0. Introduction 3

1. General screen connection principles 5

2. Earthing circuit conditions and tests 13

3. Standardized screen connection diagrams 15

4. Elements making up the screen circuit 18

5. Multiple circuits 43

6. Tests 44

7. Earthing of lightning arresters 49

1

CONTENTS

This Engineering Recommendation deals with the earthing of cable screens which constitute

an insulated three phase line, installed either in a service gallery, in an indoor installation

or directly buried. To reduce the losses on the line and optimize the transport capacity,

connection systems tend to be adopted which reduce the intensity of the currents induced

on the screens. These systems involve specific connections of the screens to each other

and to earth and give rise to permanent current and current surges on the screen circuits

which must be considered.

0.1 SCOPE

This Recommendation describes the systems and components used in the connections

of the screen circuits of three phase power lines for voltages equal to or greater than 66

kV, made up of single-core cables with metallic screen and insulating sheath. Specific

means may be required to withstand the permanent current and current surges which can

occur between different parts of the screen circuit, and between the same and the earth,

and the testing and checking of the continuity of this circuit should be allowed.

0.2 DEFINITIONS

This Recommendation uses the definitions of the International Electrotechnical Vocabulary

(publication CEI 50), and in Appendix A of "The Design of Specially Bonded Cable Systems,

(part 1)" of Electra no. 28, of May 1973.

The following definitions are moreover added:

Screen. Continuous concentric conductor which surrounds the main conductor and

insulation, aimed at confining the electric field and at conducting the possible short-

circuit currents. It can be made up of a continuous metallic covering or a corona of

wires possibly complemented with metallic tapes. It should be connected to earth

directly or indirectly.

Sheath Voltage Limiters (SVL). Devices with two terminals of strongly non-linear

voltage-current characteristic, aimed at limiting the differences of transitory potential

which, on the occasion of impulse, atmospheric or manoeuvring surges, can appear

between elements of the screen circuit with limited dielectric strength.

Connection box. Blockable box(1), built to house the connections of the screens, of

the earthing cables and the associated SVL when they exist. It should be connected

to earth if it is metallic.

0. INTRODUCTION

(1) Which requires a tool or a key to open it

3

Indirect earthing Earthing of the screens of the cables, or of a section of them, so that

the screen circulation currents that would otherwise occur are reduced or eliminated.

Other sections of screen from the same or another phase may be involved in series in

this connection between a screen and an earth electrode. Indirect earthing causes small

permanent current at some points of the screens or at the terminal.

4

In an alternating current line, the whole formed by the screens and their connections

constitutes a secondary circuit strongly coupled to the primary circuit, formed by the main

conductors subjected to mains voltage. For this reason, considerable intensities can

appear on the screen circuit during the normal operation of the line. The added losses

and heating for this reason should be taken into account, and in many cases measures

should be taken to minimize them. This Recommendation distinguishes between two

screen earthing diagram types:

Rigid earthing systems

Special earthing systems

The following conditions should be fulfilled in both systems:

During the normal operation the capacitive currents should be conducted to earth,

maintaining the screens at a potential close to that of earth.

During the time a short-circuit lasts, both external to the line and occurring in the same

or on one of its elements, the fault currents which can travel along the screen circuit

should not cause excessive voltage between screens and earth and between parts of

the screen circuit. The strength of the insulation between screen and earth (cable sheath)

and of the separation insulation between sections of screen should be sufficient to

withstand these voltages.

At the points in which the current surges of atmospheric or manoeuvring origin could

cause inadmissible dielectric stress on the screen circuit, there should be adequately

sized voltage limiting devices.

The adoption of measures to cancel or minimize the permanent intensities on the screens

associated with the operation of the line under normal conditions can cause another type

of problem, mainly the appearance on the screen circuit of high voltages during short-

circuits or current surges on the mains. This Recommendation regulates the procedures

to maintain these surges within acceptable limits.

5

1. MAIN SCREEN CONNECTION PRINCIPLES

1.1 RIGID EARTHING SYSTEMS

In these systems the screens of the three phases are connected together directly and to

earth so that, at all points of the line, the voltages of the screens between each other and

in relation to earth are maintained close to zero. No provision is adopted to prevent the

circulation of currents along the screens under a permanent regime. These currents, induced

by the main conductors, will give rise to an additional production of heat, with the consequent

reduction of the lines transport capacity.

