PROTECTION AGAINST HAZARDOUS INDUCED VOLTAGES, or don’t have thy cloak to make when it begins to rain
Автор: Александр ПОПОВ, компания «ТАХИОН»
I believe that a lot of companies have faced the situation when a global failure of a system coincided in time with welding operations at the site. And certainly during “… thunderstorm in early May…”, any installer of large external distributed systems is far from poetic sentiments and thinks how many calls will be tomorrow regarding the installed equipment failure.
The market is rapidly growing. But the laws of physics undergo no changes with time. Questions and problems addressed by the users and installers today imply that this topic will always retain its relevance. Moreover, today the relevance for CCTV monitoring market is growing as rapidly as the market itself.
Today, everyone speaks about “global monitoring systems” and “safer cities”. And certainly “all over IP” – what shall we do without it. All similar systems have long communication lines which require at least a minimum “respect”, since the physical processes as described above remain conservative as compared with market. This is the “respect” at a general understanding level in order to properly anticipate potential problems and take measures to minimize them in order to ensure correct dialogue with the customer and not to be taken hostage by the incompetence of the latter.
In addition, today a mass attack of the CCTV monitoring market takes place from other markets – both computer and media technology. Their attempt to achieve their own growth at the expense of other markets is quite clear and justifiable from the point of view of business. Nevertheless, in this attempt the products offered in these markets often cannot adapt to new application conditions; and occur to be immature for the use at hardware level, so the adaptation task often lies heavily on the installer. The specialized equipment which has undergone the trial run over time is usually designed for real operating conditions in the systems. Besides, this factor to a great extent also influences a relatively high price level of such equipment. In particular, this refers to both climatic conditions and required protection via communication lines. If we take the “native” equipment in our market, e.g. telemetry control equipment, then panels, telemetry receivers (including those as part of Speed Domes), have an interface line protection by default. Twisted-pair video transmission equipment, if it is correct, is protected on the video transmission line as part of its internal circuit; if it is absolutely correct, it is protected at the line input and output, because the equipment without such protection will be returned for repair sooner or later. The equipment which was initially designed for operation in the “sheltered” conditions of indoor computer networks, when taken alone outdoors, often is absolutely vulnerable to multiple potential external destructive factors, and the protection problem has to be solved separately.
The topic is actually infinite. It is addressed in thousands of publications.
But this is an ABC book. Therefore the explanation will be provided solely at the level of “respect” for communication lines and general idea. The discussion will be based on those real issues which we have heard from real installers and ordinary people from our market. So, I kindly ask those engineering experts who are on top of issues not to blame the author for a sort of primitive explanation. It is important for the majority of people involved in system building to have a general idea of the existing issue and thus to solve it using the professional support rather than ignoring the problem unknowingly.
The issue is relevant absolutely to all wire-based systems with the only differences in some specifications. We shall deliberately omit the term “lightning arrester” and will further speak about protection against hazardous induced voltages.
So! The main sources of hazardous voltages may include the following:
- high voltage transmission lines installed parallel to communication lines;
- electric railroad catenary lines;
- urban electric transport systems;
- electric welding units;
- adjacent radio transmitting, locating and other systems;
- atmospheric (lightning) discharges;
- malicious damage of networks (electromagnetic terrorism – such term already exists).
The widespread name of protection devices – lightning arresters – is not exactly correct. To be more precise, it is absolutely incorrect. There are many factors which have no relation to thunderstorm. When there is inherently no probability of any atmospheric discharges at the site, this doesn’t mean that there is no probability of hazardous voltages in lines. There is a plenty of other potential sources which can be faced almost everywhere, and protection against them is completely justified. What is different about it is that all other sources, apart from lightning discharges, at a certain site are known and predictable, and hence the need (or no need) in additional protection against them is obvious. Lightning discharges – are probable factors, so you begin to make thy cloak not earlier than it begins to rain. On the other hand, that are overvoltages induced by the lightning discharge which are the most dangerous due to their devastating amplitude. Generally in a CCTV system, hazardous induced voltage may “come” into the equipment via the following lines:
- 220 V power lines (in particular, when the system’s own long line is installed, and, to make things worse, as an overhead line (typical of perimeter-based systems);
- 12V/24V secondary power line (e.g. from supply units located remotely from the cameras);
- video transmission line (any wire line: coaxial cable or twisted pair);
- data transmission lines (in particular, RS-485, Ethernet, which are most often used in systems).
