44. Table 54.1 – Minimum cross-sectional area of a buried earthing conductor * Note: The covering of a green-and-yellow single-core cable is not a sheath . Protected against mechanical damage NOT protected against mechanical damage Protected against corrosion by a sheath* 2.5 mm 2 copper 10 mm 2 steel 16 mm 2 copper 16 mm 2 coated steel NOT protected against corrosion 25 mm 2 copper 50 mm 2 steel
45. 542.4 – MET connecting the earthing conductor to installation protective conductors
46.
47.
48.
49.
50. Selection of protective conductor (reference to the related line conductor and Table 54.7 Exercise: What is the csa of a protective conductor selected from Table 54.7 where a line conductor csa is (1) 6 mm 2 , (2) 25 mm 2 , and (3) 50 mm 2 (same material as the line conductor)
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61. 543.7.1.4 – Where two protective conductors are used … NOTE: The ends of protective conductors to be terminated independently; the same is true at distribution boards
Chapter 53 includes the requirements to provide compliance with the measures of protection for safety, the requirements for proper functioning for the intended use of the installation, and the requirements appropriate to the external influences foreseen.
Except as provided in Regulation 537.2.2.5, in multi-phase circuits an independently operated single-pole switching device or protective device should not be inserted in the neutral conductor. In single-phase circuits, an independently operated single-pole switching or protective device must not be inserted in the neutral conductor alone .
Although section 531is entitled Fault protection by automatic disconnection of supply (Section 531) nearly all of the requirements relate to RCDs Exercise (1) When an RCD is used for fault protection, with, but separate from, an overcurrent device, RCDs are to be capable of withstanding without damage the fault currents to which they are likely to be subjected on the load side at their point of installation (2) In a TN system, RCDs may be required where the requirements of Regulation 411.4.5 for automatic disconnection cannot be fulfilled by an overcurrent protective device, such as where the value of earth fault loop impedance (Zs) of a circuit is too high. The exposed-conductive-parts of that part of the installation must be connected to the TN earthing protective conductor or to a separate earth electrode which affords an impedance low enough to operate the RCD. In the latter case the circuit should be treated as a TT system and Regulations 411.5.1 to 411.5.3 apply.
Discrimination between circuit protective devices is achieved when, under fault conditions, the device electrically nearest to the fault operates, leaving other upstream protective devices still providing supplies to the remaining healthy circuits. Where two or more RCDs are in series, and where discrimination in their operation is necessary to prevent danger, the characteristics of the devices should be such that the intended discrimination is achieved.
RCDs in a TT system Where an installation forming part of a TT system is protected by a RCD, then either: the RCD should be located at the origin of the installation, or the part of the installation between the origin and the RCD complies with the requirements for protection by the use of Class II equipment or equivalent insulation. Where there is more than one origin, the above requirement applies to each origin.
Section 534 is unused Section 535 refers to Section 445
Section 537 provides the requirements for selection and suitability for the four different operating functions required within the term ‘Isolation and switching’
Switching or isolating devices are prohibited in combined protective and neutral (PEN) conductors and protective conductors except as permitted by Regulations 543.3.4 and as required by Regulation 537.1.5
(1) be inserted, where practicable, in the supply main circuit (2) manual operation (3) Be capable of cutting off the full load current (4) Be designed and/or installed so as to prevent inadvertent or unintentional switching on (5) Be placed and marked so as to be readily identifiable and convenient for their intended use (6) Require the open position of the contacts to be clearly visible or be clearly and reliably indicated
(1) Be capable of breaking the full load current of the relevant part of the installation. (2) Be Hand-held devices for direct interruption of the main circuit to be selected where applicable. (3) The means of operating devices (handles, push-buttons etc) for emergency switching to be clearly identifiable, preferably by colour. (4) Readily accessible at places of danger and, where appropriate, at any additional remote position from which that danger can be removed. (5) So placed and marked as to be readily identifiable and convenient for their intended use. (6) Such that its operation does not introduce a further danger or interfere with the complete operation necessary to remove the danger.
