IRONCLAD, CERTIFIED INTRINSICALLY SAFE
CERTIFICATE T/Tel./84 (methane)
CERTIFICATE No. 78 (petroleum vapour)
These telephones are certified intrinsically safe in combustible firedamp mixtures, or in petroleum vapour, and will give satisfactory service under the most arduous conditions either below or above ground.
Any number may be connected in parallel and they may be used with the Company's certified switchboards, relays, bells, etc.
The grey enamelled cast iron case has inner and outer doors and a terminal chamber below, the inner door and terminal chamber cover being locked by special screws, while the outer door, protecting the speaking equipment and generator crank, has a slam catch.
The micro-telephone on the N2976 type instrument has a cast aluminium alloy casing and mouthpiece and is held in position by springs which grip the transmitter.
The N2986 type set has a fixed transmitter with a perforated stainless steel guard, and pivoted twin receivers which move in unison and rest on rubber blocks.
The internal components, excepting batteries and terminals, are mounted on the gate hinged inner door so that when the wires have been disconnected at the five screw terminals below the ringer, the door can be lifted off, enabling adjustments to be made in any convenient place.
At the rear of the inner door are two covered compartments, one housing a powerful
"alnico" generator, and the other the induction coil, capacitors, switch springs
and, on N2986, the inset transmitter. The ringer is mounted on the outside of the hinged
cover of the upper compartment, so that the sound from the gongs can be heard through the
perforated plate at
the top rear of the instrument.
Batteries, type N3752, for speech current, are housed in a separate compartment of the main case but are not supplied unless ordered. They stand on projections formed in the metal case in order to prevent their deterioration by water from condensation collecting in the bottom of the case.
The terminal chamber has three entries for cable, two fitted with sealing glands for armoured cable and the other with a plug. Glands and plug are interchangeable. Terminal chambers suitable for conduit can also be provided.
All parts, coils, etc., are tropically finished, to withstand the corrosive effect of moisture and gases.
Both telephones were introduced in 1950.
Dimensions:- 19.25 x 11.5 x 8 inches
|Code No.||Termination Chamber||Weight without Batteries lb.|
|N2976B||3/4in Screwed Conduit||56|
|N2986B||3/4in Screwed Conduit||64|
Note - The N2976 type instrument can also be supplied with a Bakelite micro-telephone (N2976A).
In order to connect an uncertified telephone to a system of certified telephones, a
telephone coupling unit, Ministry of Fuel and Power type N8655, can be supplied.
Taken from the Ericsson catalogue, edition No. 49
Also found in Ericsson catalogue, edition No. 55, Section 5
N2976C & N2986C were terminated with Armoured Cable.
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