It shall be noted right away that there is no protection device which can protect against direct lightning strike in the line (or line equipment). And in view of this, the concept of lightning protection itself will be briefly discussed in order to ensure that plain line protection devices protecting against hazardous induced voltages obviously do not reach the lightning protection level.
The International Electrical Committee (IEC) has developed the standards to specify the concepts of surge protection of buildings and facilities. These, in particular include IEC 62305, Lightning Strike Protection. IEC 62305 requirements generate zonal protection concept with the following main principles:
- use of electrically connected building structures with steel components (reinforcement, framework, bearing components, etc.) and grounding system which form a shielding environment in order to minimize the influence of external electromagnetic impacts within the site (Faraday cage);
- proper grounding and equipotential bonding system;
- division of the site into nominal protected zones and utilization of surge protection devices;
- compliance with the procedures for installation of the protected equipment and connected conductors with reference to other equipment and conductors which can have hazardous impact or induce hazardous voltages.
Try to fit such concept to our real facilities. Usually, the first two items without which it is no good talking about any lightning protection are actually out of jurisdiction either of the system installer or of the system designer (unless it is the designer of the whole site beginning from the foundation). Also, nobody requests to refuse the order by the reason of incomplete compliance with IEC 62305. But you can assess the lightning protection level to avoid hasty obligations and warranties which will be actually impractical in the given conditions, by stipulating any additional conditions, and this is always reasonable. I don’t know how building reinforcement is arranged at out sites, but I know that very often (including giant industrial sites) there is no talk about any proper grounding system. This must be reflected in contractual obligations to avoid the situation, when the contractor gets “a planned unprofitable” project.
This is more difficult, when you need to build a system in the open fields, a sort of protection perimeter, and to start from nothing by installing video camera poles and all lines, and you have a very diligent and rich customer, because even when you have installed all required protection for the project you still cannot ensure 100% equipment protection.
We hope that now you will perceive the concept of lightning protection regarding surge protection devices as not exactly correct.
Let’s proceed to the actual question on the subject from real people from out market. The essence is that there is no need for protection devices for 220 V power line installed on poles, since the poles are properly insulated. This is not about any direct contact of an external high voltage source with our line cables. This is only about induced overvoltage.
Physics of the process will be briefly discussed below.
An external alternating magnetic field induces EMF (electromotive force) in a communication line; as a result, current flow is initiated in the line, causing difference of potentials at the line ends. The magnitude of this potential difference Uhaz. depends on the length of the exposed section and the electromagnetic field strength from the external source. Thus, the influence of the high voltage transmission lines parallel to the communication lines is characterized by long exposure section, however the field strength may be relatively low; for atmospheric discharges, there is an opposite situation – very high strength with relatively small exposure section.
In addition, the charge induced by, e.g., lightning, and spread over the ground causes a static potential which can be prevented using a shielded communication line installed in ground (cable shield, routing in steel tubes).
EMF induced in a conductor is a function of the electromagnetic field build-up rate. Thus, a lightning discharge duration will be equal to 50 µs. The number of repetitions is up to 3 with intervals up to 0.5 s. Lightning-induced EMF (induced, rather than caused by the lightning strike in the line) is up to 5,000 V (5 kV) at an average during 50 µs. It is understood that installation of fuses cannot solve the problem, since none of the fuses will even notice this during 50 µs.
The task of any line protection device is to reduce this induced voltage, which is dangerous for the main signal, to the allowable level, and to prevent the influence on the desired signal itself. In other words, there shall be somewhat no protection system for the desired signal. This is achieved by means of a multistage protection system to reduce the potential to an allowable level from the induction point to the line connection point on the equipment. In this case high potential is discharged to ground.
As the first protection stage diverters, arresters – gas discharge tubes with a certain breakdown voltage at which resistance is reduced dramatically (Fig.1) – shall be used.
