L3 U304 Session 1 Earthing and Bonding

Goal © Walsall College

City and Guilds 2357Diploma in Electrotechnical

Technology Level 3 Unit 304 Session 1Earthing & Bonding


Aims & Objectives of sessionDifferentiate between the terms Earthing and Bonding and give examples of both


Earth is a large conductor that is at 0V (zero potential)

• It is not however a good conductor, the rockier the soil the ‘poorer’ the earth.

• When we say that we have a voltage or potential of 230V, that means the voltage has been raised to a point 230V above zero.

• Earth also provides us with a means to clear phase to earth faults by operating protective devices when sufficient current flows.

• An earthed system allows all metalwork in an installation to rise to the same potential under fault conditions.

• In the UK it is the responsibility of the consumer or electrical contractor to make sure that an installation is adequately earthed. Although supply authorities often provide earth via the supply cables they are not required to, it is an act of courtesy.

What is earth?


Reference point

The star point of the supply authorities transformer on the secondary side is connected to earth which is accepted as being 0V.

The primary of the supply authorities transformer (HV) has an earth provided and this is connected to the structure of the transformer.


The earth loop is the path which begins at a fault, passes through the earth conductor then into the sub mains and then back to the point of the fault. It should have a low value of impedance.

Earth Fault Loop Path


Earth fault loop path


Reasons for Providing an Earth andBonding Together in an Installation

1. Earthing is provided to ensure sufficient current should flow under an earth fault so that the protective device (fuse or a circuit breaker) operates within a pre-defined time.

2. While the fault exists metal work could rise to a high potential.

3. Bonding occurs to enable exposed and extraneous metal work to be linked together. This gives us an equipotential zone.

Reasons for earthing


According to BS7671(2011) an exposed conductive part is:-

“A conductive part of equipment which can be touched and which is not a live part but may become live under fault conditions”.

If it is not part of a fault current circuit then it is not an exposed conductive part.

Just because something is metal it doesn't mean that it necessarily needs bonding.

Exposed Conductive Parts?


According to BS7671(2011) an extraneous conductive part is:-

A conductive part liable to produce a potential, generally an earth potential, and not forming part of the electrical installation.

If any metal comes out of the ground, and we can assume the ground is at earth potential, then that metal work is an extraneous conductive part.

Extraneous Conductive Parts?


main equipotential bonding and supplementary equipotential bonding


Revisit Aims & Objectives of session

Differentiate between the terms Earthing and Bonding and give examples of both


Earthing systems


BS 7671 lists five types of earthing system:

TN-S, TN-C-S, TT, TN-C, and IT.

T = Earth (from the French word Terre) N = Neutral S = Separate C = CombinedI = Isolated (The source of an IT system is either connected to earth through a deliberately introduced earthing impedance or is isolated from Earth. All exposed-conductive-parts of an installation are connected to an earth electrode.)


TN-S System

The TN-S system has a separate

earth arrangement throughout.

The dotted line shows where the split occurs.There is a fuse, a metering arrangement and some kind of isolation.If a consumer puts anything on the supply authorities equipment without their permission then they can be prosecuted.


TN-S System


TN-C-S System

The supply cable has the earth and neutral combined in one conductor, which can be the armouring of the cable.

The only difference between this and the TN-S system is the earthing arrangement at the intake point.


TN-C-S System


TT System

The general mass of earth has become the earthing arrangement.The consumer unit has an integral RCD.The value of the resistance of the earth electrode and the rating of the RCD will vary according to different circumstances.BS7671 permits this voltage to rise as high as 50V before the RCD must trip. In part 7 of BS 7671 :2008 for farms and construction sites this falls to 25V. maximum earth electrode resistance of 200Ω.


TT System


BS7671:(2011) states that;

A protective conductor connecting conductive parts of equipment to the main earthing terminal.

Circuit Protective Conductors (c.p.c.)?


A circuit protective conductor can be, according to Regulation 543.02.02

• A single core cable.

• A conductor in a cable.

• An insulated or bare conductor in a common enclosure with insulated live conductors.

• A fixed bare or insulated conductor.

• A metal covering such as the armouring of a cable.

• A metal conduit or other enclosure or electrically continuous support system for conductors.

• An extraneous conductive part whose continuity can be guaranteed, whose cross sectional area is sufficient, which cannot be removed and has been adapted to its new use if necessary.

Circuit Protective Conductors (c.p.c.)


