Chapter 6 - Level 2 NVQ and Technical ... - Pearson Schools

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NVQ2 Electrical Installations Remember The more chance there is of danger, the greater the precautions you will need to take 146 For this system to be effective the supply must be run through a safety isolating transformer. This device has no connection to earth, so the installation must meet the requirements of BS 7671 for a SELV (separate extra low voltage) supply. This method of protection is used when there is an increased risk of an electric shock due to body resistance being reduced, such as in damp and wet conditions. Protection against direct contact Measures to be taken to prevent electric shock by direct contact include (Regulation 412): (i) protection by insulating live parts (ii) protection by barriers or enclosures (iii) protection by obstacles, so preventing access (iv) protection by placing out of reach (v) IP codes/ratings are applied to electrical equipment to create an industry standard. These are covered below. RCDs (residual contact devices) may be used as a supplementary means of shock prevention from direct contact, but only in addition to the measures listed in (i) to (v) above. We will look further at RCDs in chapter 12, pages 307–308. Barriers or enclosures All live electrical components should be within enclosures. In some cases they will also be protected by barriers, preventing easy access from the public. In order to make sure that these requirements are followed correctly, an index of protection codes is applied to electrical equipment enclosures to create an industry standard. The first number in the code relates to the protection against direct contact in terms of the degree of accessibility, e.g. IP2X means that there is no contact with a probe of more than 12mm in diameter and less than 80mm long (approximately the length of a human finger). IPXXB is as above but for probes up to a maximum of 80mm. IP4X is used for protection against wires of 1 mm in diameter. The horizontal top surface of the barrier or enclosure, which is readily accessible, should be protected to IP4X. Protection against indirect contact �� In this situation, conductive parts, such as metalwork, have become live due to fault conditions. The potential voltage on this metalwork rises above that of earth and electric shock results when a person touches the metalwork. Measures taken to prevent electric shock by indirect contact include (Section 413 in BS 7671) the following. (i) Earthed equipotential bonding and the automatic disconnection of the supply (EEBAD).
(ii) Use of class II equipment and/or equivalent insulation. Class II usually relates to portable equipment or factory-built equipment. Equipment to this class must not have enclosing metalwork earthed. Furthermore for a class II installation, close supervision is required to ensure the installation or its equipment is not changed from class II. The symbol on the equipment that indicates that it is class II or double insulated is a square within a square. (iii) Non-conducting location, i.e. physical separation from exposed conductive parts, no earth connection. (iv) Earth free local equipotential bonding. Every part is bonded and therefore ‘no voltage difference – no shock’. (v) Electrical separation. A transformer fed secondary is not connected to earth or an isolated generator supply is used. Methods (iii) to (v) have a limited application, as close supervision is required. The design must be specified by a qualified electrical engineer. The basic limitation of these methods is the need to maintain their design condition. There should be no incorrect modifications, additions or deterioration. Overcurrent protection Protection devices In a healthy electrical system, insulation between phase conductors and neutral conductors or between live conductors and earth is sound, with current flowing from the supply, through the load and returning via the neutral to its source. Overcurrent protection is provided by means of a circuit breaker or fuse. These devices are designed to operate within specified limits, disconnecting the supply automatically in the event of an overload or fault current (short circuits or earth faults). Earth-fault protection is also provided by means of a circuit breaker or fuse, which operates under earth-fault conditions. If a high value current flows, these will safely disconnect the supply but may not be relied upon to detect and disconnect the supply in the event of low values of earth leakage current. Residual current devices (RCDs) are designed to detect and automatically disconnect the supply in the event of earth-faults. Earth-faults can exist, for example, when a live conductor touches an equipment case or if you cut through a cable when cutting the grass. They are potentially dangerous and must be eliminated in order to prevent hazards resulting from electric shock or abnormal temperature rises and fires in the electrical installation. Overcurrent protection device applies to fuses and miniature circuit breakers (MCBs). In this section we will be looking at the various means by which these devices provide protection to cables and circuits from damage caused by too much current flowing. The appropriate device must operate within 0.4 or 5.0 seconds. Chapter 6 Basic electrical circuitry and applications 147
Page 1 and 2: Basic electrical circuitry and appl
Page 3 and 4: The size of conductor used in a cir
Page 5 and 6: Insulation colours To identify cabl
Page 7 and 8: that particular circuit. The neutra
Page 9 and 10: If we now operated the switch, the
Page 11 and 12: However, by operating the intermedi
Page 13 and 14: Neutral Phase cpc replace the exist
Page 15 and 16: The requirements for a standard dom
Page 17 and 18: floor area maximum 50 m 2 Figure 6.
Page 19 and 20: ● ● Whether placed in thermal i
Page 21 and 22: Moisture Water and electricity do n
Page 23 and 24: Exposed and extraneous conductive p
Page 25 and 26: Results of a bad earth A bad earth
Page 27: Shock protection BS 7671 classifies
Page 31 and 32: FAQ Q What’s the difference betwe

Chapter 6 - Level 2 NVQ and Technical ... - Pearson Schools

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