On-site Guide

To assist electricians in their understanding of the regulations a number of guidance notes have been published. The guidance notes which I will frequently make reference to in this book are those contained in the On Site Guide. Eight other guidance notes booklets are also currently available. TTiese are ... 

The above examples show that the resistance of an aluminium cable is some 60% greater than a copper conductor of the same length and cross-section. Therefore, if an aluminium cable is to replace a copper cable, the conductor size must be increased to carry the rated current as given by the tables in Appendix 4 of the lET Regulations and Appendix 6 of the On Site Guide. 

In our own electrical industry, we have the On Site Guide and the Electricians Guide to the Building Regulations, both published by the lET. 

However, under fault conditions, the temperature and therefore the cable resistance will increase. To take account of this, we must multiply the value of cable resistance by the factor given in Table 13 of the On Site Guide. In this case the factor is 1.20 and therefore the cable resistance under fault conditions will be ... 

Table 3.1 Table II of the lET On Site Guide value of resistance/metre for copper and aluminium conductors and of R + R2 per metre at 20°C in milliohms/metres...

Reproduced from the lET On Site Guide by kind permission of the institution of Engineering and Teohnoiogy. 

Values of f i + f 2 have been calculated for copper and aluminium conductors and are given in Table II of the On Site Guide as shown in Table 3.1 of this book. 

Consumer units are now supplied which incorporate one or more RCDs, as shown in the On Site Guide Figs 3.6.3 (iv) to 3.6.3 (v). 

Figures 3.34 to 3.36 show circuits protected by RCBOs. The use of RCBOs wiii minimize inconvenience and disruption in the event of a fauit occurring, because oniy the fauity circuit wiii be disconnected by the RCBO. However, aiternative consumer unit arrangements using MCBs and RCDs are permissibie and are shown in the On Site Guide at Section 3.6.3.

Cg the grouping correction factor given in Tables 4C1 to 4C6 of the lET Regulations and Table 6C of the On Site Guide. 

Multicore cables having thermoplastic (PVC) or thermosetting insulation, non-armoured COPPER CONDUCTORS Table 4D2A of lET Regulations and Table F5(i) of the On Site Guide Ambient temperature SOX. Conductor operating temperature 70°C Current-carrying capacity (amperes) BS 6004, BS 7629... 

Where the cable is totally surrounded over a short length, the appropriate factor given in Table 52.2 of the lET Regulations or Table F2 of the On Site Guide should be applied. 

Let us solve this question using only the tables given in the On Site Guide. The tables in the regulations will give the same values, but this will simplify the problem because we can refer to Tables 3.3, 3.4 and 3.5 in this book which give the relevant On Site Guide tables. 

Appendix 4 of the lET Regulations (BS 7671) and Appendix F of the lET On Site Guide contain tables for determining the current carrying capacities of conductors which we looked at in the last section. However, for standard domestic circuits. Table 3.6 gives a guide to cable size. 

A limit must be placed on the number of bends between boxes In a conduit run and the number of cables which may be drawn into a conduit to prevent the cables from being strained during wiring. Appendix E of the On Site Guide gives a guide to the cable capacities of conduits and trunking. 

The tables in Appendix E of the On Site Guide describe a factor system for determining the size of conduit required to enclose a number of conductors. The tables are shown in Tables 3.8 and 3.9. The method is as follows ... 

The inner radius of a conduit bend should be not less fhan 2.5 times fhe outside diameter of the conduit. Adapted from the lET On Site Guide by kind permission of the Institution of Engineering and Technology. ... 

The ratio of the space occupied by aii the cabies in a conduit or trunking to the whoie space enciosed by the conduit or trunking is known as the space factor. Where sizes and types of cabie and trunking are not covered by the tabies in the On Site Guide, a space factor of 45% must not be exceeded. This means that the cabies must not fiii more than 45% of the space enciosed by the trunking. 

A small, single-point over-sink-type water heater may be considered as a permanently connected appliance and so may be connected to a ring circuit through a fused connection unit. A water heater of the immersion type is usually rated at a maximum of 3 kW, and could be considered as a permanently connected appliance, fed from a fused connection unit. However, many immersion heating systems are connected into storage vessels of about 150 litres in domestic installations, and the On Site Guide states that immersion heaters fitted to vessels in excess of 15 litres should be supplied by their own circuit (OSG Appendix H5). 

A cooker with a rating above 3 kW must be supplied on its own circuit but since it is unlikely that in normal use every heating element will be switched on at the same time, a diversity factor may be applied in calculating the cable size, as detailed in the On Site Guide. 

A cooking appliance must be controlled by a switch separate from the cooker but in a readily accessible position. Where two cooking appliances are installed in one room, such as split-level cookers, one switch may be used to control both appliances provided that neither appliance is more than 2 m from the switch (On Site Guide, Appendix 8). 

Table A1 in Appendix A of the On Site Guide deals with the assumed current demand of points, and states that for lighting outlets we should assume a current equivalent to a minimum of 100 W per lamp holder. This means that for a domestic lighting circuit rated at 5 A, a maximum of 11 lighting outlets could be connected to each circuit. In practice, it is usual to divide the fixed lighting outlets into a convenient number of circuits of seven or eight outlets each. In this way the whole installation is not plunged into darkness if one lighting circuit fuses and complies with Regulation 314.1 which tells us to divide into circuits to minimize inconvenience and avoid danger .

Because fault currents will flow around the earth fault-loop path, the measured resistance values must be low enough to allow the over-current protective device to operate quickly. For a satisfactory test result, the resistance of the protective conductor should be consistent with those values calculated for a line conductor of similar length and cross-sectional area. Values of resistance per metre for copper and aluminium conductors are given in Table 11 of the On Site Guide. The resistances of some other metallic containers are given in Table 4.2. 

The CPC for a ring final circuit is formed by a 1.5mm2 copper conductor of 50m approximate length. Determine a satisfactory continuity test value for the CPC using the value given in Table II of the On Site Guide. 

The On Site Guide describes the requirements for electrical installations ... 

Make sure you know what documents are required and expected in the exam (e.g. the On Site Guide or Wiring Regulations) ... 

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