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Extended earthing systems may suffer from different ground conditions in different parts. This can enhance the corrosion problems and requires special attention.

In order to minimize corrosion in an LPS:

• avoid the use of unsuitable metals in an aggressive environment;

• avoid contact of dissimilar metals, of substantially differing electrochemical or galvanic activity;

• use an adequate cross-section of conductors, bonding straps and conducting terminals and clamps to ensure sufficient corrosion life for the conditions of service;

• provide appropriate filling or isolating material in conductor joints which have not been- welded conductor joints, so as to exclude moisture;

• provide a sleeve or a coat or isolate metals sensitive to corrosive fumes or fluids in the location of the installation;

• consider the galvanic effects of other metallic items to which the earth electrode is to be bonded;

• avoid designs where natural corrosion products from a cathodic metal (e.g. copper) could contact and erode the LPS, such as metallic copper on an anodic metal (e.g. steel or aluminium).

To conform to the foregoing, the following precautions are cited as specific examples:

• the minimum thickness or diameter of a strand should be 1,7 mm for steel, aluminium, copper, cuprous alloy or nickel/chrome/steel alloys;

• an isolating spacer is recommended where contact between closely spaced (or touching) dissimilar metals could cause corrosion, but such contact is not electrically necessary;

• steel conductors not otherwise protected should be hot-dipped galvanized in accordance with the requirements of Tables 6 and 7;

• aluminium conductors should not be buried directly in the ground nor set in or attached directly to concrete, unless they are completely sleeved with a durable, close-fitting isolating sleeve;

• copper/aluminium joints should be avoided wherever possible. In cases where they cannot be avoided, the connections should be welded or made employing an intermediate layer of copper/aluminium sheet;

• fasteners or sleeves for aluminium conductors should be of similar metal and of adequate cross-section to avoid failure by adverse weather conditions;

• copper is suitable for use in most earth electrode applications, except for acid, oxygenated ammoniac or sulphurous conditions. However, it should be remembered that it will cause galvanic damage to ferrous materials to which it is bonded. This may require specialist corrosion advice, particularly when a cathodic protection scheme is used;

• for roof conductors and down-conductors exposed to aggressive flue gases, particular attention should be paid to corrosion e.g. through the use of high-alloy steels (>16,5 % Cr, >2 % Mo, 0,2 % Ті, 0,12 % to 0,22 % N);

• stainless steel or other nickel alloys may be used for the same corrosion resistance requirements. However, in anaerobic conditions, such as clay, they will corrode almost as quickly as mild steel;

• joints between steel and copper or copper alloys in air, if not welded, should be either fully tin plated or fully coated with a durable moisture-resistant coating;

• copper and copper alloys are subject to stress corrosion cracking in ammoniac fumes and these materials should not be used for fastenings in these specific applications;

• in marine/coastal areas, all conductor joints should be welded or effectively fully sealed.

Stainless steel or copper earthing systems can be connected directly to the steel reinforcement in concrete.

Galvanized steel earth electrodes in soil should be connected to the steel reinforcement in concrete by isolating spark gaps capable of conducting a substantial part of the lightning current (see Tables 8 and 9 for the dimension of connecting conductors). A direct connection in the soil would significantly increase the risk of corrosion. Isolating spark gaps used should conform to 6.2.

NOTE Isolating spark gaps classified N according to future IEC 62561-3 are usually suitable.

Galvanized steel should be used for earth electrodes in soil only when no steel parts incorporated in the concrete are directly connected to the earth electrode in soil.

If metal pipes are put in soil and are connected to the equipotential bonding system and to the earth-termination system, the material of the pipes, where these are not isolated, and the material of the conductors of the earth system should be identical. Pipes with a protective covering of paint or asphalt are treated as if they are not isolated. When use of the same material is not possible, the pipework system should be isolated from the plant sections connected to the equipotential bonding system by means of isolated sections. The isolated sections should be bridged by means of spark gaps. Bridging by spark gaps should also be performed where isolated pieces are installed for cathodic protection of pipework.

Conductors with lead sheaths should not be directly installed in concrete. Conductors with lead sheaths should be protected against corrosion by provision of either anti-corrosion bindings or by means of shrunk-on sleeving. Conductors may be protected by a PVC or PE covering.

Steel earth-termination conductors coming from concrete or from soil at the entry point to the air should be protected against corrosion for a length of 0,3 m by means of anti-corrosion wrappings or shrunk-on sleeving. For copper or stainless steel conductors this is not necessary.

