It is clear in standards that to make a good earthing system shape is more important than resistance value. This is why the 10 ohm value is no more mandatory. IEC 62305-3 indicates: “When dealing with the dispersion of the lightning current (high frequency behavior) into the ground, whilst minimizing any potentially dangerous overvoltages, the shape and dimensions of the earth-termination system are the important criteria. In general, a low earthing resistance (if possible lower than 10 Ω when measured at low frequency) is recommended”
At least two electrodes are needed for each down conductors. This will decrease the impedance. If it is possible to provide more paths to ground it will be better as it will further reduce the impedance.
Each of the electrodes should not be longer than 20 m as the efficiency will decrease after that length.
Better two electrodes of 10 m with enough separation to avoid coupling, than a single 20 m long electrode. This is why the “crow foot” type earthing system is popular and why it is efficient.
IEC 62305-3 also indicates: “High frequency measurement is possible at the installation stage as well as for the maintenance of the earthing system to check adequacy between the designed earthing system and the need”.
What are the benefits of high frequency measurements?
High frequency earth tester TM25
– Allow incorporation of an existing earthing system in a lightning earthing system or even use of natural earthing system for that purpose (e.g. an embedded tank)
– Determine if a part of earthing system is still able to disperse lightning current properly: this can apply to parts of earthing system that has been corroded or mechanically damaged. This is particularly true for type B earthing system (ring) where a degraded earthing system may be undiscovered with low frequency measuring technique if the ring is large but may be critical if lightning strikes where the earthing system is degraded.
Allow a better definition of the earthing system. For example, for a ring electrode (type B according to IEC 62305-3), it is very likely that the global resistance will be low if the electrode is long enough. But this will not mean that where lightning will strike, the local impedance will be low.
This really depends on soil homogeneity and local value of soil resistivity. As the current sharing between then downconductors will be uneven, this may lead to high overvoltages if the striking point is near the location where the local impedance is high.
So what are the steps to make a good lightning earthing system?
– Provide at least two electrodes for each of the down conductor: Type A electrode for local earthing system and Type B electrode for ring electrode. A mix a Type A and Type B can be used and is very efficient as it provides at the same time a better equipotentiality between the down conductors and also low impedance locally.
These electrodes should fulfill the IEC 62305-3 requirements and be of low resistance or of a minimum given length. If this is not the case, or if there is any doubt on quality of earthing system, the earth impedance should be measured to show that the earthing system will behave well in lightning conditions.
– In a few cases it is possible to reduce the number of electrodes and even to reduce it to one by measuring the earth impedance with a high frequency earthing measuring device. This is particularly useful when you decide to connect a down conductor to an existing earthing system and for example a natural earthing system.
– If the standard earthing system is not providing a satisfactory earth impedance (either obtained by following standard requirements or by high frequency measurement) there are two possibilities :
- Improve the earth impedance: this can be done by adding more parallel paths or by using earth enhancing compounds as it has been shown that they reduce both resistance and impedance. Another way to reduce impedance is to use a mesh system or even a ground plate as they are providing also good impedance (high frequency earth measurements have shown that the curve Z .vs. frequency is rather flat on range 79 Hz- 1 MHz).
- Improve equipotentiality by using by using IEC 62305-1 formulas, it is possible, based on the impedance measured with a high frequency tester to determine the percentage of current that will flow through the lightning earthing system and how much will flow through metal pipes and through Type 1 equipotential bonding SPDs. Also by using SPDs with higher Iimp values, it may not be necessary to improve the earthing system.
Paradise Electric Group offer full engineering services for grounding systems: Design, diagnostics, maintenance and construction.
The grounding systems have several objectives:
• To reduce the risk of people and animals’ death.
• To create a path with low impedance for faulted currents to the ground (short circuit) enabling effective protection.
• To improve the lightning protection system and feet preservation from contact voltage within the normal range.
• To prevent high potential differences that are dangerous for the people and the devices.
• To improve the operational reliability of the devices.
• To establish a secure path for static charges and disturbances dissipation.
Our experience in this area allows us to design and build any type of grounding for different systems, for example: for substations (medium and high voltage), for wind and photovoltaic power plants, for reducing a RF radiation (signal reference grid) in computer centers, for protection of electrostatic induction, for centers for fuel loading and unloading and fuel tanks, for exhaust pipes, for telecom stations and stations for oil wells, refineries, pipelines and water supply systems, for swimming pools, electronic sensitive devices and communication lines (telephone, radio), for hospitals and surgeries, for power transmission overhead lines (towers) of high and medium voltage, grounding protection systems for transformers, for lightning protection system and others.