PAGE No. 48

A Protected Telephone System for use along High Voltage Transmission Routes
January 1933

ERICSSON Telephones Limited have supplied a large number of telephones and associated equipment for use on Power Systems in all parts of the world and have gained considerable experience in this class of telephone service


Our well-known magneto use on high-voltage routes was introduced some years ago, and gives excellent results providing the telephone lines are of moderate length and good construction. It is absolutely safe to use, as complete protection is obtained  by means of the long rubber tubes shewn in the illustration. This design, however, with long rubber connections between the 'dummy' handset and the actual transmitter and receiver fitted in the case of the instrument, is somewhat limited as regards range of operation, especially if the telephone lines are not of good construction.

Owing to considerable development during recent years of high-voltage overhead transmission, and of the much greater lengths of lines installed, a demand has arisen for a more efficient telephone system for the use of Power Companies.

The Ericsson Company has developed a comprehensive scheme for this purpose, and has recently supplied a large amount of high-voltage telephone equipment to the Madras Hydro - Electric Development Company for its overhead system in India. A brief description of this system may be of interest to engineers engaged in high-tension transmission.

Standard Metal Cased Telephone Instruments are employed, with a very complete protective arrangement to ensure absolute safety to the persons operating the system.

In general, the arrangement consists of an equipment of fuses and arresters, together with special transformers which provide complete isolation for the actual telephone instruments during use. The switchboard is built up of white marble panels on which are mounted the various switches, fuses, etc.

The construction, it will be noted, is on the lines of a power control panel, and all connections controlled by the operator are made by means of robust quick-acting knife switches with bus-bar connecting circuits.

Calls are received at the switchboard on magneto bells fitted with drop shutters to serve as indicators. The indicators are arranged to operate a call bell, and if desired, to light a pilot lamp.

Relays of the ' mercury tube ' type are provided to control the pilot lamps which are connected to the lighting supply.

Should atmospheric or other causes render the lines noisy during a conversation, 'drainage' coils can be switched into circuit; these coils allow the lines to discharge to earth, but at the same time they are of such impedance that they do not impair the telephone transmission in any way.
Long-distance high-voltage transmission lines are at times subject to heavy potential surges, and where telephone lines run in close proximity, high voltages may be induced on the telephone system. This is one of the main causes of trouble on this class of telephone service. There is also the further risk that through gales or heavy snowstorms the overhead power conductors may come into contact with the telephone wires. This is, no doubt, a rare occurrence, but it must be allowed for in a telephone system of this type if absolute safety is to be obtained.

The method of protection in the Ericsson high-voltage telephone system is to isolate the telephone instruments by means of special transformers, and at the same time to arrange the circuits so that external connections are immediately broken should dangerous high voltages occur on the telephone lines; by this means the telephone instruments are protected from damage and any persons using them are rendered immune from danger.

The general arrangement of the various protective devices - fuses, arresters, isolating transformers, etc. - is clearly shown on the circuit diagrams, and the function of the apparatus will no doubt be readily followed; however, a brief description of the various items of equipment may be of interest.

The Horn-Type Arresters, the centre points of which are earthed, are directly connected to the external lines. A discharge to earth will occur should the voltage of the telephone lines rise to about 3,000 volts or over, due, either to a 'surge' or a direct contact with the power wires. In both cases the fuses on the telephone switchboard would blow and disconnect the apparatus from the external lines. Where a direct contact with high-tension wires occurs, the current through the horn type arrester will probably rise to such a value that the telephone line will fuse and break connection with the high-tension system. The arresters are adjustable for any (approximate) voltage of from 3,000 volts upwards.

The Isolating Switches consist of copper links which can be withdrawn by means of a long insulated rod having a hook at the end. When the links are removed the switchboard is disconnected from the line, but the 'horn' arresters, being 'outside the links', are still in circuit as a safety gap to earth for the external telephone conductors.

The High-Tension Fuses are specially designed for high voltage operation and will disrupt quickly. A discharge to earth through the carbon arresters will blow the fuses and disconnect the telephone from the high voltage.

Carbon Lightning Arresters consist of the usual arrangement of carbon blocks with a small air gap to earth. The design in this case, however, is somewhat special as the carbon blocks are so shaped and mounted that they tend to be self-cleaning as regards carbon dust. The arresters discharge to earth at about 1,000 volts.

The Rare-Gas Arresters are of similar design to the well-known 'Vacuum' type, but are filled with rare gas. Their inclusion in the circuit does not affect the transmission on the telephone lines, but should the potential on the lines rise to over 250 volts, then a discharge to earth takes place in the usual way. In the Ericsson high-voltage telephone system they constitute an extra safety gap over the carbon lightning arresters.

Isolating Transformers
In order to ensure absolute immunity from personal injury through high voltage, the telephone instruments are so arranged that all external connections are made via the windings of special transformers. The insulation of the transformers is such that they will stand a coil-to-coil pressure up to 20,000 volts, and as the fuses and arresters ensure practically instantaneous disconnection of the external circuit, should high-voltage contacts or surges occur, it will be appreciated that all risk of injury through shock is eliminated. The design of the transformers is such that the loss of efficiency in speech transmission is negligible.

Low-Tension Fuses are included on the 'inside' connections of the transformers and, as in the case of the rare-gas arresters, they are an extra precaution to the high tension fuses.

The Double-Pole Changeover Switches, which are of the quick make and break type, are for connecting the external lines to the various circuits; they are enclosed in metal cases which together with the operating handles are efficiently earthed.

The Telephone Instruments used in the main and sub-stations are the Ericsson all metal magneto bridging type as shown at the left of the switchboard illustration. Bakelite moulded hand sets are provided and the cords are specially insulated to withstand damp and rough usage.

Portable Telephones are provided for the use of linemen and patrolmen, and are of two types, namely:- For use on telephone lines or tapping points where direct contact with high voltage conductors is not possible, and a specially protected instrument, for use on telephone lines running on the same poles as the power lines.

The former is contained in a leather case with carrying straps, and in order to make connection with the sockets fitted on certain poles, watertight plugs and heavily insulated cords are supplied. If desired, suitable instruments can be provided permanently fitted in weatherproof boxes for fixing to poles.

The latter instrument is specially designed to provide extra safety should the telephone conductors become raised to a high potential. To operate the instrument, connections are made to the telephone wires by means of long insulated rods built up from sections. The top section is fitted with hooks suitable for engaging the telephone wires and special high voltage low capacity fuses are also equipped in this section. The instrument is fitted with rare-gas arresters and a terminal is provided for earthing the case. Unless otherwise available, earth connection is made by driving an earth rod into moist ground and connecting there from to the terminal on the set. This instrument is designed for safety under all conditions of working.

Both types of portable telephones are provided with a moulded Bakelite handset having a heavy rubber insulated cord.

In each Power Transmission Scheme the local conditions will naturally vary, and we shall at all times be pleased to prepare layouts and circuits to suit any requirements in this class of telephone engineering.

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Last revised: October 13, 2019