A review of research carried out by cerchar on the protection of electrical installations in underground workings

Abstract Underground electrical installations differ substantially from surface installations in their design and conditions of utilization. Furthermore the faults that may appear in underground installations can be more severe with respect to safety conditions than is the case for surface installations. It appears that protective devices which are quite convenient in conventional distribution networks give inadequate protection against faults that have been noted to occur specifically in the mining field. Since 1956, Cerchar has carried out methodical investigations on particular problems associated with the protection of underground installations. The work has resulted in the design of several devices and the purpose and description of this equipment are indicated in the following paper. It must be emphasized beforehand that in France, three phase distribution networks in underground workings generally have a voltage of 550 V and an insulated neutral-point. One type of equipment described in this paper, namely the insulation control apparatus type CPI, was specially designed for distribution networks of the above type and cannot ensure any protection for earthed neutral-point systems. This does not apply to devices that ensure protection against interphase faults, such as the DCC 3 and DCC 3S as well as the fault detector type DLT which can be installed equally on insulated neutral or earthed neutral systems. The following points are hereby examined: 1. I. Protection of three phase systems against resistive faults by means of an auxiliary high frequency current. 2. II. Insulation control of three phase insulated neutral sysems. 3. III. Insulation control of direct current networks having two insulated poles.