Earthing facts and terms you should always remember

Earthing purpose

While working as an electrical engineer, I learned that there are a dozen things you must remember, especially the ones that concern the safety. Whether you are in the electrical design field or in execution, you must admit that earthing is one of the fundamentals that you must never forget. Let’s talk about earthing.

Generally, earthing is always some reference for various concepts in electrical engineering and that’s very interesting. This technical article will put all the earthing facts on the table in order to remind all of us engineers. So, let’s start.

Earthing systems have the following general purpose: Protection of life and property in the events of 50-Hz-faults (short circuits and earth faults) and transient phenomena (lightning, switching operations).

The general layout of a complete earthing system with sections for low voltage, high voltage and buildings and building services is shown in Figure 1. The most important definitions related to earthing are grouped below.

Earth electrode is a conductor embedded in the ground and electrically connected to it, or a conductor embedded in concrete that is in contact with the earth over a large area (e.g. foundation earth).

Earthing conductor is a conductor connecting a system part to be earthed to an earth electrode, so long as it is laid out of contact with the ground or is insulated in the ground.

If the connection between a neutral or phase conductor and the earth electrode includes an isolating link, a disconnector switch or an earth-fault coil, only the connection between the earth electrode and the earth-side terminal of the nearest of the above devices is deemed to be an earthing conductor.

It does not include:

1. Earthing conductors joining the earthed parts of the single units of three-phase assemblies (3 instrument transformers, 3 potheads, 3 post insulators etc.),
2. With compartment-type installations: earthing conductors that connect the earthed parts of several devices of a compartment and are connected to a (continuous) main earthing conductor within this compartment.

Earthing system is a locally limited assembly of conductively interconnected earth electrodes or metal parts operating in the same way (e.g. tower feet, armouring, metal cable sheaths) and earthing conductors.

Specific earth resistivity ρ_E is the specific electrical resistivity of the ground. It is generally stated in Ω m²/m = Ω m and indicates the resistance between two opposite cube faces of a cube of soil with sides of 1 m.

Dissipation resistance R_A of an earth electrode is the resistance of the earth between the earth electrode and the reference earth. R_A is in practice a real resistance.

Earthing impedance Z_E is the AC impedance between an earthing system and the reference earth at operating frequency. The value of the earthing impedance is derived from parallelling the dissipation resistances of the earth electrodes and the impedances of connected conductor strings, e.g. the overhead earth wire and cables acting as earth electrodes.

Impulse earthing resistance R_st is the resistance presented to the passage of lightning currents between a point of an earthing system and the reference earth.

Protective earthing is the earthing of a conductive component that is not part of the main circuit for the protection of persons against unacceptable touch voltages.

System earthing is the earthing of a point of the main circuit necessary for proper operation of devices or installations. It is termed:

1. Direct, if it includes no resistances other than the earthing impedance.
2. Indirect, if it is established via additional resistive, inductive or capacitive resistances.

Lightning protection earthing is the earthing of a conductive component that is not part of the main circuit to avoid flashovers to the operational live conductors resulting from lightning as much as possible (back flashovers).

Where:

1. U_E – Earthing voltage
2. U_B1 – Touch voltage without potential control (on foundation earth electrode)
3. U_B2 – Touch voltage with potential control (foundation earth electrode and control earth electrode)
4. U_S – Step voltage (without control earth electrode)
5. φ – Earth surface potential
6. F_E – Foundation earth electrode
7. C_E – Control earth electrode (ring earth electrode)

Earthing voltage U_E is the voltage occurring between an earthing system and the reference earth.

Earth surface potential φ is the voltage between a point on the surface of the earth and the reference earth.

Touch voltage U_B is the part of the earthing voltage that can be shunted through the human body, the current path being through the human body from hand to foot (horizontal distance from exposed part about 1 m) or from hand to hand.

If there is a lightning stroke, the lightning current is routed through the down-conductors into the earthing system and the earth. The resistance of the down-conductor and the earth causes a voltage drop, which can lead to so-called touch voltage.

Step voltage U_S is that part of the earthing voltage that can be shunted by a person with a stride of 1 m, with the current path being through the human body from foot to foot.

Potential control consists in influencing the earth potential, particularly the earth surface potential, by earth electrodes to reduce the step and touch voltage in the outer area of the earthing system.

Earth fault is an electrical connection between a conductor of the main circuit with earth or an earthed part caused by a defect. The electrical connection can also be caused by an arc.

