No. 8 PAGE No. 42

New Ericsson Mining Magneto Telephones

Since the article on “Mining Telephones, Switchboards and Apparatus“ appeared (The Ericsson Bulletin, No. 3, July 1933), three new types of certified magneto telephones have been evolved. The two most important of these, Types N2974 and N2984, in addition to being “intrinsically safe“ are also “flameproof.” These two instruments supersede the “intrinsically safe” Types N2972 & N2982. The opportunity has been taken to include in the new production, apart from the flameproof feature, all the improvements that experience has shown could be effected.

In general external appearance, with the doors closed, types N2974 & N2984 are identical. The N2974 type is a hand microtelephone instrument. The handmicro has the latest pattern inset transmitter of robust construction and high efficiency,
and locating springs keep it securely held when not in use. The switch-hook operates through a flameproof bearing and actuates the transmitter and line circuit contacts by cam action. The N2984 type has the receivers on swing-out arms, the action of lifting the arms bringing the transmitter mouthpiece into the speaking position, and at the same time by special flameproof mechanism operating the switch contacts. This instrument has a solid-back type replaceable inset transmitter of the N7742A type, which has an extremely good frequency response and in addition to its high efficiency it is robust and as moisture resisting as possible. The remarkable frequency response of this transmitter is illustrated and those interested are referred to the article "Transmission Testing" which appeared in The Ericsson Bulletin No. 4, Jan. 1934. As an additional safeguard against interference with the diaphragm a wire gauze has been added to the grille. The high efficiency calling bell is of comparatively high impedance and the powerful generator has its protective winding incorporated in the armature and connected directly across the active winding.

N2974 N2984

The N.2984 type is also made with an aluminium case which gives an approximate reduction of 42% in weight.

A Mines Department telephone safety condenser is fitted as standard to these instruments. The handle fixing has been improved. The wiring has been carried out in a special wire, enamelled, double silk and single cotton covered, finished with cellulose acetate lacquer. This cable has a smooth, highly polished surface and is easily cleaned, dirt does not readily accumulate, the colours are easily recognised, the moisture resistance is good, consequently the electrical properties are enhanced, while the inert cellulose covering is an excellent protection against accidental contact with any corrosive substances, oils, &c.

Turning now to the flameproof features, the principle was adopted of placing in individual flameproof enclosures the points where circuit interruption can take place, namely, the generator contacts and the switch contacts. By this means the volume of atmosphere involved can be made very small and as standard flame path lengths are maintained the highest factor of safety is achieved. The terminal chamber is normally provided for the connection of armoured cable. Three points of entry are provided with two sealing glands and one cap as standard. These cable entries are not drilled, but are centred for drilling. The sealing of the armoured cable is effected in the gland in a very simple manner, as can be seen in the illustration. One advantage of this method is that where desired the appropriate lengths of cable may have the glands fitted and sealed at the surface or other convenient place, the cables then being installed and the glands bolted to the instrument on site. The terminal chamber can also be arranged to receive heavy gauge seamless screwed conduit instead of the armoured cable.

Tamper proof screws are fitted on both the terminal chamber cover and inner door. Each type of instrument carries the flameproof designation mark on the outer door and the intrinsically safe approval label upon the inner door.

These two types are thus fully equal to any requirement of service. They can be used as intrinsically safe instruments either on an all Ericsson system or in a mixed circuit using certified instruments of other makes, providing of course that the latter have been fitted with the telephone safety condenser as required by Mines Department Circular No. 75. Such circuits are perfectly safe and even if bare wires were used no risk of ignition would occur. Where, however, mixing is employed and even one instrument or subsidiary apparatus is not of certified type, then every portion of the circuit in question which comes within the danger zone (M.D. Circ. 75) must be provided with armoured cable and flameproof instruments. The new instruments, being flameproof, can equally be used on such circuits. One great advantage of this, apart from the question of maintenance stock, is that where a colliery embarks on a modernizing programme with complete intrinsic safety as their aim, these new instruments can be installed in the areas required to be “flameproof,” the older non-safe equipment being apportioned to the portions not in the danger zone. Then as replacements of these older instruments are to be made they can he replaced by certified types until the system is completely in approved form as intrinsically safe.

For the termination of armoured cable outside the danger zone a flameproof terminating box can be supplied. This is designed on similar lines to the terminal chamber of the above instruments, but the bakelite terminal plate carries four metal plates each with two connecting screws.

Where a safe circuit is connected to a surface telephone it follows that such an instrument must be certified also. The Wall Telephone, Type N2504 was already available for this, but for those cases where the executive desires the telephone on his desk a new instrument has been developed.

This Magneto Table Telephone, Type N2155B, certified 30th September 1935, is a self-contained set with the calling bell and telephone safety condenser in the base. The metal case containing the generator, etc. is surmounted by the latest bakelite pedestal and cradle carrying a modern bakelite hand micro-telephone. The set is of particularly neat design and most accessible for examination and connection. The base plate is single hole fixing, while the removal of two screws permits the main cover to be withdrawn. Telephone safety condensers are of course required on the other instruments, if not already fitted, in accordance with M.D. Circular No. 75.

Type N 2504
Magneto Mining Wall Telephone
for use on the surface under
Type N 2l55B
Magneto Mining Table Telephone
for use on the surface under


BACK Home page BT/GPO Telephones Ericsson Telephones Ericsson Home Page Ericsson PABX All Telephone Systems

Last revised: June 24, 2003