ERICSSON BULLETIN
No. 20 PAGE No. 11


Developments in Safe Communication Equipment for Coal Mines, Petroleum Refineries, etc.
JANUARY 1950

In coal mines and in certain other industrial establishments, particularly chemical and petroleum distillation plants, gases and vapours if ignited may result in a serious explosion, a flame, naked light, hot surface or an electric spark being sufficient. Some of the common industrial gases and vapours with the right proportion of air form combustible mixtures which may be ignited by sparks from even the weak currents used in bell and telephone circuits. There are two methods of safeguarding against this possibility, the first, “flameproofing” (FLP), entails limiting any sparking to a robust chamber with broad joints such that any flame from an explosion of gas inside the chamber is cooled on its passage to the outer atmosphere so that even the most inflammable surrounding mixture cannot be ignited. Special safeguards are very necessary in wiring between each instrument or junction box.
The second method is to make the circuit “intrinsically safe“ by limitations on energy and by by-pass and surge reduction devices, so that any spark which may occur is incapable of igniting any mixture of gas or vapour for which the apparatus is certified. No special line precautions are necessary in this case.
Of the two protective systems “intrinsic safety” is obviously preferable; unfortunately its application is limited to low energy devices such as telecommunication equipment. Its use for bell and telephone circuits in coal mines is compulsory. As knowledge of the spark ignition of gases is extended by research we may expect to see a widening of the field of application, which is already extending in the petroleum industry.

Fig. 1
N.1087 Type Flameproof Auto Telephone with Bakelite Micro-telephone

It should be noted that the two systems (FLP and intrinsically safe) are entirely separate and distinct, and instruments in either class must not be fitted on installations of the other class unless they are certified for both.
Power devices must of necessity be “flameproof” and the very considerable volume of research thus stimulated has resulted in the classification of a wide range of gases and vapours into four groups. (See British Standard 229). As Group I is methane (firedamp) and Group IV comprises gases for which certain protection cannot be provided, the only two for which certificates for flame-proof signal apparatus are issued are Groups II and III. This covers a large industrial range which, moreover, is still being extended by addenda to B.S. 229.

The development of safe telecommunication equipment for use in mines is a branch of the industry in which the Company has been interested for many years.
Laboratory investigations were being conducted as long ago as 1914 and led finally to the development of relays and bells which were informally approved in 1917, i.e. three years before the Home Office introduced official recognition of “safe“ equipment. This pioneering spirit has been maintained ever since and justifiable pride is taken in the fact that in almost every case the first official certificate of safety for any new apparatus has been granted to the Company.
The expansion in the chemical and petroleum industries in recent years, the export drive and research work on gases have stimulated developments some of which are briefly described.

FLAMEPROOF EQUIPMENT
Auto and C.R. Telephones

in order to extend their useful range, the design of auto and C.B. telephones, types N.1087 (Fig. 1) and N.1470 respectively, has been modified and the telephones re-certified to cover their use in Group III gases in addition to Group II. These instruments are now also certified for interconnection by means of “Pyrotenax” (a solid drawn, mineral-insulated, copper sheathed cable) in addition to the armoured cable and seamless steel conduit previously approved. A further improvement is the replacement of the built-up metal handset by a modern moulded micro-telephone.

Relay or Key Enclosure
A completely new addition to the range of FLP apparatus is a flameproof enclosure, type N.7241, (Fig. 2) designed to accommodate a relay and rectifier operating on telephone ringing current at approximately 50v, 16.6 c/s and controlling mains operated devices such as Klaxons, lamps, etc. The relay may be of normal non-locking type or can be made to self-lock and be released by a plunger provided on the side of the chamber. Heavy currents are controlled by a mercury tube switch; smaller currents by a conventional spring-set fitted with tungsten contacts. The maximum rating is 350 watts at 250 volts, which is ample for relaying any signal.

