6. Internal lightning protection

Transcription

1 6. Internal lightning protection 6.1 Equipotential bonding for metal installations Equipotential bonding according to IEC and IEC Equipotential bonding is required for all newly installed electrical power consumer s installations. Equipotential bonding according to IEC series removes potential differences, i.e. prevents hazardous touch voltages between the protective conductor of the low voltage electrical power consumer s installations and metal, water, gas and heating pipes, for example. According to IEC , equipotential bonding consists of the main equipotential bonding (in future: protective equipotential bonding) and the supplementary equipotential bonding (in future: supplementary protective equipotential bonding) Every building must be given a main equipotential bonding in accordance with the standards stated above (Figure 6.1.1). The supplementary equipotential bonding is intended for those cases where the conditions for disconnection from supply cannot be met, or for special areas which conform to the IEC series Part 7. buried installation, operationally isolated (e.g. cathodic protected tank installation) 6 metal element going through the building (e.g. lift rails) antenna remote signalling system equipotential bonding of bathroom 6 4 to PEN 230/400 V kwh Equipotential bonding bar (main equipotential bonding, in future: main earthing terminal) Foundation earth electrode Connector Lightning current arrester Terminal Pipe clamp Terminal lug Isolating spark gap heating SEB insulating element Z gas 6 6 water waste water 7 4 distribution network IT system Z foundation earth electrode/ lightning protection earth electrode terminal lug for external lightning protection Fig Principle of lightning equipotential bonding consisting of lightning and main equipotential bonding (in future: protective equipotential bonding) LIGHTNING PROTECTION GUIDE 147

2 Main equipotential bonding The following extraneous conductive parts have to be directly integrated into the main equipotential bonding: main equipotential bonding conductor in accordance with IEC (in future: earthing conductor) foundation earth electrodes or lightning protection earth electrodes central heating system metal water supply pipe conductive parts of the building structure (e.g. lift rails, steel skeleton, ventilation and air conditioning ducting) metal drain pipe internal gas pipe earthing conductor for antennas (in Germany in DIN VDE ) earthing conductor for telecommunication systems (in Germany in DIN VDE ) protective conductors of the electrical installation in accordance with IEC series (PEN conductor for TN systems and PE conductors for TT systems or IT systems) metal shields of electrical and electronic conductors metal cable sheaths of high-voltage current cables up to 1000 V earth termination systems for high-voltage current installations above 1 kv according to HD 637 S1, if no intolerably high earthing voltage can be dragged. Normative definition in IEC of an extraneous conductive component: A conductive unit not forming part of the electrical installation, but being able to introduce electric potential including the earth potential. Note: Extraneous conductive components also include conductive floors and walls, if an electric potential including the earth potential can be introduced via them. The following installation components have to be integrated indirectly into the main equipotential bonding via isolating spark gaps: installations with cathodic corrosion protection and stray current protection measures in accordance with EN earth-termination systems of high-voltage current installations above 1 kv in accordance with HD 637 S1, if intolerably high earthing potentials can be transferred railway earth for electric a.c. and d.c. railways in accordance with EN (railway lines of the Deutsche Bahn may only be connected upon written approval) measuring earth for laboratories, if they are separate from the protective conductors Figure shows the terminals and the respective components of the main equipotential bonding. Design of the earth-termination system for equipotential bonding The electrical low-voltage consumer s installation requiring certain earthing resistances (disconnection conditions of the protective elements) and the foundation earth electrode providing good earthing resistances at cost-effective installation, the foundation earth electrode is an optimal and effective complement of the equipotential bonding. The design of a foundation earth electrode is governed in Germany by DIN 18014, which, for example requires terminal lugs for the earthing busbar. More exact descriptions and designs of the foundation earth electrode can be found in Chapter 5.5. If a foundation earth electrode is used as lightning protection earth electrode, additional requirements may have to be considered; they can be taken from Chapter 5.5. Equipotential bonding conductors (in future: protective bonding conductors) Equipotential bonding conductors should, as long as they fulfil a protective function, be labelled the same as protective conductors, i.e. green/yellow. Equipotential bonding conductors do not carry operating currents and can therefore be either bare or insulated. The decisive factor for the design of the main equipotential bonding conductors in accordance with IEC and HD is the cross section of the main protective conductor. The main protective conductor is the one coming from the source of current or from the service entrance box or the main distribution board. 148 LIGHTNING PROTECTION GUIDE