Thus, after completion of the first stage (arrester), the line potential is limited at breakdown potential level. For short pulses, this potential is higher; for long-term exposure, it is lower. For further minimization of the hazardous voltage, the second protection stage is provided. It is separated from the first stage by current limiting components (chokes, resistors) (Fig.2).
The second stage is generally based on voltage regulator diodes or TRISIL diodes. They are used for further voltage limitation.
This will be sufficient in many cases.
When further voltage reduction is required, the third stage shall be provided, again with limiting resistors, which is generally based on voltage regulator diodes.
The next issue which shall be regularly addressed is often a sort of a complaint about the protection equipment quality. This is the following issue: “We have protection devices on all lines, nevertheless there was a thunderstorm and many of them have failed.” This is often accompanied with the protected equipment failure.
In general, it should be understood that, even when the protective device is “dead”, it has protected the equipment after all and has fulfilled its function, though at a certain money-equivalent cost. In these cases, equipment failure is easily corrected at a low cost. Sometimes protective devices may be absolutely helpless.
But this is all irrelevant. To understand the essence, we shall return to the zonal protection concept described in IEC 62305 which defines lightning protection zones in terms of direct and indirect lightning impact as follows:
Zone OA: site environment zone, all points of which can be exposed to direct lightning strike (be in direct contact with a lightning channel) and electromagnetic field generated by it.
Zone OB: site environment zone, points of which are not exposed to direct lightning strike, since they are in the area protected by the lightning protection system. However, this zone is exposed to a non-attenuated electromagnetic field.
Zone 1: internal zone, points of which are not exposed to direct lightning strike. In this zone, currents in all conductive parts are significantly lower than those in zones OA and OB.
Zones 2, 3 … etc. When further minimization of the electromagnetic field and discharge currents is required in order to install sensitive equipment, additional protective zones shall be designed. The higher the zone number, the lower the electromagnetic field and discharge current influence.
An example of nominal division of the site into zones is shown in Figure 3. Power supply and communication cables and other metal lines shall be included in protection zone 1 in a single point with their shield sheaths or metal parts connected to the main grounding bus at the interface of zones OA – OB and zone 1.
The site zoning mentioned above provides real successful protection of power supply systems up to 1,000 V, as well as communication and data transmission lines, computer networks and other lines within the site.
This is a brief description of site division into zones.
According to the standards (IEC 61643-1 and IEC 61643-12), surge protection devices are divided into three classes by their ability to pass various pulse currents – I, II and III (or D, С, В according to German standard Е DIN VDE 0675-6 which is almost not used any longer).
The main requirements for surge protection devices depending on the class are listed in Table 1.
|I||Protection against direct lightning strikes in the lightning protection system or overhead transmission line. Installed in switchgear or main distribution board at the building entrance. Characterized by a rated pulse current at tp. 10 / 350 µs.|
|II||Power distribution network protection against switching interference or as the second protection stage during lightning strike. Installed in distribution boards. Characterized by a rated pulse current at tp. 8 / 20 µs.|
|III||Consumer protection against residual voltage surges, protection against differential (unbalanced) overvoltage, high frequency interference filtration. Installed in the immediate vicinity of the consumer. Characterized by a rated pulse voltage at tp. 1.2 / 50 µs and current at tp. 8 / 20 µs.|
Now we can answer the question why the existing protection didn’t survive and protect the equipment.
In fact, the device class didn’t correspond to the zone where the device was installed.
Actually, Class III devices are predominantly presented in our market. Basically, only such devices are available for the system installer in the majority of cases, unless the site and all its utilities are designed and constructed not from the beginning. The designer and installer cannot influence Class I and II devices, because they already exist to a certain extent or are absent at all. It’s unlikely that somebody will offer and somebody will install a comprehensive protection system within an individual system. First, this is a rather specialized thing which requires very high level of specific knowledge. Second, the cost of an engineering system itself may be miserable as compared with such comprehensive protection system.
Besides, it’s unlikely that the level of protection will be assessed in full. As it follows from the above, it would be enough for any of zones 1, 2, 3 … to lose its “status” immediately, if somebody someday brings to the site, for example, a steel pipeline without compliance with all lightning protection requirements.
