Switched live and permanent live: what each one does in a heating system

Two kinds of live

The LIVE (L) brings the electricity to a part. The standards’ word for it is LINE. In a heating system it comes in two kinds, and every terminal wants one or the other.

• A PERMANENT LIVE has 230 V on it all the time the fused spur is on, whatever the programmer and the thermostats are doing. On a wiring centre numbered the common way, it is block 1.
• A SWITCHED LIVE (SL) has 230 V only while a switch before it is MADE: closed, so the live can go through it. The rest of the time it reads 0 volts (0 V).

Most heating controls are switches, so most of what they give out is a switched live. A programmer’s ON terminal, a thermostat’s call and a zone valve’s orange core are all switched lives.

The two names describe what a terminal carries, not the terminal itself. A switched live can read 0 V now and 230 V a minute later, on the same screw.

Which parts need which

Each part needs the right kind of live on the right terminal. The rule of thumb: permanent for what must always be ready, switched for what should run only when asked.

Put a permanent live where a switched one belongs, and that part runs all the time. A valve’s brown on block 1 would hold the valve open whatever its thermostat said. A thermostat fed from block 1 would ignore the clock, so the heating could never be timed off.

Put a switched live where a permanent one belongs, and the part stops whenever that switch opens. A programmer fed that way would go dead every time that switch opened.

The boiler’s switched live

A boiler’s SL terminal is an input. A live there is the signal to FIRE: to light its burner. Where that live comes from depends on the system.

• S-Plan: the zone valves’ orange cores, on block 10, once a valve is fully open.
• Y-Plan: block 8, where the cylinder thermostat’s call and the 3-port valve’s orange meet.
• Combi: the room thermostat’s call, on the boiler’s room-thermostat terminals. Makers name these in their own ways, so read the boiler’s manual. Some combis run that input at a low voltage and take a volt-free contact instead. See volt-free or mains-switched.

A heat-only boiler with a pump overrun takes both kinds. Its L takes a permanent live, so it can keep its pump running after the call ends. Its SL takes the switched live that tells it to fire. See pump overrun. Whether a boiler needs a permanent live at all, its manual says.

When a boiler will not fire, its switched live is one of the points the meter reads. See the call-out for a boiler that will not fire.

Controls work stops at the terminals the boiler’s manual gives for the controls. If reaching them means opening the boiler’s sealed case, or touching anything inside it, that is a job for a Gas Safe registered engineer.

There is no switched-live colour

No core colour means “switched live”. In UK fixed wiring today, BROWN is the usual live, BLUE the neutral, and GREEN-AND-YELLOW the earth. A switched live is a live, and it may run in brown. It may also run in:

• the grey or black core of a three-core-and-earth cable;
• the blue core of a twin-and-earth cable run to a switch, bringing the live back.

Grey and black are today’s line colours too. A blue used as a live looks like a neutral, so SLEEVE it brown at each end. Sleeving grey and black brown is good practice, not a rule: it tells the next engineer what the core carries.

Older wiring, from before the colours changed in 2004 to 2006, used red for the live and black for the neutral. So in an older home a black core can be either. A colour says what a core may carry, never what it does carry. You find that by test.

Telling them apart with the meter

These are live tests: only for a competent person, or a learner under the supervision of one, with test leads made to the HSE’s guidance note GS38.

Put the black probe on a neutral you know is good, such as the supply’s neutral at the spur or on block 2. Put the red probe on the terminal. Then work the control that should switch it.

• A permanent live reads about 230 V whatever you do with the controls.
• A switched live follows its switch: about 230 V while the switch is made, 0 V once it opens.

So 0 V on a switched live is not a fault on its own. First check what should be switching it: is the programmer on, and is the thermostat calling?

Dead means both kinds dead

A switched live that reads 0 V because the programmer is off is not isolated. The next timed switch-on can make it live.

So before any work, switch off at the point of isolation and lock it off. Then prove dead the permanent lives and every switched live too. Where you can, set a demand before you isolate, so that a switched live reading nothing afterwards means something. See safe isolation in five steps.

Sources

• The Electricity at Work Regulations 1989, regulations 12 to 14 (isolation, and working on or near live conductors) and 16 (competence).
• HSE guidance note GS38, Electrical test equipment for use on low voltage electrical systems.
• The DrawWarm Foundation lessons on permanent and switched lives and on core colours, and the makers’ installation instructions for each part.