TELEPHONE No. 153
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Telephone No. 153 is superseded by Telephone No. 723 for new work.
The Mark I was introduced in 1941 and the Mark II in 1961.
Telephone included (1956):-
Circuit Diagram N 253.
Made by British Ericsson, their models N2974 & N2984
Taken from The Post Office Electrical Engineer's Journal
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
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 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.)
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.
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.
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 the picture to the right, 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.
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
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.
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.
In certain classes of premises handling petroleum products the use of insulated overhead wires is being permitted as a war-time departure.
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.
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.
Last revised: September 20, 2019