(1) Coloured red and have fixed to it (or adjacent to it) a permanent durable nameplate marked with the words ‘ FIREFIGHTER’S SWITCH’ (min. dimensions req’d). (2) The ON and OFF positions to be clearly indicated by lettering which can be easily read by someone standing on the ground. (3)The switch must have its OFF position at the top and be fitted with a lock or catch to prevent it being inadvertently returned to the ON position. (4) Most importantly, the switch must be installed so as to be accessible for operation by a firefighter. (5) Where more than one such switch is used, each switch must be clearly labeled to indicate the installation it controls.
IMDs are not covered in this presentation, the following slide relates to RCMs
The term protective conductor covered in this section relates to all the conductors used for protective earthing and protective bonding
Explain that protective earthing is provided for reasons of electrical safety and is divided into two parts. The first of these is source earthing, associated with the source of supply to an electrical installation . The second is electrical equipment earthing, associated with the electrical installation itself. Source earthing, sometimes called supply system earthing, is the provision of a connection between the source of energy - normally the distribution transformer secondary winding or generator winding - and the general mass of earth (Earth), via a source earth electrode. The provision and maintenance of source earthing is normally the responsibility of the owner of the source of energy, and should be carried out to an appropriate standard, such as BS 7430, Code of practice for Earthing. The purposes of source earthing are: • To preserve the security of the supply network by limiting the potential of the live conductors (with respect to that of Earth) to a value consistent with their insulation. • In the case of a TT system, to provide a path for earth fault current and protective conductor current to return to the source of energy via Earth. Without this path, devices for protection against electric shock and fault current to earth will not operate. Electrical equipment earthing , sometimes called electrical installation earthing, is the connection of the exposed-conductive-parts of an electrical installation to an appropriate means of earthing. Electrical equipment earthing is applicable in installations where the measure for protection against indirect contact is Earthed Equipotential Bonding and Automatic Disconnection of supply (EEBAD), as used in the majority of installations in the United Kingdom. The purpose of electrical equipment earthing is to allow operation of the devices for protection against electric shock and fault current to earth, permitting automatic disconnection of the supply to the faulty circuit in the event of an earth fault.
TN-S TN-C-S TT
Earth rods or pipes, or tapes or wires Earth plates Underground structural metalwork Welded metal reinforcement of concrete (except pre-stressed concrete) embedded in the earth Lead sheaths and other metal coverings of cables (where not precluded by 542.2.5) Other suitable underground metalwork
A separate MET with link is preferable and this would certainly be expected on medium/large installation. On domestic premises where there is a single consumer unit it is normal for the MET to be the consumer unit earthing bar.
The term protective conductor is a generic term relating to earthing conductors, circuit protective conductors, main protective bonding conductors and supplementary protective bonding conductors.
The c.s.a. of the conductor is to be: Not less than 2.5 mm 2 copper equivalent, if protected against mechanical damage (e.g. by a sheath or other form of mechanical protection, as the situation demands) is provided, and Not less than 4 mm 2 copper equivalent, if protection against mechanical damage is not provided.
6 mm 2 16 mm 2 25 mm 2 Discuss the relevance of k by referring to Table 54.2, 54.3, 54.4, 54.5 and 54.6. An understanding of these tables will be required for the next slide when the adiabatic calculation is discussed.
(1) 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, for example, the sheath, screen or armouring of a cable a metal conduit or other enclosure or electrically continuous support system for conductors an extraneous-conductive-part complying with Regulation 543.2.6. (2) 10 mm 2 (3) Gas pipes, oil pipes, flexible or pliable conduit, support wires or other flexible metallic parts, or constructional parts subject to mechanical stress in normal service, must not be used as protective conductors. (4) Regulation 543.2.6
543.2.9 Where the circuit protective conductors of a final ring circuit are not formed by the metal covering or enclosure of a cable, they must be installed in the form of a ring with both ends connected to the earth terminal at the origin of the circuit. This will normally be at the distribution board or consumer unit. 543.3.1 Protective conductors must be suitably protected against mechanical and chemical deterioration and electrodynamic effects. 543.3.2 Apart from where protective conductors form part of a multicore cable or a cable trunking or conduit, protective conductors up to and including 6 mm 2 cross-sectional area must be insulated. Where the sheath of a cable incorporates an uninsulated protective conductor up to and including 6 mm 2 cross-sectional area, the protective conductor must be protected by insulating sleeving.