The screens should be joined together and to earth at both ends of the lines. If necessary,

with the aim of limiting the screen voltages which could appear in the event of a defect on

the line itself, the screens will be joined together at other points, which can also be earthed.(2)

Generally speaking, single-core cables which constitute a three phase line are arranged

in a triangle as close as possible, to reduce the currents on the screens, which increase

on increasing the separation between phases.

With this arrangement, the transpositions of phases along the line are only useful to reduce

their inductive influence on adjacent cables. When the arrangement is flat, or in some other

way asymmetric, the transposition of conductors can moreover achieve the equality of

currents on the screens, and thus obtain the minimum value of losses in this type of

connection. Under no circumstances can the transposition of conductors be used to cancel

the currents on the screens.

Joints

Fig. 1Rigid earthing system

6

(2) It is prudent to connect the screens together and to earth every 2 to 3 km, but this is only requiredif the voltages generated between phases or between phase and earth due to voltage drop becauseof the circulation of fault current exceeds the perforation voltage of the sheath.

1.2.5 Sheath voltage limiters (SVL)

The limiting devices indicated in the previous section, also called surges dischargers, are

strongly non-linear zinc oxide conductor elements. They present a very high resistance to

the reduced voltages that appear under normal operating conditions, and they do not

therefore modify the screen circuit diagram. They weakly conduct the industrial frequency

voltages originated during mains failures, without having any limiting effect on these voltages.

On the other hand, they conduct intensely for the short disturbances of atmospheric or

manoeuvring origin which would cause very high voltages at the ends and at the discontinuity

points, limiting these voltages to admissible values. This conduction is accompanied by

production of heat in the discharger, for which the latter has a reduced storage capacity.

For this reason, they are only adequate to limit surges of an order of duration lower than

1 ms, being able to be destroyed if an industrial frequency voltage, even of short duration,

exceeds their free conduction threshold.

It is necessary to limit the voltages which appear between screens and the local earth and

which subject the insulating sheath of the cable and the supporting insulators of the

terminals to dielectric stress, and the voltages that are presented between the two ends

of the screen which concur at the same joint with screen discontinuity, which must be

withstood by a very small thickness of insulating material inside the joint.

1.3 SCREEN CONNECTIONS

To allow periodic verifications of insulation and continuity of the screen circuit, the connections

between screens, to earth and to the SVL are carried out using detachable elements(3). The

connection of the SVL must also be detachable.

These connections and the SVL must be placed out of reach of unauthorized people, either

in adequate boxes, with insertion of physical barriers, or next to the external terminal base

if they are not accessible from the ground.

12

(3) The detachable connections are connected by means of blades or screws and can easily beremoved and replaced, without subjecting the elements of the circuit to stress or bending.

2.1 GENERAL CONDITIONS

All the direct connections between screens and between the latter and earth must be

carried out using detachable links. The connections between screens and between the

latter and earth carried out using SVL must be disconnectable. During the operation of the

line, these elements must be considered as live elements and the same precautions shall

be established to access them as for the live parts of the installation.

2.2 SCREEN EARTHING POINTS

The direct earthing of the screens shall be carried out taking into account the following

instructions:

In the cases in which the screens must be earthed at one of the ends of the line, the

connection will be carried out to the general earth electrode of the substation and at the

same point where the earth bonding conductor, if it exists, is connected.

In the cases in which the screens are earthed at intermediate points of the line, they will

also be connected to the bonding conductor, if it exists.

If there is no bonding conductor, and in the location there is a local earth system intended

for other uses, to carry out the connection of the screens to this system at intermediate

points of the line it will be necessary to justify that it can accept the impulses of intensity

which would originate in the event of atmospheric or manoeuvring discharge, the industrial

frequency intensities that would appear in the event of short-circuit at any point of the

line or external to it and the permanent industrial frequency intensities that may occur

due to asymmetries of the line or other causes, always ensuring the safety of people and

equipment.

2.3 EARTHING POINTS OF THE SVL

In the situations in which the SVL devices are connected in a star, the centre of the star

can be earthed in the following cases:

In the cases in which the SVL must be connected at one of the ends of the line, the

connection will be carried out to the general earth electrode of the substation and at the

same point where the earth continuity conductor, if it exists, is connected.

