Patent GB-422859-A (GB)
Title: Improvements in or relating to electric lamp mountings
IPC: F21V25/04
Filed: 1933-07-20
Abstract: 422,859. Flame-proof electric lamps ; junction boxes. REYROLLE & CO.. Ltd., A., Hebburn-on-Tyne, and TIPLADY, J., 33, Brandon Grove, Newcastle-on-Tyne. July 20, 1933, No. 20473. [Classes 39 (ii) and 75 (iv)] [See also Group XXXVI] In a flame-proof electric lamp for use in mines and other explosive atmospheres, the electric lamp bulb is surrounded by a screen of metal gauze inside or outside a light diffusing cover made of glass or like translucent material. A plug-and-socket or like connection in the bulb circuit is accommodated within a flame-proof chamber formed by interengaging members on a bulb holder and a supporting base from which the bulb and its holder can be removed, means being provided to ensure an appreciable time interval between the breaking of the electric connection and the separation of the interengaging members during removal of the holder. Locking means are provided to permit the holder being moved with respect to the base sufficiently to break the circuit so as to switch off the lamp or to isolate a faulty bulb or holder, but which prevents the holder being completely removed from the base unless it is released by a key or other independent means in the possession of an authorized person. In the form shown, the bulb H is surrounded by a metal gauze screen L and by a glass cover M having a prismatic inner surface M<1>. The open ends of the screen and cover are cemented together and to an annular channel ring N which fits closely within a ring K carrying a guard K<5>. The ring K also forms a casing which slides telescopically into a casing formed by a projecting flange A<2> on the upper part A of a two-part supporting base A, B providing a terminal box A<1> into which is looped a supply cable C. The latter is connected in the manner described in Specification 392,925. The cap H<1> of the bulb is screwed into a plug G<1> on a metal plate G which is riveted to the upper flange J<1> of a cylindrical member J having a lower flange J<2> provided with radially projecting tongues J<3> to fit into circumferential slots or recesses in the ring K. The plug G<1> screws into a socket A<4> on the base member A, which socket is coaxial with the casing A<2>, so that the central contact on the bulb cap H<1> engages a spring plunger E carried by a block of insulating material F which is also screwed into the base member A. The outer edge of a split ring K<3> of wedge-like cross section enters an internal annular groove K<4> in the ring K and tends to press the flange J<2> upwardly. A locking device is formed by means of a cupshaped member O secured to the base member A by a stud P having an end stop P<1> and a nut P<2> adapted to be loosened by a box spanner. The member 0 has a tongue O<1> which normally fits into a slot in the upper edge of the flange A<2> and is of such a length that it allows the bulb holder to be rotated to open and close the circuit, but prevents its complete removal by engaging an external shoulder K<6> on the ring K. By authorized loosening of the nut P<2>, the member O can be raised and rotated to withdraw the tongue O<1> clear of the shoulder K<6>. In a modification (Figs. 5 and 6, not shown), the telescopic casings are provided with interengaging lugs and slots so arranged that the casings can only be separated by two axial movements and an intermediate rotary movement, the lamp circuit being broken during the first axial movement whilst the rotary movement ensures a pause before the casings can be separated by the second axial movement. In another modification (Fig. 8, not shown), the holder is retained in position on the base by means of a screw-threaded bolt which serves also to secure a lid to the junction box. In both modifications slotted terminals are provided to receive the looped-in cable.