Electrolux Limited
Telephone and Staff Locator Equipment
July 1937

In the last few years Ericsson Telephones Limited have become increasingly active in the development and installation of Private Automatic Telephone Exchanges (P.A.Xs.) and many special facilities and auxiliary services have been designed as a result of studying customers' particular requirements. It is the purpose of this article to describe a typical installation, that of Messrs. Electrolux Ltd., the well known vacuum cleaner and refrigerator manufacturers.

The installation consists of a P.A.X. and a 'staff locator' or 'round call' system.

THE TELEPHONE EQUIPMENT
The P.A.X. equipment, as shown in the illustrations, consists of a 50-line unit, an automatic charging panel, a main frame and a battery. Facilities are incorporated for extending to a maximum of 400 lines by the addition of further 50-line units.

A previous issue of  'The Ericsson Bulletin ' has described in detail the register controlled P.A.X. units similar to that installed at this exchange. It will therefore be sufficient to mention here the main features.

The equipment consists of 50 line relays, 7 line finders and group selectors, 6 connectors, 2 registers, a pole changer and tone set. The line finders and group selectors, connectors and registers comprise uniselectors and relay groups.

Fuse panels, equipped with fuses of the alarm type, are fitted at the top of the unit for protection of the various circuits. At the left hand side of the unit, adjacent to the two lower shelves of uniselectors, can be seen battery and test jacks, register busy keys and connector links, all of which are utilized for routine testing purposes.

All apparatus conforms to the latest Post Office practice and the relays are the P.O. 3000 type incorporating twin contacts.

Relay groups are protected by individual dust covers.

The unit is completely enclosed by a sheet steel cabinet with removable doors at front and rear.

The power plant comprises a single battery of 24 secondary cells in sealed-in type glass boxes of 50 ampere-hour capacity, floated by an automatic charging unit. The function of the latter is to keep the battery in a sufficiently charged condition and at the same time prevent the voltage from exceeding the operation limits of the telephone apparatus.

The charging panel is a self-contained unit comprising mains transformer, metal rectifier, smoothing equipment and all control apparatus, and is mounted adjacent to the P.A.X. cabinet, as shown in the photograph.

The control meter, which is a miniature contact voltmeter, is mounted at the top left hand corner of the panel and operates when the voltage of the battery reaches the predetermined low or high limit. The charging rectifier is then switched in or out of circuit as the case may require. Should the control meter fail to function and the battery continue to discharge or overcharge, the alarm meter which is on the opposite side of the panel, and is also a contact voltmeter set to operate at slightly higher and lower limits, then comes into operation and lights the appropriate alarm lamp.

The ammeter in the top centre of the panel registers the current taken from the rectifier during charge. The top panel also mounts the keys for manual operation and routine test, supervisory lamps and battery fuses.
Immediately beneath the top panel are mounted the various relays for controlling the circuit ; these are protected by a common cover. The adjustment of the charging current is carried out by means of the regulating switch and sliding resistance mounted on the bottom panel of the unit.

The chief feature of the power plant is, of course, that it operates entirely automatically, requiring no skilled supervision and no maintenance apart from periodical attention to the battery as regards electrolyte.

The scheme can be applied for either direct or alternating current supplies. Where the supply is D.C. a small motor generator set with automatic motor starter is installed in place of the transformer and rectifier.

THE STAFF LOCATOR EQUIPMENT
The staff locator or round call system is controlled by an attendant and has no connection with the P.A.X. except that it is served by the same battery. The equipment consists of the control board and the signals, the latter being mostly of the audible type.

The control board consists of two items, viz, the operating panel and the control unit. The operating panel is attached to

the front of the manual board used for routing public exchange calls. It is equipped with a dial, dial key and two lamps. By operating the dial key and then dialling two digits the operator can set up any one of 50 different codes, the signals being situated in the various departments of the works. One of the lamps, coloured green, responds to the impulses actuating the signals, thus providing the operator with a visual check as to whether the correct code has been dialled. The other lamp, coloured red, is a pilot lamp and indicates to the operator that the dial key is operated and the equipment either ready to receive a call or that a call is being transmitted.

The control unit is a wall mounted hinged case fitted away from the operating panel ; it is equipped with three uniselectors and a relay group. The dialled impulses from the operating panel set the switches into any one of 50 different positions, according to the digits dialled, and the circuit is completed for pulsing out the required code to the remotely situated signals.

The signals consist of bells, klaxons of the buzzer type, klaxons of the hooter type, and lamps, according to the location. The audible signals, consisting of the first three mentioned, are all mains operated but the lamps are illuminated directly from the battery pulses transmitted from the control unit.

In the case of the mains operated signals it is necessary to fit a special relay in the circuit. This relay responds to the code pulses and brings in the signal or signals (in parallel) via its local contacts. The case in which the relay is fitted also includes an ironclad fuse cut-out for terminating the mains feeds. The relay contacts are twin tungsten owing to the fact that they are in circuit with the mains supply.

The equipment has proved extremely useful in finding any wanted person throughout the works and is giving entire satisfaction, thus another Ericsson product is conforming to the usual high standard of the Company.

The Electrolux Site in Luton, Beds

Last revised: October 14, 2019