To the earth continuity conductor, if it exists.

If there is no earth continuity conductor, and there is no adequate earth network, the

connection may be carried out to an electrode formed by four earth rods with a length

no shorter than 1.2 m, connected parallel and situated in the four corners of the enclosure

where the SVL are housed, provided that this electrode is electrically independent from

any other local earth system of the installation.

2. EARTHING CIRCUIT CONDITIONS AND TESTS

13

If there is no earth continuity conductor, and in the location there is a local earth system

intended for other uses, to carry out the connection to this system it will be necessary

to justify that it can accept the impulses of intensity which would originate in the event

of atmospheric or manoeuvring discharge and the industrial frequency intensities that

would appear in the event of action of one of the SVL ensuring the safety of people and

equipment.

If none of the possibilities indicated in the above hypotheses is available, the centre of

the star of the SVL will not be earthed. In these cases, the connection of the SVL in a

triangle is preferred.

14

3.1 RIGID EARTHING SYSTEMS

With this system the screens are directly joined to each other and to earth at both ends

and, possibly, at some intermediate point, without there being any interruption in their

continuity. Fig.1 illustrates the general case, in which both ends of the line and possibly

the joint points are directly earthed.

3. STANDARDIZED SCREEN CONNECTION DIAGRAMS

Joints

Fig. 1Rigid earthing system

3.2 CROSS BONDED CONNECTION SYSTEMS

Fig. 2 describes the diagram to be used in the event of a major single section and Fig. 5

in the event of two major sections. With this system surge limiting devices are only necessary

at the intermediate points of discontinuity of the screens.

Fig. 2Cross bonting system

(1 Major section, 3 minor sections)

Minor sectionMinor section Minor section

Major section

15

Cross bondinglink boxeswith SVL

The SVL can be connected in the following manners (see Fig 9):

In a triangle, each of them being parallel with the continuity interruption space of each

screen.

In a star, with its centre connected to a local earth system or to the bonding conductor

(or to one of the conductors), if it exists (see section 2.3).

In a star, with its centre insulated.

Major section Major section

Fig. 3Continuous cross bonding system

(2 major sections, 6 minor sections)

16

Cross bonding link boxes with SVL

Fig. 9Detail of SVL box and screen crossing

Cross bondinglink boxwith SVL

SVL instar

SVL intriangle

3.3 SINGLE POINT EARTHING SYSTEMS

These systems, which can be made up of one or several sections, are characterized by the

discontinuity of the screens in some (or in all) the changes of section and by the joining

of the screens to each other and to earth in a single point within each section. Surge limiting

devices assembled in a star with centre earthed must be installed at each end of section

which does not coincide with the point of its earthing. There must be an earth continuity

conductor, and the earth connections, both of the screens and of the surge limiting devices,

must be carried out directly to this conductor (Figs. 6, 7 and 8).

17

Fig. 6Single point bonded system

(Earthing at one end)

Fig. 7Single point bonded system(Earthing at the middle point)

Earth continuity conductor

SVLlink boxes

Earth continuity conductor

Fig. 8Single point bonded system

(Earthing at both ends with interruptionof screen at the middle point)

SVLlink box

SVLlink box

Earth continuity conductor SVLlink box

4. ELEMENTS MAKING UP THE SCREEN CIRCUIT

(4) The connection boxes are of a watertight horizontal type, and are used in chests or galleries.The cupboards are of vertical type protected against the elements and used fastened to walls ofsubstations or metallic structures (pylons or towers).(5) An internal short-circuit generates a very considerable excess pressure. If the box breaks, thisexcess pressure will be transmitted to the chest (of small volume) and may be sufficient to pull offthe cover and cause an accident.

18

The elements necessary to establish the connections between the elements of the screen

circuit that each system requires, and for the subsequent verification of the screens, are

described below.

4.1 CONNECTION BOXES OR CUPBOARDS (4)

The ends of the junction cables and of the SVL (except the SVL connected directly between

sectioning insulation on terminals of equipment with metallic casing or on external terminals

see section 4.5.1), will be enclosed in connection boxes capable of containing the effects

of thermal or electric failure of any of the elements housed without damage being caused

to neighbouring external elements. In the cases of boxes installed in chests or enclosures

of small volume it is recommended that they should be capable of containing the effects

of an internal short-circuit.(5)

In the case of voltages of 245 kV upwards, with their corresponding high fault levels, it is

accepted that, in certain situations, the insulation of the SVL and of the connections may

not be capable of withstanding the conditions to which they are subjected in the event of

an internal fault of the line itself. When this situation is foreseen, additional measures should

be adopted to confine the consequences of this type of fault.

