equipment and are produced by various manufacturers.
When choosing a SPD, you should be guided by its purpose:
— protection of power circuits;
— protection of data transmission lines;
— video protection, etc., as well as the required degree of protection provided.
When using a SPD, it is mandatory to ground it and comply with the rules for connecting protected circuits recommended by the manufacturer.
The question arises: why do manufacturers of fire alarm equipment not build all these elements into their products?
Firstly, the use of this protection does not provide a 100% guarantee.
Secondly, embedding security components in serial equipment significantly increases its cost.
What can be suggested in this regard? The use of distributed systems is very effective.
Let's consider an example in which 8 security detectors are located 1.5 km from the control panel.
In a traditional centralized system, it is necessary to lay 8 two-wire loop lines and one two-wire power supply line for the detectors, 1.5 km each. It is easy to imagine the noise vulnerability of such long lines and their cost.
In a distributed system, remote alarm loops can be connected not to the control panel, but to a unit remote from it.
The control panel and the unit are connected by one twisted pair of CAN network.
The transceivers of this network have sufficiently powerful protection, ensuring serviceability during lightning strikes.
And the alarm loops themselves are short and therefore less susceptible to overvoltage. And plus significant savings in cable products.
The most important measure in the surge protection system is the grounding device. Let us recall some terms and general rules in the grounding device: Grounding is a physical connection with the earth's soil. Protective grounding is grounding in order to ensure the protection of personnel from electric shock.
Protective grounding often worsens the interference environment for automation systems due to the flow of large industrial currents through its circuits.
Common wire – a conductor relative to which the electric potential is measured. In this case, the common wire can be in circuits with different current values: power (amperes and more) and signal.
Power and signal circuits must be galvanically isolated, otherwise the power circuits will affect the operation of the signal circuits.
Signal grounding is the connection of the common wire of signal circuits to the ground. Signal grounding can be screen and base.
Screen (braiding) cables, block screens, instrument housings are connected to screen grounding and serve to protect circuits from parasitic interference. Base signal grounding is used to tie the potentials of different blocks of a distributed system to one common value.
Otherwise, high potentials of different magnitudes may arise in units located far from each other under the influence of various causes (lightning discharges, industrial interference, static, etc.), which leads to breakdown of components and malfunctions.
In addition, an ungrounded cable screen increases the impact of interference, acting as an antenna. Signal grounding should be performed at one point.
Otherwise, large currents of power circuits may enter the common signal wire, which will lead to failures and accidents. It is advisable to select the connection point near the power source of the fire alarm system, located near the electrical distribution board, which has reliable grounding.
Conclusions and recommendations:
1. Thunderstorms are not the only source of surge voltages. At each specific site, specialists must assess the interference environment, the adequacy of protection measures in order to take into account the risks during the design, installation and operation of the fire alarm system
2. To ensure stable operation of the fire alarm system equipment in conditions of long lines and complex interference environments, users are recommended to take additional protective measures.
3. The concept of lightning protection cannot be reduced to the level of a single device, but is a complex set of technical measures. The implementation of protective measures must be carried out by trained specialists. Incorrectly performed protection can worsen the situation.
4. Distributed fire alarm systems are more resistant to impulse overvoltages in conditions of long connecting lines.
5. More reliable methods of protection against impulse overvoltages will require more serious additional design and installation works, material and monetary costs. These issues should be discussed with the customer when concluding contracts, highlighting them in separate clauses in order to subsequently specifically stipulate mutual claims.
Used references:
1. GOST 13109-97 Quality standards for electrical energy in general-purpose power supply systems.
2. GOST R 50571.26-2002 Electrical installations of buildings. Part 5. Selection and installation of electrical equipment. Section 534. Devices for protection against impulse overvoltages.
3. GOST R 50009-2000 Technical means of security alarm systems. Requirements and test methods.
4. RD 34.21.122-87 Instructions for the installation of lightning protection for buildings and structures.
5. A. Kiselkov, E. Kochetkov. The main reasons for the failure of video surveillance equipment. http:///dailypblshow.cfm?rid=8&pid=974