Mechanical clock timer unit for controlling an immersion heater.
Wall mounted timer unit for controlling an immersion heater. This device would switch the heating element on at night, when electricity was available at a cheaper rate. The red button would switch the heater on at other times to provide extra hot water, but at the expensive daytime rate.
The white switch (bottom left) would normally be left on 'Auto', and only switched off when no hot water was required, such as when the property was unoccupied.
Internally, the device is a mechanical clock, a relay and connection terminals.
The fixed cable from the power supply entered from the wall behind via the round hole, bottom right. The cable to the immersion element was secured under the cable clamp (bottom centre). Typically this unit would have been mounted next to the hot water cylinder.
In this example, the clock is wrongly set. This would switch on at 4am and remain on throughout the day, only switching off at midnight. This is the reverse of the intended operation. The blue marker should be at 7am, so the water is heated between 4am and 7am - this being the end of the cheap rate period.
A relay can be seen towards the bottom of the picture. This was used to switch the heater, as the contacts in the timer mechanism are only suitable for switching a small load.
The orange plastic tubular item is a neon indicator, which illuminates when the heater is on.