Cables
Heat-Resistant Flex (3 or 5 cores)
In short
Heat-resistant flex is a flexible cable whose insulation stands more heat than ordinary flex.
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
HEAT-RESISTANT FLEX is a flexible cable whose insulation stands more heat than ordinary flex. Choose it where the part gets hot: an immersion heater, a boiler or a pump.
It is drawn black here, so you can tell it from ordinary flex. On site, read the print on the sheath: the colour of a sheath is not its rating.
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.
It reads and works as ordinary flex does: a core with both ends on terminals is a wire.
3 cores: brown and blue may typically be the live (L) and the neutral (N), and green-and-yellow is the earth (E). That is enough for an immersion heater or a pump.
5 cores: brown, black, grey, blue and green-and-yellow. That is enough for a boiler that takes a permanent live, a switched live (SL) and a pump live (PL).
A motorised valve comes with a heat-resistant flex of its own.
Where it connects
Lessons that use it
- An immersion heater on its own circuit
Intermediate · Protection and extras
Practise it in the simulator.
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