Metallic connection boxes should always be earthed, by means of a connection independent

from the earthing of the elements contained inside them (connections of the screens or of

the SVL) with a sufficient level of insulation. The internal insulation of the connection boxes

should fulfil the conditions of section 6.3.

The boxes or cupboards should be equipped with some means to prevent incorrect

connection positions and shall be equipped with a label which shows the normal connection

arrangement.

The connections of the terminals, junction cables and SVL must be designed so that they

can achieve and easily maintain the contact resistances of section 6.6 (d). The terminals

and junction cables must also fulfil the short-circuit resistance conditions given in the Table

of section 4.2.

4.1.1 Watertight connection boxes

They must fulfil protection grade IP68. Those which contain SVL and which are assembled

at ground level must be of a horizontal type, with covers equipped with adequate bolts for

installation in not very deep chests under the surface of the ground. The highest point of

the box assembled in a chest (without considering the lifting eyes) should not be more than

1 m below the level of the ground, to aid handling.

The boxes foreseen for external installation must be weatherproof and adequate for assembly

on watertight terminal support structures.

The metallic boxes must be earthed by means of connection to adjacent main earth

electrodes or to the earth bonding conductor. They can also be connected just to a local

earth system, independent from the main system, if the internal insulation of the box

guarantees an adequate safety level.

Each box should contain the elements of just one circuit.

4.1.2 Protected connection cupboards or boxes

All these boxes should fulfil protection grade IP54. When they are in vertical position they

are called cupboards. The cupboards should fulfil the following conditions:

(1) The back part of the cupboard should be a fixed panel.

(2) The door(s) should be closed by means of a security lock or an alternative method. When

the cupboard has two doors, only one of them needs to be blockable; the other door can

be secured by means of internal espagnolette bolts on the upper and lower part.

(3) Each circuit should have its own cables for connection to screens and to earth, SVL

and earth cables. These elements cannot be shared by two circuits.

When the cupboard is situated within the area of an important earth network (for example

of a substation), the internal earth bar should be connected to this network. If the cupboard

is metallic, it should also be connected to this earth network, it being possible to carry out

this connection to the earth bar inside it.

When the box is not situated thus (for example at joint points all along the cable run), the

earth bar to which the connecting cables are connected (by SVL, in the appropriate cases)

in normal operation should be connected to the earthing system of the location of the joint.

The box, if it is metallic, should be connected to its own local earth. The insulation between

the internal earth bar and the box should be adequate to resist the test level between

junction cable and earth specified under Point 6.3. The insulation between the casing of

the SVL and the box should also fulfil this condition.

When the box is metallic, its earth connection terminal shall be independent from the

elements housed in it. The set of connection devices may be covered by a transparent

methyl methacrylate insulating screen, or by another suitable material.

19

If higher values are foreseen in a specific installation, suitable larger sections should be adopted.

The connecting cables to be used should be:

In cable systems with rigid connection: All the connecting cables will be single-core.

In cable systems with special connection: Except in the cases which are indicated below,

all the connecting cables shall be concentric. In a joint with screen sectioning, the conductors

on both sides of the sectioning shall be the inside and the outside of the connecting cable.

In the terminals, the junction cables connected to the screens of the cables should be the

interior conductors of the concentric cables; the exterior conductors of the aforementioned

cables shall be connected in all cases to the metallic parts of the terminal support structures.

If metallic connection boxes are used, the exterior conductors of the concentric cables of

the terminals shall be connected in all cases to the metallic box.

4.2 JUNCTION CABLES BETWEEN SCREENS AND CONNECTION BOXESAND/OR SVL HOUSING

These cables must be made up of stranded copper conductors insulated with XLPE, andshould be one of the following types:

(i ) Single-core construction, in accordance with Table 1A.(ii) Concentric construction, in accordance with Table 1B.Tables 1A and 1B show the minimum section of connecting conductor associated with thenominal voltages and with the systems short-circuit current levels. The connecting cablesmust comply with the conditions of UNE-HD-603 in everything which is applicable to them,except as regards the testing voltages, which shall be those indicated under Point 6.4 ofthis recommendation.

