The phase screened cables are usually used in the power supply of the 3.3kv coal face. Before the solid grounding and the phase fault, the insulation is first destroyed between the monitor wires of the cable and the phase shielding and the outer shielding. The installatio of the monitor wires, therefore, has the function of prevening the breakdown in advance. As a result, this can greatly reduce the possibility of the gas explosion and of the burning of the coal caused by the sparks. So the installation of the monitor wires helps ensure the safety of the power supply. The present paper analyses the three kinds of protection principie of the monitor wires adopted at present. They are the protection principle of the DC monitor wires with their terminals connected with the diodes, the protection principle of the additional DC power source monitor wires with their terminals connected with the resistance and the protection principle of the second harmonic monitor wire with their terminals connected diodes. The present paper also points out the problerms with the three kindsf protection principles. On the basis of this, this paper puts forward a kind of new monitor wire protection principle-- the protectio principle of the additional AC power source monitor wire with its terminal connected with resistance.
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