INTRINSICALLY SAFE WALL TELEPHONES
These telephones are designed for installation anywhere in the area of risk in collieries. They may also be used in petroleum installations. A diagram showing a typical application of auto telephones in a Mine P.A.B.X. network is shown on page BB3. Connection to the unsafe circuits is made via a coupling unit.
The grey enamelled cast iron case of the instrument, which is attached to two steel mounting bars, has a main compartment enclosed by a steel gate-hinged inner door as well as by the outer door, and a separate line terminal chamber below. The inner door and the terminal chamber cover are secured by shrouded triangular-headed screws for which a key is supplied.
Ingress of moisture is prevented both by the overhanging projection of the top of the main case above the outer door, and by compression of a gasket round the case when the outer door is closed.
An instruction/directory card is attached to the inside of the outer door.
The components in the main compartment consist of an auto dial having standard numbering and a 2: 1 break: make pulse ratio, a ringer, anti-side-tone induction coil, capacitors, resistors and the switch-hook springs. All are mounted on the inner door, the ringer and some other components being on a separate plate which is hinged on the inside of the inner door so that, when this plate is lowered, access to the dial and ringer mechanism is afforded. Perforations in the rear wall of the case allow the emission of ringing signals.
The handset has a robust casing and mouthpiece and is held in position by springs which grip the transmitter. The cord has an outer cover of vulcanised rubber, and is clipped to the case to prevent strain on the conductors. A moulded plastic handset can be supplied if specially ordered.
The Type N1092A telephone incorporates a push-button key for the Operator Recall facility. The drilling for this key is fitted with a dummy in the N1091B set, which has no key.
Telephones without dials can be supplied for C.B. working if required.
P.V.C. insulation is provided on all connecting wires.
Four screw terminals in the terminal chamber allow for the connection of an extension bell if required, as shown in the diagram. There are three cable entries, two normally having s.w.a. cable glands, the other being covered by a metal plate.
If specially ordered, glands for other types of cable can be supplied.
Introduced around 1934.
Taken from the Ericsson Catalogue No. 60
Last revised: October 02, 2019