• Earth is a large conductor that is at 0V (zero potential).

• An exposed conductive part is a conductive part of equipment which can be touched and which is not a live part but may become live under fault conditions.

• An extraneous conductive part is a conductive part liable to produce a potential, generally an earth potential, and not forming part of the electrical installation.

• Circuit Protective Conductors (c.p.c.) are a protective conductor connecting conductive parts of equipment to the main earthing terminal.

Recap so far


Revisit Aims & Objectives of session

Differentiate between the terms Earthing and Bonding and give examples of both


Notice the conductor from the supply intake is called the earthing conductor and everything else is a bonding conductor.

Earthing of a Small Domestic Installation


BS7671:2011 defines an earthing conductor as:-

• A protective conductor connecting the main earthing terminal of an installation to the earth electrode or other means of earthing.

BS7671:2011 defines a bonding conductor as:-

• A protective conductor providing equipotential bonding.

Earthing and Bonding Conductors

You will need this

information later


When we (the consumer) have to provide earth to an installation it will be in the form of an earth electrode.

BS7671:2011 allows for a variety of earth electrodes;

• Earth rods or pipes

• Earth tapes or wires

• Earth plates

• Underground structural metalwork embedded in foundations

• Welded metal reinforcement of concrete embedded in the earth (not pre-stressed)

• Lead sheaths or other coverings of cables

• Other suitable underground metal work

Earth Electrodes


This is the most common type of earth electrode.

It is made of copper or copper clad steel. It can be increased in length by screwing extra extensions on to it.

The electrode must be accessible so that tests can be carried out on it.

Any earth electrode you install must be at least 9m away from any supply authorities electrode or cable sheath.

The connection of the earth conductor to the earth electrode must have a warning notice attached to it.

Earth Rod


Earth electrodes

Earth rods are made of copper or copper clad steel. The rod can be increased in length by screwing on extension lengths

The connection of the earth conductor to the earth electrode must have a warning notice attached to it. This must be a permanent label.

Termination made to an earth electrode


Can you name any other types of earth electrode?

Other forms of electrodes


Connecting earthing conductors to earth electrodes can be done by

• Brazing

• Tape connectors

• Mechanical clamping devices

• special welding techniques

When a joint is made, care should be taken to ensure the joint is protected. It should be coated with grease or bitumastic paint.

Connecting Earthing Conductors to Earth Electrodes


This earth electrode is accessible via an inspection cover. The connection of the earth conductor and the earth electrode must

have a warning notice attached to it.

Connection to earth rod


Typical bonding clamp on pipework


Possible earth fault current paths in an installation

EFLP ?


The bathroom has become an equipotential zone within an equipotential zone.

The pipes , radiators etc. have been bonded to ensure that all metalwork remains at the same voltage should a fault occur.

Equipotential Zone


The following images were taken from the Building

Regulations Approved

Document ‘P’

Drawings, Charts and Installation Details


Earthing Details


Equipotential zones cannot work outside, therefore the risk of shock is high.

Any sockets likely to be used outside must be protected by RCD’s.

Outside Equipotential Zone


• Earth is a large conductor that is at 0V (zero potential).

• An exposed conductive part is a conductive part of equipment which can be touched and which is not a live part but may become live under fault conditions.

• An extraneous conductive part is a conductive part liable to produce a potential, generally an earth potential, and not forming part of the electrical installation.

• Circuit Protective Conductors (c.p.c.) are a protective conductor connecting conductive parts of equipment to the main earthing terminal.

• Earthing is provided to ensure sufficient current should flow under an earth fault so that the protective device (fuse or a circuit breaker) operates within a pre-defined time.

• While the fault exists metal work could rise to a high potential.

• Bonding occurs to enable exposed and extraneous metal work to be linked together. This gives us an equipotential zone.

Summary of session


Aims & Objectives of sessionDifferentiate between the terms earthing and bonding and give examples of both


Looking forward to next session2357-304: Task A

Question 1a) Explain the difference between earthing and bondingb) Indicate the possible outcome of the earthing conductor being

removed from the means of earthing.c) State whether earthing provides basic or fault protection. Question 2Show on drawings/copies/overlays, for the installation shown in drawings 2357-A-01/02/03,the cable routes for the earthing and bonding systems identifying each conductor. (Circuit protective conductors for all circuits need be included.)

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Documents

L3 U304 Session 1 Earthing and Bonding