The materials used for the joints between conductors in the soil should have identical corrosion behaviour to that of the earth-termination conductors. Connection by clamping is not generally permissible except in cases where such connections are provided with effective corrosion protection after making the joint. Good experience has been gained with crimped joints.

Welded joints shall be protected against corrosion.

Practical experience shows that

• aluminium should never be used as an earth electrode,

• lead-sheathed steel conductors are not suitable for use as earth conductors,

• lead-sheathed copper conductors should not be used in concrete nor in soil with a high calcium content.

E.5.6.2.2.2 Metals in concrete

The embedding of steel or galvanized steel in concrete causes a stabilization of the natural potential of the metal, due to the uniformly alkaline environment. In addition, the concrete is of uniformly, relatively high resistivity - of the order of 200 Qm or higher.

Consequently, the reinforcing bars in concrete are considerably more resistant to corrosion than when they are exposed, even if connected externally to more cathodic-electrode materials.

The use of reinforcing steel as down-conductors does not pose any significant corrosion problems provided the access points for air-terminations are well encapsulated, e.g. by epoxy resin putty of adequate thickness.

Galvanized steel strips as foundation earth electrodes may be installed in concrete and directly connected to the steel reinforcing rods. Copper and stainless steel in concrete are also accepted and may be connected to the reinforcement steel directly.

Due to the natural potential of steel in concrete, additional earth electrodes outside the concrete should be made of copper or stainless steel.

In steel fibre reinforced concrete, if it is not possible to ensure the circumfusion of concrete thickness at least 50 mm over casting earth electrodes, the use of steel earth electrodes is not permitted because during the building process the steel electrode can be pressed down, for instance by the machines used, and touch the soil. In such a case, the steel faces a serious corrosion risk. Copper and stainless steel are suitable materials for earth electrodes in steel fibre concrete.

Е.6 Internal lightning protection system

E.6.1 General

The requirements for the design of the internal lightning protection system are given in Clause 6.

The external lightning protection system and its relationship to conductive parts and installations inside the structure will determine, to a large extent, the need for an internal lightning protection system.

Consultation with all authorities and parties concerned with equipotential bonding is essential.

The LPS designer and LPS installer should draw attention to the fact that the measures given in Clause E.6 are very important in order to achieve adequate lightning protection. The purchaser should be notified accordingly.

The internal lightning protection is the same for all protection levels except for the separation distances.

The measures necessary for internal lightning protection exceed the equipotentialization measures for AC power systems in many cases because of the high current rate and current rise time occurring in the case of a lightning strike.

NOTE If protection against LEMP is to be considered, ІЕС 62305-4 should be taken into account.

E.6.2 Lightning equipotential bonding (EB)

E.6.2.1 General

In the case of an isolated external LPS, the equipotential bonding is established only at ground level.

In the case of industrial structures, electrically-continuous conductive parts of the structure and the roof may be generally used as natural LPS components and may be used in the performance of equipotential bonding.

It is not only the conductive parts of the structure, and the equipment installed therein, that should be connected to the equipotential bonding but also the conductors of the power supply system and the communication equipment. For earth electrodes inside the structure, special care should be taken to control step voltages. Adequate measures include connecting concrete reinforcement steel to the earth electrodes locally or providing an equi­potentialization mesh in the cellar or basement.

For buildings higher than 30 m, it is recommended to repeat the equipotential bonding at a level of 20 m and every 20 m above that. The separation requirements will generally be fulfilled.

This means that, at the very least, on those levels the external down-conductors, the internal down-conductors and metal parts should be bonded. Live conductors should be bonded via SPDs.

E.6.2.1.1 Bonding conductors

Bonding conductors should be able to withstand the part of the lightning current flowing through them.Conductors bonding metal installations internal to the structure normally do not carry a significant part of the lightning current. Their minimum dimensions are given in Table 9.

Conductors bonding external conductive parts to the LPS usually carry a substantial part of the lightning current. Their minimum dimensions are given in Table 8.

E.6.2.1.2 Surge protective devices

Surge protective devices (SPDs) should withstand the prospective part of the lightning current flowing through them without being damaged. An SPD should also have the ability to extinguish electrical power follow-on currents from the power supply if they are connected to the electrical power conductors.

The selection of an SPD shall be performed according to 6.2. Where protection of internal systems against LEMP is required, SPDs shall also conform to IEC 62305-4.

