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.
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.
Telephone Code No's
Introduced around 1963.
Taken from the Plessey Telephone Catalogue 1971
Last revised: October 02, 2019