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Fig. 4 x 8008Keyset KEM 3294placed beside switchboardplacing of indicators, it is a general fact that a steady light is far less apt tocatch the attention than a flashing light, so that steady light signals shouldpreferably be combined with acoustic signals. The buzzer should sound onlywhen the lamp lights, and not continue while the light is on.Keyset and lamp relay setA keyset (fig. 4) for initiating the desired signal on the lamp indicators isplaced at the switchboard. Every signal is represented by a 1, 2 or 3-digitnumber, the keyset having nine keys for this purpose. The keying of a numbersets up the corresponding lamp combination in a separate lamp relay set(fig. 5), and when the operator thereupon presses key A or B, the signal isdisplayed on all lamp indicators. Key A produces a steady, and key Bflashing, light. If buzzers are installed, both keys, by being pressed fully down,operate the buzzers as well. The signal is cancelled and the keys restored bypressing key O between keys A and B.Power requirementsA mains transformer supplies 48 or 24 volts a.c. for operation of thelamp indicators and at the same time, via a rectifier in the lamp relay set.feeds the d.c. circuits in the keyset and lamp relay set. A 48 volt supply isnormal in systems incorporating vertical type indicators, but the lattercan be fitted with lamps for 24 volts if required. The circular indicator isintended solely for the lower voltage. So. too, are the buzzers, irrespective ofthe type of indicator in which they are mounted.CablingFive separate conductors and a common return circuit are required betweenthe lamp relay set and lamp indicators. If buzzers are desired, one additionalconductor must be installed. A suitable cable is ordinary 12-wire P. V. C.