PAGE No. 44

Ericsson Automatic Mining Telephone
January 1937

The development of the N1087 type flameproof telephone had its inception in the desire of a large firm of chemical manufacturers to have a safe telephone service in all departments where a possible risk of inflammable vapours existed, yet for this service to be given via their central automatic exchange. When the first stages of experiment were commenced, a call for automatic instruments for the petroleum refining industry was received and this was shortly supplemented by the necessity of providing like communication in coal mines.

Intrinsically safe service, with the energy so limited as to be incapable of igniting the most inflammable mixture even under short circuit or bare wire conditions, is very difficult to provide from a standard automatic system, indeed it is very doubtful whether any commercial solution of the problem exists, so that a flameproof instrument has to be used, and armoured cable or sealed screwed conduit must be provided throughout the “danger zone“.

With the door closed, the instrument has the same general appearance as the already well known Ericsson flameproof magneto telephones. Opening the outer door discloses a hand microtelephone and a dial finger plate of conventional design. The instrument shewn has the terminal chamber arranged for armoured cable, but provision for screwed conduit can be provided as an alternative.

As will be seen from the picture below, the principle of placing each contact or switch point in its own flameproof enclosure has been followed. By this means the effects of ignition can be made very small, as the volume of gas involved is so low.

This point may be made clear by brief reference to the principle involved in the design of flameproof equipment. When apparatus is surrounded by an atmosphere of explosive gas for any length of time, the air in the internal spaces is gradually replaced by this. Slow gas diffusion and movement due to temperature changes are the two means by which the dangerous gas enters the apparatus. Also when gas is present it would enter if the cover were removed for inspection. Should a spark occur within the apparatus to ignite the explosive mixture, the pressure produced is released by the passage of the burning gases through the cooling flanges. These are designed so that the heat is absorbed before the outer atmosphere is reached, so that there is no possibility of the ignition being communicated.

The efficiency of the system obviously depends to a great extent on the ratio of cooling surface to total heat generated by the explosion. The small volumes of space obtained by the individual enclosures of the Ericsson N1087 type instrument result in correspondingly low heat generation and explosive pressure. In addition, the total mass of cooling metal is proportionately high in relation to the volume of possible explosive mixture.

The dial assembly is interesting in that the finger plate is independent of the dialling mechanism. Thus the safety of the enclosure does not depend on the light bearing shaft but the movement is transmitted from finger plate to mechanism by a clutch-shaft of generous proportions, coupled with a locking device which prevents damage to the dial mechanism by over-rapid dialling. Between the mechanism and the front casting carrying the flame-proof bearing a gauze is inserted as a diffuser to prevent a cone of pressure being exerted upon the bearing in the event of ignition. While this is a refinement and not an essential, it provides an additional factor of safety of particular value where hydrogen mixtures and chloroform, ether or kindred vapours are encountered/ The back cover is of massive proportions and forms a spigot joint on the front housing.

The receiver switch enclosure and the terminal chamber follow the proved design of the Ericsson flameproof magneto telephones.

The isolating switch is connected directly to the lines incoming at the terminal chamber and is operated by the inner door. Opening the inner door cuts off all connection between the lines and the internal wiring, thus any necessary examination can be conducted with safety. The switch is housed in a substantial flameproof enclosure and the contact springs and their tensioning are so arranged that in the unlikely event of any mechanical breakage the instrument is automatically disconnected. Further to protect the switch an overdrive is provided to take up any surplus movement after fitting.

The instrument has been tested for flame proofness by the Mines Department and is certified for both Group I - Firedamp (methane) and Group II, Petroleum & Acetone Vapours, by Certificate No. FLP. 853 of 21st October 1936.

It will naturally be appreciated that a similar instrument can be provided for working to a C.B. System.

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Last revised: October 14, 2019