PAGE No. 54

P. F. BURTON - Apparatus Engineering Department
July 1963

A new range of flameproof wall mounting telephones for the chemical and oil industries is described. The telephones utilize the intrinsically safe handset and amplifier previously adapted to a desk instrument, the first of its kind to receive overall flameproof certification for hazardous atmospheres in the industries. Simplification of castings has made possible the development of further ranges of ironclad telephones without additional tooling.

Although flameproof telephones have been manufactured by the Company for more than forty years their construction, irrespective of usage, has been closely related to that of mining equipment for the greater part of this time. The construction of the mining telephone, dictated by the rugged conditions of its use, is necessarily robust and formed a ready and convenient basis of design for flameproof constructions also. The tooling and production costs involved in developing a separate range of equipment less massive in proportion and more suitable in appearance to typical surface applications, needed a sizeable demand to justify the expense.

The widespread growth of oil and chemical undertakings in recent times, and the expanding need for telephone equipment safe in the hazardous gas and vapour bearing atmospheres commonly found in these industries, provided such a demand and the opportunity was taken to design equipment more in keeping with the control room, the laboratory, and the chemical production line. Considerable development work was carried out by the Company in conjunction with the Oil Companies Materials Committee, BPO engineers and the Ministry of Power, which is responsible for the certification of equipment.

Such certification, for the purposes of use in the oil and chemical industries, specifies the gases in which equipment shall be flameproof (i.e. so constructed as to contain any internally generated explosion in these gases) as being of Groups 2 and 3 in BS.229, 1957. (For a definition of these groups, and a fuller account of the flameproof method of construction see Bulletin 36, January, 1958.)

The first outcome of the development was a desk telephone, introduced in 1956 and fully described in Bulletin 36. This instrument appeared in auto and c.b. forms, and to it have now been added a range of wall mounting telephones for operation on automatic, C.B. and magneto systems, the first two with or without operator recall facilities. The desk telephone, though remaining identical in mechanical details to the 1956 instrument, has been fitted with a new type of handset and associated amplifier and re-certified for Groups 2 and 3a of BS.229, 1957. This handset has not been previously publicized and, with the amplifier, it has been adapted to, and forms an essential feature of, the whole range of flameproof instruments.

The handset fitted to the earlier desk telephone was one made ‘safe by construction’, that is, having an armoured cord and inserts in the body to exclude pockets of gas. In order to provide an even higher degree of safety, attention was subsequently directed to making the handset intrinsically safe, i.e. arranging for such limitation of electrical energy within it as to make impossible the spark-initiation of a flame in the prescribed gases. This principle, entirely new in flameproof telephone design and fully acceptable to the Ministry of Power permitted full flameproof certification to be secured for the entire telephone, the first of its kind to have been so approved.

The new handset employs two identical rocking armature receiver insets of conventional type. One is used as a high performance receiver, but the other is employed as a sound power transmitter in conjunction with the amplifier, mounted in the flameproof body of the telephone. Advantage has been taken of the intrinsically safe design to improve the appearance and handling qualities by making the handset body of the latest high-impact lightweight plastic material and using an extensible p.v.c. cord.

Figure 1
Simplified diagram of Telephone circuit

The amplifier employs transistors and its operation may be followed by reference to Figure 1, a simplified circuit of the telephone. The input transformer T3 serves to match the impedance of the microphone to the base circuit of transistor TR1. Negative feedback is applied between the collectors of TR1 and the second stage TR2. Amplified speech signals are fed by transformer T1 to a point in the telephone circuit where the transmitter would normally be connected. The telephone circuit itself is of conventional design. The microphone-amplifier combination provides an output to line approximately equal to that of a transmitter P.O. No. 13 in the same circuit. The instrument line current passing through the non-linear resistor RX provides the operating potential for the amplifier to which it is applied through a rectifier bridge, thereby ensuring correct polarity with either direction of line current. The use of a non-linear resistor reduces the range of voltage applied to the amplifier under varying line conditions, thus stabilizing the gain. The transmission loss due to this resistance is minimized by a 20uf tantalum capacitor shunt. The isolation of the microphone from the telephone circuit provided by the input transformer T3, and similar isolation of the receiver by the 1:1 transformer T2, ensure full safety in the operation of the handset.

