Introduced in 1957 the Telephone No. 702 is a table type instrument with the same physical appearance as the Telephone No. 266, and is suitable for lines of up to 1000 ohm loop resistance. The handset is intrinsically safe, as d.c. does not flow in the handset cord (Ericsson N 9622). Both the transmitter and receiver are transformer coupled to the appropriate section of the telephone circuit. A rocking-armature receiver is used in this telephone as a transmitter, transmission taking place through a two stage transistor amplifier. A Bell No. 69B must be used with this telephone. For maintenance instructions see Z 5013. Telephone No. 702 supersedes the Telephone No. 266.

Supplied by Ericsson, their Model N1881X1.

Diagram - N802.

Bell No. 69B
The Bell No. 69B is similar to the Bell No. 69A but is the superseding item, and is used with the Telephone No. 702. For maintenance instructions see Z5013. Bells No. 69A and 69B must demounted with the hinge at the left hand side and should be mounted under cover.

Relay unit CD 1982
This flameproof enclosed relay (ordered separately) is required to provide the switching facilities for mains-operated flameproof enclosed apparatus such as loud sounding bells, hooters and visual signalling devices; it should be connected in circuit as for an extension bell. The terminal chamber is provided with two 0.75in conduit side entries. Overall dimensions of the unit are 10in x 9.5in x 4in, the weight is 27 lb.

A New Flameproof Table Telephone with an Intrinsically Safe Handset

THE development of a flameproof telephone poses the problem of the safety of the handset and cord, which cannot be of flameproof construction like the telephone case. A new flameproof table telephone has been developed, in which the difficulty has been overcome by making the handset and cord intrinsically safe.

The present flameproof table telephone (Telephone No. 266) was introduced a few years ago to meet the requirements of many industries and in particular the oil industry. In the design of the Mark II version of the Telephone No. 266 a degree of safety was achieved by adopting a suitable design and standard of construction for the handset and cord without modification to the circuit design. In the interests of further safety, however, the preferred arrangement is an intrinsically safe handset with the line current excluded from the transmitter and cord, thus eliminating the possibility of fire or explosion caused by a spark should the cord be severed or the transmitter connexions become faulty.

For flameproof equipment the components are housed in a specially constructed flameproof enclosure (telephone case in this instance) which will withstand an internal explosion and prevent the transmission from within of a flame that might ignite any prescribed inflammable gas or vapour in the surrounding atmosphere.

Intrinsic safety, which is achieved by circuit design and the choice of components, implies that any sparking that might occur either during normal working or under specified fault conditions is incapable of causing an explosion of the inflammable gas or vapour. Such a handset has been made possible in the development of a new flameproof table telephone (Telephone No. 702) by the use of a rocking-armature receiver as a dynamic microphone and a transistor amplifier associated with a telephone circuit of conventional design and suitable for use on lines of up to 1,000 ohms loop resistance. A standard handset cord carrying speech currents only is used.

A 2-stage amplifier is housed within the flameproof telephone case and is mounted together with the normal telephone components on an easily removable chassis.
Speech currents from the microphone are fed to the input of the first amplifier stage via transformer T3, which both matches the impedance of the microphone to the base circuit of transistor VT1 and isolates the microphone from the telephone circuit. The first stage is resistance capacitance coupled to the second stage, VT2, which has an output transformer, T1, in its collector circuit. Amplified speech signals are fed into a standard telephone circuit at a point normally occupied by the transmitter. The microphone-amplifier combination provides an output to line approximately equal to that of a Transmitter Inset No. 13 in the same circuit. The instrument line current, passing through the 68 ohm resistor, 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 transmission loss due to this resistance is minimized by a 20 uF tantalum-capacitor shunt.
As an additional safety precaution the handset receiver is connected to the telephone circuit through a 1:1 isolating transformer, T2.

With flameproof table telephones it is necessary to have a separate bell set which also accommodates the terminals for the line wires. A new bell set (No. 69B) has been introduced for use with the new telephone.
Certification by the Ministry of Power for both Telephone No. 702 and Bell Set No. 69B covers Groups II and IIIa of B.S.229 : 1957, Flameproof Enclosure of Electrical Apparatus.

Taken from The Post Office Electrical Engineers Journal
January 1962
Vol 54, Part 4.
U.D.C. 621.395.721.4 614.83


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Last revised: December 02, 2018