1.1. Earthing of electrical systems

An electrical installation is defined as the totality of equipment assembled into a technical unit for the purpose of generating, transferring, distributing, storing or converting electrical power.

In an electrical installation earthing plays an important part for safety and functionability.

Earthing is the totality of all means and measures for connection to earth.

Earthing systems are applied to safely leak lightning, creeping and fault currents via the soil. Moreover, earthing systems are serving to warrant certain operation processes like

- return of fault currents (closing the fault current circuit)
- mastering of line-to-earth faults
- safety in case of defects in transformers.

The following definitions are given:

- lightning currents: currents originating from stroke of lightning into an electrical installation

- creeping currents: currents making current transition possible along the surface of a fixed insulation between parts being under voltage relative to each other

- fault currents: currents flowing due to an insulation fault.

The current flowing via the earthing electrode does not spread itself in the ground, but closes the fault current circuit via the earthed point of the network and the ground.

Earthing electrodes are non-insulated conductors being directly embedded in the ground. Depending on their purpose a difference is made between system earthing electrodes, protective earthing electrodes, control earthing electrodes and auxiliary earthing electrodes.

Fig. 1.1. Construction of an earthing system

1 soil, 2 secondary side of feeding transformer, 3 consumer’s installation, 4 system earth electrode, 5 protective earth electrode, 6 path of returning fault current in case of fault, 7 main fuses, 8 fuses of consumer’s installation.

Earthing systems therefore consist of:

- conductive earthing electrodes connected to each other and
- earthing lines resp. earthing concentration lines being positioned between the electrical equipment and the earthing electrodes.

Earthing is creation of a conductive connection between conductive parts of the equipment and the ground via an earthing system.

System earth electrodes serve to earth one point of the operating circuit (neutral point resp. earthed phase conductor).

Protective earthing electrodes serve to earth conductive parts of the system not belonging to the operating circuit, to which in case of fault voltage may be applied, like e.g. casings of electrical equipment.

Auxiliary earth electrodes serve for monitoring neutral conductors, for taking up measuring currents in case of earth measurements and for connection of the fault voltage circuit breaker at voltage-operated earth-leakage circuit breakers.

Control earth electrodes serve to influence the potential curve in the ground.

For transferring electrical currents into the ground all conductors are suited which have well conductive connections to the ground. Metal constructions, waterpipes or rails are fulfilling - save their actual purpose - their task as earthing electrodes as well as earth electrodes placed in the ground being determined for earthing only.

Contrary to the artificial earth electrodes placed in the ground exclusively for the purpose of earthing, conductors being well suited as earth electrodes, but primarily put into the ground for other tasks, are called natural earth electrodes.

Earthing electrodes are either dug into, driven into or placed in boreholes in the ground. Depending on their depth in the ground they are classified in surface and depth earthing electrodes. In case of a combination of these two types of earth electrodes they are called combination earthing electrodes.

Classification of earth electrodes	Depth in ground
Surface electrodes	up to 3 m
Depth earth electrodes	more than 3 m

Fig. 1.2. Types of earth electrodes

1 soil, 2 min. depth 0.8 m, 3 surface earth electrode, 4 depth earth electrode, 5 combination earth electrode, 6 strip earth electrode, 7 earthing rod. 8 earthing plate, 9+10 natural earth electrodes: pipeline and metal structure.

The advantage of natural earth electrodes as compared to artificial earth electrodes is avoiding earthwork. Earthing electrodes are named after the material used. Earthing electrodes are discerned according to their depth-position. Earthing electrodes are selected according to the condition of the ground. Hot-dip galvanized round steel being used in lightning arresting engineering since long ago is best suited as surface earthing electrode.

- At the same cross section it exposes a smaller surface for attacks by corrosion.

- Due to its round shape, its bond with the surrounding soil will be better.

When building earthing systems it is not the quality and cross section of the applied earthing material which is decisive, but a good quality of electrical connection with the surrounding soil!

Table 1.1. Materials of earth electrodes

Earth electrode	Material	Min. dimensions
Strip earth electrode (flat earth electrode)	hot-dip galvanized strip or round steel	Cross section 100 mm2 Thickness 3 mm diameter 10 mm
earthing rod	hot-dip galvanized angle	40 mm x 4 mm
	hot-dip galvanized steel pipe	ext. diameter 24 mm wall thickness 3 mm
	hot-dip galvanized round steel	diameter 10 mm to 24 mm
earthing plate	solid or perforated steel plate

When installing earthing electrodes the following principles are to be observed.

- For protecting earthing strips from dehumidifying or freezing out of the soil they are to be placed in the ground at least 70 cm under the surface.

- Earthing strips may be placed together with underground cables in cable ditches, but the heat from the cable must not be able to dry out the soil. Considering the resistance of earth they must also not be placed on gravel like underground cable.

- Since earthing electrodes will be damaged by corrosion they are not to be placed in aggressive soil, in the vicinity of rubbish or in running waters. Placing earthing electrodes beneath streets (roads) is also prohibited.

- Earthing electrodes placed in the ground are to be arranged in such a way as to prevent them from affecting each other. The distance to foundations of buildings must be 1 m at least.