Earth fault current I_F is the current passing to earth or earthed parts when an earth fault exists at only one point at the site of the fault (earth fault location). This is:

1. The capacitive earth-fault current I_C in networks with isolated neutral
2. The earth-fault residual current I_Rest in networks with earth-fault compensation
3. The zero-sequence current I”_k1in networks with low-resistance neutral earthing.

Also includes networks with isolated neutral point or earth-fault compensators in which the neutral point is briefly earthed at the start of the fault.

Earthing current I_E is the total current flowing to earth via the earthing impedance. The earthing current is the component of the earth-fault current I_F which causes the rise in potential of an earthing system.

Types of earth electrodes

Classification by location:

The following examples are distinguished:

1. Surface earth electrodes are earth electrodes that are generally positioned at shallow depths to about 1 m. They can be of strip, bar or stranded wire and be laid out as radial, ring or meshed earth electrodes or as a combination of these.
2. Deep earth electrodes are earth electrodes that are generally positioned vertically at greater depths. They can be of tubular, round or sectional material.

Classification by shape and cross section:

The following examples are distinguished: Strip, stranded wire and tube earth electrodes.

Natural earth electrodes are metal parts in contact with the ground or water, directly or via concrete, whose original purpose is not earthing but they act as an earth electrode. They include pipes, caisson walls, concrete pile reinforcement, steel parts of buildings etc.

Cables with earthing effect are cables whose metal sheathing, shield or armouring provides a leakage to earth similar to that of strip earth electrodes.

Foundation earths are conductors embedded in concrete that is in contact with the ground over a large area . Foundation earths may be treated as if the conductor were laid in the surrounding soil.

Where:

1. Foundation earth electrode
2. Ring earth electrode

Control earth electrodes are earth electrodes that by their shape and arrangement are more for potential control than for retaining a specific dissipation resistance.

Rod earth electrodes of any significant length generally pass through soil horizons of varying conductivity. They are particularly useful where more conductive lower soil horizons are available and the rod earth electrodes can penetrate these horizons sufficiently (approximately 3 m).

To determine whether more conductive lower soil horizons are available, the specific resistance of the soil at the site is measured.

The measurements are conducted in accordance with the current and voltage method in EN 61936-1 and DIN VDE 0101. The current and voltage method also allows the earthing impedance (dissipation resistance) of the installation to be calculated by measuring the potential gradient.

Use of earth testers (e.g. Megger, Fluke or similar) to measure dissipation resistance should be restricted to single earth electrodes or earthing systems of small extent (e.g. rod earth electrode, strip earth electrode, tower earth electrode, earthing for small switchgear installations).

Earthing material

Earth electrodes (under ground) and earthing conductors (above ground) must conform to specific minimum dimensions regarding mechanical stability and possible corrosion resistance as listed in Table 1.

Table 1 – Minimum dimensions for earth electrodes and earthing conductors

Where:

1. 1. Minimum thickness 2 mm
   2. For above-ground earthing conductors only
   3. For conductors protected against corrosion
   4. When laid in the soil: hot-dip galvanized (minimum coating 70 µm)
   5. Minimum thickness 3 mm (3.5 mm as per DIN 48801 and DIN VDE 0185)
   6. Equivalent to 10 mm diameter
   7. With composite deep ground electrodes: at least 16 mm diameter.
   8. Minimum wall thickness 2 mm
   9. Minimum thickness 3 mm
   10. For steel wire, copper coating: 20 % of the steel cross section (min. 35 mm²), for composite deep ground electrodes: minimum 15 mm diameter

Selection of material for earth electrodes with respect to corrosion (no connection to other materials) may be made in accordance with the following points (DIN VDE 0151):

Hot-dip galvanized steel is very durable in almost all soil types. Hot-galvanized steel is also suitable for embedding in concrete.

Copper is suitable as an earth electrode material in power systems with high fault currents because of its significantly greater electrical conductivity compared to steel. Bare copper is generally very durable in the soil.

Copper coated with tin or zinc is, like bare copper, generally very durable in the soil. Tinplated copper has no electrochemical advantage over bare copper.

Copper with lead sheath. Lead tends to form a good protective layer underground and is therefore durable in many soil types. However, it may be subject to corrosion in a strongly alkaline environment (pH values ≥ 10).

IMPORTANT! For this reason, lead should not be directly embedded in concrete. The sheath may corrode under ground if it is damaged.

Sources:

1. ABB switchgear book
2. OBO Betermann – Earthing systems