The amplifier unit is mounted on an easily removable chassis and consists of a double-sided printed circuit panel, having the three transformers of miniature design on one side and the two transistors and associated components on the other. All the components are fully tropicalized and the complete unit is connected to the telephone circuit via solder tags.

Design considerations

While the new telephones were intended to be lighter in weight and of cleaner, more modern appearance than previous flameproof equipment, it was necessary for the casework and components to be equally robust and reliable. Optional weatherproofing, giving a high degree of protection for all the external components on the front face of the instrument had to be provided, since possible situations of use could include airfield refuelling points and tanker jetties likely to be swept by heavy seas.

Figure 2
External view of Automatic Flameproof Wall Telephone

Maintenance of the telephone should be possible on site and for this purpose an isolating switch, automatic in operation on removing the instrument cover, was required both for safety reasons and to avoid affecting associated equipment.

Flameproof certification should cover gases in Groups 2, 3a and 3b of BS.229, 1957.

The telephones as designed fulfil all the above requirements.

The overall mechanical construction is uniform throughout the range and its external features can be seen from Figure 2 showing the auto instrument. The overall dimensions are 11.75” high x 13” wide x 6.75” deep (approx. 29 cm. x 33 cm. x 17 cm.).

Figure 3 shows two views of the same instrument fitted with a weatherproof cover. This cover, made of a silicon aluminium alloy chosen for its high resistance to corrosion and freedom from frictional sparking, is capable of being stayed in the horizontal position to afford some protection from rain when the telephone is in use. A neoprene gasket fits in a groove round the lip of the cover. The cover is opened by a straight pull on a stainless steel slam catch, and is then raised to the horizontal when a stainless steel latch automatically locks the cover in position. When the telephone has been used, the latch is released and the cover dropped shut, the slam catch securing the door automatically and effecting a compression of the neoprene tube on the machined face of the main body. This makes an effective seal enclosing the entire front face of the instrument. All metalwork not protected by the outer cover, such as the latches, hinges and screws are either of brass or stainless steel to avoid corrosion.

Figure 3a
Flameproof telephone with weatherproof cover in the open position
Figure 3b
Flameproof telephone with the weatherproof cover in the closed position

The main body of the telephone is of cast iron and is internally divided into three separate chambers; main apparatus, incoming line, and isolation switch, in accordance with the provisions of BS.229. The upper or apparatus chamber cover is of cast iron and the two lower covers, protecting the line and isolation switch chambers respectively, are of the same alloy as the weatherproof cover. Interchamber connections are provided between the line and isolating switch chambers and between the latter and the main apparatus chamber and are achieved by terminals moulded into a common block closely fitting in a carefully bored hole to maintain the necessary flameproof joint between adjacent compartments. The line chamber contains a moulded plate mounting three robust terminal posts of conventional design to which the incoming line is connected. The isolating switch chamber encloses a three-pole micro-switch assembly fitted to the rear face of the cover of this chamber. The main component bearing chassis of the telephone, together with the dial and ringer movement, is contained in the main chamber to which access is obtained by releasing the captive triangular-headed screws fixing the main cover. The replacement of the main cover is assisted by the provision of a locating dowel which accurately positions the cover, taking the weight prior to the refixing of the cover screws.

The cast bronze certification label is secured to the main cover by secret screws and forms a rigid plate extending over the cover of the isolating switch chamber. Into this chamber, and through the plate, passes a captive tamperproof screw which when tightened against an internal spring causes the three-pole isolating micro-switch to ‘make’, thus connecting the instrument to the line; this is the normal working condition. The arrangement is such that before opening the main cover for maintenance, the isolating switch operating screw must be retracted, thus rendering the main apparatus ‘dead’ and allowing maintenance to be carried out in complete safety. This interlock between the main apparatus cover and the isolating switch is as demanded by the regulations of BS.229.

Two four inch diameter stainless steel bell domes are located in a recess at the rear of the telephone and are actuated by a striker which passes through a flameproof bearing to the interior of the main compartment. The design of the ringer is based on the standard post office 59A 1000 ohm type incorporating a ceramic permanent magnet. This provides a vigorous movement of the striker resulting in a calling signal of considerable penetration.

