Lightning Protection System (LPS) Earthing Design Services

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Liberty Consulting Services (LCS) uses the industry leading software package – CDEGS – Current Distribution, Electromagnetic Fields, Grounding and Soil Structure Analysis, to model lightning induced earth potential rise and the impact this has on customers’ equipment. The high frequency nature of lightning current flow in the earth and earth conductors, means that specialised design techniques are required to provide low earthing system impedance, as well as the traditional low-frequency resistance – R_LF - of an earthing system.

What is a good LPS earth according to Standards?

AS1768 (Clause 4.3.4) requires 10 Ω or less for an LPS earth before bonding to other services (e.g. main electrical earth). However stating a maximum earth resistance value (measured at d.c. or low frequency) does not guarantee an acceptable LPS earth – it is only indicative of performance under lightning conditions. Other parameters have greater impact.

The impedance at 10 Ω R_LF could be much higher at typical lightning frequencies. Given that the injected lightning pulse contains a broad spectrum of frequency (see figure, data from AS1768 Table B1), the transient response of earth electrode systems is of greater concern than traditional R_LF values.

However it is difficult to measure impedance, some impedance testers are on the market but are very expensive, hence why R_LF has been traditionally used to determine the effectiveness of a LPS earth.

Considering the lightning impulse as a travelling wave, the injected current must contend with the surge impedance, defined simply as:

Understanding this helps to design LPS earths with low inductance and high capacitive coupling. Good LPS electrode design should go beyond only specifying a performance parameter based on resistance.

Effective length L_EFF of buried LPS earth conductors

The concept of attenuation of high frequency current along a buried conductor, and the definition of a maximum threshold or effective length (L_EFF) was first introduced by Gupta et al in 1980. While an electrode is subject to lightning current, extending its length beyond a certain distance does not provide any further dissipation. From this point on, the impulse current is already so much attenuated that, in spite of conductor length availability, practically no more current is injected into the soil.

For earth conductors shorter than this effective length, the impulse impedance Z is less than R_LF, and for conductors longer than L_EFF, Z becomes greater than R_LF.

Hence it is not ideal to have long, buried or counterpoise lightning earth conductors. The results of typical effective length calculations for varying soil resistivity by some researchers are shown below.

Other effects on how LP earth electrodes behave

A phenomenon known as soil ionisation occurs immediately around an earth electrode when lightning current flows through it. This has the effect of reducing the soil resistivity in this zone, and hence reducing the impedance of the electrode.

Soil ionisation effects improve performance of smaller electrode systems, reducing their impedance. This does NOT apply to long electrodes or large substation earth grids.

It should also be pointed out that soil resistivity decreases with increasing frequency, and therefore in combination with the above effects, the transient response of electrodes is favourably impacted for certain LPS earth configurations (i.e. L < L_EFF).

Step and touch potentials for lightning

Standardised limits of step & touch voltage have not been developed as they have for power frequency currents. Standards generally limit human exposure outside. ENA EG-0 states that “it is impractical to provide adequate protection of people in terms of earthing and equipotential bonding”.

IEC Standards provide detail for lightning potential control at building entries and other open areas using multiple grading rings, and limit step voltage to 25 kV. No such touch voltage limits are calculated.

However we can design LPS earthing so that EPR and step/touch voltages are minimised.

Practical earthing design for LPS

A range of suitable earthing design techniques and choices of conductor types are available to allow for the transient response of electrodes. These are:

• Short radial pattern earths for isolated systems, length less than L_EFF.
• Injection points at centre vs ends of conductors, and centrally located LPS for large grids – not on edges or corners.
• Low resistivity ground enhancement material for reducing surge impedance.
• Single point bonding to eliminate earth loops.

Earthing designs are modelled in CDEGS at lightning frequencies for optimum conductor configurations based on soil resistivity. LCS provides annual inspection and testing services for installed LPS.

Concrete encased earthing electrodes

There are two main reasons that reinforced steel concrete footings or foundation slabs are effective earth electrode systems. Firstly, this is due to the considerable steel mesh contained within the concrete slabs. It was previously thought that welding of all the steel rebar within the slab was required to make it electrically continuous, however testing has shown that the overlapping of the steel mesh sections is sufficient to provide a path for current distribution under fault conditions, as long as there are adequate connections from the above ground steel structures into this “bed” of steel.

The second reason is due to the nature of the surrounding concrete medium itself. Concrete is actually a reasonable conductor, especially for higher voltage short duration impulse currents. When wet, concrete has resistivity values under 100 Ω.m. Set concrete has resistivity values in the order of 300 Ω.m.

The feature which makes concrete such an effective ground electrode system is its ability to retain moisture and release it over long time periods. The pH of the released moisture enhances the conductivity of the surrounding soil mass. It is therefore the combination of the conductive, porous and permeable concrete medium, with the embedded conductive steel, that provides such an excellent earth electrode system.

There is a common misconception that a lightning strike will “blow up” or “spall” a concrete pad or deep driven pile. This misconception is driven by the belief that heat generated by the energy transfer into steel rebar within concrete, can turn the moisture embedded in concrete, into steam and possibly crack the concrete slab. While this may have occurred in the past where tall towers were struck by lightning, and an isolated footing was improperly bonded (i.e. above ground structural steel bolts were not adequately connected to the footing steel, and carried too high a concentration of surge current), the use of large area, flat footprint concrete slabs will provide current distribution over the many parallel steel rebar paths, and there will be no opportunity to develop the temperatures necessary to vaporise the embedded moisture.

As indicated above, it is still essential that proper bonding techniques are employed to provide a pathway for electrical currents from the above ground steel structures, into these footing electrodes. This will provide current distribution over the mass of steel rebar, across the large footprint provided by the conveyor concrete foundations.

An ideal method of connecting into the reinforcing steel in a concrete footing or foundation slab is to use dedicated rebar earth bonding plates, as shown in the figure below. These avoid any dissimilar metal corrosion between the copper and rebar steel.

These can also be retrofitted by chipping away the surface concrete, welding the steel bar section onto an exposed rebar length, and then reinstating the surface of the concrete flush with the earth point. The two holes in the earth plate can be used to bond the rebar to the above ground steel structure, as well as the buried earth system with lugged PVC insulated or bare earth cables.

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