CB887, AT1810 & AT3718

B 1080
Issue 1, 5.12.38

Details of Switchboards AT 1810, AT 3718 and CB 887

1. General
This Instruction describes the details of, and the facilities provided by, "Switchboards,  AT 1810, AT 3718 and CB 887", which are floor-type double-cord switchboards designed for use on the systems shown in Table 1.


Switchboard System for which designed Originally called
AT 1810 Automatic Switchboards, BE, AT
AT 3718 C.B. (manual) and automatic  
CB 887 C.B. (manual) Switchboards, BE, CB

2. Supersession
These switchboards are now obsolescent and are superseded for new work by "Switchboards, AT 3796" (see B 1030).

CB 887 (10+50) - picture dated 1924


Switchboard, AT1810 10+30 (picture dated 1932) Switchboard AT1810 10+50 (picture dated 1932)

3. Equipment, capacity, dimensions, and weights
Table 2 gives the equipment and capacity for the several existing sizes of each switchboard, and Table 3 the dimensions and weight.

4. Wiring
The wiring of the switchboards is complete for the capacity quoted in Table 2, except in the case of the 10 + 60 (180) sizes, in which extension wiring is only provided for 120 circuits. Wood spacing-strips occupy the unequipped indicator and jack positions.


Type Size Exch line
Exch Line Equipped Extension
Extension Equipped Cord circuit Capacity Cord circuit Equip
AT 1810    10+30 (60)     10 10 50 30 15 11
AT 3718    10+30 (65)    15 10 50 30 15 11
CB 887    10+30 (65)    15 10 50 30 15 11
AT 1810    10+50 (60)    10 10 50 50 15 15
AT 3718    10+50 (65)    15 10 50 50 15 15
CB 887    10+50 (65)    15 10 50 50 15 15
AT 3718    10+60 (180)    20 10 160 60 18 15
CB 887    10+60 (180)    20 10 160 60 18 15


Type Size Height Width Depth overall Approx. weight (lb.)
AT 1810    10+30 (60)    4' 7" 2' 3.5" 2' 2.5" 280
AT 1810    10+50 (60)    4' 7" 2' 3.5" 2' 2.5" 309
AT 3718    10+30 (65)    4' 7" 2' 1" 2' 2.5" 280
AT 3718    10+50 (65)    4' 7" 2' 1" 2' 2.5" 309
AT 3718    10+60 (180)    5' 1" 2' 2" 2' 6.5" 336
CB 887    10+30 (65)    4' 7.5" 2' 3.5" 2' 2.5" 280
CB 887    10+50 (65)    4' 7 2' 3.5" 2' 2.5" 309
CB 887    10+60 (180)    5' 1" 2' 6.5"  2' 4" 336

5. Diagrams
The diagrams relating to these switchboards are mainly in the N series.
AT 1810 - Switchboard Circuits (Explanatory) - N1090.
AT 1810 - Switchboard Circuits (Wiring) - AT1811.
AT 3718 - Switchboard Circuits (Explanatory) - N1092.
AT 3718 - Switchboard Circuits (Wiring) - AT3780.
CB 887 - Switchboard Circuits (Explanatory) - N932.
CB 887 - Switchboard Circuits (Wiring) - CB886.

6. Transmission data
The maximum permissible resistance for exchange line plus extension is given ,in the following E.I.s:-
B 3503 C.B. manual and automatic areas.
B 3504 C.B. manual and automatic areas (divided cord-circuits).
B 3505 L.B. telephone in C.B. manual and automatic areas.
B 3550/1 L.B. exchange areas.
B 3590 Inter-switchboard extensions.

7. Design and equipment
The design and equipment of these switchboards is similar to that of TELEPHONES P.B.X.s B 1080:-

  1. "Switchboard, AT3718"
    The circuit arrangements do not permit the use of a " Keysender No. 5..." without modification to the board.
  2. "Switchboard, AT 1810"
    The exchange holding and through-clearing arrangements, also the dialling keys, are in the terminations of the exchange lines instead of in the cord circuits.  The dialling keys, one per exchange line, are fitted in the face of the switchboard above the exchange line indicators.
  3. "Switchboard, CB 887"
    As for "Switchboard, AT 1810", but without dialling keys or dial.

8. Power-supply data
All the above switchboards are run on 12 volts DC.

Additional Information

These were available in the 1928 rate Book.

The 10+30 superseded the F.P.D.C., N.T. No, 11.

AT1810 was made by British Ericsson their diagram number N15400 dated 1932.


The text below is taken from GPO Technical Instructions XXIII (1930)

Refers to AT1810 and CB887 Switchboards

The switchboards are designed to stand on the floor. Sufficient clear space at the rear of the switchboard is necessary to permit of access to the interior by the removal of the rear panel.  There is no projecting apparatus on either side of the switchboard, the side cornice and beading is detachable to enable two sections to be fitted in continuous line, holes are provided in both sides for leading in the wiring.

Modifications required for working to an Automatic Exchange
Particulars of the modifications necessary when the switchboard is required to work to an automatic exchange are given in T.I. XXXIV, Part 3.

Operating Features
Hand-restored drop indicators are fitted on the exchange lines for the receipt of incoming calling signals and eyeball indicators are fitted on the extensions.  Double cord circuits are provided, each with a ringing and speaking key, a ring-back key and two supervisory signals.

The dialling keys on Switchboards, A.T. 1810 are fitted in the position occupied by the spare indicator spacing strip in the Switchboards, C.B. 887 and the capacity of the switchboard for exchange lines is thus reduced from 15 to 10 (see Part I).

The insertion of a. plug into an exchange line jack collects a loop across the exchange line via the back contact of a series relay and operates the sleeve relay of the cord circuit which disconnects the battery feeds from the tip and ring.  The exchange line is thus held during the whole time a plug is in the exchange line Jack and both supervisory signals are controlled together by the extension: `Calling-in" facilities are thus available on all  connections.  On extension to extension connections the signals are controlled separately by the two extensions.

51. Night Service
The operation of the might switching key disconnects the holding coils from the exchange lines, the battery feeds from the tip and-ring
sides of the cord Circuits and the relays from the sleeves of the plugs.  By using any pair of cords any extension may then be connected to any exchange line and when such connections are set up, there is no current drawn from the B.E. bus-bars.

In an automatic area a dial is necessary .on each extension requiring direct night service to the public exchange and if auxiliary equipment is fitted on such an extension a break jack for use on night connections should be inserted in the line; a spare extension jack, if available, should be used for the purpose and suitably labelled.

Operator's Telephone
The normal operating telephone consists of a breast-plate transmitter and head-gear receiver, no transmitter out-out key is provided; but, to avoid wastage of energy, the telephone plug should be withdrawn when the switchboard is left unattended.

Night Alarm
A bell and switch are included in the equipment of the switchboard.

Supervisory Signals
These are of the negative type, a disc being displayed while the conversation is in progress.

Ringing Circuit
Facilities for ringing by hand generator or by power ringing leads are included in the equipment of the switchboard.

Cord Test
A jack in single mounting is included in the equipment of
this type of switchboard and wired to Figure 1 of Diagram C.B. 1209 (N. 961).

This should conform to the general scheme outlined in Part 1.

The jacks are drilled for the reception of number plates on which the circuit designations should be marked. The labels for the extension jacks are supplied engraved, those for the exchange line jacks are supplied blank.

Any necessary service marking should be applied to the indicators by coded labels.


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Last revised: November 14, 2023