The chassis, bearing the induction coil, capacitors and transistor amplifier is identical to, and interchangeable with, the chassis used in the companion flameproof desk telephone. The instrument becomes operational on removal of the handset by means of a simple internal lever action and switch arrangement.

The dial movement is a modification of the P.O. No. 21 type and is operated by an improved version of the ‘flyback’ fingerplate system, incorporating the obligatory flameproof bearing to maintain the safety of the instrument.

The recall facility where provided is achieved by a pushbutton operating through a close fitting bush to operate a micro-switch located inside the main compartment. The pushbutton is replaced by a blanking plug in the normal version of the telephone.

Mounting of the complete telephone is based on a stable three-point contact with the wall, maintaining adequate clearance between the wall and the rear of the instrument. The mounting holes provide clearance for 3/8" bolts which pass through three lugs cast integral with the main body, the lower two of which are open. Therefore only the top fixing bolt need be removed if the instrument is to be replaced for maintenance purposes, the lower two being merely slackened.

The termination of the handset cord to the main apparatus is such that the handset unit can easily be removed for maintenance without disturbing the main apparatus cover.

The magneto version utilizes a rotating magnet generator in place of the dial movement. The generator is operated via a heavy flameproof bearing, by a handle on the front face of the instrument, thus being fully protected when the outer cover is closed. A feature of the design is that the generator, of standard form, is coupled to the handle assembly in such a way that the latter remains undisturbed if the generator has to be removed for maintenance at any time. This arrangement also ensures that the thrust of any violent handle rotation is taken by the heavy bearing, thus prolonging the life of the spindle bearings incorporated in the generator.

The local 3-volt dry or inert battery required for speech purposes is a single unit housed in the main compartment. Owing to the light drain current, it has a long working life and its replacement is simplified by the use of a non-reversible ‘plug-in’ connector, obviating the need for screw type terminals.

The c.b. instrument is fitted with an operator recall button located centrally. The button, operating through a flameproof bush, actuates a micro-switch in the main compartment. An alternative version without the recall facility is available, being fitted with a blanking plug in lieu of the recall button assembly.

On all types of wall telephone, the line connections are effected via sealing glands situated on the lower face of the instrument. These glands are of the modern running-coupler type incorporating a compressible sealing sleeve to provide an adequate flameproof seal over the cable without the use of sealing compound. They are common to both wall and table flameproof telephones thus simplifying the provision of spares.

Special attention has been given to the protective finish of the casework to combat oxidization under all circumstances. After an epoxy resin dip the final coating is of silver grey hammer finish stove enamel to minimize the internal temperature rise when used in strong sunlight. This is superior in appearance and quality to any previously offered on iron cased telephones.

The design of these telephones was not undertaken as an isolated development but was part of a programme aimed at simplifying the production of ironclad telephones generally. The whole range of such telephones for both flameproof (oil and chemical industries) and intrinsically safe (mining) usage has in the past involved a total of over 30 different castings, each one associated with its own pattern equipment, manufacturing tools and production problems. It was appreciated that although the reasons for robustly constructed instruments differed between the flameproof and intrinsically safe telephones, i.e. the containing of internal explosion, and meeting the rugged conditions of underground mining respectively, the same broad principles of construction had always proved satisfactory for both, and it should be possible to design a relatively small number of castings that when assembled would fulfil the requirements of both spheres of usage. More than this, a further permutation of the same few castings would enable yet a third range of ‘Weatherproof Industrial Telephones‘ to be manufactured if desired.

This rationalisation of castings means that the evolution of the proposed new ranges can proceed with relatively little further tooling or development effort, since the major part of this has already been covered in the present range of flameproof telephones.

The new range of telephones, incorporating the intrinsically safe handset circuit and transistor amplifier, gives an exceptional degree of efficiency coupled with an order of safety that is the highest yet certified by the Ministry of Power for telephones to be used in the gaseous atmospheres of the oil and chemical industries.

The author wishes to record his appreciation of the assistance received from the Chief Testing Officer of the Ministry of Power Research Establishment, Buxton, the Engineers of the Post Office, and the Oil Companies Materials Committee.

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Last revised: October 15, 2019