The dispatching communication system at RUDNA Mining Plant was characterised by relatively high hum in some subscriber's lines. Measurements of disturbance voltages in telecommunication cabling have shown occurrence of low level of symmetrical disturbance voltages and high level (peak- to- peak value of up to several volts) of symmetrical (between phase and earth) disturbance voltages. The symmetrical disturbance voltages were runs of 50 Hz basic frequency. In case of using amplifiers with differential inputs (e.g. ZLD unit in the STAR system), a coefficient of summary signals attenuation pays an important part. The paper analyses the mechanism of asymmetrical disturbances penetration onto communication system and the methods of this event reduction.
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