3 Normal Main equipotential bonding Supplementary equipotential bonding 0.5 x cross section of the largest protective conductor of the installation between two bodies between a body and an extraneous conductive part Minimum 6 mm 2 with mechanical protection without mechanical protection Possible limit 25 mm 2 Cu or equivalent conductivity 1xcross section of the smaller protective conductor 0.5 x cross section of the protective conductor 2.5 mm 2 Cu or equivalent conductivity 4 mm 2 Cu or equivalent conductivity Table Cross sections for equipotential bonding conductors In any case, the minimum cross section of the main equipotential bonding conductor is at least 6 mm 2 Cu. 25 mm 2 Cu has been defined as a possible maximum. The supplementary equipotential bonding (Table 6.1.1) must have a minimum cross section of 2.5 mm 2 Cu for a protected installation, and 4 mm 2 Cu for an unprotected installation. For earth conductors of antennas (according to IEC (EN )), the minimum cross section is 16 mm 2 Cu, 25 mm 2 Al or 50 mm 2 steel. Equipotential bonding bars Equipotential bonding bars are a central component of equipotential bonding which must clamp all the connecting conductors and cross sections occurring in practice to have high contact stability; it must be able to carry current safely and have sufficient corrosion resistance. DIN VDE : (German standard) contains details of the requirements on equipotential bonding bars for the main equipotential bonding. It defines the following connection possibilities as a minimum: 1 x flat conductor 4 x 30 mm or round conductor Ø 10 mm 1 x 50 mm 2 6 x 6 mm 2 to 25 mm 2 1 x 2.5 mm 2 to 6 mm 2 This standard also includes the requirements for the inspection of clamping units of cross sections above 16 mm 2 with regard to the lightning current ampacity. Reference is made therein to the testing of the lightning protection units in accordance with EN If the requirements in the previously mentioned standard are met, then this component can also be used for lightning equipotential bonding in accordance with IEC to 4 (EN to 4). Terminals for equipotential bonding Terminals for equipotential bonding must provide a good and permanent contact. These requirements on an equipotential bonding bar are met by K12 (Figure 6.1.2). Fig K12 Equipotential bonding bar, Part No LIGHTNING PROTECTION GUIDE 149

4 Fig Pipe earthing clamp, Part No Fig Pipe earthing clamp, Part No Fig Pipe earthing clamp, Part No Integrating pipes into the equipotential bonding In order to integrate pipes into the equipotential bonding, earthing pipe clamps corresponding to the diameters of the pipes are used (Figures and 6.1.4). Pipe earthing clamps made of stainless steel, which can be universally adapted to the diameter of the pipe, offer enormous advantages for mounting (Figure 6.1.5). These pipe earthing clamps can be used to clamp pipes that are made of different materials (e.g. steel, copper and stainless steel). These components allow also a straight-through connection. Figure shows equipotential bonding of heating pipes with straight-through connection. Test and inspection of the equipotential bonding Before commissioning the electrical consumer s installation, the connections must be inspected to ensure their faultless condition and effectiveness. A low-impedance conductance to the various parts of the installation and to the equipotential bonding is recommended. A guide value of < 1 Ω for the connections at equipotential bonding is considered to be sufficient. Supplementary equipotential bonding If the disconnection conditions of the respective system configuration can not be met for an installation or a part of it, a supplementary local equipotential bonding is required. The reason behind is to interconnect all simultaneously accessible parts as well as the stationary operating equipment and also extraneous conductive parts. The aim is to keep any touch voltage which may occur as low as possible. Moreover, the supplementary equipotential bonding must be used for installations or parts of installations of IT systems with insulation monitoring. The supplementary equipotential bonding is also required if the environmental conditions in special installations or parts of installations mean a particular risk. The IEC series Part 7 draws attention to the supplementary equipotential bonding for operational facilities, rooms and installations of a particular type. These are, for example, IEC Rooms with bathtub or shower IEC Swimming pools and other basins IEC For agricultural and horticultural premises Fig Equipotential bonding with straight-through connection The difference to the main equipotential bonding is the fact that the cross sections of the conductors can be chosen to be smaller (Table 6.1.1), and also this supplementary equipotential bonding can be limited to a particular location. 150 LIGHTNING PROTECTION GUIDE

