1

EARTHING ARRANGEMENTS OF SUPPLY SYSTEMS

6.1 EARTHING AND ELECTRIC SHOCK

● In an installation, all exposed metallic parts of electrical equipment in operation are connected to main mass of the earth or reference potential

directly. ● In the case of a short circuit to an exposed metallic part, person or animal

touching to the metallic parts of equipment will have approximately the same zero potential of the earth avoiding hazardous touch. If the metallic parts are

not connected to this common earth point, there is a risk of shock for someone touching the equipment metallic parts.

- In Fig. 6.1(a); a potential difference exists across the body. This introduces a potential difference across one arm and the feet

- In Fig. 6.1 (b), someone at the same time is accidentally touching to the radiator, which is earthed through the main hot water supply pipe.

This introduces a potential difference across two arms through upper part of the body.

● The value of the voltage in both cases is difficult to asses. It depends on how good the contact at points of touch. It may reach as much as 240 V.

66

2

N

One phase of secondarydistribution transformer

240 V

Fault current

N

One phase of secondarydistribution transformer

240 V

Fault current

Main hot watersupply pipe

Radiator

Isolated

Figure 4.1 The electric shock hazards in installation without earthing.

3

● To ensure safety in the area of a consumer electrical installation, and to avoid any possible shock and fire risk in the case of a fault, the metallic parts of equipment must be kept at any potential close to that of the earth potential i.e. at zero volt.

● In both cases of Fig. 4.1, the safe operation is achieved by connecting the metallic parts of the equipment to the earth potential. This connection brings all the metallic parts to the earth potential. This case is illustrated in Fig. 4.2. In Fig. 4.2 (a), exposed metallic parts of the equipment and in Fig. 4(b), extraneous metallic parts in the vicinity of an earthed equipment are connected to the main earthing terminal of the installation, so that all metallic parts in the installation have the same potential or equipotential.

● Joining together (bonding) all metallic parts and connecting to the earth is known as equipotential bonding.

4

N

240 V

IfE

Consumer DB

N

240 V

E

If

Consumer DB

E

Equipotentialbonding conductor

Hot watersupply pipe

Circuit protectiveconductor (CPC)Protective earthing

conductor (PE)

Extraneousconductive part

Exposed conductivepart

Figure 4.2 Earthing and bonding conductors in an installation.

5

Definitions of earthing termsIn an installation earthing exposed metallic parts of equipment and the other

extraneous metallic parts in the installation is achieved by connecting them to the main earthing terminal (MET) at the consumer's DB via a protective earth conductor

(PE). The MET is connected to the earth electrode via main earthing conductor. Description of some of the earthing terms are given in Fig. 4.2(b).

Protective earthing conductor (PE) The conductor which connects the MET to the earth electrode, or to the other means of earthing.

Exposed conductive parts (EXPC) They are accessible parts of equipment which are not live, but could be live in the case of a fault.

Extraneous conductive part (EXTC) It is a conductive part which is not part of the electrical installation, but can introduce a potential generally that of the earth.

Equipotential bonding conductor (EQP) It is the conductor which brings the exposed conductive parts of the equipment and extraneous conductive parts to the

same potential.Consumer's main earthing terminal. It is the terminal or a bar provided for the

consumer 's DB to connect PE of all final circuits, the equipotential bonding conductor, and to the conductors for functional earthing.

Functional earthing. It is earthing of one or more points in a system or items of equipment, which do not perform electrical safety function, but is necessary for

proper function of the equipment.

6

7

8

4.2 EARTH ELECTRODE SYSTEM (IEE Reg. 542-02-01)

The earth itself is not a good conductor unless it is wet. Its resistance restricts the flow of fault current. A good conducting path for the flow of fault current is

achieved by providing a good connection to the earth. There are several connection methods to the earth using:

- conductor earthing electrodes - burried earthing electrodes

(i) Conductor earthing electrodes may have the form of strip or round or stranded material, and can be installed in radial, ring, or mesh (Fig. 4.3(b)-(f)). (ii) Rod earthing electrodes are of rod type and burried in vertical position at a considerable depth. They can be installed as individual in radial, ring, or mesh

configurations for establishing an active earth conductor connection (Fig. 4.3(a)).

