47

Lightning Protection of Electronic Data Processing Systems

Kazimieras Maceika1

Abstract – Generalized knowledge about contemporary problems of lightning protection are presented. There are described parameters of lightning as source of interferences. Statistical data about overvoltages in low-voltage supply and signal lines is provided. Lightning protection zone concept is given. The surge immunity of equipment measures and testing methods are proposed. Construction, effect and applications of protective components and devices are described.

1 Introduction

Large-area networking of computers and control systems require lightning and overvoltages protection. There is now worldwide agreement that the danger radius around a point struck by lightning is about 2 km. Within this domain electronic systems are affected by conducted and radiated disturbances that may cause destruction. The future lies in the computer -integrated factory, business and administration and problem becomes more and more important. An American study [1] highlighted the seriousness of the situation: banks, sales oriented enterprises, factories, insurance companies can close if the computer network fails for two weeks. Risk can be controlled by electromagnetic compatibility measures. The best solution of the problem is Lightning Protection Zone (LPZ) concept. Application of LPZ concept requires:

- estimation of lightning impulse expose for devices, equipment and systems,

- properties of lightning protection elements and systems.

It is necessary coordination between lightning and overvoltages standards, requirements for equipment and systems surge immunity. Realization of the Lightning Electromagnetic Impulse (LEMP) protection can be provided according surge protection for electrical systems of buildings, telecommunication and computer systems standards. They are prepared for components and protective devices: arresters for power engineering, information technology, optoelectronic connections also. Usage shielding measures, equipotential bonding networks, cable routing can help in solving protection problems. 2 Lightning discharge The information about lightning strokes was obtained from measurements of the lightning currents at towers, overhead lines, triggering stations. The electromagnetic interferences caused by lightning discharge were

estimated also. At the beginning the lightning protection levels were defined:

- the efficiency of the Lightning Protection Systems (LPS),

- the radius of the rolling sphere, protection angle, mash size of net on the roof, distances between down conductors,

- the distance space between LPS and conductive elements inside and outside the building,

- the characteristics of lightning currents, - tests’ intervals of LPS. There are four protection levels: I – with efficiency of protection 0,98, II – 0,95, III – 0,9 and IV – 0,8. For the first protection level the lightning currents are taken with following parameters:

- 200 kA, - 10/350 µs - for the first positive or negative stroke to ground,

- 50 kA, 0,25/100 µs - for subsequent stroke, - 400 A, 0,5 s - long duration current.

Current waveform [1,2] can be taken from figure 1. It can be used for simulating and calculations of lightning effect and protective measures.

Figure 1. Current waveform of the first stroke to ground.

Analytic equation of the current impulse is used [3]:

).()( ttm eekItI βα −− −=

Generally overvoltage are registered in telecommunication cables. Analysis of these results permits to separate several of basic shapes induced voltages (Fig.2.). Fundamental information about values of parameters characterizing overvoltages in telecommunication lines are presented in table 1.

I,kA

200

100

0t

350 µs

Scientific Proceedings of RTU. Series 7. Telecommunications and Electronics, 2003, vol.3

PDF created with FinePrint pdfFactory Pro trial version www.pdffactory.com

48

3 Internal lightning protection zone concept

General rule of protection, described in introduced standards and recommendation, relies on creation the zones inside analysed object, in of which is defined the exposures of devices on activity: - overvoltages/overcurrents in electrical low-voltage

power nets, - overvoltages/overcurrents in signal transmission

lines, - impulse electromagnetic field (action immediately

on devices and installations in each zones).

Figure 2. Different shapes of lightning overvoltages in telecommunication lines

Parameters→ Type of line

↓

Amplitude (V)

Rise time (μs)

Time to half value (μs)

Overhead lines some kV 0,1 - 20 20 - 200

Overhead cables with pair lines

1500-2000 20-500 300 - 800

Overhead coaxial cables

800-1000 100-300 500-1000

Buried cables with pair lines

500 - 600 50 - 100 1000-1500

Buried coaxial cables

400 - 500 200-500 500-1500

Table 1. Lightning created overvoltages in telecommunication lines.

