Time switches explained: how to wire and set a DIN rail time switch

Short answer: A time switch switches a circuit on and off automatically at times you set, every day or on chosen days of the week. It usually mounts on the DIN rail in the consumer unit. Typical uses are garden and shop-window lighting, signs, circulation pumps, water heaters and ventilation. On analogue (mechanical) models you set it by pushing the tappets around the dial in 15-minute steps; digital models offer weekly programmes and finer settings. For large loads or many LED fixtures, the time switch should drive a contactor rather than the load itself.

Time switch, timer relay and photocell

• Time switch: works by clock and calendar, for example "on at 18:30, off at 23:00 every day".
• Timer relay: waits or runs for a set time after an event (for example a staircase timer or a motor star-delta change-over). It does not know the time of day.
• Photocell (dusk sensor): switches on when it gets dark and off at daylight. It follows the seasons automatically but cannot switch off at midnight.

For garden and sign lighting the two are often combined: the photocell switches on at dusk and the time switch switches off at a set time at night.

Analogue or digital?

• Analogue (mechanical): the dial turns once in 24 hours and the tappets around it set the on periods. Simple and robust, with no display or menu. The smallest step is usually 15 minutes.
• Digital: weekly (even yearly) programmes, minute accuracy, several channels, holiday mode and, on some models, an astronomical programme that calculates sunrise and sunset for your location.

What to check on the label

• Programme type: daily (24 h) or weekly.
• Minimum switching step: e.g. 15 minutes.
• Power reserve: how long the clock keeps running during a power cut, so short outages do not shift the programme.
• Contact: current rating and contact type (NO: normally open; whether it is potential-free).
• Module width: space taken in the consumer unit (1 module = 17.5 mm).

Example: the Legrand 4 127 90 analogue time switch has a daily programme, 15-minute steps and a 100-hour reserve; it is supplied at 230 V, its potential-free NO contact switches 16 A at 250 V and it is one module wide.

How to wire it

A time switch has two separate parts: the supply that runs the clock and the contact that switches the load.

1. Switch off the relevant circuit breaker and confirm by measurement that the circuit is dead.
2. Supply: connect a live from a breaker output and the neutral from the consumer unit to the clock's L and N terminals.
3. Contact: a potential-free contact is not powered internally. The live from the breaker goes to one side of the contact; the switched live leaves the other side to the load (lamp or contactor coil). The load's neutral comes straight from the consumer unit.
4. For large loads the contact drives a contactor coil and the lamps are fed through the contactor's main contacts.

Terminal numbers vary by brand; always wire to the diagram printed on the product, and have consumer unit work done by a qualified electrician.

When is a contactor needed?

A contactor protects the time switch's small contact and switches large loads safely. Use one when:

• the total load approaches the contact rating;
• many LED fixtures or floodlights are connected, because LED drivers draw a short high inrush current at switch-on that can weld the contact over time;
• motors, pumps or heaters are switched;
• a three-phase load is switched.

How to set an analogue time switch

1. Turn the dial until the current time lines up with the marker.
2. Push the tappet of every 15-minute segment during which the light should be on (e.g. 18:30–23:00).
3. If there is a selector, set it to automatic. Most models also have permanent on and permanent off positions.
4. The device needs to stay powered for a while to charge its reserve; check the programme once on the first day.

Typical uses

• Garden and facade lighting: on at dusk with a photocell, off at midnight with the time switch.
• Shop windows and signs: lit after closing time until a set hour.
• Shared areas and car parks of residential complexes.
• Circulation pumps, water heaters, ventilation fans.

• → Time switches
• → Garden lighting guide
• → Facade lighting guide
• → Street and site lighting
• → Motion sensor lighting
• → Home circuit breakers and fuse box