Ericsson Equipment at Broadcasting House
|Broadcasting House, London
EARLY in 1926 the B.B.C. decided that their premises at Savoy Hill were inadequate for their rapidly increasing activities, and commenced to search for larger headquarters which would accommodate a sufficient number of studios and house the whole of their staff of approximately seven hundred. It was originally intended to adapt an existing building, but after consideration of a number of possible properties it was realised that the many special requirements would necessitate new premises.
In 1928 a site was acquired at the corner of Portland Place and Langham Street which appealed to the B.B.C. as being ideally situated and easily accessible from all parts
of London. Building work commenced immediately after the architects plans had been approved, and eventually the final result is the magnificent structure shewn in the accompanying illustration.
It will be appreciated that an important factor in the complete co-operation between the various sections in a building of this size is an efficient telephone system, and this was given early consideration by the B.B.C. authorities. It was decided that the main requirements could best be served by an automatic exchange, while certain of the executives would also require additional facilities such as given by a combined auto and inter-communication system.
It was accepted as a compliment that Ericsson Telephone Equipment was chosen for this outstanding building.
A three-digit automatic system was provided, operating on step-by-step principles, using British Post Office pattern single and two-motion type switches. Twin contacts are fitted on all relays to minimise trouble that may be caused by dust. The equipment of the exchange was originally 200 lines but has since been increased to 270 lines. Rack capacity is available for a maximum of 300 lines. The present numbering scheme is 200-469 inclusive.
In accordance with modern practice, line finders are provided to effect an economy in space and apparatus. Fifty point
single motion type finders are employed, therefore the equipment is split into groups of 50 lines; five complete 50 line groups and one incomplete group of 20 lines making the total 270 lines. Directly connected to each finder is a selector, so that the five complete 50 line groups are each served by six finders and selectors, while the sixth and incomplete groups of 20 lines is served by three finders and selectors. The final selectors are of the 100-line two-motion type, twelve being allocated for each of the 200 and 300 groups and nine for the 400 group.
The switching equipment is mounted on 8’ 6” standard open-type racks with
channel type shelves. The line relay equipments are accommodated on a framework similar to that used for the special apparatus rack in public exchanges. All equipment, other than the line relays and miscellaneous alarm apparatus, is “jacked-in” and can be readily removed for localizing faults, etc.
|Part of the Automatic Equipment
The system operates from a 50 volt supply, and duplicate batteries, each consisting of 25 cells and having a capacity of 200 ampere-hours at the 10-hour rate, were supplied. A motor-generator having an
output of 40 amperes at 57 volts is provided for charging purposes. Ringing and tones are generated by a dynamotor operating from the 50 volt supply. Duplicate machines are fitted and arranged so that in the event of machine No. 1 failing, machine No. 2 is automatically brought into service. The dynamotor runs continuously during the day but at night the automatic operation of a time switch causes circuit changes, and the dynamotor runs only during the progress of calls.
Briefly, the switching operations during the progress of a call are as follows:-
When a receiver is removed from the switch-hook a line finder, associated with the group in which the call is originated, searches for the calling line. When the calling line is found, dial tone is transmitted to the calling party, by the group selector associated with the finder in use, to indicate that dialling may commence. The first train of dialled impulses steps the group selector to the desired hundreds group, and automatic
search then takes place for a free final selector in that group. Subsequent impulse trains step the final selector to the desired line in the selected hundreds group in the usual manner, ringing tone or busy tone being transmitted to the calling party to indicate the condition of the called party’s line.
First-party release is provided in the final selector circuit, so that the called party’s line is not held busy in the event of the calling party failing to replace the receiver at the termination of a call.
To ensure that common equipment is not held unnecessarily, permanent loops due to receivers being left off, etc. are automatically disconnected from the switching equipment after a period of between 30-60 seconds, by a special fault relay. The removal of the loop from the line after a forced disconnection has occurred, automatically releases the line circuit and allows further calls to proceed in a regular manner.
Visual and audible alarms are provided to indicate blown fuses, ringing failure, and switches which fail to release after the termination of a call. The fuse alarm and ringing failure alarm are immediate, but release alarm is retarded for a period of between 9-18 seconds.
As already stated, some of the executive offices required certain peculiar communication facilities which were satisfactorily met by means of a special intercommunication system arranged to work in conjunction with the automatic exchange. There are seven master stations each of which can communicate directly with the others, with five associated side stations and one exchange line. When communicating
on an exchange line a "hold exchange" feature is incorporated enabling a master station to call any other station even if the latter is engaged, while under other conditions an engaged line would give an “engaged test.”
A side station is a simple extension from a master station and can only reply to its particular master station, i.e., it cannot originate a call on the intercommunication system. It can, however, originate and reply to exchange calls. The side station instruments are ordinary automatic bakelite table telephones with a means of
switching from the exchange to the master line and vice versa.
|The Master Station Instrument
The master station instrument comprises an automatic table telephone mounted on a special base containing the necessary
auto-reset keys for the various lines. The flexible cable from the instrument terminates on a plug which, when in position, totally encloses all connections, as may be seen from the illustration.
The telephone equipment was successfully brought into service at the latter end of 1931 but was not subjected to heavy traffic, however, until the whole of the B.B.C. staff was transferred from the old headquarters at Savoy Hill in 1932.