In object, which is divided on zones, at passage from one zone to another the overvoltages, overcurrents and impulse electromagnetic fields are limited to admissible values in given zone. Technical equipments in given zone should be chosen in such manner, that their surge resistances were greater

in comparison with admissible peak values of surges in considered zone. In accepted standards most threatened areas are marked as zone 0A and 0B. General rules in partition of object in zones and the dis-tribution of protective elements are represented in Fig.3. The number of necessary lightning protection zones depends on the surge immunity of electronic and electric equipment. Correctly projected and installed lightning and overvolt-ages protection should limit jumps of potentials in ob-ject and to aim to travelling of potentials inside of ob-ject. The requirements for arresters are different depend-ing on the specific interface between zones. There were:

- lightning current arrester between LPZ 0A and 1, - overvoltages arresters between LPZ 1 and 2 as

between LPZ 2 and 3 and subsequent interfaces.

Figure 3. Lightning protection zones Inside building, each from separated zones characterizes admissible values overvoltages and overcurrents which reached devices working in given zone. Below, the values, characterizing parameters of impulse electric and magnetic fields and surges in power and signal nets are presented.

Zone 0A

Device and electronic systems working in this zone are subjected on direct influence of impulse fields and lightning currents with non-attenuated amplitudes. Threatened are devices or systems working on free air. The values of parameters which characterize lightning currents in zone 0A are represented. The amplitudes of overvoltages results from strength of insulators or isola-tion of cables and carry out from tens to hundreds kV. The parameters characterizing impulse electromagnetic field carry out: - electric field intensities until 500 kV/m, - magnetic field intensities until 10 kA/m.

Zone 0B

Devices are subjected on direct influence of electromagnetic field, similarly as in zone 0A , but not to

PDF created with FinePrint pdfFactory Pro trial version www.pdffactory.com

49

direct lightning strikes and the lightning induced surges. The values of overvoltages in: - electric low-voltage nets - 10 kV, - signal transmission lines - 6 kV. Surge current coming to the zone 0B are characterized by rise time 8 µs time to half values 20 µs.

Zone l

Electric and electronic equipments working in zone 1 are protected before: - impulse electromagnetic fields by single screen,

which most often is created by LPS or reinforcing rods of a building,

- overvoltages in every low-voltage power and signal lines by lightning current arresters. The transient voltages in zone 1 should be limited to levels below: - 6 kV in power networks, - 4 kV in signal lines. The shape of surge current in this zone is analogous how in zone 0B. The values of electric and magnetic fields intensity were properly some kV/m and several hundred A/m. Creation the following zones requires the system of overvoltage arresters, which protect before voltage/current surges and introductions the additional screens :

- reinforced concrete walls of rooms inside of object, - solid screens of rooms, - metal housing of the devices.

In real objects with complex and sensitive electronic systems practically were used two-or three- steps of overvoltage protection system. Proposed values basic parameters characterizing overvoltages in each zone are presented in table 2.

Zone → Parameter ↓ Zone 2 Zone 3 Zone 4

Overvoltages in supply nets 4 kV 2,5 kV 1 1,5 kV

Overvoltages in signal lines 2kV 1 1 kV 0,5 kV

Electric field 5kV/m 50V/m 5V/m

Magnetic field 1OO A/m 1 A/m 0,l A/m Table 2. Surges in different lightning zones. . It is proposal to introduced additional protective zones. In these zones the values of parameters characterizing im-pulse disturbances are fixed between interested sides for examples between producers and users. 4 Protection measures The lightning protection system consists of external and internal lightning protection. External protection has air termination system, the down conductors and the earthing systems. It is well described in [1] for

four protection levels. Internal lightning protection includes all additional measures to avoid electromagnetic interferences in protected volume. Lightning protection equipotential bonding is part of the internal protection, which reduces the potential differences caused by lightning current. It is realized by bonding the conductors of the external lightning protection system with the metal frame of the structure, installations and with the power and information technology equipment in the volume to protect. Bonding measures include bonding lines and arresters. A difference is made between lightning current arresters (tested by surge currents of wave shape 10/350 μs) and surge arresters (tested by surge currents of wave shape 8/20 μs). Arresters for power engineering are based on a spark gap. Surge arresters for low-voltage overhead lines are constructed as a series connection of spark gap and voltage dependent resistor designed for surge current 8/20 μs with 5 kA peak value.

