The heating of domestic pendant lamp fittings and their connecting leads

摘要

This paper is an investigation of the temperature-rise of the rubber insulation of leads connected to pendant fittings using lamps up to 200 watts as now made for domestic use, and of methods for reducing existing values to safe limits. On account of the higher filament temperature of the gas-filled lamp than the vacuum lamp, and the presence of the gas in the bdlb, the temprature of the lamp cap and of the surface of the glass is much higher in the former case than in the latter. This causes the connecting leads to become unduly hot and their deterioration to be considerably accelerated. As a result, after a limited period of service there is a risk of short-circuit or burning of the flexible cord occurring adjacent to or within the holder. There is the further possibility of metallic holders becoming alive. Although these risks exist, it is by no means common for serious trouble to occur. Four general methods are available for mitigating these effects, viz.:? (i) By improving the heat-resisting properties of the rubber insulation. (ii) By replacing rubber by a material capable of withstanding, existing working temperatures. (iii) By modifying the design of fitting and lampholder so as to reduce the working temperature of the rubber to 50?C. (iv) By modifying the design of the lamp so as to reduce the working temperature of the rubber to 50?C. Methods (I) and (ii) have both been under consideration by the Cable Makers' Association, and as a result the E.R.A. has been advised, since the preparation of this paper, that in regard to method (i) it would be possible to supply an improved type of rubber-insulated flexible cord which would give satisfactory operation at 60?d C. for a reasonable life under reasonable usage; and, in respect to (ii), flexible cords of this description are available and are being used in dry situations. This paper is based on the 50? C. limit, but the modifications necessary to fittings, etc., so that a temperature of 60? C. will not be exce-neded, will easily be seen from the test results given. Method (ii) is to some extent obligatory in the U.S.A. and Canada. This paper is restricted to methods (iii) and (iv), and it is shown that a satisfactory solution can invariably be obtained by a combination of them and, frequently, by method (iii) alone. Tests show that in nearly all domestic pendant lamp fittings up to 200 watts, the temperature-rise of the connecting lead at ? in. from the terminal is in excess of the maximum permissible, viz 23?4 deg. C, and this temperature-rise is attained at points varying from 2? in. to 9 in. from the terminal. At the terminal the value varies from 51 to 117.5 deg. C. for the fittngs tested. Theory and experiments show that the temperature-rise depends fundamentally on two factors, viz, the temperature of the source of the heat which is copducted fromthe lamp to the holder, and the temperature of the air surrqunding the holder, which in a large measure controls the dissipation of heat from the holder, surface. Other subsidiary factors are the thermal resistance in the heat path between the lamp and the holder, and the radiation emissivity of the holder. The lamps used for the tests conform to the British Standard Specification for Tungsten Filament Electric Lamps, No. 161, 1932. Investigational work has been extended to shop-window, office, and, factory lighting fittings, and will be the subject of a later report.