Testing & Teaching
Changeover Switch
In short
A changeover switch has three terminals: the common (COM), normally open (NO) and normally closed (NC). Its terminals: COM, NO, NC.
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 CHANGEOVER SWITCH has three terminals: the common (COM), normally open (NO) and normally closed (NC).
At rest, COM is joined to NC. Operate it, and COM lets go of NC and joins NO. Click it with Select to operate it.
It carries whatever is put on COM: it is a VOLT-FREE switch.
Most thermostats and every programmer channel are one of these. A plain room thermostat (stat), a frost thermostat and a 2-port valve's end switch are single contacts: a changeover with one way out unused.
Terminals
| Terminal | What it is for |
|---|---|
| COM (Common) | The common (COM): the way in. |
| NO (Normally Open) | Normally open (NO): joined to COM when the switch is operated. |
| NC (Normally Closed) | Normally closed (NC): joined to COM at rest. |
What the meter reads
Feed the common (COM), and put a lamp on each contact: one lamp is always lit, never both.
Where it connects
Lessons that use it
- How a switch works: COM, NO and NC
Foundation · Reading a circuit
- Reading a valve
Foundation · Reading a circuit
- The colours, and sleeving
Foundation · Electricity for heating controls
- Permanent and switched lives
Foundation · Electricity for heating controls
- Why we work dead
Foundation · Safe isolation and the earth
- Prove dead
Foundation · Safe isolation and the earth
- The 2-port valve
Foundation · The parts
- The 3-port valve
Foundation · The parts
Also in testing & teaching
Practise it in the simulator.
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