LPS Earth-Termination: Type A and Type B Electrodes

Short answer

IEC 62305-3 defines two electrode arrangements: type A (horizontal or vertical electrodes connected to each down-conductor, minimum two) and type B (a ring in contact with the soil for at least 80 % of its length, or a closed-loop foundation electrode). Low resistance is recommended, and what matters most is equipotential bonding.

Short answer

The LPS earth-termination disperses the lightning current into the soil and sets the reference potential of the structure. IEC 62305-3 defines two arrangements: type A, with horizontal or vertical electrodes connected to each down-conductor (minimum two in total), and type B, with an outer ring in contact with the soil for at least 80 % of its length or a closed-loop foundation electrode. The standard recommends a low resistance (if possible below 10 Ω, measured at low frequency), but warns that equipotential bonding is what matters most. Type B is preferred for air-termination meshes and for several down-conductors.

What lightning earthing aims for

Lightning current is a high-frequency impulse. For that reason the shape and dimensions of the earthing system matter more than the resistance value at 50/60 Hz: a well-distributed system controls dangerous overvoltages. The standard gives the earth-termination three tasks: conduct the lightning current into the soil, make the equipotential bonding between down-conductors and control the potential near conductive walls.

For lightning, a single integrated earthing system for the structure is preferable and serves all purposes: lightning protection, power system and telecommunications. Earth-termination systems are bonded following the requirements of equipotential bonding. The conditions for separation and bonding with other earth-termination systems are usually set by the national authorities. Where dissimilar metals are joined, serious corrosion can occur.

This is different from a substation earthing grid (IEEE 80), which is calculated for the 50/60 Hz fault current and for tolerable step and touch voltages. For that grid, see the guide on earthing grids.

Type A arrangement

These are horizontal or vertical electrodes installed outside the structure and connected to each down-conductor, or foundation electrodes that do not form a closed loop. The total number of electrodes cannot be fewer than two.

The minimum length of each electrode at the base of a down-conductor is:

l₁ for horizontal electrodes;
0.5 · l₁ for vertical or inclined electrodes.

l₁ is read from Figure 3 of the standard according to the class and the soil resistivity: for classes III and IV it is a fixed length and does not depend on resistivity, while for classes I and II it increases once the resistivity passes certain values. For combined electrodes the total length is counted. The lengths in the figure can be omitted if the earth resistance of the system is below 10 Ω, measured at a frequency different from the power frequency and its multiples to avoid interference.

Two practical notes from the standard: reducing the resistance by lengthening electrodes is practical up to about 60 m, and with resistivity above 3,000 Ω·m, type B or soil enhancers are recommended. Type A suits low and existing structures, with rod or catenary air-terminations, or with an isolated LPS.

Type B arrangement

This is a conductor ring outside the structure, in contact with the soil for at least 80 % of its length, or a closed-loop foundation electrode. It can be meshed. Even though up to 20 % may not touch the soil, the ring must be fully connected along its whole length.

The mean radius re of the area enclosed by the ring must not be less than l₁:

re ≥ l₁ (Eq. 1)

If l₁ is greater than the radius available, horizontal or vertical electrodes are added with lengths

lr = l₁ − re (horizontal) (Eq. 2)

lv = (l₁ − re) / 2 (vertical or inclined) (Eq. 3)

The number of electrodes should be no fewer than the number of down-conductors, with a minimum of two, connected to the ring at the down-conductor points and, where possible, at equal distances. Type B is preferred for air-termination meshes and for LPS with several down-conductors, for bare rock and for structures with extensive electronic systems or a high fire risk. It fulfills the three tasks of the earthing system; the radial electrodes and deep vertical electrodes of type A do not handle equipotential bonding or potential control well.

Installation

The type B ring, preferably at 0.5 m or more depth and about 1 m from the outer walls.
Type A electrodes, with their top end at least 0.5 m down and distributed as uniformly as possible to reduce coupling between them.
Electrodes must be inspectable during construction.
Depth and type are chosen to limit corrosion, soil drying and frost. Under frost, the top part of a vertical electrode equal to the freezing depth is not counted as effective.

Foundation as electrode

The interconnected reinforcing steel of foundations or other buried metal structures is preferably used as an electrode. With good-quality concrete covering the electrode by at least 50 mm, it is reasonably protected from corrosion. The standard warns that earth resistance may increase over time, and that in prestressed concrete the mechanical effect of the current must be considered. If the foundation is the only electrode, no test joint is needed on the natural down-conductors connected to it. The guide on reinforcing steel of the concrete covers the joints.

Resistance and measurement

The standard speaks of earth resistance, but also acknowledges that, once the works are complete and services connected, it is often impossible to measure during maintenance. One option is to install one or more reference electrodes near the structure to follow the evolution of the system. If you want a measurement, the guide on earth resistance testing explains the test and the earthing resistance tool helps with the calculation.

Common mistakes

Looking only at the ohms. Equipotential bonding is the most important measure.
A single electrode. Type A requires at least two.
Lengthening electrodes without limit. Beyond about 60 m the reduction does not pay off.
Mixing metals carelessly. Galvanic corrosion damages the joints.
Confusing it with the IEEE 80 grid. Here the criterion is impulse, not 50/60 Hz fault.

Guides in the same series

Electrode materials are in LPS materials and dimensions, the connection with down-conductors in LPS down-conductors, and the three-dimensional earthing of the building in bonding network and earthing system in 3D.

What we do

We size the arrangement type to your soil and structure, and verify the existing earthing with field measurement in Colombia, or elsewhere on request.

References

IEC 62305-3:2010, Part 3: Physical damage to structures and life hazard: Cl. 5.4 (earth-termination: 5.4.1 to 5.4.4), Figure 3, Eq. (1) to (3), and Annex E.5.4.

Frequently asked questions

What earth resistance does the lightning standard ask for?

IEC 62305-3 recommends a low resistance, if possible below 10 Ω measured at low frequency. But it also states that equipotential bonding is what matters most and that, for type A, the minimum lengths of Figure 3 can be omitted if an earth resistance below 10 Ω is achieved.

When is type B preferred?

When the type A length requirements cannot be met, on bare rock, on structures with extensive electronic systems or a high fire risk, and in soils with resistivity above 3,000 Ω·m if soil enhancers are not used.

Can the reinforced concrete foundation be used as an electrode?

Yes. The standard states that the interconnected reinforcing steel of the foundation should preferably be used as an electrode, with special care at the joints so as not to crack the concrete. It is a form of type B if it makes a closed loop.