Although all buildings should be protected against lightning strikes sensitive electronic equipment used in installations may require additional measures to be used to counteract electromagnetic effects.
Tim Williams and Keith Armstrong co-authors of EMC for Systems and Installations look at designing against the threat of lightning.
For lightning protection safety is the overriding concern. The appropriate standards for safety and the protection of buildings must always be followed. But for the purposes of this article we are only looking at lightning from an EMC viewpoint and it is assumed that an adequate protection system is already in place.
Best practice for EMC in installations requires the use of cable trays conduits and heavy gauge conductors as parallel earth conductors (PEC) to divert earth loop power currents away from cables and their screens. Different types of PEC are shown in order of high frequency effectiveness (top left to bottom right)
Designing a lightning protection system (LPS) in accordance with the basic safety standards will not be sufficient to protect the electronic apparatus inside a structure from the direct or induced effects of a strike. A completely enclosed metal structure would create the ideal LPS with no internal fields at all during a strike. Where electronics absolutely must survive lightning or similar external surges a seam-welded all metal structure may turn out to be required. In the absence of seam-welded metal structures all authorities recommend increasing the number of vertical LPS down conductors to share the surge currents so that each down conductor carries less current and hence creates lower magnetic fields. This also reduces the magnetic fields inside the structure. Natural components such as metallic facades conductive roof layers concrete reinforcement bars window and door frames may all be pressed into service as part of the enhanced LPS to further reduce its mesh size and hence the induced fields it emits when struck. Where a structure is made of reinforced concrete that uses welded or tied reinforcement bars and when the reinforcement bars are also welded to the metal window and door frames that break into the concrete using these reinforcement bars as part of an LPS can help create a protected structure.
Direct strikes
Exposed equipment such as antennas satellite dishes cameras and so on must be protected from direct strikes for their own sake. It is best to design nearby air terminations so they create zones of protection intercepting the lightning strike before it reaches the exposed equipment. Metal support structures for exposed equipment could be used as part of this air termination network. All exposed metal parts of equipment should also be bonded to the LPS and where they cannot they should be designed to withstand a direct strike or else be expendable. Exposed wiring should be run in metal conduit bonded to the LPS or in a protected zone such as the inside angles of steel girders. Cables associated with metal masts should run inside them. Where these measures still do not bring the surge voltage down to the immunity level of the equipment being protected fit surge protection devices (SPD). Bonds from SPDs to the LPS should be direct metal-to-metal contact where possible or via conductors no more than a few cm long.
How lightning strikes electronic equipment
Lightning can damage electronic equipment in a number of ways. The resistance of the soil and earthing networks becomes significant during lightning strikes and this gives rise to potential differences between areas normally considered to be at the same potential. Long cables are particularly at risk of causing damage due to this effect which is sometimes known as ground potential rise or ground lift. The rate of change of the discharge currents from a lightning strike may cause excessive voltages to be induced in conductors up to 100 metres away due to radiated magnetic fields. Rates up to 200kA/ms are seen in the arc channel itself with lower values where the lightning discharge current is shared between a number of conductors. The explosions and arcing flashovers associated with a direct strike to external equipment often results in damage to connected internal equipment but can also cause damage to unrelated equipment by flashovers in shared cable routes or cabinets. Immediately prior to a lighting strike electric fields of up to 500kV/m (the breakdown voltage of air) are generated over an area up to 100m from the eventual strike point. Fluctuating fields of 500kV/m.ms occur during a strike. These fields will induce voltages and currents into conductors and devices but are usually effectively dealt with by the measures taken to protect equipment from the other threats. Lightning electromagnetic pulse (LEMP) is a far-field phenomenon caused by cloud-to-cloud lightning and distant cloud-to-ground lightning. It is usually only a problem for exposed external conductors and is effectively dealt with by the measures taken to protect equipment from other lightning threats. A typical lightning event consists of many discharges of which the second one usually contains the most damaging energies. Multi-stroke flashes can exceed 10 strokes and last for over a second. This is of great importance in the design of software for error-correction and for the recovery of systems.
Metallic cables and services entering a building should preferably have travelled underground for their entire length (even telephone wires). Overhead cables and services are exposed to lightning threats and need special consideration.
