Tele No. 153 Mk1 Tele No. 153 Mk2

Telephone No. 153 is a wall mounted weatherproof, flameproof, enclosed type telephone for magneto working. Overall dimensions of the telephone are 20in x 12in x 10in; the total weight is 92lb including batteries. Telephone No. 153 is superseded by Telephone No. 723 for new work.

Made by British Ericsson, their models N2974 & N2984

Flameproof Telephone Apparatus


The author discusses briefly some general features of the above subject with a reference to the Department’s policy, and then describes the various types of apparatus available with details of its installation and maintenance

IT has always been the policy of the Post Office to discourage the use of any telephone apparatus or the provision of telephone circuits in any situation where there is likely to be a risk of explosion. Nevertheless the Post Office will agree to supply certain flameproof apparatus, provided it is installed in positions where the risk is not continuous (i.e. in positions where a concentration of explosive vapours would be present only occasionally), and on condition that the subscriber will accept the full responsibility for any consequences. This article describes the special types of telephone apparatus which the Post Office has available for this purpose.

It is well known that telephone apparatus (such as the dial, gravity switch and trembler bell contacts), like other electrical apparatus which has to break reactive circuits, will produce sparks during its normal operation. The amount of energy dissipated in such a discharge is dependent upon the voltage employed, the electrical characteristics of the apparatus, and the mechanical frequency of the circuit interruptions. It has been established by the Ministry of Fuel and Power (Coal Division) that the sparks caused by telephone apparatus using the standard telephone voltages would cause an explosion if allowed to occur in certain concentrations of explosive vapours (such as petroleum and acetone fumes) hence the need for some form of protection on such apparatus. It should perhaps be mentioned that when using telephone apparatus in ordinary premises and under normal conditions the sparks produced by this apparatus do not constitute a hazard.
To facilitate the design and subsequent testing of flameproof apparatus, the explosive gases generally encountered in industry are classified into three groups:-

Group I. - Methane (fire damp).

Group II. - Acetone, benzene, pentane, etc.

Group III. - Coal gas, coke-oven gas, etc.

Some of the Post Office apparatus is suitable for all three groups, but others are suitable for only Groups I and II. The apparatus is not suitable for use in premises where explosive powders are handled.
Before detailing the various pieces of flameproof apparatus it will, no doubt, be of interest to mention that there are two distinct and different principles employed to make telephone apparatus and circuits safe, and it is essential to recognise that the two systems are not interchangeable. These two methods of obtaining safety are known as:-

(a) Flameproof protection.

(b) Intrinsic safety.

Flameproof protection consists of enclosing all parts of the circuit and apparatus where sparks would normally occur during operation, so that any resulting flame is so cooled in its passage to the outside atmosphere that it is incapable of igniting the most sensitive mixture of the gases within the group for which the apparatus is to be used.
Intrinsic safety consists of the limitation of the energy which can be liberated in any spark to below that required for ignition of the most sensitive gases
The apparatus described here is all of flameproof design, and Tele. No. 153 is, in addition, designed for intrinsic safety. Intrinsically safe apparatus cannot be used effectively on the standard Post Office systems because the voltage and circuit conditions of these systems are outside the limits allowed for intrinsic safety.
The types of flameproof apparatus at present available for use in premises where some protection is desirable are very limited, and consist of:-

(a) A telephone suitable for use in auto and CB areas (Coded Telephone No. 149). This telephone can also be used in C.B.S. areas by fitting an auxiliary unit (Unit Auxiliary C.B.S. 536) at the exchange to provide a battery feed and signalling facilities.

(b) A telephone suitable for use in magneto areas (Coded Telephone No. 153).

(c) A flameproof relay - suitable for connection in the extension bell position of the above two telephones and used for controlling devices operated by the electric supply mains such as bells, hooters, etc. (Coded Relay-Unit C.D. 393.)

(d) A flameproof box to house a Protector and Fuse No. 1 2/2 when the latter is fitted in the danger area. (Coded Box Protector C.D. 408.)

