Summarize of lightning and surge protection devices
Failure of technical installations and systems in residential and functional buildings is very unpleasant and expensive. Therefore, faultless operation of devices must be ensured both during normal operation and thunderstorms. The number of annually registered lightning activities in Germany maintained at a constantly high level over many years. Damage statistics of insurance companies clearly show that there are deficits in terms of lightning and surge protection measures both in the private and commercial sector (Figure 1).
A professional solution allows to take adequate protection measures. The lightning protection zone concept, for example, enables designers, constructors and operators of buildings and installations to consider, implement and monitor different protection measures. All relevant devices, installations and systems are thus reliably protected at a reasonable expense.
Sources of interference
Surges occurring during a thunderstorm are caused by direct / nearby lightning strikes or remote lightning strikes (Figure 2 and Figure 3). Direct or nearby lightning strikes are lightning strikes to a building, its surroundings or electrically conductive systems entering the building (e.g. low-voltage supply, telecommunication and data lines). The resulting impulse currents and impulse voltages as well as the associated electromagnetic field (LEMP) are particularly dangerous for the devices to be protected with regard to the amplitude and energy content involved. In case of a direct or nearby lightning strike, surges are caused by the voltage drop at the conventional earthing impedance Rst and the resulting potential rise of the building in relation to the remote earth (Figure 3, case 2). This means the highest load for electrical installations in buildings.
The characteristic parameters of the impulse current present (peak value, rate of current rise, charge, specific energy) can be described by means of the 10/350 μs impulse current wave form. They have been defined in international, European and national standards as test current for components and devices protecting against direct lightning strikes (Figure 4). In addition to the voltage drop at the conventional earthing impedance, surges are generated in the electric building installation and the systems and devices connected to it due to the inductive effect of the electromagnetic lightning field (Figure 3, case 3). The energy of these induced surges and of the resulting impulse currents is far lower than the energy of a direct lightning impulse current and is therefore described by a 8/20 μs impulse current wave form (Figure 4). Components and devices that do not have to conduct currents resulting from direct lightning strikes are therefore tested with such 8/20 μs impulse currents.
Lightning strikes are called remote if they occur at a farer distance to the object to be protected, strike medium-voltage overhead lines or their surroundings or occur as cloud-to-cloud lightning discharges (Figure 3, cases 4, 5, 6). Similar to induced surges, the effects of remote lightning strikes on the electrical installation of a building are handled by devices and components which have been dimensioned according to 8/20 μs impulse current waves. Surges caused by switching operations (SEMP) are, for example, generated by:
– Disconnection of inductive loads (e.g. transformers, reactors, motors)
– Arc ignition and interruption (e.g. arc welding equipment)
– Tripping of fuses
The effects of switching operations in the electrical installation of a building can also be simulated by impulse currents of 8/20 μs wave form under test conditions. To ensure continuous availability of complex power supply and information technology systems even in case of direct lightning interference, further surge protection measures for electrical and electronic installations and devices based on a lightning protection system for the building are required. It is important to take all causes of surges into account. To do so, the lightning protection zone concept as described in IEC 62305-4 is applied (Figure 5).
Lightning protection zone concept
The building is divided into different endangered zones. These zones help to define the necessary protection measures, in particular the lightning and surge protection devices and components. Part of an EMC compatible (EMC: Electro Magnetic Compatibility) lightning protection zone concept is the external lightning protection system (including air-termination system, down-conductor system, earth-termination system), equipotential bonding, spatial shielding and surge protection for the power supply and information technology systems. Definitions apply as classified in Table 1. According to the requirements and loads placed on surge protective devices, they are categorised as lightning current arresters, surge arresters and combined arresters. The highest requirements are placed on the discharge capacity of lightning current arresters and combined arresters used at the transition from lightning protection zone 0A to 1 or 0A to 2. These arresters must be capable of conducting partial lightning currents of 10/350 μs wave form several times without being destroyed in order to prevent the ingress of destructive partial lightning currents into the electrical installation of a building. At the transition point from LPZ 0B to 1 or downstream of the lightning current arrester at the transition point from LPZ 1 to 2 and higher, surge arresters are used to protect against surges. Their task is both to reduce the residual energy of the upstream protection stages even further and to limit the surges induced or generated in the installation itself.
