Intrinsicaly Safe (Sparkless) Electrical Signalling Relay
Definition of Intrinsic Safety
An Intrinsically Safe piece of equipment is an electrical device that is incapable of causing an ignition of the prescribed flammable gas, vapour, or dust, regardless of any spark or thermal effect that may occur in normal use, or under any conditions of fault likely to occur in practice.
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One of the first uses of electricity in mines was in the form of batteries which powered basic signalling devices.
A typical application was on steam or compressed air operated rope haulages which were used to move drams of coal and materials to and from the coal face area and pit bottom. The haulage motor operator required signals to start and stop, from remote locations which may have been many hundreds of yards away and, to facilitate this, two bare wires, which were insulated from each other and ground, were connected to a battery and bell and were run the length of the haulage plane. The electrical circuit was completed when the two wires were shorted out by a piece of metal, usually made in the form of a T, the vertical section being held in the hand whilst the horizontal section was placed across both wires.
Whilst this was a simple yet very practical method, it had one major drawback in that, when the electrical circuit was completed, a spark was produced and when the electrical circuit was broken an even larger spark was generated due to the stored energy in the bell coils being dissipated.
Furthermore, the location of the wires was mostly at the top end of the roof supports in an area where methane gas accumulates thus presenting pockets of potentially explosive atmospheres.
In order to obviate this hazard Henry Davies designed and patented a sparkless relay.
Events Relating to the Relay
The first item of documentation concerning this relay is dated 29 April 1914 and is in the form of a draft specification.
With all inventions, unless the inventor has the necessary finance and manufacturing facilities, it is necessary for a backer to be found, and in this instance it was the British General Electric Company Ltd (G.E C.) which is not to be confused with its American namesake.
Henry Davies already had experience in the manufacturing of electrical induction coils in his work on the design of magnetos for the generation of a spark required to light miners’ safety