5 Fig DEHNbloc NH lightning current arrester installed in a busbar terminal field of a meter installation (refer to Fig ) 6.2 Equipotential bonding for low voltage consumer s installations Equipotential bonding for low voltage consumer s installations as part of the internal lightning protection, represents an extension of the main equipotential bonding (in future: protective equipotential bonding) according to IEC (Figure 6.1.1). In addition to all conductive systems, this also integrates the supply conductors of the low voltage consumer s installation into the equipotential bonding. The special feature of this equipotential bonding is the fact that a tie-up to the equipotential bonding is only possible via suitable surge protective devices. The demands on such surge protective devices are described more detailed in Annex E subclause of IEC (EN ) as well as in subclause 7 and Annexes C and D of IEC (EN ). Analogous to the equipotential bonding with metal installations (see Chapter 6.1), the equipotential bonding for the low voltage consumer s installation shall also be carried out immediately at the point of entry into the object. The requirements governing the installation of the surge protective devices in the unmetered area of the low voltage consumer s installation (main distribution system) are described in the directive of the VDN (Association of German Network Operators) Surge protective devices Type 1. Directive for the use of surge protective equipment Type 1 (up to now Class B) in main distribution systems (see subclauses and 8.1) (Figures and 6.2.2). Fig DEHNventil ZP combined arrester directly snapped on the busbars in the terminal field of the meter cabinet 6.3 Equipotential bonding for information technology installations Lightning equipotential bonding requires that all metal conductive components such as cable lines and shields at the entrance to the building shall be incorporated into the equipotential bonding so as to cause as little impedance as possible. Examples of such components include antenna lines, (Figure 6.3.1) telecommunication lines with metal conductors, and also fibre optic systems with metal elements. The lines are connected with the help of elements capable of carrying lightning current (arresters and shielding terminals). A convenient installation site is the point where cabling going LIGHTNING PROTECTION GUIDE 151

6 α α air termination tip α α feeding point isolated air-termination system (DEHNconductor) sealing unit range connection equipotential bonding insulating pipe 230 V~ DATA 230 V~ antenna isolated down conductor (HVI-conductor) s =^ 0.75 m in air s =^ 1.5 m in brickwork s = separation distance equipotential bonding to BTS earth-termination system Fig Lightning equipotential bonding with isolated air-termination system, type DEHNconductor, for professional antenna systems according to IEC (EN ) Fig Isolated construction of a lightning protection system at a cell site outside the building transfers to cabling inside the building. Both the arresters and the shielding terminals must be chosen to be appropriate to the lightning current parameters to be expected. In order to minimise induction loops within buildings, the following additional steps are recommended: cables and metal pipes shall enter the building at the same point power lines and data lines shall be laid spatially close but shielded avoiding of unnecessarily long cables by laying lines directly Antenna installations: For reasons connected with radio engineering, antenna installations are generally mounted in an exposed location. Therefore they are more affected by surges, especially in the event of a direct lightning strike. In Germany they must be integrated into the equipotential bonding in accordance with DIN VDE 0855 Part 300 (German standard) and must reduce the risk of being affected through their design, (cable structure, connectors and fittings) or suitable additional measures. Antenna elements that are connected to an antenna feeder and cannot be connected directly to the equipotential bonding, as this would affect their functioning, should be protected by arresters. Expressed simply, it can be assumed that 50 % of the direct lightning current flows away via the shields of all antenna lines. If an antenna installation is dimensioned for lightning currents up to 100 ka (10/350 μs) (Lightning Protection Level III (LPL III)), the lightning current splits so that 50 ka flow through the earth conductor and 50 ka via the shields of all antenna cables. Antenna installations not capable of carrying lightning currents must therefore be equipped with air-termination systems in whose protection area the antennas are 152 LIGHTNING PROTECTION GUIDE

