The IHT/GPT timeswitch is a synchronous motor driven timer, which has been specifically designed to meet the requirements for controlling a standard 3 kilowatt immersion heater, of the type which can be fitted to a domestic copper hot water storage cylinder.

Within this timeswitch assembly, Grasslin make use of their FM/1 synchronous timeswitch module - a similar FM/1 is used in the STA200 plug-in timer. Hence the IHT/GPT timeswitch has some familiar features - 96 captive tappets which control the output switch for 15 minute periods. All tappets between the ON and OFF times must be moved to the outer edge of the dial. Tappets left at the inner edge of the dial produce an ‘OFF’ output. A three-position manual override is built in to the dial of the timeswitch, giving Timed, Fixed on, and Fixed off control. For ease of setting (and reading) the current time, this unit features clock hands. The IHT/GPT has a 24 hour dial. There is a seven day version, the IHT/GPW - the dial of this unit is divided into seven sectors, corresponding to the days of the week.

The IHT/GPT, and the seven day version, are BEAB (British Electrotechnical Approvals Board) approved for the specific use of controlling an immersion heater, which being a 3 kilowatt resistive load, takes a current of about 12.5 amps. The unit has therefore been tested by BEAB for this particular application. Hence the product is marked with a 13 amp current rating. The BEAB approval only applies to the product’s use as an immersion heater timer. The switch built into the FM/1 module however, is rated at 16 amps.

When fitting the timeswitch as an immersion timer, installation must be in accordance with the instructions supplied with the product, which have also been subject to the control (and approval) of BEAB. Features provided to facilitate installation as an immersion timer are cable clamping, and an earth ‘park’ terminal (the timeswitch is double insulated, and does not need an earth connection, but earth continuity must be provided for the immersion heater’s wiring, hence the term earth park).

The unit is termed Immersion/General Purpose Timeswitch, since it can control a variety of mains or other voltage loads up to the rated current. The FM/1 module has a volt-free changeover contact. This means that the switch opens and closes under the control of the dial with its’ tappets, or the manual override, but until a voltage is applied to the switch, an electric current cannot flow. When installing as an immersion timer (or for any mains switching application), there is a brass link already fitted between terminals 2 and 3, and this link should be left in place, since it is doing the job of connecting the incoming live supply to the ‘common’ contact of the switch. The advantage of providing the brass link, which can be removed, is that the timer can be easily converted to a volt-free version. This is appropriate for SPECIFICATIONS where 240 volt mains is available to run the timer, but a different voltage is required to be switched. Within industrial control panels it is commoplace to have voltages other than 240, either for safety reasons - a serious electric shock is unlikely to occur from a 110v supply. Or for control reasons - some components are only available in, for instance, 110, 48 or 24 volts because of the way they work, or because of their location.