Cables
Twin and Earth, 3-Core and Earth (flat cable)
In short
Twin and earth is a flat, grey cable with solid cores: each core is one solid copper wire.
If this is your home: this page is written for heating engineers, and for learners under the supervision of one. It explains how the wiring works; it is not training to do the work. In England and Wales, electrical work in a home must meet the Building Regulations (Part P), and some of it must be notified to building control or done by a registered electrician; Scotland and Northern Ireland have their own rules. For the wiring, call a registered electrician or a heating engineer competent in controls. Work inside a gas boiler is for a Gas Safe registered engineer only. If you smell gas, leave the building and call the gas emergency number: 0800 111 999 in Great Britain, 0800 002 001 in Northern Ireland.
What it is
TWIN AND EARTH is a flat, grey cable with SOLID cores: each core is one solid copper wire. It is for the FIXED WIRING: the supply to the fused spur, or a run to a programmer or a room thermostat (stat).
Its earth core is BARE copper. Where the bare core is connected, it wears GREEN-AND-YELLOW SLEEVING: right-click the core and fit it.
On the canvas it comes short, with its cores loose at both ends.
- Take a core with the Wire Tool, and click a terminal inside its reach. When both ends of a core are on terminals, that core is a wire.
- The Wire Tool also drags an end or a bend by its handle, and drags the sheath to move the whole cable.
- Bend adds a bend.
- Erase lifts a core off its terminal, or cuts back one that is not needed.
What the meter reads
Live tests. On site, any reading taken with the supply on is work on or near live parts. Regulation 14 of the Electricity at Work Regulations 1989 allows it only when working dead is unreasonable, as when a fault shows only while the controls work, and only with suitable precautions: test leads made to the HSE’s guidance note GS38, fingers behind the guards. Regulation 16 allows it only for a competent person, or a learner under the supervision of one. Wiring and repairs are always done dead. In the simulator, nothing is live.
Twin and earth: brown, blue, and the bare earth between them.
3-core and earth: brown, black, grey and the bare earth. The black and the grey may typically be switched lives, sleeved brown at each end.
The bare core, with its sleeving on at both ends, is an earth conductor like any other.
Where it connects
Practise it in the simulator.
Runs in the browser on a laptop or desktop. Cancel any time.