Principles of cable fault detection

Cables with intrinsically safe circuits (see Section 6.9) are explosion protected and cannot act as an ignition source in case of mechanical damage or cable fault. However, the power transferred per circuit is limited to some 10° W or 10aW. 

With the assumption that a three-phase AC grid with industrial frequency (50-60 cps) represents the state of the art of a power grid in a coal mine, two features shall be considered in parallel to establish a network and 

L3 phase conductors 1/1, 1/2, 1/3 voltages phase to neutral C1, C2, C3 capacitances phase to earth PE protective earthed conductor RF earth fault resistance UN, PE voltage between neutral N and PE in case of an earth fault 

A comprehensive treatment of these points is given in [35]. So, some focal points only are discussed here. 

With the exception of terminal compartments in e or d or within an enclosure in case of direct cable entries into d or p, such an IT system shows totally screened phase conductors which are not subject to an immediate short-circuit condition in case of cable damage or an internal insulation fault. Even in the earth fault condition the network may remain operational. And this is the great advantage of these systems compared with, e.g., a TN system 

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