HOW DO YOU PROTECT AGAINST LIGHTNING
The first thing to emphasize is that due to the unpredictable nature of lightning no lightning protection system can be 100% effective. In the IEC documents the best stated efficiency is 98%.
LIGHTNING PROTECTION OF BUILDINGS
with all things concerned, with lightning protection the protection of structures
is a field where you can get every thing from recognized protection systems
to the “snake oil” type of cure. For this reason it is important
that a lightning protection system is designed and installed in accordance
with recognized codes of practice. It must also be noted that a lightning
protection system works by attracting the lightning and discharging it safely
to earth, therefore an incorrectly designed lightning protection system may
be more dangerous than no system.
The recognized codes of practice in South Africa are:
10313:1999 The protection of structures against lightning.
SABS IEC 61024-1:1990 Protection of structures against lightning Part 1: General principles
SABS IEC 61024-1-1:1993 Protection of structures against lightning Part 1: General principles Section 1: Guide A – Selection of protection levels for lightning protection systems
SABS IEC 61024-1-2: 1998 Protection of structures against lightning Part 1-2: General Principles – Guide B – Design, installation, maintenance and inspection of lightning protection systems.
SABS IEC 61662:1995 Assessment of the risk of damage due to lightning
10313 has been based on the IEC documents with certain changes mainly in the
protection of thatched structures, to suit South African Conditions.
For a lightning protection system to comply with either the IEC documents or the SABS document first a risk assessment of the site has to be conducted. During the risk assessment the expected annual frequency of direct lightning flashes to a structure is calculated along with the Accepted annual frequency of lightning flashes. From this it is possible to calculate the required efficiency of lightning protection system required, and therefore the protection level. At present the risk assessment in SABS 0313 does not take into account any flashes to ground near the structure. The draft IEC for the assessment of risk for structures if adopted will have a far more rigorous method of risk assessment.
the protection level is known the lightning protection system can be designed.
One, or a combination of the following three methods may determine the zone
of protection of the air termination system:
The air termination is positioned such that all parts of the structure to be protected are with in the cone of protection formed by the angle a to the vertical. There are limitations to the use of this method.
The rolling sphere method may be used to identify the protected parts and areas of a structure where use of the protective angle method is excluded. The method is applied by rolling a sphere with radius R over the structure to be protected; the sphere should only touch the ground and air termination system.
This is generally used for flat surfaces.
should be noted that the following are not recognized by IEC or the SABS:
Enhanced streamer emission terminals
Lightning elimination devices.
conductors must be positioned at appropriate intervals as specified in the
codes of practice. As the lightning current is shared by several down conductors
the risk of a side flash to the structure and electro magnetic disturbances
within the structure will be reduced. The impedance of a down conductor should
be as low as possible as;
The typical inductance of a 35 mm2 down conductor is 1,5 x 10-6 H/m with a resistance of 0,7 mO/m if a lightning flash of peak current 30 kA and a di/dt of 25 x 109 A/sec, over one meter of conductor the voltage rise is 37,521 kV/m.
From the above it can be seen that for a mast, distance has to be maintained between the mast and structure.
As the purpose of a lightning protection system is to discharge the lightning flash safely to earth the earth termination system is of vital importance. The earth electrode should be designed and installed in accordance with SABS 0199:1985 The design and installation of an earth electrode. As well as the constraints of the lightning protection codes of practice. The touch and step voltages should be taken into account during the design of the earth electrode.
LIGHTNING PROTECTION OF EQUIPMENT
For full lightning protection the electronic equipment should also be protected. This involves the fitting of surge protection on all incoming lines. A building may divided in to zones of lightning protection. At the entry to each zone all conductors are either bonded directly to earth or via surge diverters. It is recommended that all services enter a building at a common point. If this is not possible bonding bars must be fitted at each location. The bonding bars must be connected to the earth termination and interconnected by means of an internal ring (type A earthing). According to the new wiring code all new installations will have to be fitted with surge suppression at least at the incoming distribution board.
The codes of practice call for periodic inspection and maintenance to be conducted on a lightning protection system. The frequency of which will depend upon the weather, actual lightning damage, the protection level and any legislation (e.g. the explosives act). A typical program would entail, A visual inspection, checking of all conductors and system components, checking the equipotential bonding, measuring of earth resistance checking the surge protective devices, and ensuring the lightning protection system has been extended to cater for any structural changes.