The same enclosure is also certified to take a normal telephone relay or a telephone type push key and the terminal chamber can be supplied to accommodate any one of the approved types of wiring; i.e. seamless conduit, steel wire armoured and Pyrotenax cables. The enclosure is certified for use in Group II gases for the relays and for both Groups I and for II keys.

Fig. 3
N.2121.Z Magneto Table Telephone for use
Under Cover on the Mine Surface

INTRINSICALLY SAFE EQUIPMENT
Magneto Table and Wall Telephones
In order to keep mining instruments up to date and in line with the latest developments in telecommunications, certification has recently been obtained for a new table telephone, type N.2121Z, shown in Figs. 3. This instrument, intended solely for surface use under cover, now replaces the previous office desk set N.2120Z and its separate bell set N.3100Z. The new instrument incorporates the bell, latest alnico generator and all other components in one housing. The moulded casework is of the same pleasing design as the latest British Post Office type telephone. In order to provide an easier and smoother turning action the generator drive has been placed on the right-hand side, instead of on the front as in the previous model. Telephone N.2121Z is certified intrinsically safe for use in petroleum vapours as well as methane. The complementary magneto wall telephone for surface use is the type N.2518, which is generally similar to the type N.2504 which it supersedes. It also replaces the N.2202 wall telephone which did not incorporate a battery compartment.

Fig. 5
N.1846 Portable Magneto Telephone for
Mines

Portable Magneto Telephone
The range of intrinsically safe instruments has been further extended by a portable magneto telephone type N. 1846. The design, which is very robust and compact, is based upon the wartime development of a lightweight instrument for the Services and is notable for the high grade of speech transmission it provides. It is intended principally as a linesman’s or electrician’s telephone, but in an emergency it would provide a temporary station where a standard instrument was not immediately available. Type N. 1846 portable telephone is certified for use in both methane and petroleum vapours and is illustrated in Fig. 5.

Magneto Switchboards
Pyramid type and ordinary cordless boards have been re-certified in order to incorporate under one certificate various improvements and additions which have been introduced since the original certificate was granted in 1933.

The opportunity was taken still further to modernize the equipment and also to simplify operation of the boards by using the calling indicators for supervision. A modern moulded bakelite hand micro-telephone and the latest alnico generator have also been fitted. A 10 line board, type N.555, with three connecting circuits, is shown in Fig. 6.

Magneto Extension Bell
in certain circumstances a bell is required in a different location from that of the telephone instrument. For this purpose a loud-ringing magneto bell has been developed, type N.3111. This bell, suitable for underground and outdoor use, can be wired in parallel with the bell in the instrument, or may actually replace it, the instrument bell being disconnected.

Mains Signalling Bell
Type N.3111C. bell is similar to the above magneto bell, but is intended for operation on a system energized via a certified intrinsically safe signalling transformer as defined in B.S. 1259. The bell coils are suitably proportioned to the voltage and frequency and the original 16.6 c/s movement has been re-designed with a lightweight stem and hammer to respond to the 50 c/s standard mains frequency. It has been certified for use in methane and petroleum vapours.

Fig. 6
N.555 Type Magneto Cordless Switchboard
Fig. 8
Mines Rescue Telephone

Mine Rescue Telephone (Fig. 8)
The only modification to this equipment, which was described in Bulletin No. 14, January 1939, is in the signalling apparatus and arises from a request by the Mines Rescue Superintendents for a buzzer which would remain in stable adjustment for a longer period of service than was given by the original type, consequently a new design based on wartime experience with field telephones has been developed. This buzzer has a double-acting, double polarized, twin contact vibrating reed and is found to be extremely stable. It is almost entirely unaffected by mechanical shocks and vibration.
The safety devices, consisting of spark quench and battery reversal rectifiers and voltage limiting resistors, have been suitably proportioned to the new design.

 

BACK Home page BT/GPO Telephones Ericsson Telephones Ericsson Home Page Ericsson PABX All Telephone Systems

Last revised: June 22, 2003