(e) A flameproof switch plug purchased specially as required for certain ship-to-shore terminations.

An effort is also being made to obtain a flameproof magneto loud-sounding extension bell of which none is available at the moment. The availability of a bell of this type will reduce the demands for the flameproof relay.
Items (a)-(d) above are covered individually by flameproof certificates as issued by the Ministry of Fuel and Power (Coal Division). This Government Department undertakes to test all flameproof apparatus, and if satisfactory in design issues what is called a flameproof certificate. Each piece of apparatus is required to bear a distinguishing mark followed by the certificate number.
Although the switch plug is of flameproof design, it has not been certified by the Ministry of Fuel and Power. This is because its design permits the use of a flexible lead which has never been regarded as satisfactory from a flameproof point of view.

Tele. 149 and Tele. 153 are identical in external appearance and are of very robust construction, being completely enclosed in a substantial iron case. The weight of each telephone is approximately 85 lbs. The front portion or cover of the telephone, which is in fact an access door, is normally kept closed for protection from weather, dirt, etc., and has to be opened to gain access to the receiver, transmitter, etc.
Both telephones were originally designed for use in coal mines, and therefore, in addition to their flameproof properties, they were required to withstand fairly rough usage. They are also made weatherproof.
Although designated as being of flameproof design the flameproof properties are confined to those parts of the telephone such as. the gravity switch, dial contacts, and generator contacts where sparking occurs during normal operation. These parts of the telephone circuit are completely enclosed in small separate enclosures designed for flameproof protection in accordance with B.S.S. 229, and known as flameproof enclosures. The terminal chamber located at the base of the telephone housing is also a flameproof enclosure.
It will, of course, be appreciated that it is impossible to make the telephone completely flameproof due to the fact that all working parts cannot be completely enclosed.

Flameproof Enclosure
All flameproof enclosures are distinguishable by the wide, machine fiat, flange on their periphery. The width of the flange is 0.5 in. as specified in B.S.S. 229 “for bells and accessories and parts of telephones.” Larger enclosures have 0.75 in. or 1 in. flanges, depending on whether they are less than 100 cu. in. or not.

The definition of a flameproof enclosure per B.S.S. 229 is as follows:-
“A flameproof enclosure for electrical apparatus is one that will withstand, without injury, any explosion of prescribed inflammable gas that may occur within it under practical conditions of operation within the rating of the apparatus (and recognised overloads, if any, associated therewith), and will prevent the transmission of flame such as will ignite the prescribed inflammable gas which may be present in the surrounding atmosphere.”

The following essential points of manufacture are also worth mentioning: all the joints of the flameproof enclosure shall be either flanged joints, spigoted joints or screwed joints, or a combination of such joints without the intervention of any loose or perishable packing; all such joints shall fit as closely as the methods of manufacture
permit, and in no case shall the gap between joints exceed certain limits ; the maximum permissible gap allowed will depend upon the gas in which the flameproof enclosure is required to work, and these limits vary from 0.046 in. for methane to 0.001 in. for acetylene.

It will be seen that some of the internal wiring for the telephone components, also certain terminals, are not within the special enclosures. This in itself constitutes a hazard, and should one of the wires become disconnected during a conversation or while the bell is being rung a spark might occur, and an explosion might be caused. It is for this reason the two telephones are not 100 per cent. safe.

The electrical circuit for the Telephone No. 149 is similar to the standard astic circuit, and that for the Telephone No. 153 similar to the old local battery circuit, and neither calls for any special comment. There are, however, one or two mechanical features not normally found on ordinary telephones, and these will now be dealt with in greater detail.
The main physical difference between the Telephone No. 149 and the No. 153 is that the former incorporates a hand microphone, whereas the latter has twin receivers fixed to rigid arms with the transmitter located centrally between these arms. The striker for the magneto bell on both telephones passes through a watertight gland on the top of the casing.
There is a special attachment to the dial mechanism on the Telephone No. 149, which is commonly known as a “slipping clutch device,” and is fitted to prevent the mis-operation of the dial, which might otherwise produce a condition for which the telephone had not been tested. This provides that when the dial has been engaged and drawn round to the finger stop, the finger plate, when released, is disengaged from the dial mechanism and returned to its normal position very quickly under the control of a coil spring.

