CLOCK SYSTEM

These clock system are used where a number of clock throughout a building are required to show the sometime, or else used to operate time-recorders for stamping time-card which indicate when work has been started or finished.

Most clocks found in small installation are independent units, run by a synchronous motor fed from mains voltage. Impulse-clock system are independent of mains and operate from extra-low voltage supplies. The master clock is the name given to the primary unit which controls all other clocks in the installation. It is pendulum-operated and has an impulse transmitter which transmits electrical impulses of alternate polarity at one-minute intervals over a two-wire circuit to the subsidiary or slave clocks. The slave which locks have movement which accept these impulses and alter their clock hands accordingly.

The mechanism of one type of master clock consists of a pendulum of half-second beat operated by an electrically-wound spring through a dead-beat escapement. 

At each one-minute interval, while a small synchronous motor is rewinding the main spring, an impulse is transmitted to the subsidiary clocks. The main a.c supply is transformed to 48V for operating the synchronous motor and again reduced and rectified to provide 24V d.c for the transmitted impulses. Should the mains supply to interrupted for any reason, the main spring has a sufficient reserve to operate the escapement movement and hands for about 10 hours through no impulse will be transmitted to the subsidiary clocks. The movement of a subsidiary clock is a one-minute polarized movement with a rotating armature, and incorporates a flywheel to render the hands dead-beat.

The usual master-and-slave installation can cater for up to 60 clocks. To add clock to the system, it is only necessary to connect a clock in parallel with the remainder. The clock load and the connecting cables should total a certain value of resistance so that the farthest-away-clock has sufficient voltage at its terminals.

The impulse current is around 220mA. In series-impulse clock system, the voltage required for the installation is calculated as the total resistance of the clocks plus the line resistance multiplied by the impulse current of 220mA, i.e.,(R_C  + R₁) * 0.22A = voltage required.

 Sixty volt is the recommended maximum.Should the required operating voltage be above this, the installation should be sub-divided. As it is occasionally required to remove a clock from a series system, `shorting-blocks` are provided.