E.6.2.2 Equipotential bonding of internal conductive parts

Bonding should be provided and installed in such a way that the internal conductive parts, the external conductive parts and the electrical power and communication systems (for example computers and security systems) can be bonded by short bonding conductors. Internal and external conductive parts that have no electrical function should be bonded directly. All electrical connections (power and signal) should be bonded by means of SPDs.

Metal installations, i.e. water, gas, heating and air pipes, lift shafts, crane supports etc. shall be bonded together and to the LPS at ground level.

Sparking can occur in metal parts not belonging to the structure if those parts are close to the down-conductors of the LPS. Where this is considered dangerous, adequate bonding measures according to 6.2 should be used to prevent sparking.

A bonding bar arrangement is shown in Figure E.43.

/ЕС 2731/10

Key

1. power to user

2. power meter

3. house connection box

4. power from utility

5. gas

6. water

7. central heating system

8. electronic appliances

9. screen of antenna cable

10. equipotential bonding bar

11. SPD

12. ISG

M meter

Figure E.43 - Example of an equipotential bonding arrangement

The bonding bars should be located so that they are connected to the earth-termination system or to the horizontal ring conductors with short conductors.

The bonding bar is preferably installed at the inner side of an outer wall near ground level, close to the main low-voltage power distribution box and closely connected to the earth­termination system comprising the ring earth electrode, the foundation earth electrode and the natural earth electrode such as the interconnected reinforcing steel, where applicable.

In extended buildings, several bonding bars may be used provided that they are interconnected. Very long connections can form big loops leading to large induced currents and voltages. To minimize these effects, a meshed interconnection of those connections, the structure and the earthing system according to IEC 62305-4 should be considered.In reinforced concrete structures conforming to 4.3, the reinforcement may be used for equipotential bonding. In this case, an additional meshed network of welded or bolted terminal joints, described in E.4.3, should be installed in the walls, to which the bonding bars should be connected via welded conductors.

NOTE In this case, keeping a separation distance is not necessary.

The minimum cross-sections for a bonding conductor or a bonding connector are given in Tables 8 and 9. All internal conductive parts of significant size, such as elevator rails, cranes, metal floors, pipes and electrical services, should be bonded to the nearest bonding bar by a short bonding conductor at ground level and at other levels if the separation distance according to 6.3 cannot be maintained. Bonding bars and other bonding parts should withstand the prospective lightning currents.

In structures with reinforced walls only a minor fraction of the total lightning current is expected to flow through the bonding parts.

Figures E.44, E.45 and E.46 illustrate bonding arrangements in structures with multiple-point entries of external services.

Key

1 external conductive part, e.g. metallic water pipe

2 electric power or communication line

3 steel reinforcement of the outer concrete wall and the foundation

4 ring earthing electrode

5 to an additional earthing electrode

6 special bonding joint

7 steel-reinforced concrete wall, see Key, 3

8 SPD

9 bonding bar

NOTE The steel reinforcement in the foundation is used as a natural earth electrode.

Figure E.44 - Example of bonding arrangement in a structure with multiple point entries of external conductive parts using a ring electrode for interconnection of bonding bars

EC 2733/10

Key

1. steel reinforcement of the outer concrete wall and foundation

2. other earthing electrode

3. bonding joint

4. internal ring conductor

5. to external conductive part, e.g. water pipe

6. ring earthing electrode, type В earthing arrangement

7. SPD

8. bonding bar

9. electric power or communication line

10. to additional earthing electrode, type A earthing arrangement

Figure E.45 - Example of bonding in the case of multiple point entries of external
conductive parts and an electric power or communication line using an internal ring
conductor for interconnection of the bonding bars

Key

1. electric power or communication line

2. external horizontal ring conductor (above ground)

3. external conductive part

4. down-conductor joint

5. steel reinforcement in the wall

6. bonding joint to construction steel

7. bonding bar

8. SPD

Figure E.46 - Example of bonding arrangement in a structure with multiple point entries
of external conductive parts entering the structure above ground level

E.6.2.3 Lightning equipotential bonding for external conductive parts

No additional information available.

E.6.2.4 Lightning equipotential bonding for electrical and electronic systems within the structure to be protected

Details for lightning equipotential bonding for internal systems are given in IEC 62305-4.

E.6.2.5 Equipotential bonding of external services

Preferably, the external conductive parts and the electrical power and communication lines should enter the structure near ground level at a common location.

Equipotential bonding should be performed as close as possible to the entry point into the building. In the case of a low-voltage power supply, this is immediately downstream of the service entrance box (subject to approval of the local power company).The bonding bar at this common location of entry should be connected with short bonding conductors to the earth-termination system.