Referring to Fig. 5, A is the normal finger plate which is directly coupled to detail B. Detail C is directly coupled to the dial mechanism. Detail B engages with C when the dial is being operated and drawn round to the finger stop. Immediately the finger plate is released it will be returned to normal, in advance of the dial mechanism, by the spring D. A locking arrangement E is provided so that B is locked on its return to normal and cannot be operated again until the dial mechanism is returned to zero and the lock on B removed by C.
A metallic gauze cover is fitted over the front of the dial mechanism as an additional safeguard against flame being transmitted from the dial direct to the outer atmosphere via a worn finger plate spindle and bearing.

Telephone No. 153 incorporates a hand generator in place of the dial in the Telephone No. 149. The generator is a special 3-magnet type of rather more robust construction than the standard No. 4C. It has a flameproof cover over the generator contacts.

This telephone, in addition to having a flameproof certificate, is certified as intrinsically safe. This latter feature is maintained only when the telephone is used on an intrinsically safe circuit i.e., a circuit in which the energy is controlled below the incendive limit.

Briefly, the difference between a piece of apparatus designed for intrinsic safety is the incorporation of shunts across such items as generators to limit the voltage and condensers connected in shunt with inductive coils such as the magneto bell, to absorb the back E.M.F.

Flameproof Relay (P.O. Code Relay Unit C.D. 393)
This item has been designed for the purpose of relaying calling signals (such as magneto ringing current) to devices operated by electric supply mains. The reason for its introduction was the demand for a suitable loud sounding flameproof bell or visual device for use in particularly noisy situations where flameproof apparatus was desirable. As mentioned previously, no satisfactory flameproof loud sounding bell, suitable for operation on the standard telephone circuit, and by the normal ringing current, was available, and the only alternative was to provide some device similar to a relay switch which could be used to operate an electric supply mains device, of which several were available.
The relay unit comprises a 3,000 type relay with heavy duty contacts, a rectifier element 1 /12A, and a Condenser MC No. 170; a strap is provided on the top of the rectifier element to facilitate disconnecting the condenser when the relay unit is used with Telephone No. 149. The condenser is required only when the unit is used with Telephone No. 153.
All the components are housed in a substantial metal casting which, together with a terminal chamber, comprises a complete flameproof unit. The wide flanges called for per B.S.S. 229 are easily recognised from the illustration.

Protector and Fuse
The flameproof enclosure to house the Protector and Fuse No. 1 2/2 as illustrated in Fig. 7 requires no special comment. The complete unit is a properly designed flameproof enclosure with a separate input terminal chamber. Five terminals, insulated from the metal case, are provided, one of which is to facilitate bringing out the earth connection which may have to be separated from the conduit earth.

Switch Plug and Socket
The flameproof switch plug and socket is of special design. In addition to being of flameproof design, a locking device is provided in association with the switch so that the plug cannot be withdrawn with the switch in the “on” position.


To provide a reasonably safe installation in a danger area where the use of flameproof apparatus is considered necessary, special precautions have to be taken to prevent damage to the cabling. This is achieved by using (1) armoured cable, (2) Pyrotenax cable, or (3) enclosing the wiring in solid-drawn or seamless conduit. System (3) is the one used by the Post Office.
The type of conduit used for this purpose should be either solid-drawn or seamless-welded in accordance with B.S.S. No. 31. The conduit must be screwed to the telephone or other apparatus, and run to a point outside the danger area, where it is coupled up by the normal methods of construction employed for that installation. Where the circuit is being led into the premises from an O.H. line, the conduit would be terminated in a Box Weatherproof CD 364, which is provided to house the protectors. (Certain slight ,departures from this method are, however, being allowed for the duration of the war.) The ends of the conduit must be effectively sealed where it leaves the danger area, or at its termination, whichever is the more convenient, to prevent the travel of gas.

