PABX No. 4
Type ET4 apparatus racks in a 1500-line exchange
The ET4 private automatic branch exchange is an attractive alternative to the PABX 3. It operates on the step-by-step principle and is for use when more than 50 extensions are required. As there is no fundamental limit to the capacity of the exchange, initial requirements may be readily expanded to any desired number of extensions, exchange or inter-PBX lines and manual positions. The manual positions are of console type and use lamp signalling and press button control throughout.
The system may be used for a single exchange or for a main and satellite scheme having a central group of manual positions and a common numbering plan. Exchange and inter-PBX circuits can be arranged to meet all normal signalling requirements.
Extension-to-extension calls dialled direct.
Outgoing exchange calls obtained by dialling the access digit, then the subscriber's number (or instructing the distant operator if the exchange is manual). Alternatively, calls may be set up by the PABX operator.
Incoming exchange calls received at the switchboard. Ring-when-free conditions are applied if wanted extension engaged.
Barred access to exchange lines applied completely or partially to selected extensions.
Right-of-way for selected extensions on (a) local calls and (b) exchange calls. Note: (b) is not permissible in the U K.
Differing ringing signals on incoming exchange and extension calls. Also differing busy tones to indicate whether an extension is engaged on an exchange or an extension call.
Secrecy on all calls with intrusion tone given when a right-of-way extension or the PABX operator breaks into an established connection.
Inter-PBX working arranged for manual/manual, auto/manual and auto/auto circuits.
Assistance from the PABX operator by dialling access digit.
Night service in two versions:
Switch quantities are normally calculated on a grade of service of one lost call in 200
at the busy hour, excepting calls to the switchboard.
The alarm equipment rack houses ringing equipment and alarm and miscellaneous circuits.
Standby pulse relay sets are provided for installations of more than 100 lines, and a
duplicate ringing machine with automatic changeover where the number of lines exceeds 400.
The exchange-line and position equipment rack has capacity for 14 exchange-line circuits, eight connecting circuits, night service, outgoing auto '0' level and other miscellaneous circuits. Supplementary relay-set racks may be required for exchange direct-access, and inter-PBX relay sets. The main distribution frame may be double-sided floor mounted, or single-sided wall and floor mounted. It accommodates fuses, fuse dummies, test jacks, and also IDF-type connection strips if necessary.
The console woodwork is light oak, and the plinth and writing shelf are faced respectively with black and grey Warerite. The steel control panel, with its lamps and keys arranged to facilitate speedy operation, is matt-finished in dark green. Its front is hinged, giving full access to all internal apparatus.
The control panel includes a waiting-calls meter for registering a maximum of ten incoming exchange calls, and three line identification indicators, the particular number and letters being illuminated when the display key is pressed.
At the top left are alarm lamps and keys. Below these are seven columns of red (answer) and white (outgoing) buttons and lamps, each column being associated with a particular class of call, such as enquiry, incoming exchange, or operator recall. The bottom horizontal row of press button keys in this group are controls common to the position and, like all other common keys, have black tops. Each of the eight columns of keys and lamps in the centre is associated with a connecting circuit. On the right, beside the dial, are the key-sender control keys which have black tops.
One operator's lightweight headset is supplied for each position. A chair is not supplied unless ordered. A supervisor's desk can be provided if necessary.
With a single battery float system, the choice of battery capacity depends upon the period of reserve supply required in the event of mains failure. This period is normally 24 hours. The 24-cell battery is 'floated' across the closely regulated output of an automatic float-charge rectifier unit. Voltage regulation of the charging unit is by transductor control, and the output is smoothed to CCITT standard.
Taken from the Plessey Telephone Catalogue 1971
Last revised: October 09, 2019