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.

Numbering
A 3-, 4-, or mixed 3 and 4-digit numbering scheme can be used. Selector levels are normally allocated:

Level 1 -spare;

Levels 2 to 8 -extensions and inter-PBX lines via second selectors if necessary; also night service;

Levels 9 and 0 -access to public exchange, and PABX-operator assistance as required.

Facilities
Exchange

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.

Enquiry calls made to any extension during an exchange call by single operation of instrument button and dialling wanted extension's number. A repeat button operation returns the original extension to the exchange line. Automatic call transfer to any non-barred extension or the PABX operator; after set-up of the enquiry connection, transfer is immediate on replacement of handset. Serial-call working (to customer order only) ensures that the outside caller is always reverted to the PABX operator after each completed conversation, enabling the caller to be connected to a number of extensions in succession.

Night service in two versions:
(a) Automatic switching of incoming exchange calls to predetermined extensions.
(b) Signalling of incoming calls a tone or more separate bells placed at strategic points, and calls answered by any non-restricted extension by dialling an appropriate digit.

Switchboard
Separate lamp signals for incoming calls, operator recall and different circuit groups.
Waiting-call display indicates the number of unanswered exchange calls.
Automatic call parking at any stage during an attempt to set up a connection.
Display of exchange or inter-PBX route and the number of the circuit to which the PABX operator is connected.
Connecting circuits normally used only during call set-up, but can be used for call duration.
Lamp and tone supervisory signals including visual indication of type of call in progress when a busy extension is keyed.
Keysender or dial for originating any call.
Extension of alarms to switchboard.
Automatic call concentration as positions are vacated, and automatic night switching when all positions are unstaffed.

Exchange Equipment
This is mounted on open-type racks which are supported by overhead ironwork and braced to a ceiling or wall. The racks have a standard height of 93in (2362mm) and require an upper clearance of not less than 18in (457mm) for cabling purposes. If preferred, the racks may be totally enclosed by dustproof transparent panels held in position on a rack frame by magnetic attraction.

BPO-approved components are used throughout, and include major and minor relays (Types 3000 and 600), heavy-duty uniselectors (Type 2) and 100-outiet 2-motion selectors (Type 2000).

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 line rack accommodates 100 extension line circuits, 20 line-finders and 20 final selectors. The group selector rack houses up to 60 selectors (1st, 2nd and call-back types) and also incorporates grading facilities.

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.
This duplicate equipment can be provided for smaller exchanges if specified.

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.

Click here to go to PABX 4 equipment racks

Manual Switchboard

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.

Telephones
Etelphones are recommended for extension telephones. However, any telephone may be used provided it is equipped with an non-locking press-button and incorporates a dial with a 2:1 break/make ratio and pulsing speed of 10 p/s.

Power Equipment
The working voltage is normally 50V. For loads over 100A (i.e. exchanges with 800 extensions approx.) duplicate battery float system is used. Below 100A loading, a single battery float system is normally supplied, but a duplicate battery system can be provided for greater security or quick recharging after supply failure. Equipment provision is tailored to the requirements of each exchange.

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.

Line Limits
Maximum extension line loop resistance is 600 ohms, excluding the telephone, but may be extended to 2000 ohms by long-line equipment. 
Exchange-line loop resistance may be up to 1000 ohms, depending upon the limits imposed by the remote exchange equipment. Inter-PBX line limits vary with the type of signalling employed; for example, a loop-disconnect auto/auto circuit to a 50V exchange is limited to 1200 ohms.

Accommodation
Because of variable factors, it is not possible to state the accommodation required for exchanges of any specified number of lines, but a typical layout is shown for a PABX serving 200/400 extensions.

Taken from the Plessey Telephone Catalogue 1971

Click for an article from the Ericsson Bulletin on the ET4H (Hotel Variant)

Click for an article from the Ericsson Bulletin on the ET4H

Operators positions with two enquiry desks on the left (Ericsson ET4 type)

Apparatus racks (Ericsson ET4 type)

Last revised: November 24, 2025