PAGE No. 20

Mining Telephones, Switchboards and Apparatus
July 1933

Pioneers in the field of telephone and signalling apparatus designed for use in the Coal Mining Industry, Ericsson Telephones Ltd. have been well to the fore in the considerable advances of the past twenty years in this field. They have produced a complete range of equipment which is safe, fulfils its function with efficiency and convenience, is robust, and proof against malicious tampering.

As an instance of attention to safety, it is interesting to note that the original D.C. mining bell with a short-circuited winding above and below each active winding as safety device, introduced over 20 years ago and certified in 1920, was almost as safe as the most modern bells.

Early in 1924 certification was obtained for an improved design of bell, and immediately afterwards a new certified relay was introduced which replaced the previous relay.

Ericsson telephone instruments were the first to obtain a certificate for magneto working and again for use on the battery call system, also at the present time their switchboards are the only ones for which a certificate has been issued.

Telephones installations in mines have to conform to the appropriate sections of the Coal Mines Regulations. Until a few years ago the mining industry had to rely almost entirely on the manufacturer to ensure that signalling apparatus was safe. It will be readily understood that even where a manufacturer had the requisite equipment and personnel for safety tests such as were provided in the Ericsson laboratories, there was no generally recognised standard of safety. In 1919 the Mines Department commenced the test and certification of D.C. bells and relays for bare wire working, and in 1926 opened the Sheffield Testing Station for investigation of signalling problems and the test of commercial apparatus. Thus they have been enabled to formulate testing methods and indicate desirable features in apparatus. Ericsson Telephones Ltd. realised at the outset the value of this work and as testing facilities became available they applied for test and certification of their apparatus, so that they now have a full range of equipment certified safe for use in fiery mines.

Apparatus may be divided into two groups, (1) flameproof, (2) intrinsically safe.

Flameproof apparatus depends upon its design and construction to ensure that if ignition of a mixture inside the case occurs, the resulting flame is so cooled on its passage to the exterior that it cannot produce any ignition of the surrounding methane atmosphere, however explosible the latter may be. The usual methods are, to provide wide machined flanges for cooling, to limit the unoccupied internal space and to pay special attention to strength of materials and construction. All apparatus associated must be flameproof and the lines fully insulated and well maintained, as a line fault immediately introduces the element of risk.

The alternative method is to design the electrical circuit so that if an electric spark occurs, inside or outside the apparatus, whether in normal working or as a result of any fault in the wiring system, such a spark shall be incapable of igniting the most sensitive methane mixture. This intrinsic safety is doubtless the most satisfactory, as purely flameproof apparatus has always to contend with the ultimate possibility of a dangerous fault in the wiring.

Good maintenance may make this improbable, it can never make it impossible.

Magneto telephones have a considerable application in mines and during the past fifteen years the original Ericsson design has received those additions and modifications which experience and research have shown to be of service. The present instruments, certified for parallel working, are available in two types. Type N2982 shown with door open for use, has a stout cast case with inner door on which are mounted the speaking equipment and generator. The ringer gongs are protected from damage by a cowling, and access to the interior is by special key. The inner door is waterproof including the generator crank gland. A protected water-proof transmitter inset is used. The outer door fastens with a slam catch and the rounded corners of the set are another feature of design. The case may be obtained cast in iron or aluminium. When the speaking unit is swung out for use the battery circuit for the transmitter is automatically switched on.

The other type N2972 is of similar design but is fitted with a hand micro-telephone instead of the swing-out unit. Both instruments have the same electrical characteristics and are intrinsically safe for any number in parallel, and when used with Ericsson mining switchboards types N510, N515 and N550. These switchboards, certified for use with the telephones mentioned, are of very robust construction and simple to operate. Types N510 and N515 are “pyramid” boards for up to 5 circuits, while the connecting keys in the case of the N550 type are arranged in horizontal rows and a hand micro-telephone and calling generator are provided for the operator. These keys are of plunger type with engraved metal tops. The smaller, pyramid boards have no integral speaking and calling equipment, it being usual to fix the board adjacent to a telephone at which attendance is regular, thus there is no need to have duplicate equipment. Drop shutter indicators are used on these switchboards unless indicators with individual dust covers are desired, when a flap indicator, of the type illustrated in page 9 is provided, which has a restoring button mounted under the indicator disc to release the mechanical holding device on the shutter. This latter indicator in a wood case and provided with contacts is available as an indicator-relay for providing a visual signal as additional to the telephone ringer.

