Ask the experts/Technical
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Fully charged Cables to BS 8436 Protective devices for circuits wired with BS 8436 (foil screened) cable must meet certain requirements given in that standard. This subject was previously covered in Issue 171 of Connections (Autumn 2009). However, a new edition of BS 8436 was published in 2011. This contains revised requirements for the protective devices, which are covered in this article.
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oil screened cables to BS 8436 are listed Fig 1 Example of a fourin Regulations 522.6.101 and 522.6.103 core BS 8436 cable of BS 7671 as one of the types of cable incorporating an earthed metallic covering that may be: • concealed in a wall or partition at a depth of less than 50 mm from the surfaces, without being run Uninsulated circuit in one of the zones identified in indent (v) of protective conductor Regulation 522.6.101 and without having to be (to be sleeved green-and-yellow provided with additional protection by an RCD for at terminations) the purposes of Regulation 522.6.102, or • concealed in a wall or partition having an internal Metallic foil screen construction that includes metallic parts (other than fixings such as nails, screws or the like), without having to be provided with additional protection by an RCD for the purposes of indent (vi) of Regulation 522.6.103. Oversheath The protective device for a circuit wired with BS 8436 cable must be selected to meet certain conditions given in that standard (and detailed later in this article). This is necessary to prevent the metallic foil screen of the cable from burning away from a nail or screw, or similar that has penetrated the cable and made contact with the line conductor, as this could leave the nail or screw at line conductor voltage (typically 230 V to Earth), posing a risk of electric shock. Description of cable Cables conforming to BS 8436 (see Fig 1) are rated at 300/500 V and are intended for use concealed in walls, partitions and building voids. They are available in two-core, three-core and four-core, with conductor sizes of 1 mm2, 1.5 mm2, 2.5 mm2 and 4.0 mm2, and have
Fig 2 Example of marking denoting energy limiting class number
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Autumn 2012 NICEIC Connections
42_technical cables to BS8436.CC2.indd Sec1:42
low emission of smoke and corrosive gases when affected by fire. The cables incorporate a metallic foil screen that is in contact with an uninsulated protective conductor within the cable. The protective conductor has the same nominal cross-sectional area as the insulated conductors. Requirements of BS 7671 relating to automatic disconnection of supply When a nail, screw or similar penetrates a BS 8436 cable and comes into contact with a line conductor, the connection formed by the nail between the line conductor and the foil screen allows earth fault current to flow to operate the circuit protective device to disconnect the supply. For the protective device to operate within the maximum time permitted by BS 7671, the earth fault loop impedance (Zs) in the circuit must not exceed the limiting value for the device, as required by Regulation 411.4.5 (TN system), or 411.5.3 and 411.5.4 (TT system). Selection of the circuit protective device for a BS 8436 cable To prevent the screen burning away from a nail, screw or similar that has penetrated the cable, the circuit protective device must have characteristics given in a), b) and c) below, which are specified in clause 12 of BS 8436: 2011. a) The protective devices must be either Type B circuit-breaker to BS EN 60898 or a Type B RCBO to BS EN 61009-1. b) The rated current of the protective device must not exceed: • 16 A for the 1 mm2 cable, • 20 A for the 1.5 mm2 cable, • 32 A for the 2.5 mm2 cable when forming part of a ring final circuit (with or without non-fused spurs), or • 32 A for the 4.0 mm2 cable. c) The maximum let-through energy (I2t) of the device must not exceed: • 42,000 A2s for the 1 mm2 and 1.5 mm2 cables, or • 60,000 A2s for the 2.5 mm2 and 4.0 mm2 cables. Condition c) is likely to be met if the Type B circuitbreaker or RCBO is of energy limiting class 3 and the prospective earth fault current in the circuit does not exceed 5,000 A anywhere in the circuit. This corresponds to an earth fault loop impedance (Zs) of not less than 0.046 Ω at the origin of the circuit, at a nominal line-to-Earth voltage (U0) of 230 V. The energy limiting class is sometimes marked on the circuit-breaker or RCBO by the method shown in Fig 2. The protective device must also be selected in the normal way in accordance with BS 7671 for automatic disconnection of supply (see earlier) and protection against overcurrent (Chapter 43).
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01/10/2012 09:20