The lightning and surge protective measures at the boundaries of the lightning protection zones described above equally apply to power supply and information technology systems. All measures described in the EMC compatible lightning protection zone concept help to achieve continuous availability of electrical and electronic devices and installations. For more detailed technical information, please visit www.lsp-international.com.
LPZ 0: Zone where the threat is due to the unattenuated lightning electromagnetic field and where the internal systems may be subjected to full or partial lightning surge current.
LPZ 0 is subdivided into:
LPZ 0A:Zone where the threat is due to the direct lightning flash and the full lightning electromagnetic field. The internal systems may be subjected to full lightning surge current.
LPZ 0B: Zone protected against direct lightning flashes but where the threat is the full lightning electromagnetic field. The internal systems may be subjected to partial lightning surge currents.
Inner zones (protected against direct lightning flashes):
LPZ 1: Zone where the surge current is limited by current sharing and isolating interfaces and/or by SPDs at the boundary. Spatial shielding may attenuate the lightning electromagnetic field.
LPZ 2 … n: Zone where the surge current may be further limited by current sharing and isolating interfaces and/or by additional SPDs at the boundary. Additional spatial shielding may be used to further attenuate the lightning electromagnetic field.
Terms and Definitions
Breaking capacity, follow current extinguishing capability Ifi
The breaking capacity is the uninfluenced (prospective) r.m.s. value of the mains follow current which can automatically be extinguished by the surge protective device when connecting UC. It can be proven in an operating duty test according to EN 61643-11:2012.
Categories according to IEC 61643-21:2009
A number of impulse voltages and impulse currents are described in IEC 61643-21:2009 for testing the current carrying capability and voltage limitation of impulse interference. Table 3 of this standard lists these into categories and provides preferred values. In Table 2 of the IEC 61643-22 standard the sources of transients are assigned to the different impulse categories according to the decoupling mechanism. Category C2 includes inductive coupling (surges), category D1 galvanic coupling (lightning currents). The relevant category is specified in the technical data. LSP surge protective devices surpass the values in the specified categories. Therefore, the exact value for the impulse current carrying capability is indicated by the nominal discharge current (8/20 μs) and the lightning impulse current (10/350 μs).
A combination wave is generated by a hybrid generator (1.2/50 μs, 8/20 μs) with a fictitious impedance of 2 Ω. The open-circuit voltage of this generator is referred to as UOC. UOC is a preferred indicator for type 3 arresters since only these arresters may be tested with a combination wave (according to EN 61643-11).
Cut-off frequency fG
The cut-off frequency defines the frequency-dependent behaviour of an arrester. The cut-off frequency is equivalent to the frequency which induces an insertion loss (aE) of 3 dB under certain test conditions (see EN 61643-21:2010). Unless otherwise indicated, this value refers to a 50 Ω system.
Degree of protection
The IP degree of protection corresponds to the protection categories
described in IEC 60529.
Disconnecting time ta
The disconnecting time is the time passing until the automatic disconnection from power supply in case of a failure of the circuit or equipment to be protected. The disconnecting time is an application-specific value resulting from the intensity of the fault current and the characteristics of the protective device.
Energy coordination of SPDs
Energy coordination is the selective and coordinated interaction of cascaded protection elements (= SPDs) of an overall lightning and surge protection concept. This means that the total load of the lightning impulse current is split between the SPDs according to their energy carrying capability. If energy coordination is not possible, downstream SPDs are insufficiently
relieved by the upstream SPDs since the upstream SPDs operate too late, insufficiently or not at all. Consequently, downstream SPDs as well as terminal equipment to be protected may be destroyed. DIN CLC/TS 61643-12:2010 describes how to verify energy coordination. Spark-gapbased type 1 SPDs offer considerable advantages due to their voltage-switching
characteristic (see WAVE BREAKER FUNCTION).
The frequency range represents the transmission range or cut-off frequency of an arrester depending on the described attenuation characteristics.
With a given frequency, the insertion loss of a surge protective device is defined by the relation of the voltage value at the place of installation before and after installing the surge protective device. Unless otherwise indicated, the value refers to a 50 Ω system.
