Intrinsically safe telephone systems

#This invention relates to telephone systems. The invention has been developed primarily in relation to the requirements which arise in respect of a telephone system in which certain of the stations to and from which calls require to be made are situated in a hazardous area, that is to say an area in which the gaseous environment is of a composition such that there is a risk of ignition, as, for example, occurs in underground mine workings.

#It will, however, be understood that the invention is of general application in environments in which like or analogous requirements arise.

#In underground mine workings, and more especially those in which mining operations are conducted by what is known as long wall mining method, the coal or other mineral to be extracted is removed from a long face. At one end there is an inlet tunnel or roadway leading up to the face along which incoming supplies are brought and at a position spaced along the coal face, an outlet roadway or tunnel leads away from the coal face, the mined coal or mineral material normally being conveyed away along the outlet roadway.

#The invention will now be described, by way of example, with reference to the accompanying drawings wherein:

#FIG. 1 is a schematic circuit diagram of the telephone system installed in an underground mine working shown in plan;

#FIG. 2 is a circuit diagram (mainly schematic) of one form of telephone system in accordance with the invention;

#Connection points from the individual circuits of FIGS. 3 to 6 to other parts of the circuit shown schematically in FIG. 2 are designated by references a to p which are also shown in FIG. 2.

#As shown in FIG. 1 the hazardous area is an underground mine working such as a coal mine in which the coal face is indicated at 10, the coal face area extending along the coal face at 11, and the inlet and outlet tunnels or roadways at 12 and 13. The area 14 contains a stowed material, or, in some cases in this area, the roof of the mine working will have been allowed to cave fully onto the floor of the mine working.

#The inlet roadway 12 is shown as extended by a main tunnel or roadway 15 to the lower end of the mine shaft at the upper end of which is a station 16.