Page 665 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 11 Electrical principles and processes for building services engineering

Unfortunately, you will not always be working on a new type of consumer
unit. Figure 11.14 shows an old consumer unit which is still very common in
older buildings/dwellings. Sometimes it can be unclear how to isolate specific
circuits so you should stop work and seek expert advice. If in doubt a qualified
electrician must always be called to carry out the work.

Types of wiring used in electrical systems
All cables that run throughout a dwelling will have copper inside – copper is one p Figure 11.14 Old fused
of the best metals available for keeping resistance to a minimum. It’s very cheap consumer unit
in comparison with silver, which also has a very low resistance.

Thermoplastic (PVC) cables (twin and cpc)
‘Twin and cpc cable’ is normally the grey-coloured cable you will see when you
remove floorboards to install pipe runs. It comprises two insulated current-
carrying conductors and an uninsulated protective conductor (earth). Line and KEY TERM
neutral are individually insulated, with different colours for identification – the
protective conductor is bare but is still mechanically protected by the main grey Sheath: insulation around
PVC sheath. The protective conductor (cpc) does not have a PVC covering in the main copper cable,
order to keep the manufacturing cost down. Anyone working on the circuit must made from PVC plastic.
ensure that the bare protective conductor is sleeved to enable identification.
Twin and cpc cable comes in many thicknesses and in single lengths up to 150 m
long. The most common sizes in domestic installations are 1.5 mm for lighting
2
and 2.5 mm for power circuits, but you can get a large cable for electrical
2
shower units in domestic dwellings up to 10 mm in size. This needs to be large
2
to accommodate the draw of electricity (up to 10.5 kW) from the consumer p Figure 11.15 Old twin and cpc
unit to the shower unit. A smaller cable would be liable to melt due to the heat cable
produced and risk starting a fire.

Flex and heat-resistant flex cables
Flexible (sometimes shortened to ‘flex’) cables and heat-resistant flexible cables
are designed – as the name suggests – to be flexible. (As opposed to flat profile p Figure 11.16 New colour code
cable, which is sturdier, and is designed to remain where the electrician places for twin and cpc
it.) Flex cable looks like the kind of cable you would find on an appliance. Flex
cables are primarily used to connect appliances to fused connection units – for
example boilers, control valves and macerators.
Heat-resistant flex can be used as standard flex, but it is particularly suitable
in high-temperature zones – for example in an airing cupboard to connect the
immersion heater. These cables look very similar, so you will need to read the
information on the cable itself, which will show the temperature it can withstand.
p Figure 11.17 Heat-resistant
cable
Types of cable protection

When installing cables, you must ensure that all your hard work is not wasted KEY TERM
by following the relevant regulations. For example, sometimes a cable cannot Chased: in the case of
be chased in a wall or under the floor, and in this case you would have to use pipework or cables, this
either conduit or trunking to protect it. Cables must always be protected when means they are fitted inside
buried inside walls. a cut made in a wall.

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