PLESSEY TELEPHONE
No's N1473A, N1058A, N1059A & N2987A

Flameproof 
Wall Telephones
Auto and magneto

These telephones are designed for use in areas subject to fire hazard and are produced in two  main patterns for connection to automatic, c.b. or magneto systems. For c.b. systems the auto telephone (N1059A, shown on the right) is used, with its dial locked. Both have the same general construction and, compared with previous types, offer reduced size and weight in addition to improved transmission performance.

Certification
Because it is impracticable to flameproof the handset and connecting cord, these components are made intrinsically safe by transformer coupling of the handset transducers with the remainder of the transmission circuit. The handset circuits have been satisfactorily tested in hydrogen by the Ministry, but flameproof certification is limited to flameproof groups 2 and 3, BS229-1957.

General Features
Each telephone is housed in an ail-metal, silver-grey case equipped with a weatherproof cover.  The main apparatus, line-termination field and line-isolation switch are accommodated in separate compartments fitted with stove-enamelled, hammer-finished covers. These are fixed by triangular-headed screws to restrict access to authorised personnel issued with the special key provided with the telephone.  Installation is facilitated by the provision of open-ended wall mounting lugs in the lower end of the case.

The main body of the instrument and the upper (main apparatus) cover are of cast iron, whilst the lower compartment covers are of silicon aluminium alloy, a material noted for its high resistance to corrosion and freedom from frictional sparking.
The weatherproof door is opened by a straight pull on a stainless-steel slam catch, and can be raised to the horizontal and automatically latched in this position to give partial protection against the weather when the instrument is in use. If the telephone is installed under cover, the door can be dispensed with as this in no way affects certification.
The isolation switch comprises a 3-pole microswitch interlocked with the main apparatus cover.  Withdrawal of a captive screw in the isolation-switch cover causes the microswitch to release the interlock, so disconnecting the line and allowing the instrument to be serviced in complete safety in its fixed position.

The handset, moulded in high impact resistant material, incorporates rocking-armature transducers which are used with a sensitive anti-side-tone induction coil to ensure high-quality transmission on lines of up to 1000 ohm loop resistance, including the telephone. The handset cord, which is high-temperature resistant, is the coiled extensible type, and may be disconnected for maintenance without opening the main cover. Connecting wires are insulated with p.v.c.  Three cable entries in the under-side of the case allow for the connection of line wires and an extension bell if required. The entries are threaded to accommodate various connecting glands , and plugs for two entries are supplied as standard.

Special Features
The auto telephone features a trigger dial fitted with a 'fly-back' fingerplate; the dial has standard numbering and 2:1 break/make
pulse ratio unless otherwise ordered. A press button for operator recall or line-intrusion purposes is an optional feature.
The 1000 ohm ringer has two stainless steel loud-ringing gongs of 4in (102mm) diameter fitted on the outside rear of the case; the striker arm projects through a flameproof bearing. The magneto telephone (N2987A, shown right) is equipped with a hand generator of rotating-magnet type and a 2000 ohm ringer. A 3V intrinsically-safe battery, or inert, can be accommodated in the main compartment; the batteries are arranged for plug-in connection.

NOTE: Only intrinsically-safe batteries with integral current-limiting resistor should be used with magneto telephones.

Dimensions
Height. 13.5in (341 mm)
Width: 13in (330mm)
Depth: 7.5in (181 mm)

Weights (approx.)
74lb (33.5kg) net

Telephone Code No's

Introduced around 1963

Taken from the Plessey Telephone Catalogue 1971
Publication No. 7158/1

Click here for an article from the Ericsson Bulletin

Last revised: July 28, 2003