EARTHING

Definitions:‘Connection of the exposed-conductive parts of an installation to the main earthing terminal of that

installation.’ BS 7671

Exposed-Conductive-Part: A metallic part of an Installation or Appliance

which can be touched by persons and which is not normally live but may become live due to a fault

condition.

Extraneous-conductive-part: a metallic part, structure or any metalwork within a Premises which is not part of, and is unrelated

to, the electrical Installation and which is not designed to carry current, but which may become

livedue to a fault condition.

For example:Non-insulated floors or walls, Metal framework of buildings

Metal conduits and pipework (not part of the electrical installation) for water, gas, heating,

compressed-air, etc. and metal materials associated with them

Electric Shock of persons A person subjected to an electrical voltage is electrified.

According to the gravity of the Electric Shock, this person may experience:

Discomfort * Muscular contraction * Burn * Cardiac arrest (Electrocution)

Standards and regulations distinguish two kinds of dangerous contact,

Direct contact

Indirect contact

Direct contact This is accidental contact of persons with a live conductor (phase or neutral) or a normally live

conductive element

Protection measures consist: in placing these live parts out of reach or in insulating them by means of insulators, enclosures or barriers..

Indirect contactContact of a person with accidentally energized metal frames is

known as indirect contact

Common Reasons: insulation fault

Protection measures consist: 1st level:

The earthing of all exposed-conductive-parts of electrical equipment in the installation & the constitution of an

equipotential bonding network.

2nd level: Automatic disconnection of the supply

Why Earthing is required?

The most important reason is Safety/Protection of Persons & Property;

So as to protect against electric shock by indirect contact

(Discomfort/Muscular contraction/Burn/Cardiac arrest)

&Avoid overheating & fire

Earthing of Exposed-Metallic-Parts of an Installation and of Appliances in a Premise is

required to serve the following functions of safety:

Allow the passage of fault current: in the event of a live conductor touching an Exposed-Metallic-Part;

Ensure that the magnitude of fault current is sufficient to operate Protective Devices:including Earth Leakage Protection devices,

Ensure that, the less resistance from the fault to the Earth:so as to avoid overheating or fire.

GENERAL PRINCIPLES OF EARTHING

SYSTEMS OF EARTHINGToday 3 system earthings such as defined in

IEC 60364:

TN TT ITThe purpose of these three earthings is identical as regards protection of persons and property:

They are considered to be equivalent with respect to safety of persons against indirect contacts.

Identification of the system types is thus defined by means of 2 letters:

The first one for transformer neutral connection (2 possibilities) T for "connected" to the earth I for "isolated" from the earth

The second one for the type of application frame connection (2 possibilities)

T for "directly connected" to the earth N for "connected to the neutral" at the origin of the installation

The TT system

One point at the supply source is connected directly to earth.

All exposed- and extraneous-conductive-parts are connected to a separate earth electrode at the installation.

This electrode may or may not be electrically independent of the source electrode.

Protection & Operation Technic in TT system

The residual current devices (RCDs) are used

Interruption for the first insulation fault

The source is earthed as for the TT system (above).

In the installation, all exposed and extraneous-conductive-parts are connected to the neutral conductor.

The TN system

The several versions of TN systems are shown below.

TN-CIf the N and PE neutral conductors are one and the same (PEN)

TN-SIf the N and PE neutral conductors are separate

Interruption for the first fault using overcurrent protection (circuit-breakers or fuses)

interruption for the first insulation fault

Protection & Operation in TN system

No intentional connection is made between the neutral point of the supply source and earth.

Exposed- and extraneous-conductive-parts of the installation are connected to an earth electrode.

The IT system

Indication of the first fault by an insulation monitoring device (IMD)

Interruption for the second fault using overcurrent protection (CB or fuses)

Monitoring of the first insulation faultMandatory location and clearing of the fault

Interruption for two simultaneous insulation faults

Protection & Operation in IT system

Fault Current & Automatic disconnection for different Earthing systems

Automatic disconnection & Protective Devices for different Earthing systems

CHOOSING THE EARTHING SYSTEM

Choice may be determined by normal practice in the country.

In terms of the protection of persons, the three system earthing arrangements (SEA) are equivalent if all

installation and operating rules are correctly followed.

Consequently, selection does not depend on safety criteria.

The applicable regulations which in some cases impose certain types of SEA

Secondly, the decision of the owner if supply is via a private MV/LV transformer (MV subscription) or the

owner has a private energy source (or a separate-winding transformer)

If the owner effectively has a choice, the decision on the SEA is taken following discussions with the network designer (design office,

contractor)

Fault Current & Automatic disconnection for TT system

The resistance of the earth electrode of substation neutral Rn is 10 Ω.

The resistance of the earth electrode of the installation RA is 20 Ω.

The earth-fault loop current Id = 7.7 A.

The fault voltage Uf = Id x RA = 154 V and therefore dangerous,

But IΔn = 50/20 = 2.5 A so that a standard RCD will operate without intentional time delay and will clear the fault where a fault voltage

exceeding appears on an exposed-conductive-part.

Fault Current & Automatic disconnection for TN system

The resistance of the earth electrode of substation neutral Rn is 10 Ω.

The resistance of the earth electrode of the installation RA is 20 Ω.

The earth-fault loop current Id = 7.7 A.

