Importance

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Abstract

A signalling device applies a pulsating signal to supply lines when a head is removed from a base. A monitoring system includes a plurality of such signalling devices and is responsive to the pulsating signal to indicate the absence of a head from its base. When a pulsating signal is already on the supply lines, its waveform is preserved independently of the number of heads removed from respective bases. A timer is triggered either to ensure synchronisation of pulsating signals generated by two or more bases, or to prevent any second base from generating a pulsating signal, if a first base is already generating one. The timer can be an oscillator where a capacitor is prematurely discharged by a pulsating signal already present on the supply lines. The base can include additional means for inhibiting the pulsating signal when a voltage applied to the signalling device either exceeds a predetermined threshold, or changes in polarity, or both. An end of line device can be used to connect a load across the lines if a head is removed and to disconnect the load from the lines if the voltage rises above a threshold. A signalling device is also described which signals a fault if a head of one type is inadvertently fitted to a base which requires a different type. The invention can be used in fire detection systems.

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First Claim