TELEPHONE No. 702
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.
Relay unit CD 1982
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.
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.
Taken from The Post Office Electrical Engineers Journal
Last revised: May 02, 2010