PAGE No. 38

The Ericsson Mine Rescue Telephone
January 1939

The Coal Mines General Regulations (Coal Mines Act 1911) require owners of mines to which the regulations apply, to make
definite provision for rescue work in mines after an explosion or a fire, or otherwise in any atmosphere which may be dangerous to life.

Coal mining being an extra hazardous industry there are many occasions when the services of trained rescue workers are required, in addition to those cases of major disaster which bring vividly before the general public the risks involved.

Central Rescue Stations are maintained at positions convenient to serve a group of mines and affiliation to such a station is compulsory, except that the Divisional Inspector of Mines may exempt a mine situated more than 15 miles from a central rescue station, subject to such conditions as he may require, or where effective independent rescue provisions have been made. Such exceptions are, however, rare.

There are 30 associations or authorities controlling in all some 36 rescue stations and in addition there is one private rescue station. These stations are so placed as to cover the coalfields of England, Scotland and Wales. The permanent rescue corps are trained and equipped to undertake the rescue and fire fighting underground at the pits affiliated to their station.

Each pit is in telephonic communication with its rescue station and calls on it in case of fire, explosion, or other accidents requiring the services of rescue men trained in the use of breathing apparatus.

At fourteen central rescue stations a permanent corps of from eight, the minimum, to eighteen men, including instructors, is provided. With the exception of one station, the men live on or near the premises and do not work underground. At the remaining twenty-three stations the only persons employed are from two to four instructors and the men work regularly underground.

Eight central rescue stations are each provided with a fire-engine for surface work; some, but not all, have fire fighting apparatus and equipment for use underground.

All mines employing 100 or more men underground must have certain of their men trained in mine rescue work; the actual number varies according to scales included in the General Regulations (Rescue) 1928 First Schedule. All the colliery men are trained at the central rescue stations. These trained rescue workers, who are selected for their knowledge of the mine in addition to coolness, powers of endurance and general suitability, co-operate with the permanent corps, who form the spear-head of the attack in cases of emergency. Similarly the professional corps is assisted in fire fighting by the pit fire brigade, where one exists.

At the central rescue station the statutory equipment consists, among other things, of a minimum of twenty suits of breathing apparatus, with supplies of oxygen or liquid air for two days continuous use, four smoke helmets, four reviving apparatus, 20 electric and 6 flame safety lamps, motor transport, first aid boxes, etc., caged birds

for carbon monoxide testing and two portable signalling devices. From this list it will be seen that a considerable amount of the work has to be undertaken in irrespirable atmospheres. The duties include dealing with underground fires, exploration and recovery of mines after fires and explosions, and the taking of samples of gas, all 0f which entail entering and working in foul, noxious or inflammable atmospheres. The men are trained regularly in active work for continuous periods of at least two hours in a hot and irrespirable atmosphere in order to accustom them to their duties.

When a rescue corps is called and it commences operations, a base is established as soon as possible in fresh air but as near the irrespirable zone as possible.

Communication is obviously of considerable importance. The base should be in telephone communication, if possible, with the surface or the shaft bottom. Often the existing mine telephone system will provide for this or an instrument can be connected on to a line. For communication between the rescue brigade and the base, however, greater difficulties exist. As the brigade or team, of five to eight men, will be wearing breathing apparatus, speech is impracticable and communication has to be established by other means. Uniform codes of signals are prescribed, one for use between members of a brigade, for which each man carries a bulb hooter, and one for electric signalling. Each code consists of five simple signals only. It will readily be appreciated that while the existing telephone or bell system may provide a signalling medium to the base, it cannot be depended upon and it would but rarely prove convenient.

The provision of continuous communication was under consideration by the rescue apparatus Advisory Committee of the Mines Department as long ago as 1933. The commercial possibilities of equipment for this purpose were so obviously limited as to preclude manufacturers undertaking development on a competitive basis. Finally, Ericsson Telephones Limited were invited to collaborate with the Mines Department Testing Station and Mine Rescue Stations in the development and to undertake the production of a Mine Rescue Telephone at a price within reach of all rescue stations and low enough to encourage equipment of mine brigades. They accepted this invitation and initially the essential requirements and secondary desirable features were decided. Following this the circuit principles were considered, the electrical components and their associated safety devices designed. As the equipment was to be used under conditions of very real danger as regards ignition, specially keen attention was paid to safe operation, and the suggested electrical arrangements were checked for safety in hydrogen also by the Sheffield Testing Station of the Mines Department.

Safety considerations and the technical performance once settled, the service form was developed and the first model made. This was then tested out under working conditions both at rescue stations and in mines. This work was completed, and early in 1937 the various detail requirements and suggestions resulting from these practical trials were discussed and the final form agreed upon and engineered. Patterns and tools were put in hand and finally production models were made and put forward for official test and certification. The Certificate (T/Tel 28A) was granted on 18th June 1938 and production was then authorized.

The Mine Rescue Telephone consists of two units, a base station and an advance station, connected continuously by a light but tough cable. Fig. 1. In the constructional features of the design, particular attention has been paid to strength with low weight and to making the equipment convenient to carry and handy to operate, so as to add as little as possible to the arduous task of the rescue party. Both the case of the base station and the frame of the advance station are cast in aluminium-silicon alloy to obtain a robust yet light housing for each unit.

