AN AC CURRENT SENSOR FOR MEASURING ELECTRIC AC CURRENT IN A CONDUCTOR AND AN INDICATOR SYSTEM COMPRISING SUCH A SENSOR

摘要

The invention concerns a current sensor (22, 24; 26) for measuring electric current in a conductor. It is adapted for a short circuit indicator system (1). It comprises current responsive means, such as a current transformer (2210), adapted for mounting adjacent to said electric conductor: and sensor circuitry (2220) means for emitting a non-electrical wave signal as an output: said sensor circuitry (2220) comprises current level indication circuitry (2222) comprising a current-to-frequency converter adapted to provide said non-electrical wave signal as a current level indication pulse signal (CS 1 ) having a pulse frequency (Pf ca ) which is proportional to said electric conductor current, when the latter is within a predetermined current interval (l 1 - l 2 ). Thus, a low cost sensor is provided. The invention further relates to a short circuit indicator system (1 ) for an electric conductor. It comprises at least one of the above mentioned current sensors (22; 24; 26); a communication link (32: 34; 36) for relaying at least said non-electrical wave signal from said current sensor; voltage measurement means (40; 2240) for performing a voltage measurement over said electric conductor; a detection relay (51) operable for sensing at least the frequency of at least a current pulse signal and translating it to a current corresponding to the frequency of said current pulse signal; current threshold means operable for determining whether the current corresponding to the frequency of said current pulse signal (CS) is above a predetermined current threshold (I max ): and voltage threshold means operable for determining whether the voltage measured by said voltage measurement means (40; 2240) is below a predetermined voltage threshold (V min ) after a predetermined time period (t 1 ) after said current threshold means has made a positive determination: and short circuit display means (532; 534; 536) operable for indicating that a short circuit has occurred on said electric conductor when said voltage threshold means has made a positive determination. Thus, a rapid and reliable short circuit indicator system is provided, which may be adapted for use for electric conductors in LV, MV and HV power lines.