LL L 2w

z

zb 2ab

(b)

(d) (f)

2a

2a

b

a

b>a

(a) (c)

(e)

Figure 4.3 Earthing electrodes:(a) Earthing rod(b) Burried wire. (c) Burried strip. (d) Thin disk plate near the soil surface.(e) Ring. (f) Mesh.

9

Earth electrode ResistanceThe resistance of any electrode system, illustrated in Fig. 4.3, varies with soil

resistivity, humidity and temperature as well as with the type of earthing electrode system, and can be estimated simply using the following formulas.

a

L

LR

2ln

2

)L

w

L(L

Rz2

ln2

ln2

bR

4

a

blnb

R8

8 2

)z

Lln

a

L(ln

LR

22

(a)Rod electrode

(b) Burried wire

(e) Ring (in infinite medium)

(f) The expression for earthing resistance of a mesh is complex, but may be approximated as a semi-sphere having a diameter of b.

(c ) Burried strip

(d) Thin disk

bR

4

10

MaterialsThe conductor materials to be used as earthing electrode should resist

corrosive conditions or to be protected from corrosive conditionsThe materials which fulfil these requirements are:

- copper,- aluminium

- copper-clad aluminium The conductors to be used for an earthing system should be continuous with

no joints, and may preferable be driven into soil as bare solid rod, connections between driven rods as stranded.

Size of Earthing Electrode(IEE On-Site Guide)

For TN-S and TN-C-S supplies the minimum size of buried earthing conductor 16 mm2 copper or greater. For TT supplies both conductor sizes also

applies. 25 mm2 copper or 50 mm2 steel if not protected against mechanical damage or corrosion. See A10 of IEE Guide for the sizes PE and bonding

conductors.

11

Table4.1 Reference values for the soil resistivity for various soil types.

Soil type Soil resistivity, (m)

Marshy ground (Wet organic soil)Clay (Moist soil)Dry soilSand GravelSandstoneBed rockGraniteGround waterSea water

5 to 4020 to 200500 to 1000200 to 25002000 to 30002000 to 30005000 to 10 000 to 50 000100-2000.01-1

Earth soil resistivity The earth resistance of a grounding electrode mostly depends on the

soil resistivity and type of grounding electrode. The soil resistivity varies with (i) type of soil and its granular structure,

(ii) the moisture content and (iii) the amount of dissolved salts in the soil. (Table 4.1).

Because of the importance of the moisture, it is preferable to bury the earth electrode to a depth where the condition for dampness achieves a

stable value. The resistivity of soil may be measured using the earth electrode using

the expression given for a rod

a

LRL2

ln

2

12

4.3 SUPPLY SYSTEM ARRANGEMENTS

There are several types of arrangements for the supply systems that the consumer’s MET is connected to the supply earth point.

These are classified in IEE Reg. 312-03-01 as applied in practice as:TN-S system, TN-C-S system, TT system, TN-C system, and IT system.

This grouping is based on earthing of the supply systems and defined by the letters.

- The first letter T (Terra =Earth): It indicates that at least one point of the supply is connected directly to the earth or isolated from the earth.

- The first letter I (Isolated): It indicates that the earth point of the supply is isolated from the earth in an industrial plant.

- The second letter N (Neutral): It shows whether exposed conductive parts in the installation are connected directly to the earth point at the source supply or not.

- The third and final letters (C, C-S, S): These indicate the arrangement of the equipment protective conductor and neutral conductor serving TN system.

13

The letter C shows both protective and neutral conductors are combined together. This could be formed in a single or multicored cable and known as

PEN conductor. The letter S shows both protective and neutral conductors are separate from

each other. All of the following power supply systems except IT system are used for

public supplies.