N510 N550

Where a telephone, situated above ground, at the bank, in the winding house, in the offices, etc., is connected, or may be connected by switchboard, to an underground circuit it is essential that it should be safe also. It is important that this point be appreciated. The same equipment can, of course, be used, but where a wood cased instrument is preferred, an instrument certified for surface use under cover is available in type N2504 (shown to the right), which has the same electrical circuit and equipment as the magneto telephones just described.

Battery operated bells, relays and telephones form another important section of communications in mines.

The Ericsson battery call mining telephone type N1150 (pictured below) is certified as intrinsically safe. The simple and robust cast case is waterproof and carries the waterproof inset transmitter inside the front, protected by a metal grid. A loud-speaking type of receiver is housed inside also ; a flexible metallic tube with earpiece serves as listening tube and a thumb operated switch of substantial proportions controls the ringing and speaking. An external certified bell is used for calling and where a bare
wire system is used the signalling battery must, of course, be of a certified type and common to all the telephones on the system.

 By the addition of a condenser in the speaking circuit, any number of these instruments can be connected in parallel on a three-wire signalling circuit with a common battery. The condenser with strap is not normally fitted but can be readily added by a maintenance man, the drillings for the strap being provided.

The Ericsson D.C. bell type N3030 used for haulage roads, with telephones, or wherever a certified bell is required, is supplied in two resistances - 20 ohms and 30 ohms. The 20 ohm bell finds favour for local working with say up to 12 volts, while the 30 ohm bell is more used for the longer circuits with up to 25 volts. Either is, of course, approved for use with any number in parallel up to the full 25 volts permissible by regulations.

Where long distances have to be encountered or local battery working is preferable or more economical, relays are used, closing the contact of a local bell circuit.

A new magneto-telephone relay type N7236, has been developed and is now available, which will operate in connection with Ericsson mining magneto telephones and switchboards, and provides one, two or three make contacts as desired. This relay, of unusual but highly efficient design, is housed in a stout cast case with both the relay and the terminal cover secured by tamper-proof screws.

The Ericsson D.C. Relay N7237A, certified in 1924, replaced the original certified relay, then discontinued. This relay, 100 ohms with an anti-spark winding of 500 ohms, and giving one make contact, is housed in a cast case with flanged cover of the same general design as the bell.

In addition a new type of D.C. relay is now introduced, type N7240, available with one, two or three make contacts as required, in a case of the same construction as the new magneto-telephone relay. This multi-contact relay represents all that is most modern and desirable in a relay of this nature, incorporating as it does those features which have been proved in arduous service in telephone equipment all over the country, retaining for each purpose only the best in materials and design.

All D.C. bells, relays, etc., when used for parallel working on a bare wire system must use a common battery of certified type as a source of energy. The Mines Department have made this ruling because, whatever the apparatus in circuit, it is highly desirable to limit the energy available to a safe maximum. In the case of a battery this can be effected by selecting a type which has an inherent internal resistance which is maintained at a suitable value under load, such as the 3-pint Leclanché cell. It will be seen that accumulator batteries are unsuitable by reason of their low internal resistance.

In connection with signalling systems a safety push and a pull are available, both being certified by the Mines Department.

The certified push contains Morse key contacts operated by a special drop handle, making accidental operation impossible. The stout cast case has a separate terminal chamber.

The certified pull, for such use as on haulage roads where wet or other conditions interfere with the satisfactory working of bare wire signalling, provides contacts in a cast case with separate terminal chamber, the contacts being operated by pulling on stranded wire attached to the eye of the pull and carried on pulleys along the road to an anchorage at the end of the run.

No mention of coal mines equipment would be complete without reference to rescue work, and we are pleased to announce that, in co-operation with the leading manufacturer of rescue apparatus, we have designed and are commencing production of portable signalling equipment for this purpose.


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