Integrated backup fuse
According to the product standard for SPDs, over-current protective devices / backup fuses must be used. This, however, requires additional space in the distribution board, additional cable lengths, which should be as short as possible according to IEC 60364-5-53, additional installation time (and costs) and dimensioning of the fuse. A fuse integrated in the arrester ideally suited for the impulse currents involved eliminates all these disadvantages. The space gain, lower wiring effort, integrated fuse monitoring and the increased protective effect due to shorter connecting cables are clear advantages of this concept.
Lightning impulse current Iimp
The lightning impulse current is a standardised impulse current curve with a 10/350 μs wave form. Its parameters (peak value, charge, specific energy) simulate the load caused by natural lightning currents. Lightning current and combined arresters must be capable of discharging such lightning impulse currents several times without being destroyed.
Mains-side over-current protection / arrester backup fuse
Over-current protective device (e.g. fuse or circuit breaker) located outside of the arrester on the infeed side to interrupt the power-frequency follow current as soon as the breaking capacity of the surge protective device is exceeded. No additional backup fuse is required since the backup fuse is already integrated in the SPD.
Maximum continuous operating voltage UC
The maximum continuous operating voltage (maximum permissible operating voltage) is the r.m.s. value of the maximum voltage which may be connected to the corresponding terminals of the surge protective device during operation. This is the maximum voltage on the arrester in
the defined non-conducting state, which reverts the arrester back to this state after it has tripped and discharged. The value of UC depends on the nominal voltage of the system to be protected and the installer’s specifications (IEC 60364-5-534).
Maximum continuous operating voltage UCPV for a photovoltaic (PV) system
Value of the maximum d.c. voltage that may be permanently applied to the terminals of the SPD. To ensure that UCPV is higher than the maximum open-circuit voltage of the PV system in case of all external influences (e.g. ambient temperature, solar radiation intensity), UCPV must be higher than this maximum open-circuit voltage by a factor of 1.2 (according to CLC/TS 50539-12). This factor of 1.2 ensures that the SPDs are not incorrectly dimensioned.
Maximum discharge current Imax
The maximum discharge current is the maximum peak value of the 8/20 μs impulse current which the device can safely discharge.
Maximum transmission capacity
The maximum transmission capacity defines the maximum high-frequency power which can be transmitted via a coaxial surge protective device without interfering with the protection component.
Nominal discharge current In
The nominal discharge current is the peak value of a 8/20 μs impulse current for which the surge protective device is rated in a certain test programme and which the surge protective device can discharge several times.
Nominal load current (nominal current) IL
The nominal load current is the maximum permissible operating current which may permanently flow through the corresponding terminals.
Nominal voltage UN
The nominal voltage stands for the nominal voltage of the system to be protected. The value of the nominal voltage often serves as type designation for surge protective devices for information technology systems. It is indicated as an r.m.s. value for a.c. systems.
Surge protective devices exclusively designed for installation between the N and PE conductor.
Operating temperature range TU
The operating temperature range indicates the range in which the devices can be used. For non-self-heating devices, it is equal to the ambient temperature range. The temperature rise for self-heating devices must not exceed the maximum value indicated.
Protective circuits are multi-stage, cascaded protective devices. The individual protection stages may consist of spark gaps, varistors, semiconductor elements and gas discharge tubes (see Energy coordination).
Protective conductor current IPE
The protective conductor current is the current which flows through the PE connection when the surge protective device is connected to the maximum continuous operating voltage UC, according to the installation instructions and without load-side consumers.
Remote signalling contact
A remote signalling contact allows easy remote monitoring and indication of the operating state of the device. It features a three-pole terminal in the form of a floating changeover contact. This contact can be used as break and / or make contact and can thus be easily integrated in the building control system, controller of the switchgear cabinet, etc.
Response time tA
Response times mainly characterise the response performance of individual protection elements used in arresters. Depending on the rate of rise du/dt of the impulse voltage or di/dt of the impulse current, the response times may vary within certain limits.
In high-frequency applications, the return loss refers to how many parts of the “leading“ wave are reflected at the protective device (surge point). This is a direct measure of how well a protective device is attuned to the characteristic impedance of the system.
Resistance in the direction of the signal flow between the input and output of an arrester.
Relation of the power fed into a coaxial cable to the power radiated by the cable through the phase conductor.