5 Standards and normative

Basically new in lightning protection standards are methods for the assessment the risk of damage due to lightning and overvoltages. Field measuring stations register the radiated electromagnetic interference fields of lightning discharges, lightning currents with their extreme values are simulated in the laboratory and influence on information technology equipment is investigated. A lightning protection system is settled [1,2]. The International Electrotechnical Commission produces standards on the following:

- electromagnetic interference of lightning discharge and its statistical distribution,

- methods to estimate the risk of determining the protection levels,

- measures to discharge the lightning current and screening electromagnetic fields,

- measures to discharge conducted lightning interference,

- protective concepts and components, requirements and tests.

Lightning discharges are relatively rear and very short duration , but they are dangerous for the control systems of nuclear power stations, alarm systems and military installations, telecommunications and electronic data processing systems. In the case of lightning discharge there must be no error signals and protection means are necessary. Table 3 shows internationally agreed lightning protection standards, proposed zonal protection concept.

PDF created with FinePrint pdfFactory Pro trial version www.pdffactory.com

50

Standard IEC 61204 Protection of structures against lightning

Part 1. General principles

IEC 61204-1 1990

Section 1. Selection of protection levels for lightning protection 1993

Section 2. Design, installation, maintenance and inspection of

lightning protection systems 1998

Standard IEC 61312

Protection against LEMP

Part 1. General principles

IEC 61312-1 1995

Part 2. Shielding of structures, bonding inside structures and earthning

IEC 61312-2 1999

Part 3. Requirements of surge protective devices

IEC 81/121/D 1999

Standard IEC 61663 Lightning protection – telecommunication

lines

Part 1. Fiber optic instalations

IEC 61663-1 1999

Part 2. Subscriber lines using metallic conductors

IEC 81/128/CD 2000

Table 3. Standards proposed by Technical Committee of the IEC

The standards were supplemented by Technical Reports: IEC 61662 – Assessment of risk damage due to lightning, IEC 61819 – Test parameters simulating the effects of LPS components. The protective measures and methods are for external and internal protection, shielding, surge limitation, which

complement one another. According European Member State law on the electromagnetic compatibility of devices ( Council Directive 89/336/EEC of 1989 ) the equipment must have a sufficient immunity also against lightning interferences.

6 Conclusions

1. The parameters of lightning current impulse: peak value, front time, time to half are specified in standards having one meaning. It is shown that shapes of lightning overvoltages in telecommunication lines are a very different.

2. Use of many kinds of electronic systems: computers, telecommunication facilities, control systems in the protected volume caused that building must be subdivided into lightning protection zones. The different protection zones OA, OB, 1 and etc are described and defined having one meaning.

3. There were not possible in paper to introduce a great number of protection measures: arresters of various classes, nonlinear resistors, metal oxide varistors, Zener diodes and decoupling elements. Applications and systems of these protective elements are taken in literature.

4. System of standards is completed for EMC- oriented lightning protection measures and sufficient immunity of equipment against lightning interferences.

References

[1] P. Hasse, : Overvoltage protection of low voltage systems, The Institution of Electrical Engineers, London, 2000.

[2] A.W. Sowa, “Coordination between lightning protection concept and EMC requirements”, Conference EMD’2002, Proceedings of conference, 2002, Palanga, pp. 9 – 14.

[3] K. Aniserowicz, “Application of discrete Fourier transform to analysis of overvoltages in transmission lines”, VIII international conference “Overvoltages in power-, electronic- and computer engineering”, Proceedings, Bialystok, 1997, pp. 5 -12.

1 Department of Radioengineering, Vilnius Gediminas technical university, Naugarduko str. 41, LT-2006, Vilnius, LITHUANIA. E-mail: kmaceika@el.vtu.lt

PDF created with FinePrint pdfFactory Pro trial version www.pdffactory.com