Certified flameproof for groups II & III gases.
Casework of iron and Aluminium.
Model No. N2981X1 was supplied to the GPO as Telephone
Flameproof Wall Telephones
Auto, C.B. and magneto
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 undercover, 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 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 underside 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.
The 1000 ohm ringer has two stainless-steel loud-ringing gongs of 4in. (102 mm) diameter fitted on the outside rear of the case; the striker arm projects through a flameproof bearing.
The C.B. telephone is identical with the auto type apart from the exclusion of the dial and its mounting aperture. The inclusion of a press button is optional.
The magneto telephone is equipped with a hand generator of rotating-magnet type and a 2000 ohm ringer. A 3V intrinsically-safe battery, dry 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.
All telephones are to Certificates FLP 4683 Factory dept IS 3186 (hydrogen, ethylene, pentane).
Taken from Plessey Technical document (7094) - 1969
Last revised: October 04, 2019