And what about the above mentioned outdoor perimeter-based system with pole-mount equipment and overhead communication lines in the open field? According to the zoning concept (Fig. 3), we have for our system zone O as it is. A direct lightning strike is quite likely for this zone. The protective device is of Class III. So, you can make your conclusions. When the whole comprehensive lightning protection system is made, then no perimeter-based system is probably required, because an adequate steel structure fence will be provided. And the comparative cost of the protected system and protection system is out of the question.
Therefore, a compromise solution should be always sought to avoid any absurd situation.
First of all, assess the vulnerability of the equipment and lines. For example, underground lines are significantly less vulnerable than overhead lines.
In addition, if you look at the thunderstorm activity world map, Russia is in the minimum comparative probability zone. The probability of direct lightning strike in the line is even lower by several orders of magnitude, in particular if there are high-rise buildings, masts, trees, etc. in the vicinity.
The main selection criterion of the protection level is certainly the cost criterion: high cost of the protected equipment may be a convincing argument for a more complicated protection system and vice versa.
All these related particularly to a lightning protection system. And now let’s go back to the beginning to the main hazardous induced voltage sources. Lightning discharges are only one item in a sufficiently large list of hazards. And if for this hazard which is of probabilistic nature, when there are no other protection, Class III protection also functions on the “win-or-lose” basis, then for the remaining list of hazards which are obvious and measurable, the protection is efficient and sufficient for a properly built system. In particular, this relates to the grounding system. Anyway, when an electric shock device was deliberately inserted in the video transmission line, our Class III protection was still functioning. It shall be also noted that according to our experience the causes associated with atmospheric phenomena are by no means leading in the equipment failure statistics, when there are no surge protection devices. Industrial interferences were the most widespread causes, and huge industrial facilities were the most unfavourable sites. Therefore, the use of Class III protection equipment is always reasonable, when there is a fundamental hazard of induced voltage in the line. Especially as the price factor considering the cost of such devices always speaks in favour of such application.
The next frequently asked question is where the protection device shall be installed?
Since in this context we discuss Class III protection devices, then, in accordance with Table 2 and the logic of reasoning, the place shall be near the protected equipment. The logic is such that other hazardous voltages will be no longer induced on the way from the protection device to the equipment to be protected.
From this also the answer to the next question is derived – how many protection devices shall be installed in the line?
As many as there are components connected to the line to be protected. And though the protection equipment itself is often called the “line protection device for protection against hazardous induced voltages”, it actually protects the equipment connected to the line, rather than the line (it is the line that poses a threat as a receiver of hazardous induced EMF!). All the protected equipment will survive. Hazardous voltage will be applied to both line ends, and if there is no protection on the other end, everything will be affected there.
What is the minimum line length where protection equipment is required?
Let’s go back to the beginning of the publication: the magnitude of this potential difference depends on the length of the exposed section and the external electromagnetic field strength. A minimum line length beginning from which the use of protection devices is reasonable will depend on the potential field strength in this very point. This field strength can be only determined directly on site. According to our practical experience, lines as long as 2 m were successfully protected. There is also a general rule for any protection: be closer to the hazard… The prices for Class III equipment are not so high to economize on it.
And one more very frequently asked question which provokes the main disputes even among competent market experts – do we have to ground protection devices, and if yes, how shall we do it properly, and what will be, if there is no grounding?
As can be seen from the protection device layout (Fig.1 and 2), hazardous potential is discharged to ground. It means that the grounding shall be provided anyway; the system cannot be suspended in the air.
Compared to a washing machine which will continue to work whether there is grounding or not (grounding will only fulfil the electric shock protection function in case of equipment failure), the protection device will not work at all without grounding. The device would not have been installed, if there were no grounding.
The grounding may (and must) be installed immediately in the installation point together with protection devices of other categories, if any. Since the grounding was made using an arrester for the first stage (Fig. 1) or diodes for the second stage (Fig. 2), this grounding will only arise in case of line overvoltage and tripping, and will disappear, when there are no more hazardous induced voltages. Thus, in normal operating conditions, such layout will not cause multiple grounds and interference associated with them.
Sometimes it might not be possible to provide grounding directly in the installation place. Then a separate wire from the protection device grounding terminal shall be installed to the nearest possible grounding point.
