Protection of fire alarm systems from static electricity. Lightning protection of objects.
After the fire alarm system is installed, configured and successfully delivered to the customer, it (the system) begins its working days.
But when performing such a noble task as protecting the life and health of people, as well as their property, the security system itself is subject to various dangers.
Perhaps the greatest troubles are caused by various external electromagnetic influences. In this article, we will consider the impact of static electricity and lightning on the performance of fire alarm systems.
The key to successfully countering these adverse phenomena is in the competent design of the system, proper installation and subsequent operation.
Static electricity.
Static electricity is a set of phenomena associated with the generation, preservation and relaxation of a free electric charge on the surface or in the volume of dielectrics or on insulated conductors.
This is the definition according to GOST 12.1.018-93 «Fire and explosion safety of static electricity».
How does a charge arise?
Basically, static electricity is generated by friction of objects – the triboelectrification effect. Triboelectricity (from the Greek tribos – friction) is the phenomenon of the formation of electric charges during friction and subsequent separation of materials.
Examples of formation can be the most basic things: walking is one of the largest sources of triboelectric charge.
When walking, the sole of the shoe comes into contact with the floor covering, and then they are separated.
This action occurs repeatedly.
The human body is a good conductor, which allows it to conduct and accumulate charges that form during the separation of two materials. When walking on a carpet, a potential of up to 15,000 V can form on a person.
How to deal with electrostatics.
Means of protection against static electricity are divided into the following types according to the principle of operation:
— grounding devices;
— neutralizers;
— humidifying devices;
— anti-electrostatic substances;
— shielding devices.
First of all, the fire alarm system equipment must be properly grounded.
The ground leakage circuit operates satisfactorily if its resistance does not exceed 106 Ohm.
Grounding is effective only for materials with a specific resistance of no more than 1010 Ohm•m.
Thus, if the surface of the devices is plastic, grounding may not always be effective. In this case, other methods of combating static should be used.
Air ionizers capable of generating ions of both polarities are used to discharge dielectric surfaces.
Such ionizers are used for local neutralization of charges directly at workplaces or they are added to ventilation systems so that the flow of filtered air is ionized and charges on walls, ceilings, equipment surfaces, etc. are neutralized.
Electrification of dielectric materials decreases sharply with increasing air humidity, but this worsens the operating conditions of the equipment.
Therefore, as a rule, humidity should not exceed 40%.
In addition, to prevent electrification when walking, as well as to organize an additional path for the “drainage” of electrostatic charges, the room where the control and monitoring equipment is located should be equipped with an antistatic floor covering.
The simplest thing is to lay a special conductive linoleum with an electrical resistance of about 107 Ohm relative to the ground, at which the charges on them decrease to safe values within 0.02 s.
It is highly desirable to protect the operator's workplace itself, if any. Tables should have a conductive coating made of carbon-impregnated plastic, conductive divinyl or antistatic material.
These coatings are usually grounded using buses laid on the tables under the coating. Chairs can have similar coatings.
If all the above conditions are met, we get guaranteed protection of the fire alarm equipment from electrostatic discharge.
And the losses from one such blow can many times exceed all the costs of preventive measures.
Lightning protection.
Atmospheric electricity discharges during thunderstorms are an extremely beautiful, but equally dangerous phenomenon.
In addition to the fact that lightning directly threatens human life, in the absence of a properly designed and installed lightning protection system, it can cause damage or even complete destruction of both electrical equipment and the property as a whole.
The entire history of mankind has been shaped by the inevitable celestial danger: a lightning strike was equated with God's punishment, there was no escape from it.
But in 1753, Benjamin Franklin, yes, the same one depicted on the $100 bill, invented the lightning rod.
This discovery introduced a new word into the language — lightning protection.
Modern standards provide everything necessary to ensure lightning protection of buildings and electrical equipment.
The main regulatory documents for the development of a set of lightning protection tools at the design stage are RD 34.21.122-87 «Instructions for the installation of lightning protection for buildings and structures», «Instructions for the installation of lightning protection (lightning protection) for buildings, structures and industrial communications» (SO-153-34.21.122-2003), PUE-7 edition of GOST R 50571.19.
In accordance with these standards, appropriate lightning protection against direct and indirect lightning strikes must be provided.
The level of protection depends on the importance of the facility and the possible consequences of lightning strikes. For example, conventional industrial enterprises are divided into 4 categories (I, II, III and IV), the reliability of lightning protection for which must be at least 0.98; 0.95; 0.90 and 0.80, respectively.
For special industrial facilities, the protection level is set within the range from 0.9 to 0.999 in agreement with state control authorities.
Protection against direct lightning strikes is carried out using lightning rods installed on building structures or free-standing lightning rods.
The protection complex includes a lightning rod (lightning conductor), current leads and grounding devices.
Grounding devices can consist of artificial and natural grounding devices.
To organize protection against direct lightning strikes, metal elements of objects, connected to each other, should be used as much as possible.
In some cases, the use of plaster containing powders of various metals is quite effective.
Reliable protection against secondary effects of lightning is no less important.
To reduce induced interference, external shielding of cable lines and shielding of power and communication lines are widely used.
Cables must have metal shields, grounded at both ends and connected to the lightning protection system, including at the boundaries of zones.
In open street wiring, power supply cables and communication lines must be laid in grounded pipes.
In addition, low-current objects, such as personal computers, must have network filters installed that significantly reduce the level of pulses coming through the phase, neutral wire and ground.
And, finally, to increase the reliability of protection from secondary effects, active hardware protection against overvoltage must be provided.
For this, various types of «coarse» and «fine» protection are used. The first one includes gas dischargers, surge arresters, etc., the second one is a set of various protective devices, including powerful Zener diodes.
These devices should be installed at the intersection of power supply, control, communication, telecommunications lines, the border of two shielding zones, as a rule, this is the input to the building.
Once again, I would like to emphasize that the foundations of protection should be laid at the design stage of the system.
It is equally important to convince the customer of the need for all measures and additional costs.
After all, penny savings, unfortunately, can result in major losses.