INVENTORS
At the present time with few exceptions, the Battery power used on relays is in excess of that sufficient to give a dangerous break-flash at the Signal Wires. The commonest kinds of relays are dangerous when used with a Battery of more than 4 wet Leclanche Cells or with any current in excess of the current given by such a Battery.
The Report shows in detail how the Bare Wire system of electrical Signalling as commonly employed can be rendered quite secure from any danger of the Ignition of inflamable gases by the break-flash at the signal Wires or at the Contacts of the Signalling Instruments.
In order to secure a safe Bare Wire signalling System it is necessary in the first place that the Battery Power employed on any one circuit should be limited, and in the second place, that the signalling Instrument, whether Bell or Relay, should comply with certain requirements set out in the Report.
The maximum pressure allowed by the General Regulations (No.134a) and from a point of view of safety a pressure of 25 volts for a signalling circuit is 25volts is not excessive provided that, as is usual Wet Lechlanche Cells (quart size) are employed. Systems in which the Battery used consists of dry primary cells, and systems in which continuous, or alterating current generators are employed, can be rendered safe if, in addition to the use of anti-sparklng devices, sufficient non-inductive resistances are permanently installed in the circuit.
Both Bells and Relays should have flame-tight covers (preferably locked) and should be so constructed and maintained that when included in a Circuit with a Battery at 25 volts the break-flash produced when Bare Signal Wires are separated after giving a Signal is incapable of igniting an 8 per cent methane-air Mixture.
The results of the researches of Dr. Wheeler and Professor Thornton is to show that most types of bells and Relays in ordinary use In Coal Mines do not comply with the provisions of General Regulation 132 and that there is no difficulty in constructing Bells and Relays which will comply with the Regulation. In Mines therefore to which Regulation 132 Applies, steps must be taken without delay to bring into compliance with the Regulation any existing installations which do not already comply; and no new apparatus must of course be installed except such as complies strictly with the Regulation.
I am, Gentlemen
Your obedient servant
(signed) R.A.S.Redmayne