Lightning Protection - test before a strike
Most building services would simply not function correctly if faults or defects were present but the correct operation of a lightning protection system only becomes obvious when it is called upon to protect a structure. For this reason it is even more vital to ensure that fully trained and qualified electrical engineers undertake regular testing and maintenance works on vulnerable structures and sites. The current in a lightning strike is likely to be in the range of 2,000 - 200,000A and so an effective operational system is vital to ensure the protection of assets.
The provision of a lightning conductor system will not prevent the occurrence of a lightning strike. The purpose of the installation is to direct the current discharged from the strike to earth safely, protecting the structure and its occupants from the effects of the strike.
The vast majority of structures in the UK use BS6651 to inform their design, testing and maintenance works in relation to lightning protection.
As large parts of the lightning protection systems may be hidden or inaccessible after completion, it is particularly important, and indeed a requirement of the regulations, that each component of a lightning protection system should be inspected during the construction stages of an installation. Special attention must be given to any part of the system that will be concealed upon completion. These components may be hidden for aesthetic reasons or the component may be an integral part of the structure.
Inspections should be carried out not only during the installation process but also upon completion and at regular intervals thereafter.
Visual inspection of an installation should take into account the following key points and observations recorded in the detailed inspection report:
- Inspections should be repeated at fixed intervals, preferably not exceeding 12 months. If the intervals are fixed at 11 months, the system will have been inspected throughout every season of the year over a period of 11 years
- The mechanical condition of all conductors, bonds, joints and earth electrodes should be checked and any observations noted
- If a part is unable to be inspected, this should be noted
- The bonding of any recently installed/added services should be checked
This section deals with testing the earth electrodes on the system, although reference is made to a visual or measured test of any joints or bonds. In practice, it is usual for inspections of components to be undertaken rather than for testing to be carried out.
Electrode testing requires experience and expertise to ensure that any test carried out is meaningful and reflects the resistance of the electrode under test. Too frequently, we are handed client information presenting resistance readings that are obviously continuity tests and not true earth-resistance tests.
There are two appropriate methods of testing lightning protection earths: ‘Fall off Potential/the 61.8% method’ and ‘Dead Earth’.
‘Fall of Potential’ is the recommended method and involves the electrode under test; two reference electrodes, a set of leads and a four-pole test meter. The electrode under test is isolated and connected to the meter for the ‘Fall of Potential’ or for the ‘61.8%’ method. In turn, the test meter is connected to the two reference electrodes, which are driven approximately 300mm into the ground and located typically 25 and 50 metres away from the electrode under test.
A test is made and the direct resistance of the electrode under test is recorded on the meter. This method, however, is only practical if the electrode to be tested is located adjacent to virgin ground where test electrodes can be driven. In reality, in town and city centres for example, this is very often not the case. The presence of buried services and pipes may also have an influence on the test current and the final test value may be corrupted as a result of these external influences. Reference electrodes should therefore be sited away from such potential disturbances.
Where practical conditions dictate that the ‘Fall of Potential’ method cannot be used, the ‘Dead/Known Earth’ method is really the only practical alternative. However, it is important to be aware this method is open to error and misrepresentation if the test engineer is not competent to determine an appropriate dead earth or interpret the readings, which is why it is essential to use an experienced engineer to undertake tests of this nature.
The ‘dead earth’ can be any low-resistance earth not directly or fortuitously connected to the earth under test. A connection is made from a suitable earth to the test meter, which is in turn connected to the electrode under test, which shows the lightning protection system acting as the known ‘dead/known’ earth. A reading is then taken and the ohmic value achieved is effectively the series resistance of the electrode under test and the dead earth.
The ‘Dead Earth’ method has some advantages when using the lightning protection system as the low-resistance ‘dead/known’ earth, as, due to the equipotential bonding required to other incoming services, it should provide a low-resistance earth path. Test clamps, or the clamp to the rod in the inspection pit, should be opened and the meter connected to the rod/rod side of the test clamp and the other side of the test meter connected to the system side of the test clamp.
A reading can then be taken, which will show the series resistance of the electrode under test and the rest of the system together with other connected parallel electrical and other earth paths.
As these other parallel paths usually have a relatively low combined resistance, the meter reading is effectively the resistance of the electrode under test as, if correctly selected, the ‘dead’ earth that is used is normally of such low value that it has little impact on the final result.
In addition to providing an ohmic value for the electrode under test, this method also verifies the circuit to the dead earth source and by virtue of this, the electrical condition of the joints in the system. If the connections from the top of the test clamp to the air termination through to the other earths on the system and other parallel paths were loose or damaged, they would provide a high resistance, which the meter reading would reflect. This situation should then be investigated so that any high-resistance joints can be addressed.
Where no access to an electrode is possible and, for example, the pile foundations have been utilised as the earth termination, it is recommended that individual reference rods are installed around the structure and tested upon completion. These do not necessarily form a part of the installation but may be used as comparisons against the original pile foundation test results. In short, if the reference rod values have not increased year on year then it can be assumed neither has the resistance of the pile foundations.
The ‘Dead/known earth’ test method also applies to clamp-on CT type testers where disconnection is not required, although this type of testing is not always practical.
At least two types of test are recommended, one for each of the individual electrodes in isolation and a second for a combined value. The requirements of BS6651 are an overall system resistance (excluding bonding to any services) of 10Ω and each electrode not exceeding 10 times the number of earth electrodes on the system. Any disconnection of the system should be preceded with a test to ensure that it is not ‘live’ and no testing should be carried out during storm conditions.
Failure to keep up to date, accurate records can result in hidden parts of a system not being adequately attended to and potentially unnecessary remedial works being proposed and executed, as a full assessment of the installation has not been made. At the time of the annual test and inspection, the following records are needed either on site or in an accessible place.
BS6651 states that the following records should be kept:
- drawings of the lightning protection system
- details of the geology (nature of the soil and details of any special earthing arrangements)
- type and position of the earth electrodes
- test conditions and results obtained
- details of any alterations to the system, including additions and repairs
- the name of the person responsible for the system
In order to comply with the Construction Design and Management Regulations, these records should be provided at the completion of the original installation for inclusion in the project Health and Safety file.
The person responsible for the upkeep of the building should recover the lightning protection system records from this file and present them to the engineer undertaking the first post installation inspection and test. Details of the inspections should be recorded so that the required information can be updated and maintained. The programe of tests and inspections will identify what, if any, maintenance is needed. BS6651 states that attention should be given to the following:
- evidence of corrosion or conditions likely to lead to corrosion
- alterations and additions to the structure that may affect the lightning protection system (e.g. changes in the use of the building, the installation of crane tracks, erection of radio and television aerials)
Statistics show the UK alone is subjected to around two million strikes per year and, in order to ensure your lightning protection system is operational when called upon, bearing in mind you have no way of determining when that may be, any maintenance work should be carried out with appropriate expediency.
In the hands of experienced engineers, proper testing and maintenance of lightning protection systems can become a routine, but very necessary, part of a comprehensive safety programe. At the very least the consequences of not taking a thorough approach could incur unnecessary costs but, given the destructive potential of a lightning strike, those consequences could be much worse.
For further information please contact XS Engineering. We are based in Paphos and cover the whole of Cyprus.