CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 18 – Earthing and Bonding
Earthing & Bonding Unit Aims By the end of the unit participants should be able to: Differentiate between the terms earthing and bonding and give examples of the usage of each. (Syllabus Reference: 2.3.04) State the purpose of earthing and the function of earth protection. (Syllabus Reference: 2.3.06) List possible exposed conductive parts and extraneous conductive parts of other metallic structures or services. (Syllabus Reference: 2.3.05)
Earthing & Bonding Purpose of Earthing The three main reasons for earthing electrical systems are as follows: To maintain the potential of any part of the system at a defined value with respect to earth; To allow current to flow to earth in the event of a fault, so that the protective equipment will operate to isolate the faulty circuit; To ensure that, in the event of a fault, apparatus normally „dead‟ cannot reach a dangerous potential with respect to earth (earth is normally taken as 0 volts, „no volts‟).
Definitions Exposed Conductive Part “Conductive part of equipment which can be touched and which is not normally live, but which can become live when basic insulation fails” (from BS 7671 „Definitions‟). This refers to conductive parts that form part of an item of electrical equipment or electrical system that are not normally live unless the basic insulation fails. It includes the following: Metal trunking Steel trunking Metal enclosure of an electrical appliance
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 18 – Earthing and Bonding Extraneous Conductive Part “A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation” (from BS 7671 „Definitions‟). This refers to conductive parts of equipment or items that do not form part of any electrical system or equipment but which could become live. It includes the following: Water pipes Gas pipes Oil pipes Structural metalwork Heating and ventilation ducting Basic Protection “Protection against electric shock under fault-free conditions” (from BS 7671 „Definitions‟). Basic Protection is provided to prevent a person or livestock touching parts that can normally be expected to be live under normal circumstances, e.g. live terminals, conductors, bus bars, etc. The example right shows a person coming in to contact with the line terminal of a socket outlet; the current returning via the person through earth back to the supply. This was previously referred to as “Protection against direct contact”. Fault Protection “Protection against electric shock under single-fault conditions” (from BS 7671 „Definitions‟). Fault Protection is provided to prevent danger to a person or livestock touching conductive parts, whether forming part of electrical equipment (exposed conductive parts) or not (extraneous conductive parts), that would not normally be expected to be live but have become live due to a fault, e.g. gas pipes, water pipes, equipment enclosures, metal bath tub, structural steelwork, etc. The example below shows a person coming in to contact with a radiator that has become live due to it coming into contact with a line conductor.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 18 – Earthing and Bonding
The most common method of providing fault protection in the UK is „Protective equipotential bonding and automatic disconnection of supply‟ (PEBADS) (BS7671 413.3.1 and 413.3.2) By bonding all exposed and extraneous conductive parts to each other and to earth, when an earth fault occurs a large fault current will flow causing appropriate protective devices to operate quickly thereby removing the danger quickly and effectively.