Huge differences in earth voltage exist between two structures when one of them is struck by lightning and power or signal cables passing between structures will therefore inject severe surges into electronic equipment. Surge voltages are also generated if there are earth faults in one of the structures. To help protect the cables and the electronics they interconnect the earthing systems of the structures should be interconnected by many parallel metallic paths preferably forming a mesh. The cables should be laid within parallel earth conductors (PEC) a technique for reducing interference where signals and their earth are run along almost identical routes preferably by totally enclosing the cable in a metal conduit which forms the earth. The PECs are bonded to the earth mesh between the structures and the equipotential earth bonding bars in the structures at either end. Metalwork and metal services (such as gas and water) passing between structures should similarly be bonded to the interconnecting earth mesh and the equipotential bonding bars at each end. Cables between structures should have any spare cores bonded to the earth at both ends as long as those cores have sufficient cross-sectional area to carry the anticipated continuous and surge currents. One of the most important issues is that all external metallic cables (power telephone data) and services using metallic ducts or pipes (water gas steam) should enter the building in one small underground area and there be bonded to a single large equipotential bonding plate. Recommendations for protecting telecommunication installations say that none of the bonds and SPDs for cables and services should be more than 2m from the neutral of the main incoming supply disconnector giving the entry area a maximum width of 4m. This is good general advice for any electronic installation and is easiest to imagine as a star point between the structure to be protected and the rest of the world - it helps prevent external surges from travelling through the structure. Where something metallic cannot be bonded directly to earth it should be connected via SPDs also installed on the same single equipotential bonding plate. This plate should be inserted into the line of the ring earth electrode and bonded to it at both ends. It should also have multiple connections to the internal bonding ring conductor (BRC) of the structure's common bonding network and to concrete reinforcing or foundation electrodes. Foundation strip electrodes or foundation earth electrodes may be used in place of the ring earth electrode.
Lightning surge protection devices
Surge protection devices (SPD)s are most effective when used with a carefully designed and constructed protective structure. They need to be chosen on the basis of the relevant standards or codes of practice for the waveforms and peak values of current and energy they must handle. Co-ordinating them with the equipment they are intended to protect also requires data on their let-through voltages. SPDs can be connected between a power (or signal) conductor and earth to suppress line-to-earth surges or between two power conductors to suppress line-to-line surges. Apparatus will have different immunity ratings for line-to-earth and line-to-line surges so the SPDs used for the relevant modes may not be the same. Surge arrestors are designed to provide a clamping effect when the voltage across them exceeds a certain level rather like the action of a zener diode.
There are four basic types: Gas discharge tubes (GDT) are essentially just spark gaps slow but with high power rating and negligible leakage. Metal-oxide varistor (MOV) a bulk semiconductor fast but less rugged than a GDT. Avalanche devices are semiconductors with a zener type of action. They are very fast but have limited power capabilities. Thyristor devices are also semiconductors but are slow with high current handling properties. The GDT and thyristor devices have to be self-triggered before clamping and during the initial period they let through surge voltages which may be potentially damaging. They also have a foldback characteristic which means that once they fire off and are carrying current the voltage across them drops to well below the level they were previously able to block. Careful design is needed to make sure that if connected to a source of DC current they do not remain in continuous conduction once triggered by the surge. This characteristic is not shared by MOV and avalanche devices but their disadvantage is a higher clamping voltage and dissipation for a given normal operating voltage. Modern installations often use a combination of different types to achieve the overall performance desired. MOV types used to be considered rather short-lived but in recent years they have been developed so that they now appear to be robust as long as they are rated correctly for their anticipated worst-case surge currents. Some of the test standards for SPDs are becoming tougher too. For instance IEC61643-1 stresses the SPD under test with 20 surges at its maximum rated current and then checks that its electrical parameters have not changed by more than 10%. If the lightning risk analysis has been done correctly each SPD will only rarely have to carry its full rated current and it will be a long time before they see 20 full-current surges. However all SPDs are prone to failure so they should be checked regularly and replaced if found to be degraded or failed. Some manufacturers are now offer units fitted with indicators which warn of degraded performance and impending failure. SPDs are installed first at the equipotential bonding plate (or main earthing terminal in less sensitive structures) on all incoming or outgoing conductors that are not directly bonded to the earth at the same place. Other SPDs may be fitted at lightning protection zone boundaries in a similar way within the structure and some may be fitted at the equipment itself. What they all have in common is a need to keep their lead lengths short and to ensure that their earthing is relative to what is being protected . The total let-through voltage is a combination of the SPD's voltage clamping action plus the transient voltage drop in its connecting leads due to their inductance (usually reckoned to be 1mH/metre). For the rates of change of current associated with lightning strikes if an SPD has an earth lead longer than 1 metre much greater voltage surges can be let through to the equipment supposedly being protected. Thus SPDs for lightning suppression should have lead lengths of 500mm or less. Binding the leads to an SPD together also helps reduce lead inductance considerably. Always make sure that SPD protected cables are segregated by at least 150mm on a parallel run from any cables which have not been protected.