1. TN-S Supply SystemThis is one of the popular types of supply system in that the

electricity authority provides the supply to the building by underground cables. Fig. 4.4 illustrates the circuit wiring for TN-S

supply arrangement to consumer's DB. In this system of connection:

14

L N

M

MBProtective earthingconductor (PE)

Earthing clamp (earthing bymeans of cable armour)

Servicecut-out

box

Supply cable

NL N

M

MB

PEL L

L1L2L3N

L

PE

Figure 4.4 Single-phase TN-S supply system. Notice the function of cable metal sheath as PE conductor in (a) single-phase and (b) three-phase distribution systems.

15

- Neutral and PE conductors run separate throughout the system. - PE conductor is the metal sheath or armour of the underground

cable or a separate conductor and is connected to the consumer's main earthing terminal (MET) in DB.

- All exposed conductive parts of the installation are connected to MET in consumer DB.

R

Y

B

DB

Three-phaseequipment

Local 11 kV/ 415 VTransformer or source

Single-phase equipment

NPE

DB

Exposed metallicparts

PE

Figure 4.5 TN-S supply system serving single and three-phase equipment.

16

N

PD incut-out

M

MB

N

E

PD

DB

LOAD

L

E

N

N

Supply earthpoint PE

conductor

Local 11 kV/415 V, 240 Vsecondary transformer

Exposedconductive parts

Figure 4.6 Single-phase TN-S supply system. (a) Connection of PE conductor to cable armour. (b) Wiring diagram.

17

2. TN-C-S Supply System (PME)This type of underground supply arrangement has been recently started to

supply installations because of its proven safety. This arrangement is also known as protective multiple earthing (PME).

- The functions of both protective and neutral conductors are combined in the supply cable (PEN conductor) (Fig. 4.7(b)).

- The consumer's MET is formed by connecting the neutral of supply cable in the consumer's installation or DB.

- All exposed metallic parts such as installation gas, water pipes etc. are then connected to MET and the neutral terminal. These terminals are joined together.

- Thus, phase-to-earth faults are effectively converted into phase-to-neutral faults.

- The combined and neutral conductor (PEN) should be earthed many times throughout the supply authority's distribution system.

18

L N

M

MB

Servicecut-out

box

Supply cable

PEN

PEN conductor(PEN should be earthed

at several points)

N

M

MB

N

E

PD

DB

LOAD

L

E

N

N

CPCconductor

Figure 4.7 TN-C-S supply system.

19

N

M

MB

N

E

PD

DB

LOAD

L

E

N

N

PEconductor

ECPEarthingelectrode

RCCB

3. TT Supply SystemThis type supply arrangement should be applied when the installation is fed from overhead lines. The supply authorities do not provide protective earthing to the premise. In this case a residual current device RCCB must be included in in the installation (Fig. 4.8) +- All the exposed conductive parts of the installation are connected to the earth electrode through MET of the installation and is independent of the supply earth.

Figure 4.8 TT-supply system.

20

N

M

MB

N

PD

DB

LOAD

L

N&E

N

N

PENconductor

ECP

4. TN-C Supply SystemIn this system of connection the neutral and protective functions of the supply are

combined in a single conductor (PEN) throughout the system (Fig. 4.9). - This supply system is applied to privately owned generating plants or

transformers where there is no connection between the TN-C system and the publics supply. All exposed conductive parts of the installation are connected to

the PEN conductor. As both protective and neutral conductors are combined, RCCB cannot be used with this type of installation.

Figure 6.9 TN-C supply system

21

5. IT Supply SystemThe supply (transformer) is isolated from the earth with a highly ohmic earthing resistor ( Fig. 6.10).

- In this case any fault involving the earth there is no shock or fire risk. - If a fault occurs, type of earth may be detected and identified using

monitoring equipment, and an audible or similar warning may be produced. - This type of supply arrangement is used in mines, quarries and

chemical factories where interruption of the operation or processes may create hazardous situations.

- This system of connection must not be used for public supplies and its usage is limited to private generating plants.

