This article provides guidance for the electrical contractor when the installation design calls for screened cables to BS 8436: 2011 to be installed. In addition to meeting the requirements of BS 7671, certain conditions given in BS 8436 must be met when installing this particular type of cable.
Cables conforming to BS 8436: 2011. Electric cables – Specification for 300/500 V screened electric cables having low emissions of smoke and corrosive gases when affected by fire, for use in wall, partitions and building voids – Multicore cables are primarily intended for use in walls, partitions and building voids.
Cables conforming to BS 8436 have low emissions of smoke and corrosive gases when exposed to fire and are available with 2, 3 and 4-cores having conductor sizes of 1.0 mm2, 1.5 mm2, 2.5 mm2 and 4.0 mm2. A number of manufacturers produce cables having larger cross-sectional areas, for example, 6 mm2 and 10 mm2. In such cases, it will be necessary to refer to the specific manufacturers’ instructions in respect of their selection and use.
The cables incorporate a thin metallic foil screen which is in direct contact with an uninsulated protective conductor within the cable. Typically, the protective conductor has the same cross-sectional area (csa) as the insulated conductors.
Requirements
Cables conforming to BS 8436 are listed in Regulation 522.6.204 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 surface, without being run in one of the prescribed zones and without having to be provided with additional protection by an RCD, (indent (i) of Regulation 522.6.202).
● Concealed in a wall or partition having an internal construction which includes metallic parts, other than metallic fixings such as nails, screws and the like without having to be provided with additional protection by an RCD, (indent (i) of Regulation 522.6.203).
Automatic disconnection of supply (ADS)
Typically, the protective measure automatic disconnection of supply (ADS), in accordance with section 411 of BS 7671 is the intended means of protection against electric shock.
Where this is the case, when a sharp metallic object such as a nail or screw penetrates the foil screened cable, a connection is formed automatically between the inner foil screen, including the cable’s protective conductor, and the internal live conductors. As a result a fault current will flow which should cause the protective device to operate within the required time.
In order for the protective device to operate within the maximum permitted time of Table 41.1 or those stated in Regulation 411.3.2.3 and 411.3.2.4, the earth fault loop impedance (Zs) for the circuit must not exceed the maximum earth fault loop impedance values for the protective device given in Tables 41.2 to 41.4. However, where cables to BS 8436 are used, only Table 41.3 shall apply, as discussed below.
Overcurrent protective device for a BS 8436 cable
In addition to meeting the requirements of BS 7671, the overcurrent protective device for a circuit wired with BS 8436 cable must be selected to meet the particular conditions given in that standard. This is necessary in order to prevent the metallic foil screen of the cable from burning away during the penetration of the cable by a nail, screw or similar, making simultaneous contact with a live conductor. This could leave the nail or screw potentially at a line conductor voltage (230 V to Earth), creating a shock risk.
The protective devices used for cables conforming to BS 8436 shall be either a:
● Type B circuit-breaker to BS EN 60898 of energy limiting class 31 , or
● Type B RCBO to BS EN 61009 of energy limiting class 3.
This precludes the use of any other type of protective device where cables to BS 8436 are utilised.
Furthermore, the circuit protective device must have the following characteristics, specified in Clause 12 of BS 8436: 2011:
(i) the maximum energy let-through (I2t) of the device must not exceed 42,000 A2s for cable sizes of 1.0 mm2 or 1.5 mm2, and 60 000 A2s for a cable size of 2.5 mm2 or 4.0 mm2, and
(ii) the maximum current ratings used must not exceed the values detailed in Table 1.
The let-through energy of a device can be obtained using the time-current characteristics found in Appendix 3 of BS 7671. From this characteristic, the operating time (t) of the device, in seconds, corresponding to the value of prospective fault current for the circuit (If), in amperes, is found. The value of let-through energy can then be calculated by multiplying the square of (If) by (t) to give (If2t).
However, for fault currents of a very short duration causing the device to operate in less than 0.1 s, the time current characteristic curves in Appendix 3 of BS 7671 are not applicable. Alternative information regarding the let-through energy (I2t) of the device should be obtained from the manufacturer or product standards (Regulation 434.5.2).
Conditions (i) and (ii) above are likely to be met where the protective device is a Type B circuit-breaker to BS EN 60898 or a Type B RCBO to BS EN 61009, having an energy limiting class 3, as shown Fig 2.
Installation of cable
Whilst BS 7671 provides requirements for the safe installation of electrical installations, Annex A of BS 8436 provides additional guidance on certain installation practices necessary when working with cables to this standard.
Cables to BS 8436 are designed to be installed in air, or closed systems including trunking, in thin partitions and building voids. Where installation falls outside of these reference methods, the cable manufacturer’s guidance should be sought.
Summary
When using foil screened cables to BS 8436: 2011, consideration must be given to the maximum size of conductor in relation to the nominal current rating of the protective device as mentioned in its standard, in addition to the requirements of BS 7671.
Where it is expected in a circuit that under fault conditions a high level of prospective fault current may exist due to low values of earth fault loop impedance, there may be a greater risk of damage to the metallic foil screen.
Type B circuit-breakers to BS EN 60898 or Type B RCBOs to BS EN 61009-1 must be used to provide overcurrent protection for cables to BS 8436. No other means of overcurrent protection is acceptable.
References
1 The energy limiting class of an overcurrent protective device is the (I2t) characteristic; a measure of energy let-through under fault conditions occurring within a time period of less than 0.1 s. This characteristic is used typically to obtain selectivity between upstream and downstream overcurrent protective devices.
2 Cables with a csa greater than 4.0 mm2 are not considered in BS 8436. For cables having a larger csa, it will be necessary to refer to the specific manufacturer’s instructions in respect of their selection and use.
3 Maximum current ratings may need to be further reduced depending on installation method and correction factors.
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