PAGE No. 24


January 1957

It is questionable whether the activities of any organization are more dependent upon telecommunication than are those of the British Broadcasting Corporation.

The article describes in general terms a P.A.B.X. No.3 equipment installed in Bush House last year to replace a C.B. manual system and provide for extra lines. Details of our P.A.B.X. No. 3 units were given in Bulletin No. 31, July, 1955.

The accommodation occupied by The British Broadcasting Corporation at Bush House, London, was extended in 1955 to cater for the future regrouping of their External Broadcasting Services. This created a need for more telephone lines. The capacity of the existing C.B. manual switchboard was insufficient for the requirements and in August 1955 we were invited to tender for the supply and installation of a 1000 line automatic exchange of the P.A.B.X. No. 3 type to replace the manual system. The necessity for early delivery was stressed.

We were able to meet this requirement (it is our policy to stock a range of standard equipments for such eventualities) and in November 1955 the contract was received. Power plant was to be supplied by the British Post Office, this being their prerogative for all P.A.B.X’s within their jurisdiction.

The B.B.C. agreed that 1200 lines could be considered the maximum ultimate requirement ; an important point because the changes in specification which are stipulated for exchanges of more than 1200 lines would in this case have necessitated special engineering work and modification of the stock equipment which together could have caused delay.

A certain amount of engineering work had to be done. Firstly, machine ringing rather than the usual vibratory ringing supply was considered desirable for an exchange of this size, more especially as machine ringing may become the P.O. standard alternative for all sizes of P.A.B.X. No. 3. Secondly, terminating equipment was required for 40 tie lines to Broadcasting House and associated premises which, at present served by manual exchanges at
Egton House and some of the other premises, may have its own P.A.B.X. No. 3 equipment in the future.

Ringing circuits complying with the known proposals of the Post Office were prepared and approved.

A complication with respect to the tie line terminating equipment was that manual inter-switchboard relay sets to P.O. Diagram SA.8162 would be required initially, but these might subsequently have to be replaced by larger auto. inter-switchboard relay sets to P.O. Diagram SA.816l, together with the associated incoming selectors. It was agreed in consultation with the B.B.C. engineers that we should provide accommodation and wiring on the relay set rack to permit easy replacement of the smaller relay set by the larger, and they would amend the current order to include the necessary incoming selectors, thereby facilitating the change-over.

Fig. 1 - The Exchange Trunking

Fig. 1 shows the exchange trunking. It provides for a four-digit numbering scheme employing second group selectors for extension to extension calls. First selector level 2 is allocated for extensions, levels 6 to 8 afford access for dialling out to other exchanges, level 9 is for calls to the public exchange, and level 0 for assistance calls to the P.A.B.X. switchboard. The B.B.C. consider the "Hold for Enquiry" and "Operator Recall" facilities to be of particular value to them.

The accommodation provided for the private telephone exchange equipment at Bush House is satisfactory in all respects as two rooms of adequate size were reserved for the automatic equipment and the manual switchboard respectively (Fig. 2). It made possible a clear layout of the racks in cabling sequence, and rendered angle sections unnecessary in the manual room.

Fig. 2 - The Exchange Layout

Attention is drawn to the absence of a trunk distribution frame from the floor plan. Facilities for grading were provided instead at the rear of the group selector racks. The arrangement is exclusive to the Company’s version of P.A.B.X. No. 3 and has the advantages that much less cable and less floor space are needed than would be required when using a separate T.D.F. The scheme imposes no limitations and is just as flexible as the more orthodox arrangement.

All the switching equipment racks are of standard size, i.e. 7 feet 9 inches high and 4 feet 6 inches wide. A departure from normal practice was made in using bus bars instead of cables for power distribution, whilst the rack lighting equipment which for P.A.B.X’s is normally provided by the customer in accordance with Post Office requirements, was in this instance supplied as part of the exchange order.

For all our contracts for P.A.B.X. No. 3 equipment we now provide cables sheathed in cream p.v.c., the overhead ironwork also being coloured cream. By this means the appearance of the switchroom is much improved. The visitor seeing the switchroom at Bush House will at once notice its brightness.

Fig. 3 - Auto Units and part of the Distribution Frame

The distribution frame is equipped with standard type fuse mountings and protector mountings, but a few of the verticals carry connection strips as shown in Fig. 3. Some explanation of the large cables seen above the connection strips is warranted. The Post Office had underground cables and internal distribution cables terminated at a frame in the basement of Bush House. It was necessary that the circuits be extended to the new frame. For this purpose the Post Office supplied and installed a 1400-pair, paper cored lead cable which, at the joint seen in the illustration, was divided into quad cables to permit distribution to the fuses and connection strips. The individual conductors are enamelled and silk and wool covered.

Fig. 4 - General view with Relay Set Racks in the foreground

In the foreground of Fig. 4 are three relay set racks, the centre one of which accommodates the Egton House manual inter-switchboard circuits which eventually may be replaced by auto circuits. It should be explained that both of the Post Office standard circuits concerned include a relay which performs the switching for ancillary calling lamps. The circuits current at the time provided for this particular relay to be external to the relay set. whereas it was known that by the time auto-to-auto working might be required, the Post Office will have standardized a circuit which includes the relay within the set. Accordingly it will become necessary to dispense with the external relays. To enable this to be done conveniently the wires to the relays concerned, which are mounted at the top of the rack, are segregated from the rack local cable. Later on, the shelf complete with relays and wiring can quite easily be removed and the new circuits connected, since the wires for the latter are incorporated in the rack form.

Fig. 5 - The Manual Switchboard

The ten-position manual switchboard is shown in Fig. 5. It might be thought that ten positions is a high rate of provision for an exchange of this type and size. In fact it is slightly in excess of the calculated traffic requirement, but with good reason.

Fig. 6 - Interior view of the Switchboard

B.B.C. engineers have learned from past experience that there are conditions under which the volume of switchboard traffic can assume unusually heavy proportions and therefore the B.B.C. prefer to have a margin of safety in the number of operating positions.

The switchboard construction conforms with the Post Office standards for P.A.B.X. No. 3 but distinction is given to its appearance by using light oak timber finished in the natural shade, green Warerite facings with black edges and fillets for the keyshelves, and green spacing strips in the panels. The operators’ chairs also are green. Non-glare illumination of the board is obtained from diffused lighting units under the metal cowls seen in the illustration. These units were designed and supplied by the B.B.C.

Each switchboard position has panel equipment for exchange line, inter-switchboard line and extension line circuits respectively, in ascending order, the jack multiple being repeated every four panels.

Installation posed no problem other than that of getting the equipment up to the third and fourth floors. The lifts are designed only for passengers, so every item had to be carried up the stairs, with no small exertion and at the expense of a few skinned knuckles. The final call-through test was completed on the 4th September, 1956, exactly ten months from the day that the order was placed. This was quite an achievement, bearing in mind the size of the exchange. The completion of the contract calls to mind another occasion when we had the privilege of assisting the work of the B.B.C. In 1931 we supplied a private automatic exchange and a special intercommunication telephone system for Broadcasting House, prior to the transfer of staff from Savoy Hill. Both systems are still operating satisfactorily and there is every reason to forecast an equally long and efficient service life for the new equipment at Bush House.

This article is published by kind permission of the British Broadcasting Corporation.

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