The fault voltage Uf = Id x RA = 154 V and therefore dangerous,

But IΔn = 50/20 = 2.5 A so that a standard RCD will operate without intentional time delay and will clear the fault where a fault voltage

exceeding appears on an exposed-conductive-part.

Automatic disconnection for TN system

SYSTEMS OF EARTHINGCustomers must be provided with a supply operating

under either of the following earthing systems:

Customer Earthed System (TT): the Customer provides a Main Earth Terminal for the Installation, which is connected to a sufficient number of local Earth Electrodes to provide a maximum Earth Resistance measured at the Customer’s Main Earth Terminal of no greater than 10 Ohms (referred to in BS 7671 as a ‘TT’ system);

Distribution Company Earthed System (TN-S): the Distribution Company provides a connection to the Customer’s Main Earth Terminal, using the distribution network Earthing system, normally via the armouring or metallic sheath of the incoming supply cable (referred to in BS 7671 as a TN-S system). The Distribution Company system is earthed at the distribution transformer (11kV/400V) and separate earth and neutral conductors are used throughout the distribution network; or

Distribution Company Earthed System with additional Customer Earth (TN-S-TT):the Distribution Company provides an earth connection to the Customer’s Main Earth Terminal and a local Earth provided by the Customer is also connected to the same MET, having an Earth Resistance value of no greater than 10 Ohms.

TN-S

TT

TT-S-TN

Main Earth Terminal The Main Earth Terminal must be provided

close to or within the Main Distribution Board or other switchgear at the Supply Intake.

A means of removing the connection to Earth (such asby a detachable metal link) must be provided at the MainEarth Terminal to facilitate testing of the earthing of the

installation and the source of supply,

Main Earth TerminalIllustration shows Distribution Company Earthed System with

additional Customer Earth (TN-S-TT)

For Premises consisting of more than one building, where either a TT or TN-S-TT system is used, Earth Electrodes must be provided at each building that

is more than 30 m distance from the Supply Intake.

Earth Electrodes Earth Electrodes must be provided at Premises whichare supplied by a Customer Earthed System (TT) or aDistribution Company Earthed System with additional

Customer Earths (TN-S-TT).

Earth Electrodes will normally comprise of 9 to 15 mm diameter steel cored copper rods driven to a minimum depth of 1.2,1.5 to 3 m. The top of each Earth Electrode must be housed inside a 300 mm x 300 mm x 300 mm inspection pit which is clearly

labelled and accessible for routine testing,

Alternative types of Earth Electrodes may be used with prior approval of the Distribution Company

(e.g. plate or wire mesh type, or structural steelwork in a

building)..

Where more than one Earth Electrode is required to

achieve the required Earth Resistance value these must be separated at a distance

greater than their mutual resistance zone.

The condition of the ground in which an Earth Electrode

is placed must be taken into account to ensure its long-term performance. In

particular, potential corrosion effects and ground moisture content is of critical

importance. Proprietary chemical or salt materials may be used around the Earth

Electrode to maintain moisture content but these must be stable and sustain their

chemical properties over time.

For supplies of 500 A rating and above, at least two

independent Earth Electrodes must be provided, regardless of the Resistance Value

achieved for each Earth Electrode, and connected to the same Main Earth Terminal,

The number of Earth Electrodes required at a Premises will be determined primarily by the

value of Earth Resistance that can be achieved from each. However, the minimum

number in any case shallbe as shown below:

However, the minimum number in any case shall

be as shown below:

Main Earth Conductors (MEC): conductors connected between Distribution

Boards, Earth Electrodes and Main Earth Terminals;

Earth Conductor

Circuit Earth Conductors (CEC): connected from Final Distribution Boards and

associated with Final Circuits, including Appliance Earth Conductors such as within a

flexible cord. Outside these Regulations these may also be known as the Circuit

Protective Conductor (CPC) or Earth Continuity Conductor (ECC).

For metal sheathed or armoured cables, the sheath orarmouring may be used as the Earth Conductor where

it is rated to the equivalent of the conductor sizes shown

The metal sheath or armouring must be terminated by suitable cable glands with

appropriateconnection to an Earth Conductor,

For Main Earth Conductors between Earth Electrodes and the Main Earth Terminal of an Installation, S should be taken as the cross-

sectional area of the conductors of the incoming supply cable. For other Earth

Conductors S should be taken as the cross-sectional area of the circuit phase

conductors.

Sizing of earth conductors and equipotential bonding conductors

All Exposed-Metallic-Parts of an Installation, includingAppliances, must be connected to Earth via EarthConductors in accordance with the requirements

Exposed-metallic-partsMetallic part of an Installation

or Appliance which can be touched by persons and which is not normally live but may become live due

to a fault condition. Exposed-Metallic-Parts are required to be connected to Earth,

All connection points in an Installation must include an

Earth Conductor for future use.

The following items must be connected to the Circuit Earth Conductor, which should be

provided separately for each Final Circuit, insulated with green/yellow PVC sheath, be of

the appropriate size…

The requirement for connection of Extraneous-Metalic-

Parts to Earth Conductors depends on the type of protection system used in an Installation

All connection points in an Installation must include an

Earth Conductor for future use.

Earth Fault Loop ImpedanceThe Earth Fault Loop Impedance (Zs) is defined as the totalimpedance presented to an earth fault current, comprising

the impedance of the following parts of a system,

The requirement for connection of Extraneous-Metalic-

Parts to Earth Conductors depends on the type of protection system used in an Installation

All connection points in an Installation must include an

Earth Conductor for future use.