SWITCHBOARD - NT Single Cord types

All the information below is taken from the GPO Technical Instructions XXIII (1930)
Type N.T. Number Capacity
W.P., S.C. 1 5 3
W.P., S.C. 2 5 5
W.P., S.C. 3 10 10
W.P., S.C. 5 16 16
W.P., S.C. 6 20 20
W.P., S.C. 7 30 30


Switchboard W.P., S.C. NT No. 1 (Picture dated 1924) Switchboard W.P., S.C. NT No. 2 (Picture dated 1924)
Switchboard W.P., S.C. NT No. 6 (Picture dated 1924) Switchboard W.P., S.C. NT No. 7 (Picture dated 1924)

Transmission Data
The allowance for the losses in the switchboard apparatus on an exchange to extension connection is 0.5db, or when calling in facilities are provided 1.2 db.

These switchboards are designed for fixing to a wall or other flat vertical surface.  Wiring may be led in from either side through holes provided in the backboard.  The terminals are fixed to the backboard and protected by a cover which is hinged on the left.  The front portion of the board carries all the apparatus and is hinged on the left to give access to the interior.

Operating Facilities
Each line is connected to an indicator, jack and plug; the indicator (1000 ohms) acts as a calling signal and as a ring-off signal.

When working to a non-condensered magneto exchange the operator's telephone is connected to a plug only and connection with any circuit is obtained by inserting the telephone plug into the jack of the desired line.

When working without calling-in facilities to a condensered magneto exchange or to a C.B. exchange, the operator's telephone is also connected to a jack.  The generator connections are arranged to ring out on the plug only and, if the plug of a calling line is inserted into the telephone jack, any required line may be rung through the plug without disconnecting the calling line.  This method of operating is adopted to avoid giving false clears while setting up an exchange connection, and a holding jack is also provided for use in the same circumstances when, for any reason, it is necessary to leave an uncompleted connection.

For working to a condensered magneto exchange or to a C.B. exchange, the above arrangements are departed from in detail when calling-in facilities are provided, particulars of the modifications are given on Diagrams T.L. 969 (N. 917) and T.L. 971 (N. 919).

Night Service
The standard diagrams allow for any extension to be connected to any exchange line for direct night service.

Calling-in Facilities
For the mounting of the eye-ball indicators necessary for the receipt of calling-in signals a "Case Indicator Mounting" should be fitted.  The case is a standard item and, for single-cord switchboards, is provided in three sizes designed for fitting in the place of the terminal cover.

The details are as follows:-

Case-Indicator No. For use on Switchboards Capacity (inds) Height (in) Width (in) Depth (in)
3 W.P., S.C., N.T. 3 4 3.2 10.25 6
4 W.P., S.C., N.T. 5 and 6 6 3.2 16 6
5 W.P., S.C., N.T. 7 6 3.2 23 6

Where 7- or 8-point jacks are necessary for the exchange lines, single jacks (Rate Book description "Jack, Switchboard, N.T. 12") should be fitted in the woodwork below the jack field.  Wiring should be transferred to these jacks from those in the bottom strip and the disused jacks pegged up.

The remainder of the special apparatus should be mounted in a convenient manner apart from the switchboard.

Operator s Telephone
The standard operator's telephone to be used with this type of switchboard is a "Telephone, No. 59" which includes a hand micro-telephone.  When a breast-plate transmitter and head-gear receiver are required they should be wired to Figures 6 and 7, Diagram C.B. 1208 (N 960).

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

Cord Test
The "Hold" jack should also be used for cord testing.  Switchboards No's 5, 6 and 7 are fully equipped in this respect when issued.



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Last revised: January 01, 2019