N

R

Y

B

DBDB

Three-phaseequipment

Installationearth electrode

Earthingimpedance

Local 11 kV/ 415 VTransformer or source

Two-phase equipment

Figure 6.10 IT supply arrangement.

22

6.5 EARTHING CONNECTION IN MAIN SWITCHBOARD AND DISTRIBUTION BOARDS

In large scale domestic or commercial installations supplied by the local private transformer, having a set of SDB or DBs, directly supplied from the

main substation switchboard or from the main distribution board, the earthing connection usually follows TN-S supply arrangement (Fig. 6.11)

- The main earthing in the MDB or substation switchboard is usually a busbar, called main earthing busbar (ME busbar).

- In installations using two SDBs, as in workshops or factory halls, PE conductors of both SDBs are separately connected to ME busbar in main DB

and finally to ME busbar of the substation switchboard.

23

11 kV/415 V, 240 Vsecondary distribution transformer

Main (substation)switchboard (cubicle)

Distribution board (DB)

Sub-distribution board (lighting)

Sub-distribution board (power)

Main-distribution board (MDB)

ME busbar

MET ME busbar

MET MET

PE

PEPE

ME Busbar- Main earthing busbar

Figure 6.11 TN-S supply arrangement in a TPN distribution system

24

6.6 ELECTRICAL BONDINGThe purpose of bonding is to keep all the exposed metalwork in the interior of a

premise at the same earth potential as the metalwork of the electrical installation, so that no potential difference exists between them to cause an

electric shock. This connection or bonding of all exposed metalwork in an installation is known as equipotential bonding. Depending upon types of exposed metalwork involved in bonding the Regulations imposes different

restrictions.

6.6.1 MAIN EQUIPOTENTIAL BONDING To avoid possible potential difference between exposed metallic parts of

earthed electrical equipment and metalwork of other services must be effectively connected to the main earthing terminal (MET) of the installation. The other

electrical and non-electrical services described in the Regulation are:- main water and gas pipes,

- other service pipes and ducting,- risers, ducts of central heating and air conditioning systems,

- extraneous metal parts of the building structure- lightning protective conductors.

25

Gassupply pipe

Watersupply pipe

Safety labelsupply

Antenna

Main earthingterminal (MET)

Sewegepipe

Cut-outfuse box

Heatingpipe

Bondingclamp

Earthingelectrode

Final circuits

MEBconductor

MEBconductor Safety label

Gas meter

60 cm

60 cmEarthingclamp

Figure 6.12 An example of main equipotential bonding arrangements of water and gas supply pipes in a TN-S connected supply arrangement.

26

Main equipotential bonding (MEB) should be made to water and gas services at their entry point to the building as shown in Fig. 6.12.

- The connecting conductor, known as MEB conductor, should be insulated

- The minimum permitted size ,6 mm2≤ (AC)min <25 mm2.

- In domestic and small commercial premises the bonding clamp must be fitted on the consumer side of meters (gas and water) before any branch

pipework, but within 60 cm of the meter. A permanent label also must be fixed at or near the point of connection of MEB conductors for safety warning.

6.6.2 SUPPLEMENTARY EQUIPOTENTIAL BONDING This bonding requires all exposed extraneous conductive parts within close

vicinity of a non-electrical metalworks (water pipes etc) already being connected with MEB conductor to maintain the equipotential zone created by

the MEB (Fig. 6.13).

27

Bath taps andwaste pipe

Basin taps andwaste pipeWater

supply

Showerwaste pipe

Showerfitting

Radiantheater

SEBconductor

SEBconductor

to MET

Figure 6.13 An example of supplementary equipotential bonding arrangement in a bathroom area.

28

Supplementary bonding conductors to have a minimum cross-sectional area of AC=2.5 mm2 when mechanically protected (within conduit or mini-trunk ) or

- if placed in an inaccessible position, and 4 mm2 if unprotected.

