MINING TYPE, CERTIFIED SAFE FOR MINES
These instruments are specially designed for use in mines, and have been approved by the Mines Department, as being both intrinsically safe and flameproof. They may be used on any type of line circuit, provided all other apparatus used is similarly certified safe, and that other makes of telephones are also fitted with a safety condenser. These robust instruments, wired throughout a danger zone with armoured cable, provide full flameproof protection even although they may be connected to uncertified equipment outside the danger zone.
The best characteristics of our original intrinsically safe telephones are retained, but, in the re-design, various mechanical and electrical improvements are incorporated, and the switch, generator and terminal chamber have individual flameproof enclosures which provide a generous factor of safety against danger from any outside source.
The watertight case is made in cast iron as indicated in the table below. Two wrought iron bars are fitted to the back for fixing purposes. The inner door is secured by tamper-proof screws for which a suitable key is provided.
The outer door is fitted with a slam catch and protects the speaking equipment and generator crank from damage and dirt.
The ringer hammer is operated through a greased bearing and the gongs are housed under the protective cowling on top of the case.
The generator has two extra large magnets-equal to five of the ordinary size-and the handle is strongly constructed and works through a watertight gland.
The micro-telephone fitting is firmly held on the switch hook by means of projecting pads which bear on the transmitter case, thus making the switch more positive in action. The separate receivers and transmitter unit, when raised for use, automatically operate the switch springs.
The casework and fittings, the apparatus, coil windings and wiring, are specially treated to prevent the detrimental and corrosive effects of moisture and gases.
Terminals for the lines, extension bell, and also for external speaking battery if desired, are mounted in a flameproof chamber which is provided with three junctions for the entry of cables. Two sealing glands for armoured cables are provided as standard, the third entry being covered by a cap.
The Telephone Safety Condenser used in these instruments is N3673.
Note - Equivalent sets with terminating chamber suitable for Pyrotenax cable are also available.
In order to connect an uncertified telephone to a system of certified telephones, a Patented Telephone Coupling Unit, Mines Department Type N8655 can be supplied. Further particulars and illustration on application.
For surface use in petroleum vapours, etc., certified Magneto Telephone No. N2974A
(shown right) can
be supplied, the equivalent of N2974, but with Bakelite micro-telephone.
N2974A - Armoured Cable termination
Last revised: October 04, 2019