Bonding at the entry to buildings for maximum protection from lightning effects involves a main earthing terminal - a large plate welded to several structural reinforcing bars. Direct bonding is shown here but where surge protection devices or filters are used they should also bond to this plate
Large bonding plates
To handle all the bonding and SPDs the equipotential bonding plate will often need to be quite large probably greater than 1m2. For larger installations it may be acceptable to have a number of large plates side-by-side each connected to the ring earth electrode at both ends. A structure made of reinforced concrete could interconnect all its vertical and horizontal reinforcing bars and use them as natural components of an effective LPS. The re-bars could be all that is needed for the down conductors and foundation earth electrode and give much better performance than an LPS made of copper conductors fixed to the outside or buried in the ground. The equipotential bonding plate for the star point of such a structure needs to be set into the vertical plane of the wall reinforcing bars
Small mesh protection
The mesh size of the LPS for this structure would be so small and the number of down-conductors so large that no re-bar would carry large lightning currents and the current and field penetration inside the structure would be low. The only structure significantly better than this for lightning protection would be one completely skinned in welded metal sheeting. As well as providing an equipotential earth a fully meshed earth inside a structure provides useful shielding against the magnetic fields created by lightning currents. A mesh with 3 to 4m spacing is recommended to help protect electronics. Vertical meshes are said to give the greatest protection from lightning fields. Where there are no existing metallic components large cross-section conductors or steel rods may be added.
Alternative earthing
Some argue that creating fully meshed earths with adequate performance at the frequencies required is expensive and recommend instead isolated mesh earths. Star-earthed systems are suitable for smaller IT systems but to protect them from lightning magnetic fields their interconnecting cables must follow their star-earthing conductors at all times to avoid induction loops. The concept of zoning involves identifying or creating zones within structures where there is less exposure to some or all of the effects of lightning and to co-ordinate these with the immunity characteristics of the equipment installed in them. Generally speaking in a properly earth-meshed structure the central volume is least exposed to the effects of lightning so this is the best place to install the most sensitive apparatus. Installation of electronics should be avoided on roofs or the top floors of buildings (especially tall ones) or near outside walls corners down conductors masts or chimneys. However in all-metal structures location of equipment is not generally critical. It is possible to extend the more protected zones by careful design of the LPS earth bonding and cabling.
Zoning problems
Although zoning is good in theory it may be difficult to achieve in practice since few immunity standards include lightning related tests. However the telecoms ITU Recommendations K.20 21 and 22 require the calculation of failure probabilities based upon the resistibility of equipment and the protection measures implemented for the zone it is used in. Appendix C of BS6651 has a zoning approach which begins with a modified form of lightning risk assessment. It also identifies (as does IEEE C62.41-1991) three distinct zones with differing surge exposure categories. For each category BS6651 provides a table listing the surge protection device test specifications required for each of the risk assessment levels: high medium or low. The surge current test levels in this table may be applied to the rating of direct earth bonds as well as to SPDs. The surge voltage and current test levels can be used for testing electronic apparatus for its ability to withstand lightning surges on its mains inputs. The shape of the test voltage and current waveforms is also specified by BS6651 and corresponds to the combination wave generator which is now on the list of IEC EMC immunity test standards as IEC61000-4-5. The exposure levels suggested by Appendix C of BS6651 are based on lightning risk assessment only. If transients of other origin are present consider upgrading any SPDs or specifying apparatus with higher surge immunity.
| Contacts: Tim Williams and Keith Armstrong are joint authors of EMC for Systems and Installations from which this article is an extract Tim is a consultant with Elmac Services Chichester UK +44 (0)1243 533361 Keith is with Cherry Clough Consultants Oldham UK +44 (0)1457 871605 This article was first published in APPROVAL Magazine the engineering guide to quality standards and regulations. For more information on APPROVAL visit the website www.approval.co.uk or contact Adrian MacLeod |
