Abstract: A method and a circuit arrangement (10) for the measurement of earth leakage in intrinsically safe current feeds to electronic control units (4) for underground mining installations and machines, whereby at least one earthed control unit is connected to the two intrinsically safe potentials of an intrinsically safe direct current supply (1) and at least one earth leakage sensor (20) is provided for earth leakage measurement with which steps of the periodical application of a test potential (Umess) from an intrinsically safe switchable test voltage source (21) to an RC-element (22), isolated from earth (8) by means of decoupling capacitors (C1, C2), measurement of the charging or discharging current behaviour of the RC-element (22) after each switching of the test voltage source (21) and comparison of the measured charging or discharging behaviour with stored reference data.

Abstract: The warning tone transmitter (10) according to the invention has a piezoelectric horn element (12) with two input connectors (29, 30) for the application of the supply voltage at a determined frequency and has a circuit arrangement (14) fed from the current supply, which generates the supply voltage. In order to be able to receive confirmation that the horn signal is actually generated by the warning tone transmitter (10) and in order to be able more easily to calibrate the warning tone transmitter (10) such that it is driven at its resonant frequency the piezoelectric horn element (12) has an additional output connector (34), which, when the horn element (12) is fed with the supply voltage, generates an answering signal taken to the circuit arrangement (14) if the piezoelement is actually oscillating. The magnitude of the answering signal is a value for the sound volume of the warning tone transmitter (10).

Abstract: The present invention relates to a method and to a circuit arrangement (10) for the measurement of earth leakage in the current feeds in the intrinsically safe current supply (1) to the electronic control units (4) for underground mining installations and machines, whereby at least one earthed control unit is connected to the two intrinsically safe potentials of an intrinsically safe direct current supply (1) and at least one earth leakage sensor (20) is provided for earth leakage measurement with which steps of the periodical application of a test potential (Umess) from an intrinsically safe switchable test voltage source (21) to an RC-element (22), isolated from earth (8) by means of decoupling capacitors (C1, C2), measurement of the charging or discharging current behaviour of the RC-element (22) after each switching of the test voltage source (21) and comparison of the measured charging or discharging behaviour with stored reference data are made by means of a microprocessor (24) using stored reference da

Abstract: The warning tone transmitter (10) according to the invention has a piezoelectric horn element (12) with two input connectors (29, 30) for the application of the supply voltage at a determined frequency and has a circuit arrangement (14) fed from the current supply, which generates the supply voltage. In order to be able to receive confirmation that the horn signal is actually generated by the warning tone transmitter (10) and in order to be able more easily to calibrate the warning tone transmitter (10) such that it is driven at its resonant frequency the piezoelectric horn element (12) has an additional output connector (34), which, when the horn element (12) is fed with the supply voltage, generates an answering signal taken to the circuit arrangement (14) if the piezoelement is actually oscillating. The magnitude of the answering signal is a value for the sound volume of the warning tone transmitter (10).