Alternatively to the effective sealing by the compound, a proper flameproof barrier between the wiring in the danger area and the safe area can be provided. This method has not, up to the present, been adopted for use on Post Office circuits.
All conduit bends must be of the solid type; inspection elbows and tee pieces must not be used. Where access to the conduit is required for drawing in or testing it should be provided by properly designed flameproof junction boxes or inspection bends. Lock nuts are usually provided at, the points where the conduit enters each piece of apparatus, but for telephone installations these are considered unnecessary.

Earthing of the Conduit
To avoid the possibility of sparking between the conduit and other earthed subjects, and between the conduit and the internal circuits due to the presence of a high potential induced on the conduit from the effects of lightning, special attention has to be paid to the efficient earthing of the conduit. The earth lead connecting the conduit to the earth electrode should be as short as possible - not more than five yards in length. The protector earth should, if possible, be separated from the conduit earth by a distance of at least 10 ft.

The lead-in for the telephone circuit to a danger building should normally be run underground, employing standard methods of construction, as determined by the circumstances of the particular site. Certain firms and large undertakings may insist upon the provision of special construction and in some cases offer the use of their own cables.
In certain classes of premises handling petroleum products the use of insulated overhead wires is being permitted as a war-time departure.
In areas where overhead distribution is employed, the protectors for each circuit would normally be fitted outside the danger area in a Box Weatherproof CD 364. Where, however, the overhead wires are permitted to be run in the danger area the protector also has to be fitted in the danger area, and is then located in a Box Protector CD 408. The provision of these boxes has been insisted upon by one of the larger groups of subscribers, who agreed to the use of the overhead wires in the danger area.
Although there is no authoritative specification for a flameproof installation at present, it is understood that consideration is being given to the issue of a code of practice for electrical installations (including telecommunications in dangerous premises), early in the post-war period. It is expected that both flameproof and intrinsic safety protection will be considered.


The efficacy of a flameproof installation in preventing an explosion lies not only in its correct installation to the recognised standards, but also in providing regular’ and efficient maintenance. Periodic and regular inspections of all flameproof installations should be made as a matter of routine, all flameproof apparatus, being examined for loose parts, frayed or damaged cords, loose material’ on the flat surface of the flameproof enclosures, etc. The conduit should be inspected for loose connections, damage by rust Or corrosion, efficiency of the earth, connections, etc.
The necessity for the prompt reporting of all faults should be impressed upon the subscriber, otherwise faulty parts, not given immediate attention, may create an undesirable hazard.

All Post Office staff whose duties involve their working in dangerous premises must comply with the regulations appropriate to the type of premises concerned, as issued by the owner of the premises. It is the duty of the officer concerned, when visiting this type of premises, to first contact a responsible representative of the firm and inform them of the type of work proposed to be undertaken, the tools,, etc., it will be necessary to use, and the apparatus to be fitted.

An endeavour has been made to give readers an insight into the main features of flameproof protection and the associated apparatus which the Post Office is able to supply for telephone purposes.
The subject of flameproof protection is so wide in scope that it is quite impossible to give but the briefest reference to many aspects of this important subject. Questions such as the “design and testing of electrical apparatus,” “types of gases, materials and premises involved,” and “intrinsic safety” would each in themselves provide material for lengthy articles.

The author acknowledges the very informative discussions which have taken place with both Messrs. Ericsson of Beeston, and Mr. J. A. B. Horsley of the Ministry of Fuel and Power (Coal Division), and which have proved extremely valuable in the preparation of ‘this article. Thanks are due to Mr. E. S. Ritter for some helpful suggestions.

Taken from The Post Office Electrical Engineer's Journal
October 1944
Vol 37 Part 3
U.D.C. 614.841 : 621395.6


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