The section of the connecting cables, both for the interior and exterior conductors of theconcentric cables and for the single-core cables in any type of connection, shall be determinedby the maximum value of the short-circuit current foreseen for the installation. Dependingon the highest voltage for the material, the minimum levels of this current to be taken intoaccount in the design of the connecting conductors are established in Tables 1A and 1B,as follows:

20

U 72,5 kV 16 kA for 1 second

72,5 kV < U 145 kV 25 kA for 1 second

145 kV < U 245 kV 40 kA for 1 second

245 kV < U 63 kA for 1 second

The SVL and their detachable connections can be situated directly on the pylon or metallic

support structure.

The connections should be designed to minimize the length of the cables. Whenever

possible, no concentric type connecting cable should be more than 10 m long.

Joints will not be accepted in the connecting conductors in new installations, but they can

be accepted in subsequent interventions, for example in maintenance or repair work.

The exceptional cases in which single-core connecting cables can be used in systems with

special screen connection are:

Junction cables used for the earthing of the screens of the cables at the common earthing

point of two sections with single-point connection, as shown in Fig. 7.

21

Fig. 7Single point bonded system(Earthing at the middle point)

Earth continuity conductor

Junction cables used in terminals normally earthed.

Junction cables used for earthing or other connections in the terminals in equipment

with metallic casings, as in Figs. 20, 21 and 22.

SVLlink box

SVLlink box

Fig. 10 Screen connection diagram in terminals with metallic casing

(Direct earthing)

22

Connections close to metallic casings

Blockableconnection box

assembledon frame

Main earth of the system orearth continuity conductor

The bridges must be connected directly to both sides of theseparation insulation and the metallic casings must beconnected to the main earth of the system.

NOTE:

Connection function:Element:Applicable to:

Direct earthingTerminal of the cableFor direct earthing in systems with class SF61insulated screen.

23

Fig. 11Screen connection diagram in terminals with metallic casing

(Direct earthing)

Connections far from metallic casings

Blockableconnection box

on frame

Connecting cableto pass through theCT (if appropriate)

Main earth of the system orearth continuity conductor

The SVL must be connected directly to both sides of theseparation insulation and the metallic casings must beconnected to the main earth of the system.

NOTE:

Connection function:Element:Applicable to:

Direct earthingTerminal of the cableDirect earthing in systems with class SF62insulated screen.

46

6.5 SHEATH VOLTAGE LIMITERS

The routine and maintenance tests on finished assemblies shall be as follows:

Zinc oxide units

The voltage test at AC 50Hz must be adapted to give a peak test intensity of

1.6 mA and the voltage measured must be within the range given below.

In the finished installation tests the visual examination must show that the junction cables

and the external surface of the unit or of the container are free from important protuberances

or from other signs of deterioration.

(a) Internal insulation resistances for SVL enclosures with earth cables.

The resistance between the earth cables and any metallic case should not be less than

10 M measured with a 1000 V megaohmmeter.

Type of limiter Peak voltage Peak voltage/2

(kV) (kV)

CPA-03 5,05 5,56 3,57 3,93

CPA-06 10,10 11,12 7,14 7,86

6.6 COMPLETE CIRCUITS ONCE INSTALLED

(a ) After installing a complete circuit tests shall be performed to ensure that the

screen and the sectioning insulation of the screen at all points are capable of

withstanding a rigidity test at direct voltage of 10 kV for 1 minute. The SVL should

be disconnected on performing these tests.

The connections and the earthing of the screens should also be checked.

(b ) It is recommended, at regular intervals, to test the integrity of the screen-earth

insulation at a direct voltage of 5 kV for 1 minute. The SVL should be checked

at the same time, in accordance with Point 6.5.

(c ) If there is an internal defect in a cable system with special connection, the tests

referred to in section (b) above should be carried out and the continuity of the

screen-earth metallic path should be checked before putting the cable back in

service.

(d ) After the installation a measurement should be taken of the resistance of all the

contacts of the junction cables of the screens in each connection box or cupboard,

using a digital microhmmeter. The contact resistance should not be greater than

20 .

