The control of electrical equipment in hazardous atmospheres, such as occur in oil refineries, lubricating oil works, etc., has hitherto been a problem never completely solved ; for although a lamp, for example, may be enclosed in a flameproof housing, the cable supplying current to the lamp could become accidentally damaged, whereby a spark may be emitted. A method which completely eliminates the danger of electrical equipment in contact with inflammable gases or vapours in the atmosphere is the pressurised system recently introduced by the General Electric Company Ltd. In this, the electrical installation within the danger area is hermetically sealed and carries an internal air pressure of about 5 p.s.i. If at any time the installation is damaged, the release of the internal air pressure momentarily blows away the inflammable gas or vapour from the point of breakage, and simultaneously the current is cut off. This is effected by reason of the fact that the current supply in the danger area is only maintained as long as the internal air pressure exists, for the air pressure serves to maintain a special switch in the “on” position.
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