If the services entering the building are shielded lines, the shields shall be connected to the bonding bar. The overvoltage reaching the active conductors is a function of the size of the partial lightning current over the screen (i.e. according to Annex B) and the cross-section of the shield. Annex E of IEC 62305-1:2010 provides a method to estimate this current. SPDs are necessary if the expected overvoltages exceed the specification of the line and connected objects.

If the services entering the building are not shielded, the partial lightning current will flow on the active conductors. In this case, SPDs with lightning current capabilities should be placed at the entry point. PE or PEN conductors may be connected to the bonding bar directly.

When the external conductive parts, the electrical power and communication lines have to enter the structure at different locations, and therefore need several bonding bars to be installed, the bonding bars should be connected as closely as possible to the earth­termination system, i.e. the ring earth electrode, to the reinforcement of the structure and to the foundation earth electrode of the structure, if applicable.

When a type A earthing arrangement is utilized as a part of the LPS, the bonding bars should be connected to individual earth electrodes and, in addition, they should be interconnected by an internal ring conductor or an internal conductor forming a partial ring.

For entries of external services above the earth surface, the bonding bars should be connected to a horizontal ring conductor inside or outside the outer wall bonded to the down­conductors of the LPS and to the metallic reinforcement of the structure, if applicable.

The ring conductor should be connected to the steel reinforcement, and other metallic elements of the structure, at regular subdivisions of the distance between the down­conductors as stated in Table 4, typically every 5 m to 10 m.

In buildings principally designed for computer centres, communication buildings and other structures requiring a low level of LEMP induction effects, the ring conductor should be connected to the reinforcement typically every 5 m.

For the bonding of external services in reinforced concrete buildings which contain large communication or computer installations, and for structures where EMC demands are severe, a ground plane with multiple connections to the metallic reinforcement of the structure or other metallic elements should be used.

E.6.3 Electrical isolation of the external LPS

E.6.3.1 General

An adequate separation distance, determined according to 6.3, should be maintained between the external LPS and all conductive parts connected to the equipotential bonding of the structure.

The separation distance may be evaluated by Equation (4) shown in 6.3.

The reference length, /, for the calculation of the separation distance s (see 6.3), should be the distance between the connection to the nearest equipotential bonding point or earth­termination network and the point of proximity along the down-conductor. The roof and down­conductors should follow a route as straight as possible to keep the necessary separation distance low

.

The length and the path of the conductor within the building running from bonding bar to point of proximity is generally of little influence on the separation distance, but when this conductor runs close to a lightning current-carrying conductor the necessary separation distance will be lower. Figure E.47 illustrates how the critical length, /, used for calculation of the separation distance, s, according to 6.3, is measured on an LPS.

EC 2735/10

Key

1. metallic radiator/heater

2. wall of brickwork or wood

3. heater

4. equipotential bonding bar

5. earth-termination system

6. connection to the earth-termination system or to the down-conductor

7. worst case

d actual distance

I length for evaluation of separation distance, s

NOTE The structure consists of insulating bricks.

Figure E.47 - Directions for calculations of the separation distance, s, for a worst case
lightning interception point at a distance, /, from the reference point according to 6.3

In structures where the building components are used as natural down-conductors, for example steel reinforcement in concrete, the reference point should be the connection point to the natural down-conductor.

Structures with outer surfaces that do not contain conductive elements, such as structures of wood or brickwork, should use the shortest possible overall distance along the lightning protection conductors / from the most unfavourable lightning strike point to the nearest earth­termination or the point where the equipotential bonding system of the internal installation is connected to the down-conductor or the earth-termination system, for calculation of the separation distance, s, according to 6.3.

When it is not possible to maintain the distance greater than the separation distance s along the whole length of the considered installation, bonding of the installation to the LPS should also be performed at the furthest point from the reference bonding point (see Figure E.47). Therefore, the electrical conductors should either be re-routed in accordance with the requirements of the separation distance (see 6.3) or they should be enclosed in a conductive shield bonded to the LPS at the furthest point from the reference bonding point.

When bonding of installations to the LPS in buildings lower than 30 m is performed at the reference point and the furthest point, the separation distance is fulfilled along the whole path of the installation.

The following points are often critical and require particular consideration:

- In the case of larger structures, the separation distance between the LPS conductors and the metal installations is often so large that it cannot be implemented. This involves additional bonding of the LPS to these metal installations. Consequently, a portion of the lightning current flows via these metal installations to the earth-termination system of the structure.