Surge protective devices (SPDs)
Surge protective devices mainly consist of voltage-dependent resistors (varistors, suppressor diodes) and / or spark gaps (discharge paths). Surge protective devices are used to protect other electrical equipment and installations against inadmissibly high surges and / or to establish equipotential bonding. Surge protective devices are categorised:
- a) according to their use into:
- Surge protective devices for power supply installations and devices
for nominal voltage ranges up to 1000 V
– according to EN 61643-11:2012 into type 1 / 2 / 3 SPDs
– according to IEC 61643-11:2011 into class I / II / III SPDs
The changeover of the Red/Line. product family to the new EN 61643-11:2012 and IEC 61643-11:2011 standard will be completed in the course of the year 2014.
- Surge protective devices for information technology installations and devices
for protecting modern electronic equipment in telecommunications and signalling networks with nominal voltages up to 1000 V a.c. (effective value) and 1500 V d.c. against the indirect and direct effects of lightning strikes and other transients.
– according to IEC 61643-21:2009 and EN 61643-21: 2010.
- Isolating spark gaps for earth-termination systems or equipotential bonding
- Surge protective devices for use in photovoltaic systems
for nominal voltage ranges up to 1500 V
– according to EN 50539-11:2013 into type 1 / 2 SPDs
- b) according to their impulse current discharge capacity and protective effect into:
- Lightning current arresters / coordinated lightning current arresters
for protecting installations and equipment against interference resulting from direct or nearby lightning strikes (installed at the boundaries between LPZ 0A and 1).
- Surge arresters
for protecting installations, equipment and terminal devices against remote lightning strikes, switching over-voltages as well as electrostatic discharges (installed at the boundaries downstream of LPZ 0B).
- Combined arresters
for protecting installations, equipment and terminal devices against interference resulting from direct or nearby lightning strikes (installed at the boundaries between LPZ 0A and 1 as well as 0A and 2).
Technical data of surge protective devices
The technical data of surge protective devices include information on their conditions of use according to their:
- Application (e.g. installation, mains conditions, temperature)
- Performance in case of interference (e.g. impulse current discharge capacity, follow current extinguishing capability, voltage protection level, response time)
- Performance during operation (e.g. nominal current, attenuation, insulation resistance)
- Performance in case of failure (e.g. backup fuse, disconnector, failsafe, remote signalling option)
Short-circuit withstand capability
The short-circuit withstand capability is the value of the prospective power-frequency short-circuit current handled by the surge protective device when the relevant maximum backup fuse is connected upstream.
Short-circuit rating ISCPV of an SPD in a photovoltaic (PV) system
Maximum uninfluenced short-circuit current which the SPD, alone or in conjunction with its disconnection devices, is able to withstand.
Temporary overvoltage (TOV)
Temporary overvoltage may be present at the surge protective device for a short period of time due to a fault in the high-voltage system. This must be clearly distinguished from a transient caused by a lightning strike or a switching operation, which last no longer than about 1 ms. The amplitude UT and the duration of this temporary overvoltage are specified in EN 61643-11 (200 ms, 5 s or 120 min.) and are individually tested for the relevant SPDs according to the system configuration (TN, TT, etc.). The SPD can either a) reliably fail (TOV safety) or b) be TOV-resistant (TOV withstand), meaning that it is completely operational during and following
Surge protective devices for use in power supply systems equipped with voltage-controlled resistors (varistors) mostly feature an integrated thermal disconnector that disconnects the surge protective device from the mains in case of overload and indicates this operating state. The disconnector responds to the “current heat“ generated by an overloaded varistor and disconnects the surge protective device from the mains if a certain temperature is exceeded. The disconnector is designed to disconnect the overloaded surge protective device in time to prevent a fire. It is not intended to ensure protection against indirect contact. The function of
these thermal disconnectors can be tested by means of a simulated overload / ageing of the arresters.
Total discharge current Itotal
Current which flows through the PE, PEN or earth connection of a multipole SPD during the total discharge current test. This test is used to determine the total load if current simultaneously flows through several protective paths of a multipole SPD. This parameter is decisive for the total discharge capacity which is reliably handled by the sum of the individual
paths of an SPD.