The contact resistance of the SVL connections in the boxes or cupboards should

be measured in a similar manner and should not be greater than 50 .

When possible the contact resistance of the external earth cable should be

measured in a similar manner, and should not be greater than 50 .

Note

If a fault current has circulated through a cable system with special connection,

it is recommended at the first opportunity available to check the integrity of the

screen-earth insulation, the integrity of the connection cables and the characteristics

of the SVL.

47

48

Table 1A

CONSTRUCTION OF SCREEN CONNECTION SINGLE-CORE CABLES

*Also for connections of 132kV and 275kV systems not subjected to the short-circuit current, for example connections to SVL.

Table 1B

CONSTRUCTION OF SCREEN CONNECTION CONCENTRIC CABLES

12,8 13,3 17,8 18,7 20,2 21,0 26,4 28,0

1,2+2,0 19,2 20,7 1,7+1,0 23,4 25,3 1,8+1,0 25,8 27,9 2,2+3,0 36,8 39,7

24,4 26,4 33,6 36,3 36,0 38,9 47,4 51,2

1,8 28,4 30,7 1,8 37,6 40,6 2,1 40,6 43,8 2,4 52,6 56,8

Dimensions(mm)

Nominal sectionof the conductor

Voltage of the as-sociated system

Central conductor

XLPE Insul. +PVC internal sheath

30 up to 66 kV 132 kV 275 kV 400 kV

120* mm2 240 mm2 300 mm2 500 mm2

Concentricconductor

PE external sheath

Thickness Diameter

Min Max

Thickness Diameter

Min Max

Thickness Diameter

Min Max

Thickness Diameter

Min Max

12,8 13,3 17,8 18,7 20,2 21,0 26,4 28,0

1,2+1,5 18,2 19,7 1,7+1,7 24,6 26,6 1,8+1,8 27,4 29,6 2,2+2,0 35,3 38,1

Thickness Diameter

Min

Dimensions(mm)

Nominal sectionof the conductor

Voltage of the as-sociated system

Conductor

XLPE Insul. +PVC Sheath

Max

30 up to 66 kV 132 kV 275 kV 400 kV

120 mm2 240 mm2 300 mm2 500 mm2

Thickness Diameter

Min Max

Thickness Diameter

Min Max

Thickness Diameter

Min Max

49

7. EARTHING OF LIGHTNING ARRESTERS

7.1. INTRODUCTION.

The object of this chapter is to determine how lightning arresters should be connected to

earth in high voltage circuits.

The earthing is standardized by the substation regulation MIE RAT 13 Earthing installations

of the Regulation on Technical Conditions and Safety Guarantees in Electric Power Plants,

Transformer Stations and Substations. To summarize, this regulation says:

- In an installation there cannot be a live accessible point which can damage people,

whether under conditions of normal operation or in short-circuit. Consequently, any

cable which conducts current should be insulated.

- The earthing circuit cables which connect the electrodes should be bare, resistant

to corrosion and preferably visible.

- The dischargers have to be connected to the earth of the accessory that they

protect.

- The earth circuits should be straight without forced bends and as short as possible.

The earthing of lightning arresters (insulated cable or bare conductor) is not determined.

There is no reason to use one or another type of conductor.

Taking the case of a lightning arrester at the top of a support. The earth connection can

be carried out with an insulated cable or with a bare conductor, and in actual fact is carried

out in both ways depending on what the customer decides. It should be taken into account

that:

- It is preferable to use an insulated cable, which is on the safety side.

- In any case the customer can make any decision without it affecting the safety of

the circuit.

- We can opt for a bare cable, linked electrically and mechanically to the support

in short sections, which will not have problems of perforation of insulation in the event

of an important impulse.

50

7.2. CONNECTION TO THE EARTH CIRCUIT.

The connection of the different earthings indicated (direct earthing, earthing of the

discharger, earthing of the lightning arresters) will be carried out as follows:

The lightning arrester earthing cable can be the same for the three lightning arresters of

a circuit.

The same cable cannot be used for lightning arresters of different circuits.

The earth connection will be carried out to the earth cable indicated by the customer.

The connection elements must be suitable for the section of the conductor connected.

The project completion documentation must indicate where the connection of each circuit

is carried out.