Cable, normally of metallic braided, p.v.c. insulated type, is supplied to order. A flameproof cable plug, with associated socket for mounting on the jetty, can also be supplied.
Description of Telephones used
The instruments are suitable for 1000 ohm loop working and are interchangeable with the earlier 600 ohm loop flameproof type. When required, extra facilities such as operator recall or transfer are catered for by a press button situated in front of the handset. The bell unit associated with each instrument is for permanent wall mounting and accommodates screw terminals for line connection. Before shipment, the unit is connected to the telephone by armoured, plastic-sheathed cable of sufficient flexibility to allow reasonable movement by the user.
Since it is impracticable to flame-proof the handset, it is made intrinsically safe by transformer coupling of the handset transducers with the remainder of the transmission circuit. The handset and cord have been satisfactorily tested in hydrogen by the Ministry, but flameproof certification for both types of set is limited to flameproof groups 2 and 3, BS 229-1957.
The telephone and bell-unit cases are made of silicon aluminium alloy and, to prevent
unauthorised access, are secured by shrouded triangular-headed screws released by the
special key provided.
The apparatus comprises a dial, transistor amplifier, plunger and press button springsets, together with induction coil and capacitor. The dial mechanism, fixed to the telephone body, is coupled to the finger-plate by a spindle through a flameproof bearing. All other apparatus is unit-assembled on a bracket secured to the case by four screws. The telephone can be used as a c.b. instrument by locking the dial. The handset rests on two roller-type plungers. When the handset is lifted, the rollers move over a plate which operates the spring-sets, the whole movement being practically frictionless.
Safety and high-quality transmission are assured by the use of rocking-armature transducers in conjunction with isolating transformers and an amplifier within an improved anti-side-tone circuit. The coiled, high-temperature resistant, p.v.c. handset cord, and the 4-way cord to the bell are connected via glands to screw terminals in the terminal chamber at the back of the telephone.
Bell Terminal Unit
Last revised: October 07, 2019