Voltage protection level Up
The voltage protection level of a surge protective device is the maximum instantaneous value of the voltage at the terminals of a surge protective device, determined from the standardised individual tests:
– Lightning impulse sparkover voltage 1.2/50 μs (100%)
– Sparkover voltage with a rate of rise of 1kV/μs
– Measured limit voltage at a nominal discharge current In
The voltage protection level characterises the capability of a surge protective device to limit surges to a residual level. The voltage protection level defines the installation location with regard to the overvoltage category according to IEC 60664-1 in power supply systems. For surge protective devices to be used in information technology systems, the voltage protection level must be adapted to the immunity level of the equipment to be protected (IEC 61000-4-5: 2001).
Planning of internal lightning protection and surge protection
Lightning and surge protection for Industrial Building
Lightning and surge protection for Office Building
Lightning and surge protection for Residential Building
Requirements for External Lightning Protection Components
Components used for installing the external lightning protection system shall meet certain mechanical and electrical requirements, which are specified in the EN 62561-x standard series. Lightning protection components are categorised according to their function, for example connection components (EN 62561-1), conductors and earth electrodes (EN 62561-2).
Testing of conventional lightning protection components
Metal lightning protection components (clamps, conductors, air-termination rods, earth electrodes) exposed to weathering have to be subjected to artificial ageing/conditioning prior to testing to verify their suitability for the intended application. In accordance with EN 60068-2-52 and EN ISO 6988 metal components are subjected to artificial ageing and tested in two steps.
Natural weathering and exposure to corrosion of lightning protection components
Step 1: Salt mist treatment
This test is intended for components or devices which are designed to withstand exposure to a saline atmosphere. The test equipment consists of a salt mist chamber where the specimens are tested with test level 2 for more than three days. Test level 2 includes three spraying phases of 2 h each, using a 5% sodium chloride solution (NaCl) at a temperature between 15 °C and 35 °C followed by a humidity storage at a relative humidity of 93% and a temperature of 40 ±2 °C for 20 to 22 hours in accordance with EN 60068-2-52.
Step 2: Humid sulphurous atmosphere treatment
This test is to evaluate the resistance of materials or objects condensed humidity containing sulphur dioxide in accordance with EN ISO 6988.
The test equipment (Figure 2) consists of a test chamber where the specimens
are treated with a concentration of sulphur dioxide in a volume fraction of 667 x 10-6 (±24 x 10-6) in seven test cycles. Each cycle which has duration of 24 h is composed of a heating period of 8 h at a temperature of 40 ±3 °C in a humid, saturated atmosphere which is followed by a rest period of 16 h. After that, the humid sulphurous atmosphere is replaced.
Both components for outdoor use and components buried in the ground are subjected to ageing / conditioning. For components buried in the ground additional requirements and measures have to be considered. No aluminium clamps or conductors may be buried in the ground. If stainless steel is to be buried in the ground, only high-alloy stainless steel may be used, e.g. StSt (V4A). In accordance with the German DIN VDE 0151 standard, StSt (V2A) is not allowed. Components for indoor use such as equipotential bonding bars do not have to be subjected to ageing / conditioning. The same applies to components which are embedded
in concrete. These components are therefore often made of non-galvanised (black) steel.
Air-termination systems / air-termination rods
Air-termination rods are typically used as air-termination systems. They are available in many different designs, for example with a length of 1 m for installation with concrete base on flat roofs, up to the telescopic lightning protection masts with a length of 25 m for biogas plants. EN 62561-2 specifies the minimum cross sections and the permissible materials with the corresponding electrical and mechanical properties for air-termination rods. In case of air-termination rods with larger heights, the bending resistance of the air-termination rod and the stability of complete systems (air-termination rod in a tripod) have to be verified by means of a static calculation. The required cross sections and materials have to be selected based
on this calculation. The wind speeds of the relevant wind load zone also have to be taken into account for this calculation.
Testing of connection components
Connection components, or often simply called clamps, are used as lightning protection components to connect conductors (down conductor, air-termination conductor, earth entry) to each other or to an installation.
Depending on the type of clamp and clamp material, a lot of different clamp combinations are possible. The conductor routing and the possible material combinations are decisive in this respect. The kind of conductor routing describes how a clamp connects the conductors in cross or parallel arrangement.
