Power Supply
Immersion Spur (the immersion heater's own supply)
In short
The same fused connection unit (FCU) as the Fused Spur, on a silver plate marked 3 kW. Its terminals: L, N, E.
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
The same fused connection unit (FCU) as the Fused Spur, on a silver plate marked 3 kW. It feeds an IMMERSION HEATER: an electric element that heats the water in the hot water cylinder.
A 3 kilowatt (3 kW) immersion heater draws about 13 amps (13 A). So this spur has a 13 A fuse, and a circuit of its own from the CONSUMER UNIT, the home's fuse box.
It is never fed from the heating controls' 3 A spur.
Terminals
| Terminal | What it is for |
|---|---|
| L | The live (L) out to the immersion heater. It runs through one side of the switch, then through the 13 amp (13 A) fuse. |
| N | The neutral (N) out. It runs through the other side of the same switch. |
| E | The earth (E) out. It goes straight through: it is never switched and never fused. |
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.
Switched on, its live (L) and neutral (N) feed the immersion heater's flex.
On site the earth is never switched. In this simulator the spur is the supply itself: switched off, it gives no live, no neutral and no earth. The earth path is still there: with the power off, a continuity test proves it.
Switched off, the immersion heater is dead, and the heating controls keep their own supply. So there are two supplies and two POINTS OF ISOLATION: switch off both before work on the cylinder.
Where it connects
Lessons that use it
- An immersion heater on its own circuit
Intermediate · Protection and extras
Also in power supply
Practise it in the simulator.
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