6.7 EARTH FAULT IMPEDANCE In an earthed and bonded installation in order to avoid shock and fire risks

under fault conditions, it is required to ensure operation of protective devices at disconnecting times less than the permissible values (0.4 s or 5 s depending on

circumstances).■ This depends on the magnitude of the fault current, which is determined

by the earth loop impedance (EFLI) in a faulty circuit. ■The value of this impedance in relation to the impedance defined for the

protective device under short circuit conditions establishes disconnecting times for protection against;

- shock for indirect contact - temperature rise in conductors

The fault current is limited by the EFLI, Zs. In the case of a short circuit EFLI of the path in a TN-S supply arrangement involves;

29

N

MB

N

E

PD

DB

LOAD

L

E

NECP

L

F (Fault between line and E)

Z1Z4

Z3

Z0

Z2

Protective conductors from theother final circuits or extraneous

conductive partsFigure 6.14 Definition of earth-loop fault impedance path.

Z1 - Impedance of the circuit protective conductorZ2 - Impedance of phase conductor of the final circuit from fault point

to the consumer DB.Z3 - Impedance of protective earthing conductor

Z4 - Impedance from the consumer DB to the sourceZ0 - Impedance of the source

The total impedance of the short circuit path isZs= Z0 + Z1+Z2 +2Z3 +Z4

30

ZE= Z0 +Z3 +Z4

orZS= ZE+

Z1+Z2 ZE may be measured using a phase-to-earth loop impedance tester. However, at the design stage of an installation, if there is no other

information about the supply transformer and the size of the supply cables, the following maximum values given by the supply authority may be used for

different type of supply arrangements:TT : 21 , TN-S: 0.80 ,

TN-C-S: 0.35

EXAMPLE 6.1A BS 3871 type 1 20-A MCB is used to protect a radial socket-outlet circuit in a kitchen environment. The cable length is 30 m and it incorporates 2.5-mm2

PVC cable and a 1.5-mm2 PE conductor. If the EFLI of the supply measured at DB is 0.45 ohm, determine the total EFLI of the circuit. Assume that the

ambient temperature is 30 OC.

31

Solution. ZS = ZE + (Zline + ZPE ) The value measured at DB position, ZE= 0.4 Ohm

The resistance of the line conductor at 30 OC or at the corresponding table temperature of 115 OC can be found using Table 6.1 as

Zline = Rline=(10.226 x 30 m) x10-3 = 0.307 and of PE conductor at 30 OC or at the corresponding table temperature of 95

OCZPE = RPE=(15.73 x 30 m) x10-3 = 0.472

The total EFLI is, therefore,ZS= 0.400 + (0.307+ 0.472) = 1.179

The value of Zs should be as low as possible in order to achieve effective protection against direct contact, when a fault occurs between the phase

conductor and the protective conductor or an exposed metallic parts of the equipment,. to disconnect the supply within a specified time. That is, in order the PD to operate sufficiently rapidly, EFLI limit given for a PD, ZPD, should be less

than or equal to EFLI of the final circuit, Zs, i.e.,

ZPD Zs

IEE Tables 41B to 41D given for EFLI values under Regulations (413-02-09) to (413-02-13) are specified for the disconnecting time limits for PDs. The specified

time limits for a PD is related to the type of final circuit and may be classified as follows:

32

A - The disconnecting time should not exceed 5 s for:(i) A final circuit supplying socket outlet or portable equipment

(including hand-held Class I equipment) and(ii) A final circuit supplying only stationary equipmentB - The disconnecting time is 0.4 s (Table 41A) for:

A final circuit supplying socket outlets and to other final circuits, which supply portable equipment, intended for manual movement use or

hand-held Class I equipment (Bathroom or kitchen condition)

EXAMPLE 6.2In Example 6.1 determine whether it is safe to operate the MCB (BS3871 type 1)

for a kitchen (within 0.4 s) in this circuit or not.

Solution. The maximum permitted value given for the MCB (Table 41B2) is 3 . Since Zs is less than this value, the PD will operate within the specified time limit of 0.4 s (kitchen condition). This means the sizes of conductors are sufficient for

shock safe operation under short circuit conditions.