Power Supply
Fused Spur (fused connection unit)
In short
A fused spur is a switch and a fuse in one small box on the wall. Its proper name is a fused connection unit (FCU). Its terminals: L, N, E.
If this is your home: this page is written for heating engineers. Mains wiring and gas appliances are not DIY jobs; call a Gas Safe registered engineer. If you smell gas, leave the building and call the National Gas Emergency Service on 0800 111 999.
What it is
A FUSED SPUR is a switch and a fuse in one small box on the wall. Its proper name is a fused connection unit (FCU). The whole heating system is fed from it.
It is also the one place where the system is switched off, and locked off, before work.
Inside is a DOUBLE-POLE switch: it breaks the live (L) and the neutral (N) together. A FUSE sits on the live. For a gas boiler's controls it is 3 amps (3 A): the appliance's manual gives the size.
Terminals
| Terminal | What it is for | On the S-Plan example | On the Y-Plan example | On the Combi example |
|---|---|---|---|---|
| L (Live) | The live (L) out. It runs through one side of the switch, then through the fuse. | Wiring centre block 1 | Wiring centre block 1 | Combi Boiler L |
| N (Neutral) | The neutral (N) out. It runs through the other side of the same switch. | Wiring centre block 2 | Wiring centre block 2 | Combi Boiler N |
| E (Earth) | The earth (E) out. It goes straight through: it is never switched and never fused. | Wiring centre block 3 | Wiring centre block 3 | Combi Boiler E |
Read from the wired example circuits: S-Plan, Y-Plan, Combi boiler with a wireless thermostat. A wiring-centre block carries whatever is landed on it; the numbers are the habit the lessons teach.
What the meter reads
Switched on, its live (L) and neutral (N) feed the wiring centre, and from there the rest of the system.
On site the earth is never switched. Its earth (E) stays joined to the supply's earth, with the spur on or off.
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 live and the neutral are dead at every part the spur feeds. That is why it is the POINT OF ISOLATION: the place you switch off before work.
Call-outs with this part on the path
- Everything dead: no heating, no hot water, the controls dark
“Nothing works. No heating, no hot water, and the controls are dead.”
- Radiators hot with everything off at the timer
“The radiators get warm when nothing's asking for heat - it's all off at the timer and they're still hot.”
- Boiler fires with everything off, radiators cold
“The boiler keeps firing with everything off at the timer, but the radiators stay cold.”
- Hot water scalding: the cylinder keeps on heating
“The hot water's scalding - far hotter than it used to be, whatever the dial on the cylinder says.”
Where it connects
Lessons that use it
- The parts list and the canvas
Foundation · Using the simulator
- Wires: draw, colour, bend, erase
Foundation · Using the simulator
- The words, and why they are what they are
Foundation · Reading a circuit
- A circuit: source, path, load, return
Foundation · Reading a circuit
- The colours, and sleeving
Foundation · Electricity for heating controls
- The fused spur
Foundation · Electricity for heating controls
- Why we work dead
Foundation · Safe isolation and the earth
- Prove dead
Foundation · Safe isolation and the earth
Wiring diagrams it is part of
- S-Plan
S-Plan
- S-Plan with a digital room stat
S-Plan
- Wireless S-Plan
S-Plan
- S-Plan with a frost thermostat
S-Plan
- S-Plan Plus
S-Plan
- Y-Plan
Y-Plan
- Y-Plan with a frost thermostat
Y-Plan
- Combi boiler with a wireless thermostat
Combi
- Combi boiler with two heating zones
Combi
Also in power supply
Read the explanation
- Safe isolation in five steps: prove, confirm live, isolate, test every pair, prove again
Heating wiring is done dead. Prove the tester, confirm the supply is live, isolate and lock off, test every pair, prove the tester again. Why each step matters.
Practise it in the simulator.
Runs in the browser on a laptop or desktop. Cancel any time.