When energized and in normal use, some electrical equipment can cause current to flow in the circuit protective conductors. This process is referred to as functional earthing as the equipment concerned requires the current to flow in the protective conductor to function or operate normally. The requirements of BS 7671 relate to individual equipment and circuit protective conductor currents. The requirements depend on the value of these currents. Regulation Group 543.7 Earthing requirements for the installation of equipment having high protective conductor currents was originally considered to be a special installation (Section 607 High protective conductor currents of the 16 th edition). These requirements are detailed and it may be difficult for students to remember all the different requirements relating to different leakage current values. The most common requirement to be met probably relates to ring final circuits.
(1) Equipment which in normal service, will have a protective conductor current exceeding 3.5 mA, but less than 10 mA must be either: permanently connected to the fixed wiring of the installation without the use of a plug and socket, or connected by means of a connector complying with BS EN 60309-2 (that is, an industrial-type connector) (2) Five options are provided in Regulation 543.7.1.3. (i) A single protective conductor having a c.s.a. of not less than 10 mm 2 . (ii) A single copper protective conductor having a c.s.a. of not less than 4 mm 2 , the protective conductor being enclosed to provide additional mechanical protection. (iii) An earth monitoring system to BS 4444 (iv) A double-wound transformer or equivalent in which the input and output circuits are electrically separate (v) Two individual protective conductors, each one complying with the requirements of Section 543. It is permitted for the two protective conductors to be of different types (for further information see 543.7.1.3 (iii)).
The label should be so positioned as to be visible to a person modifying or extending the circuit.
Main protective bonding conductors are required to establish the requirements for protective equipotential bonding (an indispensable part of the most commonly used protective measure, Automatic Disconnection of Supply (ADS)).
Exercise (1) 10 mm 2 (2) 25 mm 2 (3) The main bonding connection to any gas, water or other service must be made as near as practicable to the point of entry of that service into the premises. Where there is an insulating section or insert at that point, or there is a meter, the connection should be made to the consumer’s hard metal pipework (not to a lead pipe or a flexible pipe for example) and before any branch pipework. Where practicable, the connection is required to be made within 600 mm of the meter outlet union or at the point of entry of the service to the building if the meter is external.
The requirement for supplementary bonding where disconnection times could not be met is covered by Regulation 411.3.2.6. However this would be an unusual situation.
Chapter 55 covers is in some ways different from the other chapters of BS 7671 in that it relates to electrical equipment. The Learning Guide has two sections, one covering Sections 551, 552, 553, 554 and 555, the other covering 559. The requirements of 559 now include requirements that were originally considered to be a special installation or location in the 16 th edition (Section 611 Installation of highway power supplies, street furniture and street located equipment) although this is only part of 559.
Requirements are included for: supply to an installation which is not connected to a system for distribution of electricity to the public, supply to an installation as an alternative to a system for distribution of electricity to the public, supply to an installation in parallel with a system for distribution of electricity to the public, appropriate combination of the above. When designing or installing generators special consideration needs to be given to the prospective short-circuit current and prospective earth fault current for each source as well as: Capacity and operating characteristics Overcurrent protection Fault protection Synchronising (if operating in parallel) Locking-off/interlock devices
A system of locks with a single transferable key A three-position break-before-make changeover switch An automatic changeover switching device with a suitable interlock Other means providing equivalent security
(1) Socket-outlets mounted on a wall or similar structure should be mounted at a height above the floor or any working surface to minimize the risk of mechanical damage, including damage that may occur to the flexible cord caused during insertion or withdrawal of the plug. Note: there are also requirements for the mounting height of accessories in Part ‘M’ of the Building Regulations (2) Where portable equipment is likely to be used, provision should be made so that the equipment can be fed from an adjacent and conveniently accessible socket outlet, taking into account the length of flexible cord normally fitted to appliances and luminaires.