And finally, regarding the grounding issue: the use of a cable shield as a grounding wire.
If this is a balanced line shield (e.g. TPP cable shield), which only fulfills the function of protection against external induced voltage, then all shields of such system cables will be grounded at the receiving end in a single point and can be used as grounding for protection devices. Such protection will be slightly less efficient than in case of protection device grounding on site, since the grounding conductor resistance is increased in proportion to the wire length (e.g. instead of wire length of 5 m we will obtain 1,000 m – the resistance will be 200 times greater – with only one grounding point per system). But such system is better than nothing.
For an unbalanced line, where the protection device is connected between the line and shield, such shield grounding is equivalent to the absence of protection device at all.
Class III protection device circuit diagrams for closed-circuit television systems
Figure 4 shows a circuit diagram of two-stage equipment protection device in balanced and unbalanced video transmission lines and secondary power supply lines.
The first stage consists of a gas discharge tube. The second stage consists of an electronic surge arrester based on TVS protective diodes. Both for the video transmission line and secondary power line. Of course, limiting voltages in these lines are different.
Figure 5 shows a circuit diagram of two-stage surge protection device in RS-485 lines, which is essentially similar to the above mentioned device. Also, stage 1 consists of a lightning arrester, and stage 2 consists of an electronic surge arrester based on TVS protective diodes.
Figure 6 shows a circuit diagram of a three-stage protection device for Ethernet data ports. The second stage consists of galvanic isolation, where common-mode interference induced in line conductors is suppressed. The first stage consists of a lightning arrester; the third stage – of TVS protective diodes.
Figure 7 shows a circuit diagram of residual voltage surge and induced voltage protection devices in 220V power lines. I is a gas discharge tube connected between the neutral and grounding conductor. II is a varistor-based electronic surge arrester connected between the line wire and neutral. Unlike all previous devices, a 220V power line protection device is connected to the line in parallel.
The appearance of the protection devices is shown in Figure 8. UZL-K is a protection device for balanced (twisted pair) and unbalanced (coaxial cable) video transmission and secondary power line (12V, 24V); UZL-I is a RS485 line protection device; UZL-Е is an Ethernet line protection device; UZP-220 is a 220V primary power line protection device.
Circuit diagram of two-stage equipment protection in balanced and unbalanced video transmission lines and secondary power supply lines.
Circuit diagram of a two-stage surge protection device in RS-485 lines
Circuit diagram of a three-stage protection device for Ethernet data ports
Circuit diagram of residual voltage surge and induced voltage protection devices in 220V power lines
The most significant parameters for selection of Class III protection devices are as follows:
- maximum pulse discharge current at Tpulse = 8/20 µs – the peak test current pulse which may be passed by the protection device without failure;
- limiting voltage – the voltage level up to which the line overvoltage will be limited after passing through the protection device regardless of overvoltage magnitude; nominal pulse discharge current at Tpulse = 8/20 µs –the peak test current pulse, which can be repeatedly withstood by the protection device;
- maximum continuous operating voltage – the maximum voltage, which may be continuously (throughout the service life) applied to the protection device outputs;
- protection level – maximum voltage drop across the protection device without pulse discharge current flow through the protection device; characterizes the device capability to limit overvoltages arising on its terminals; the response time for varistors is usually maximum 25 ns, for lightning arresters – from 100 ns to several microseconds.
Protection against hazardous induced voltages, even when it strictly meets all requirements, doesn’t ensure 100% protection against any damage or malfunctions due to such induced voltages. Particularly when the whole protection is limited to installation of Class III devices. At the same time, the absence of such devices in long communication lines almost 100% leads to failure and/or malfunctions – it is only a matter of time. Therefore, the use of such devices is always reasonable in systems consisting of long lines, where tangible electromagnetic interference is present or possible, at industrial facilities and outdoor overhead line sites.
The presence of a safety belt by no means prevents either road traffic accidents or injury and death. Nevertheless, the necessity of the safety belt is currently codified by law.
There is also a whole list of small details which, when neglected at the CCTV system design stage, in the ideal case may turn the system into a “planned unprofitable” project, and in the worst case – turn it in inherently non-operational. These will be addressed in the next chapter.