In case of a lightning current load, clamps are subjected to electrodynamic and thermal forces which highly depend on the kind of conductor routing and the clamp connection. Table 1 shows materials which may be combined without causing contact corrosion. The combination of different materials with one another and their different mechanical strengths and thermal properties have different effects on the connection components when lightning current flows through them. This is particularly evident for stainless steel (StSt) connection components where high temperatures occur due to the low conductivity as soon as lightning currents flow through them. Therefore, a lightning current test in compliance with EN 62561-1 has to be carried out for all clamps. In order to test the worst case, not only the different conductor combinations, but also the material combinations specified by the manufacturer have to be tested.
Tests based on the example of an MV clamp
At first, the number of test combinations has to be determined. The MV clamp used is made of stainless steel (StSt) and hence can be combined with steel, aluminium, StSt and copper conductors as stated in Table 1. Moreover, it can be connected in cross and parallel arrangement which also has to be tested. This means that there are eight possible test combinations for the MV clamp used (Figures 3 and 4).
In accordance with EN 62561 each of these test combinations has to be tested on three suitable specimens / test set-ups. This means that 24 specimens of this single MV clamp have to be tested to cover the complete range. Every single specimen is mounted with the adequate
tightening torque in compliance with normative requirements and is subjected to artificial ageing by means of salt mist and humid sulphurous atmosphere treatment as described above. For the subsequent electrical test the specimens have to be fi xed on an insulating plate (Figure 5).
Three lightning current impulses of 10/350 μs wave shape with 50 kA (normal duty) and 100 kA (heavy duty) are applied to every specimen. After being loaded with lightning current, the specimens must not show signs of damage.
In addition to the electrical tests where the specimen is subjected to electrodynamic forces in case of a lightning current load, a static-mechanical load was integrated in the EN 62561-1 standard. This static-mechanical test is particularly required for parallel connectors, longitudinal connectors, etc. and is carried out with different conductor materials and clamping ranges. Connection components made of stainless steel are tested under worst case conditions with a single stainless steel conductor only (extremely smooth surface). The connection components, for example the MV clamp shown in Figure 6, are prepared with a defined tightening torque and then loaded with a mechanical tensile force of 900 N (± 20 N) for one minute. During this test period, the conductors must not move more than one millimetre and the connection components must not shown signs of damage. This additional static-mechanical test is another test criterion for connection components and also has to be documented in the manufacturer’s test report in addition to the electrical values.
The contact resistance (measured above the clamp) for a stainless steel clamp must not exceed 2.5 mΩ or 1 mΩ in case of other materials. The required loosening torque has to be ensured.
Consequently installers of lightning protection systems have to select the connection components for the duty (H or N) to be expected on site. A clamp for duty H (100 kA), for example, has to be used for an air-termination rod (full lightning current) and a clamp for duty N (50 kA) has to be used in a mesh or at an earth entry (lightning current already distributed).
EN 62561-2 also places special demands on conductors such as air-termination and down conductors or earth electrodes e.g. ring earth electrodes, for example:
- Mechanical properties (minimum tensile strength, minimum elongation)
- Electrical properties (max. resistivity)
- Corrosion resistance properties (artificial ageing as described above).
The mechanical properties have to be tested and observed. Figure 8 shows the test set-up for testing the tensile strength of circular conductors (e.g. aluminium). The quality of coating (smooth, continuous) as well as the minimum thickness and adhesion to the base material are important and have to be tested particularly if coated materials such as galvanised steel (St/tZn) are used.
This is described in the standard in the form of a bending test. For this purpose, a specimen is bent through a radius equal to 5 times of its diameter to an angle of 90°. In doing so, the specimen may not show sharp edges, breakage or exfoliation. Moreover, the conductor materials shall be easy to process when installing lightning protection systems. Wires or strips (coils) are supposed to be easily straightened by means of a wire straightener (guide pulleys) or by means of torsion. Furthermore, it should be easy to install / bend the materials at structures or in the soil. These standard requirements are relevant product features which have to be documented in the corresponding product data sheets of the manufacturers.
Earth electrodes / earth rods
The separable LSP earth rods are made of special steel and are completely hot-dip galvanised or consist of high-alloy stainless steel. A coupling joint which allows connection of the rods without enlarging the diameter is a special feature of theses earth rods. Every rod provides a bore and a pin end.
EN 62561-2 specifies the requirements for earth electrodes such as material, geometry, minimum dimensions as well as mechanical and electrical properties. The coupling joints linking the individual rods are weak points. For this reason EN 62561-2 requires that additional mechanical and electrical tests have to be performed to test the quality of these coupling joints.