Bonding The main purpose of the bonding regulations is to keep all exposed metalwork of an installation at the same earth potential as the metalwork of the electrical installation, so that no currents can flow and cause an electric shock. For a current to flow there must be a difference in potential between two points, but if the points are electrically connected together, there can be no potential difference. This bonding or connecting together is known as „Protective Equipotential Bonding‟. Main Equipotential Bonding Where earthed electrical equipment may come into contact with the metalwork of other services, they too must be effectively connected to the main earthing terminal of the installation. Other services are described by BS7671 Regulation 411.3.1.2 as:
(i)
Water installation pipes
(ii) Gas installation pipes (iii) Other installation pipework and ducting (iv) Central heating and air conditioning systems (v) Exposed metallic structural parts of the building.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 18 – Earthing and Bonding Size and Position of Main Bonding Conductors Main equipotential bonding to gas, water and other services should be made as near as possible to the point of entry using bonding conductors with a cross-sectional area not less than half the cross-sectional area of the earthing conductor; the minimum size being 6mm2 but need not exceed 25mm2 (Regulation 544.1.1) except where PME conditions apply when Table 54.8 should be used. Table 4.4 (i) of the On-Site Guide gives clearer guidance on this. Bonding of gas service pipes should be made on the consumer‟s side of the meter between the outlet union and any branch pipe work, but, where practicable, within 600mm of the meter outlet union or at the point of entry to the building if the meter is external (Regulation 544.1.2). Similarly, connection to the water should be on the consumer‟s side of the stopcock before any branching pipe work but within 600mm of the stopcock. These conditions are shown below:
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 18 – Earthing and Bonding Connection of the bonding conductor to pipework must be made using an approved earth clamp. A permanent label complying with BS 951 must also be fixed at or near the point of connection of the bonding conductor with the words: „SAFETY ELECTRICAL CONNECTION - DO NOT REMOVE‟ (BS7671 Regulation 514.13.1); such a clamp and label is shown below:
Supplementary Bonding According to Section 4.6 of the ON-Site Guide, the purpose of supplementary equipotential bonding is to reduce the voltage between the various exposed-conductive-parts and extraneous-conductive-parts of a location during a fault to earth. If the required disconnection time cannot be achieved supplementary bonding shall be applied (Regulation 415.2). The sizing of supplementary bonding conductors is dealt with in Regulation 544.2 (1-5). Table 4.6 of the On-Site Guide gives clearer guidance on this. Section 4.7 of the ON-Site Guide is very important as it dispels some of the myths relating to supplementary bonding and is reproduced in full below: Supplementary equipotential bonding is required in some of the locations and installations of Part 7 of BS 7671. If the installation meets the requirements for earthing and bonding there is no specific requirement in BS 7671 for supplementary equipotential bonding of: Kitchen pipes, sinks or draining boards Metallic boiler pipework Metal furniture in kitchens Metallic pipes to wash hand basins and WCs Locations containing a bath or shower, providing the requirements of 701.415.2 are met. Note: Metallic waste pipes in contact with Earth must be bonded to the main earthing terminal as they are extraneous-conductive-parts. Supplementary bonding is not required to metallic parts supplied by plastic pipes. The full requirements of Regulation 701.415.2 is given below: Local supplementary bonding in accordance with Regulation 415.2 shall be established connecting together the terminals of the protective conductor of each circuit supplying Class I and Class II equipment to the accessible extraneous-conductive-parts, within a room containing a bath or shower, including the following: (i)
Metallic pipes supplying services and metallic pipes (e.g. water, gas)
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 18 – Earthing and Bonding (ii)
Metallic central heating pipes and air conditioning systems
(iii)
Accessible metallic structural parts of a building (metallic door architraves, window frames and similar parts are not considered to be extraneous-conductive-parts unless they are connected to metallic structural parts of the building).
Supplementary bonding may be installed outside or inside rooms containing a bath or shower, preferably close to the point of entry of extraneous-conductive-parts into such rooms. Where the location containing a bath or shower is in a building with a protective equipotential bonding system in accordance with Regulation 411.3.1.2, supplementary equipotential bonding may be omitted where all of the following conditions are met: (i)
All final circuits of the location comply with the requirements for automatic disconnection according to Regulation 411.3.2
(ii)
All final circuits of the location have additional protection by means of an RCD in accordance with Regulation 701.411.3.3
(iii)
All extraneous-conductive-parts of the location are effectively connected to the protective equipotential bonding according to Regulation 411.3.1.2. Note: the effectiveness of the connection of extraneous-conductive-parts in the location to the main earthing terminal may be assessed, where necessary, by the application of Regulation 415.2.2. Supplementary bonding is NOT required to metal parts supplied by plastic pipes, such as metal hot and cold water taps supplied from plastic pipes. Similarly, a metal bath not connected to extraneous-conductive-parts (such as structural steelwork) with plastic hot and cold pipes and plastic waste pipes does not require supplementary bonding.
For more information see pages 140 to 141 of „Electrical Installations Level 2 2330 Technical th Certificate‟ – revised for the 17 Edition IEE Wiring Regulations (ISBN 978 0 435401 09 2).
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