7 located. Choosing a suitable cable, the respective partial lightning current share must be determined for each antenna line involved in down conducting. The required cable dielectric strength can be determined from the coupling resistance, the length of the antenna line and the amplitude of the lightning current. According to the current standard IEC (EN ), antenna installations mounted on buildings can be protected by means of air-termination rods elevated wires or spanned cables In each case the separation distance s must be maintained in the areas protected against lightning strikes. The electrical isolation of the lightning protection system from conductive components of the building structure (metal structural parts, reinforcement etc.), and the isolation from electric lines in the building, prevent partial lightning currents from penetrating into control and supply lines and hence protect sensitive electrical and electronic devices from being affected or destroyed (Figure and Figure 6.3.2). Fibre optic installations: Fibre optic installations with metal elements can normally be divided into the following types: cables with metal-free core but with metal sheath (e.g. metal vapour barrier) or metal supporting elements cables with metal elements in the core and with metal sheath or metal supporting elements cables with metal elements in the core, but without metal sheath. For all types of cable with metal elements, the minimum peak value of the lightning current, which adversely affects the transmission characteristics of the optical fibre, must be determined. Cables capable of carrying lightning currents must be chosen, and the metal elements must be connected to the equipotential bonding bar either directly or via an SPD. Metal sheath: termination by means of shield terminals e.g. SAK, at the entrance of the building Metal core: termination by means of earthing clamp e.g. SLK, near splice box Prevention of potential equalising currents: connect indirectly via spark gap e.g. DEHNgap CS, base part BLITZDUCTOR CT, rather than directly Telecommunication lines: Telecommunication lines with metal conductors normally consist of cables with balanced or coaxial cabling elements of the following types: cables with no additional metal elements cables with metal sheath (e.g. metal dampproofing) and/ or metal supporting elements cables with metal sheath and additional lightning protection reinforcement The splitting of the partial lightning current between IT lines can be determined using the procedures in Annex E of IEC (EN ). The individual cables must be integrated into the equipotential bonding as follows: a) Unshielded cables must be connected by SPDs which are capable of carrying partial lightning currents. Partial lightning current of the line divided by the number of individual wires = partial lightning current per wire. b) If the cable shield is capable of carrying lightning currents, the lightning current flows via the shield. However, capacitive/inductive interferences can reach the wires and make it necessary to use surge arresters. Requirements: Fig The shield at both ends must be connected to the main equipotential bonding to be capable of carrying lightning currents (Figure 6.3.3). SAK shield connection system capable of carrying lightning currents LIGHTNING PROTECTION GUIDE 153

8 APL Fig Fig BLITZDUCTOR 3 OUT 4 BCT MLC BD 110 No IN 2 Telekom TAE customer BLITZDUCTOR CT BCT MLC BD ka (10/350 μs) IT installation Lightning equipotential bonding for connection of a telecommunications device BLITZDUCTOR CT (application permitted by Deutsche Telekom) DEHN equipotential bonding enclosures (DPG LSA) for LSA-2/10 technology, capable to carry lightning current c) If the cable shield is not capable of carrying lightning currents, then: for the terminal connected at both ends, the procedure is the same as for a signal wire in an unshielded cable. Partial lightning current of the cable divided by the number of individual wires + 1 shield = partial lightning current per wire if the shield is not connected at both ends, it has to be treated as if it were not there; partial lightning current of the line divided by the number of individual wires = partial lightning current per wire If it is not possible to determine the exact wire load, it is recommendable to take the threat parameters from IEC For a telecommunications line hence results a maximum load per wire of 2.5 ka (10/350 μs). Of course not only the used SPD must be capable of withstanding the expected lightning current load, but also the discharge path to the equipotential bonding. By means of a multi-core telecommunications line for example this can be demonstrated: A telecommunications cable with 100 double wires coming from LPZ 0 A is connected in an LSA building distribution case and shall be protected by arresters. The lightning current load of the cable was assumed to be 30 ka (10/350 μs) The resulting symmetrical splitting of lightning current to the individual wire is 30 ka/ 200 wires = 150 A/wire. In both buildings where the cable ends, the lightning protection zone concept must be applied, and the active wires must be connected in the same lightning protection zone (usually LPZ 1) If an unshielded cable is laid in a metal pipe, this must be treated like a cable with a cable shield which is capable of carrying lightning currents. At first this means no special requirements to the discharge capacity of the protective elements to be used. After the discharge elements have flown through, the partial currents of all wires add up to 30 ka again to load in the downstream discharge path, for example clamping frames, earthing clamps or equipotential conductors. To be safe from any damage in the discharge path lightning current tested enclosure systems can be used (Figure 6.3.5). 154 LIGHTNING PROTECTION GUIDE