For this test, the rod is put into a guide with a steel plate as impact area. The specimen consists of two joined rods with a length of 500 mm each. Three specimens of each type of earth electrode are to be tested. The top end of the specimen is impacted by means of a vibration hammer with an adequate hammer insert for a duration of two minutes. The blow rate of the hammer must be 2000 ± 1000 min-1 and the single stroke impact energy must be 50 ± 10 [Nm].
If the couplings have passed this test without visible defects, they are subjected to artificial ageing by means of salt mist and humid sulphurous atmosphere treatment. Then the couplings are loaded with three lightning current impulses of 10/350 μs wave shape of 50 kA and 100 kA each. The contact resistance (measured above the coupling) of stainless steel earth rods must not exceed 2.5 mΩ. To test whether the coupling joint is still firmly connected after being subjected to this lightning current load, the coupling force is tested by means of a tensile testing machine.
The installation of a functional lightning protection system requires that components and devices tested according to the latest standard are used. Installers of lightning protection systems have to select and correctly install the components according to the requirements at the installation site. In addition to mechanical requirements, electrical criteria of the latest state of lightning protection are to be considered and complied with.
50 Hz Ampacity of Earthing Conductors, Equipotential Bonding Connections, and Connection Components
Equipment of different electrical systems interacts in electrical installations:
- High-voltage technology (HV systems)
- Medium-voltage technology (MV systems)
- Low-voltage technology (LV systems)
- Information technology (IT systems)
The basis for a reliable interaction of the different systems is a common earth-termination system and a common equipotential bonding system. It is important that all conductors, clamps and connectors are specified for the various applications.
The following standards have to be considered for buildings with integrated transformers:
- EN 61936-1: Power installations exceeding 1 kV a.c.
- EN 50522: Earthing of power installations exceeding 1 kV a.c.
Conductor materials and connection components for use in HV, MV and LV systems have to withstand the thermal stress resulting from the 50 Hz currents. Due to the prospective short-circuit currents (50 Hz), the cross sections of the earth electrode material have to be specifically determined for the various systems / buildings. Line-to-earth short-circuit currents (normative requirement double earth fault current I“kEE) must not inadmissibly heat of the components. Unless there are special requirements of the network operator, the following is taken as a basis:
- Duration of the fault current (disconnection time) of 1 s
- Maximum permissible temperature of 300 °C of the earthing conductor and connection component / clamp materials used
The material and the current density G (in A/mm2) in relation to the fault current duration are decisive for the selection of the earthing conductor cross section.
Calculation of the Line-to-Earth Short-Circuit Current
System configurations and the associated currents to earth Medium-voltage systems can be operated as systems with isolated neutral, systems with low-impedance neutral earthing, solidly earthed neutral systems or inductively earthed neutral systems (compensated systems). In case of an earth fault, the latter allows to limit the capacitive current flowing at the fault location to the residual earth fault current IRES by means of a compensation coil (suppression coil with inductance L = 1/3ωCE) and is thus widely used. Only this residual current (typically up to max. 10 % of the uncompensated earth fault current) stresses the earth-termination system in case of a fault. The residual current is further reduced by connecting the local earth-termination system to other earth-termination systems (e.g. by means of the connecting effect of the cable shield of the medium-voltage cables). To this end, a reduction factor is defined. If a system has a prospective capacitive earth fault current of 150 A, a maximum residual earth fault current of about 15 A, which would stress the local earth-termination system, is assumed in case of a compensated system. If the local earth-termination system is connected to other earth-termination systems, this current would be further reduced.
Dimensioning of earth-termination systems with respect to the ampacity
For this purpose, different worst case scenarios must be examined. In medium-voltage systems, a double earth fault would be the most critical case. A first earth fault (for example at a transformer) may cause a second earth fault in another phase (for example a faulty cable sealing end in a medium-voltage system). According to table 1 of the EN 50522 standard (Earthing of power installations exceeding 1 kV a.c.), a double earth fault current I’’kEE, which is defined as follows, will flow via the earthing conductors in this case:
I“kEE = 0,85 • I“k
(I“k= three-pole initial symmetrical short-circuit current)
In a 20 kV installation with an initial symmetrical short-circuit current I’’k of 16 kA and a disconnection time of 1 second, the double earth fault current would be 13.6 kA. The ampacity of the earthing conductors and the earthing busbars in the station building or tansformer room must be rated according to this value. In this context, current splitting can be considered in case of a ring arrangement (a factor of 0.65 is used in practice). Planning must always be based on the actual system data (system configuration, line-to-earth short-circuit current, disconnection time).