Fig. 1
Mine Rescue Telephone Units

Signals in both directions are effected by high-note buzzer, using the code set out in the regulations. The note is distinct and penetrating, the comparatively high frequency avoiding any chance of confusion or obliteration by other noises. in addition the base party can speak to the advance party, both speech and signals being received on the loud speaker at the advance station unit. Key signalling only is provided from advance party to base. Much thought was given to the facilities provided and many considerations were reviewed before deciding to limit to key signals only from the advance party. Among these may be mentioned technical considerations of efficiency, increased weight and cost, and the reduced mobility of the advance party if both-way speech were provided. When oxygen breathing apparatus is worn, clear and reliable speech is not practicable. Even if special transmitter and receiver were fitted, as smoke helmets would permit, the use would tend to be restricted to one man and would probably need the services of an extra man in the advance party. Also the emergency value of the code of signals is such that it is undesirable that any apparatus should tend to cause its disuse.

The base station unit is about 9.5 in square with a depth of 6.25 in. The weight is 15 lbs. On its front is fitted a sensitive transmitter, a small loud speaking receiver and a high frequency buzzer. Provided in the buzzer mounting is a small key which when depressed connects the buzzer, via the induction coil, to the adjacent receiver, so that the former can be tested to confirm that signals are being sent out. A convenient handle provides for adjustment of the buzzer when the test indicates this to be desirable. A hinged metal flap protects test key and adjustment handle from dust and accidental damage. On the left of the unit is a 'speak' key which must be operated to speak to the advance party. This connects the transmitter battery and at the same time disconnects the local receiver circuit, thus ensuring the highest efficiency of operation. To be seen on the right hand side is a similar key marked 'signal' for calling the advance station. This operates the buzzer and a high-note call is transmitted via the induction coil.

As this coil transmits both speech and signals no direct current is passed to line. The keys are sealed, against the entry of dust, by flexible leather diaphragms.

When the keys are in their normal position the loudspeaking receiver is connected directly to the trailing cable and hence to the advance station unit. Any signal sent out by the advance party is thus immediately reproduced by this receiver.

The line cable is connected by plug and socket on the right of the case. This cable is of special design having a composite conductor of copper and steel with tough, impregnated insulation.

Fig. 2'A Rescue Brigade ready for Service

The strong alloy frame of the advance station unit bears the cable drum and the totally enclosed case containing the communication apparatus. The size is slightly less than an 11 in cube, and the weight, with a full reel of 280 yards 0f cable, about 25 lbs. Particular attention has been directed to producing a form as convenient as possible to carry, easy to operate and immune from minor mechanical damage, so that the advance party enjoy the advantages of communication without having their activities impeded. During the advance, while the cable is being unreeled from the drum, the line is continuously connected to both sets and speech and signals can be transmitted. The communication apparatus is similar to that of the base station except that no transmitter is fitted. The same facilities for test and adjustment of the buzzer are also provided.

The batteries are of the ordinary cycle lamp type so that replacements of the correct type are readily obtainable. To cover the possibility of use abroad under tropical conditions, approval has been given to the alternative use of a standard type of inert cell. Battery voltages are 6 V and 3 V respectively for base and advance party units. The safe operation of the system in dangerous atmospheres is a feature of the electrical design. The well known high sensitivity obtainable with a good receiver is used to advantage and by combining high note buzzer signals with speech there is no direct current flowing in the line. The line currents themselves are of small dimensions, incapable of producing ignition in the most dangerous of atmospheres. As regards the local circuits at each unit, the buzzers are fitted with non-inductive shunts and with condensers as an additional safeguard the induction coil primary of the base unit has a safety shunt provided.

If during rescue operations it is desired to extend beyond the first cable length, a further advance party instrument can be connected by a simple plug and the circuit transferred from the first to the second unit.

The illustrations, shewing a permanent rescue corps' regular practice of at least two hours, in various stages, conveys a good general idea of the equipment to be carried, and some impression of the service use of the rescue telephone. Fig. 2 shows a brigade leaving their ambulance ready for immediate service in an irrespirable atmosphere. Oxygen breathing apparatus is in use and the nose is clipped so that respiration is entirely through the mouth. Across the front of the harness is the hooter for signalling between members of the brigade. Among the equipment can be seen a case containing reviving apparatus, stretcher, canary for carbon monoxide detection, the two units of the telephone, flame safety lamp, etc.

Fig. 3'Advance Party about to Commence Operations

In Fig. 3 a fresh air base has been established and the advance party are going forward to commence operations. Fig. 4 shews the men at work in a practice gallery. The operations are devised to represent those underground and include the repeated handling of heavy weights, building and removing temporary stoppings of stone, brick, etc., including transport of material, removing debris as from a fall of roof, setting roof supports, practice with a loaded stretcher, etc. The advance party unit is seen in operation, and it will be noticed that the loud speaker call avoids the necessity for any connection between the rescue party and the set. Thus all can hear the incoming messages and any one may answer as convenient. This is an advantage as no question of loss of communication due to incapacitation of the operator can arise.

Fig. 4
Advance Party at work in a Practice Gallery

It is hoped that the introduction of this certain, continuous and speedy means of communication will both materially assist the work and add to the confidence of the advance party in the course of their rescue and exploration duties.

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