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Tuesday, 9 August 2016


The erection of aerials for the reception of radio and TV broadcasts is usually undertaken by the specialist.In building which consists of blocks of flat,communal pick-up services are provided, being fed from a communal pre -amplifier.This unit is installed as near as possible to the

 aerial site that any interference picked up by the intervening feeder is reduced to a minimum. The contractor`s interest in these services is mainly confined to the provision of conduit or socket-outlet facilities. In a multi-point television installation up to twenty receiver points may be connected to one cable which is looped through the socket-outlets.


These systems are either internal or are connected to the Post Office public telephone facilities. All installations which have public connections are subject to the supervision and approval of the Post Office, which body undertakes the final connecting-up. The electrical contractor is generally required to install conduit or trunking to facilities the wirring of the building for telephone outlets.In large buildings a main switchboard is installed to receive incomming calls which are then  switched to the required extension phone. There are two types of private installations: PMBX (private manual branch exchange) and PABX (private automatic branch exchange)

In the PMBX system each extension phone is wired to the main switchboard and connection is made by socket called “jacks”.There are certaim in disadvantages associated with this system which usually required an additional internal phone system.

In the PABX system all incomming calls are terminated at the manual switchboard and are answered by the telephone operator. 

All extension-to-extension calls are set up automatically and direct out dialing on certain extension is possible. All extension phones can call the operator who can identify the extension on a lamp per line basis. Direct access to the local Fire Brigade can be incorporated in the system a special code being allocated for this purpose.

 A cordless switchboard(PMBX)is a more recent development of the PABX system. It has a switchboard and with a translucent screen or lamp signalling.It enable the operator to supervise ans connect all calls with full control given by a few levers and keys.

 When a call is transferred to an extension it disappears from the switchboard and is then under the full control of the extension this is a feature not available with the older and Post Office approved PABX system known as PABX 3.
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