The EN 50522 standard specifies the maximum short-circuit current density G (A/mm2) for different materials. The cross section of a conductor is determined from the material and the disconnection time.
he calculated current is now divided by the current density G of the relevant material and the corresponding disconnection time and the minimum cross section Amin of the conductor is determined.
Amin=I”kEE (branch) / G [mm2]
The calculated cross section allows to select a conductor. This cross section is always rounded up to the next larger nominal cross section. In case of a compensated system, for example, the earth-termination system itself (the part in direct contact with earth) is loaded with a considerably lower current namely only with the residual earth fault current IE = r x IRES reduced by the factor r. This current does not exceed some 10 A and can permanently flow without problems if common earthing material cross sections are used.
Minimum cross sections of earth electrodes
The minimum cross sections with regard to the mechanical strength and corrosion are defined in the German DIN VDE 0151 standard (Material and minimum dimensions of earth electrodes with respect to corrosion).
Wind load in case of isolated air-termination systems according to Eurocode 1
Extreme weather conditions are on the rise all over the world as a result of global warming. Consequences such as high wind speeds, an increased number of storms and heavy rainfall cannot be ignored. Therefore, designers and installers will face new challenges particularly with regard to wind loads. This does not only affect building structures (statics of the structure), but also air-termination systems.
In the field of lightning protection, the DIN 1055-4:2005-03 and DIN 4131 standards have been used as dimensioning basis so far. In July 2012, these standards were replaced by the Eurocodes which provide Europe-wide standardised structural design rules (planning of structures).
The DIN 1055-4:2005-03 standard was integrated in Eurocode 1 (EN 1991-1-4: Actions on structures – Part 1-4: General actions – Wind actions) and DIN V 4131:2008-09 in Eurocode 3 (EN 1993-3-1: Part 3-1: Towers, masts and chimneys – Towers and masts). Thus, these two standards form the basis for dimensioning air-termination systems for lightning protection systems, however, Eurocode 1 is primarily relevant.
The following parameters are used to calculate the actual wind load to be expected:
- Wind zone (Germany is divided into four wind zones with different base wind speeds)
- Terrain category (the terrain categories define the surrounding of a structure)
- Height of the object above ground level
- Height of the location (above sea level, typically up to 800 m above sea level)
Other influencing factors such as:
- Position on a ridge or top of a hill
- Object height above 300 m
- Terrain height above 800 m (sea level)
have to be considered for the specific installation environment and have to be calculated separately.
The combination of the different parameters results in the gust wind speed which is to be used as a basis for dimensioning air-termination systems and other installations such as elevated ring conductors. In our catalogue, the maximum gust wind speed is specified for our products to be able to determine the required number of concrete bases depending on the gust wind speed, for example in case of isolated air-termination systems. This does not only allow to determine the static stability, but also to reduce the necessary weight and thus the roof load.
The “maximum gust wind speeds” specified in this catalogue for the individual components were determined according to the Germany-specific calculation requirements of Eurocode 1 (DIN EN 1991-1-4/NA:2010-12) which are based on the wind zone map for Germany and the associated country-specific topographic particularities.
When using products of this catalogue in other countries, the country-specific particularities and other locally applicable calculation methods, if any, described in Eurocode 1 (EN 1991-1-4) or in other locally applicable calculation regulations (outside Europe) must be observed. Consequently, the maximum gust wind speeds mentioned in this catalogue only apply to Germany and are only a rough orientation for other countries. The gust wind speeds have to be newly calculated according to the country-specific calculation methods!
When installing air-termination rods in concrete bases, the information / gust wind speeds in the table have to be considered. This information applies to conventional air-termination rod materials (Al, St/tZn, Cu and StSt).
If air-termination rods are fixed by means of spacers, the calculations are based on the below installation possibilities.
The maximum permissible gust wind speeds are specified for the relevant products and have to be considered for selection / installation. A higher mechanical strength can be achieved by means of e.g. an angled support (two spacers arranged in a triangle) (on request).