Abstract: An inductive coupler (1) comprises a rectifier (2) to rectify an input alternating current power supply, a high frequency oscillator (3) driven by the rectified power supply, a modulator (4) to modulate the output of the high frequency oscillator (3) with a lower frequency alternating signal, a split transformer (6) having physically separable primary and secondary sections, and a demodulator (7). The output of the modulator (4) is coupled to the primary section of the transformer (6), and the secondary section of the transformer (6) is coupled to the demodulator (7) which demodulates the modulated high frequency signal to produce a low frequency power output. An auxiliary connector (8) connects a reference from the low frequency modulation signal imposed by the modulator (4) to the demodulator (7) to maintain synchronism.

Abstract: An explosive device comprises a charge of explosive material which can be fired by feeding power from a firing circuit to an igniter circuit which includes an electrically-energizable fuse. In order to check the integrity of the igniter circuit, a monitoring circuit feeds a continuous stream of electrical pulses to the fuse circuit at such a low energy level that the pulses cannot cause ignition of the explosive charge, but they are sufficient to cause a light-emitting diode to emit short bursts of light. The production of these bursts is monitored by a receiver circuit via an optical fibre, and provided that corresponding signals are received continuously by a timer, a lamp indicates that the igniter circuit is ready for operation. If there is a break in the received signals, for example due to open-circuiting of the fuse, an alarm is operated. The connections between the igniter circuit and the monitoring circuit may be made via a demountable transformer.