- After installation the soil surrounding the earthing electrode is to be carefully stamped or to be jetted in layers in case of dry, not bonding soil.

- Earthing electrodes must not be contaminated. Besides appearance of corrosion also tar, paint etc. are to be considered as contaminations.

- In order to keep the user informed about the exact position of the earthing system a precise layout plan of the earthing equipment has to be drawn.

Connection of earthing electrodes among each other are being made by keyed joints, welded joints or bolted joints. Welded joints are to be preferred.

Fig. 1.3. Keyed joint of a strip earth conductor

1 passage, 2 junction.

In case of bolted joints a bolt M 10 has to be taken at least. For joining the earth lead to the auxiliary earthing electrode in case of applying the protective measure “voltage-operated earth-leakage protection” a bolt M 6 will suffice (Always hardened and tempered bolts with hexagonal head are to be used).

Fig. 1.4. Bolted joint of an earth lead

1 earth lead, 2 hex, head bolt, 3 washer, 4 spring washer, 5 nut.

Fig. 1.5. Pipe clip for earthing connection to strip steel

1 pipeline, 2 pipe clip, 3 bolt M 8, 4 bolt M 10 for strip steel joint.

Connections to natural earthing electrodes are preferably to be made outside the soil. At points where this is impossible and at joining faces being not metallic-bright, toothed lock-washers are to be used. At joining faces being metallic-bright, joints between earthing electrodes may be made by applying spring lockwashers resp. plain lockwashers. At the joints of earthing electrodes protection against corrosion is of utmost importance. It must be durable and fully effective.

By welding and drilling the zinc layer on the steel is damaged leading to stronger corrosion at the defective points.

Protection against corrosion is safely warranted by application of anti-corrosive tape being wound firmly and semi-overlapping, in case of aggressive soils twice semi-overlapping, around and approx. 100 mm beyond the joint. In case of aggressive soils it is recommended to seal the anti-corrosive tape by sealing compound.

Welded joints are to be thoroughly cleaned from scale by means of a welder’s hammer prior to applying the anti-corrosive tape. Earthing connections to pipes are sealed by a plastic sealing compound against humidity prior to applying the anti-corrosive tape.

Before connecting natural earthing electrodes they are to be checked for their usability.

Using gas pipes as earthing electrodes is generally forbidden due to the very high transition resistances of the sealing materials and the hazards of explosion. On the other hand, lead sheaths of underground cables, reinforcing steels in concrete, rail track systems and waterpipe networks may be used for earthing. However, tracks used in earthing must not serve for operation of vehicles driven by direct current.

Prior to using the waterpipe network for earthing purposes the approval of the water supply utility will have to be obtained. This is important, because water meters and locking valves must reliably be bridged. These bridges must under no circumstances be removed when repairing waterpipe networks.

The material for bridges at water meters and locking valves is to be rain. 10 mm2 Cu, 25 mm2 Al or 50 mm2 hot-dip galvanized strip steel!

Fig. 1.6. Bridging of a water meter

1 water meter, 2 water pipeline, 3 stop valves, 4 pipe clips according to fig. 1.5, 5 bridging.

Lines connecting parts of the plant to the earthing electrode are called earth leads. When preparing earth leads care has to be taken to cut hot-dip galvanized materials by shears or saw only.

Hot-dip galvanized strip steel is aligned on simple straightening machines or on a parallel by hammer. Processed ends of hot-dip galvanized strip steel are to be deburred by filing. Bores are to be deburred properly by a countersink or bigsize drill.

Fig. 1.7. Aligning hot-dip galvanized strip steel by hammer

1 straightening rail, 2 hot-dip galvanized strip steel, 3 hammer, 4 blow points for alignment in case of horizontal (lateral) bends.

For installing earth leads on walls, special clamps are employed. They firmly accommodate the earth leads and are easily mounted. They are directly inserted in the wall or screwed to the wall. Joints and junctions of earth leads and earthing concentration leads are to warrant a durable, safe and electrically well conductive connection.

Permitted are: bolted, welded and pressed joints, but also soldered and notched joints to stranded conductors. Also in this case welded joints are being preferred* Joints must be protected from corrosion!

Fig. 1.8. Holders for earth leads of flat steel

1 directly to be inserted into wall, 2 screwable to wall.

The earth resistance of an earthing system may be ascertained by different methods with varying precision. The expert will select from a multiple of available measuring methods that one, supplying him with the most exact result possible, while saving time and expense and applying as little equipment as possible. Fig. 1.9. shows such a measuring arrangement at which the earth resistance may be directly read on the earthing measuring instrument fed by alternating current. It has, however, to be observed that the auxiliary earthing electrodes, earthing electrodes and sensing probe must at least be in a distance of 20 m from each other.

Fig. 1.9. Measuring arrangement for ascertaining earth resistance

1 earthing measuring instrument, 2 earth electrode, 3 probe, 4 auxiliary earth electrode.

The system and protective earth leads are marked by black transverse stripes at all ends and junctions. Blank earth leads in installations like e.g. copper conductors et socket outlets with earthing contact need not be marked. Insulated earth leads are to be marked green-yellow like protective conductors.