(1) They are used for: Underfloor electric heating in dwellings and other premises (the requirements of Section 753 must also be met in this case) Heating of the playing area at open-air sports stadiums Heating roads and pavements to prevent icing Soil warming in agricultural and horticultural premises Industrial heating applications Trace heating of pipes and vessels. (2) Section 753 When installing heating conductors and cables care must be taken to prevent mechanical damage, damage from corrosion, thermal effects to adjacent material and to ensure the system temperature is not exceeded
Where a step-up transformer is used, a linked switch must be provided for disconnecting the transformer from all live conductors of the supply.
• Equipment of the owner or operator of a system for distribution of electricity to the public (distributors) • high voltage signs supplied at low voltage (such as neon tubes) • signs and luminous discharge tube installations operating from a no-load rated output voltage exceeding 1 kV but not exceeding 10 kV ( BS EN 50107 ) • temporary festoon lighting
Luminaire suitable for direct mounting on non-combustible surfaces only Luminaire suitable for direct mounting on normally flammable surfaces Luminaire with limited surface temperature Electronic convertor for an extra-low voltage lighting installation Transformer –short-circuit proof (both inherently and non-inherently)
(1) the maximum power dissipated by the lamp (2) the fire resistance of adjacent material (3) the minimum distance to the combustible materials, including those in the path of spotlight beams
A ceiling rose to BS 67 A batten lampholder A luminaire to BS EN 60598 A suitable socket-outlet to 1363-2, BS 546 or BS EN 60309-2 A plug-in lighting distribution unit to BS 5733 A connection unit to BS 1363-4 Appropriate terminals enclosed in a box A device for connecting a luminaire (DCL)
Where the fixing is intended to support a pendant luminaire it must be able to carry a mass of not less than 5 kg. If the mass of the luminaire is greater than 5kg, then the installer must ensure the fixing means can support the weight of the pendant luminaire. Remember the TV programme Only fools and horses (this was down to them releasing the wrong winch but it’s still a good story!)
16 A (2) Bayonet lampholders B15 and B22 should comply with BS EN 61184 and have the temperature rating T2 (lamp cap temperatures up to and including 210 0 C) (3) Except for E14 and E27 lampholders to BS 60238 , circuits connected to TN or TT systems must have the outer contact of every ES or centre bayonet cap type lampholder connected to the neutral conductor. This requirement also applies to track mounted systems.
Through wiring is only permitted if the luminaire is designed for that purpose. For luminaires complying with BS EN 60598 , but with no temperature markings, heat resistant cables are not required. Luminaires to this standard with temperature marking will require suitable heat resisting cables. Where no information is provided, heat resisting cable (or conductors insulated in accordance with Regulation 559.6.2.2) must be used.
(1) 6 mm 2 (and not less than the supply neutral) (2) 5 seconds (3) 2.50 m
It is normally the case that safety service systems will be required to operate at times of “mains failure” and also continue their effective operation through the harsh environment of a fire condition. It is imperative that these potential life saving installations have adequate consideration given for their design, installation and continued verification (via regular inspection and testing) to ensure that they have and maintain, the appropriate level of operational integrity.
The integrity of the electrical source is of paramount importance, and as such must be of appropriate capacity capable of supplying the total load and installed as fixed equipment, located in an area that is only accessible to appropriate personnel. The majority of safety sources take the form of either a dedicated, constantly charged battery, or a combination of battery and generator set. However, the less common system of separate independent feeder supplies is also recognized, providing appropriate assurance is obtained from the supplying network or networks that these supplies are unlikely to fail concurrently. Exercise. (1) storage batteries primary cells generator sets independent of the normal supply a separate feeder of the supply network effectively independent of the normal feeder.
Short break. An automatic supply available between 0.15 s and 0.5 s Medium break. An automatic supply available between 5 s and 15 s
The required minimum operational design life of batteries should be in accordance with BS EN 50171 , with a minimum declared life of (1) 10 years for central power supply sources and (2) 5 years for low power supply sources.
