PLESSEY TELEPHONE No. N29407

Flameproof
portable
Telephones
Auto

Portable Telephone
The portable telephone is used primarily for temporary ship-to-shore communication and consists of a telephone and bell terminal unit as described. Both are contained in an oiled and waxed hardwood case having a woven-fabric shoulder-carrying strap and brackets on the outside for storing the flexible cable. The lid of the case is stayed so that the telephone can be used while in the case.
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
These lightweight telephones (type N1881A shown above) comprise a table and portable type approved by the Oil Companies Material Committee in the UK for use in the petroleum and allied industries. Typical areas of application include offices, laboratories, control rooms, and tankers requiring temporary ship-to-shore  communication.
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.

Table Telephone
The telephone has four rubber feet secured by conventional screws to the base. This mates with the instrument body and is fixed by six triangular-headed screws.
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
This unit is in two parts, hinged and fastened together by four screws. It is arranged for 3-point fixing and must be mounted vertically with the hinges to the left. Screw terminals for line connections are in the base of the unit; the ringer is inside the cover,
with the core irons and magnet extended through the metal to allow the armature and strike hammers to be under the dome. No adjustment is necessary with this type of mechanism. Four cable entries are provided, one having a gland for the telephone connecting cord, the remainder being threaded to suit a variety of connecting glands for the line cables, and an extension bell if required. Plugs for two entries are supplied as standard.

Models

Taken from part of Publication No. 7149 (1971)

Last revised: April 07, 2000