SWITCHBOARD - NT Double Cord types


All the information below is taken from the GPO Technical Instructions XXIII (1930)
 

General Capacities

Type N.T. Number Capacity
Exchange Lines
Capacity
Extensions
Equipment
Exchange Lines
Equipment
Extensions
Capacity
Cord Ccts
Equipment
Cord Cct
F.P., D.C. 3 10 50 5 50 12 10
F.P., D.C. 7 20 100 10 100 18 14
W.P., D.C. 19 5 10 5 - 5 3
W.P., D.C. 20 5 10 5 5 5 4
W.P., D.C. 21 5 10 5 5 5 5
W.P., D.C. 22 6 40 6 40 10 10
W.P., D.C. 23 6 40 6 20 10 6


Switchboards F.P., D.C. No's 3 and 7

Switchboard F.P., D.C. No. 3 (Picture dated 1924) Switchboard F.P., D.C. No. 3 (Picture dated 1932)
   
Switchboard F.P., D.C. No. 7 (Picture dated 1924) Switchboard F.P., D.C. No. 7 (Picture dated 1932)

Transmission Data
The allowance for the loss in the PMBX apparatus on a connection between an extension and a non-condensered magneto exchange is 0.5 db; between an extension and a magneto call-automatic clear exchange is 0.5 db; between an extension and a condensered magneto or C.B. exchange is 2.0 db; or between an extension and any of the above types of exchange when calling line facilities are provided is 1.2 db.

Design
These 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 or cornice on either side, Switchboards may therefore be fitted side by side without alteration.

The connecting rack is above the indicators and enclosed back and front by removable panels.  A hole for leading in the wiring is provided in the top of the switchboard.

Operating Features
Normally, each exchange line and extension is equipped with a calling indicator and one jack, but when connected to a condensered magneto exchange or to a C.B. exchange separate day and night jacks are required on the exchange lines; the circuit being "held" as long as a plug is in the former. Double cord circuits are provided each with a ring-off indicator and a speaking and ringing key.  A Common ring-back key is fitted.

Night Service
When working to a non-condensered magneto exchange, any extension may be connected to any exchange line by any pair of cords and left through for night working.  When working to a condensered magneto exchange or to a C.B. exchange, special jacks for such connections are provided on the exchange lines and condensers should be connected in series with all ring-off indicators.  Condensers should also be fitted in the bell Circuits of extensions requiring direct night service to the main exchange.

"Calling-in" Facilities
The indicators for the receipt of "calling-in" signals should be fitted in a "Case Indicator Mounting" No, 6 or 7, details of which are as follows:-

"Case Indicator Mounting" No, 6, is fitted to F.P., D.C. No. 3 which has a capacity of 10 indicators.

"Case Indicator Mounting" No, 7, is fitted to F.P., D.C. No. 7 which has a capacity of 15 indicators.

The case should be fitted on top of the switchboard, the pediment being removed and refitted above the "Case Indicator Mounting."

The remainder of the Special apparatus should be mounted locally in a convenient manner.

Operator's Telephone
A head-gear receiver and breast-plate transmitter should normally be provided, but if the traffic is very small or the local conditions require it a hand micro-telephone may be provided instead.  The operator's telephone is not included in the equipment of the switchboard.

Night Alarm
No bell or switch is included in the equipment of the switchboard.  These should be fitted locally.

Generator
A generator will be included in the equipment of future issues of these switchboards.
 

 

Switchboards W.P., D.C. No's 19, 20 and 21

Switchboard W.P., D.C. NT No. 19 (Picture dated 1924) Switchboard W.P., D.C. NT No. 20 (Picture dated 1924)

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

Design
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 front portion of the board, which accommodates the whole of the apparatus, is hinged on the left to give access to the interior.

Operating Features
Each line circuit is equipped with a calling indicator and a jack.  Double cord circuits are provided, each with a ring-off indicator and a double-speaking key, of which one position (the answering) connects the operator's telephone in parallel with both Plugs, and the other position (the Calling) connects the operator's telephone to the calling plug only, leaving the answering plug connected to the ring-off indicator.  On an incoming call, this arrangement is necessary to prevent a premature clear being given to the main exchange by the Earth on the bell of the extension telephone.  As the generator is in the operator's telephone circuit, this arrangement of the speaking key also serves to isolate the ringing current from the calling line while the required circuit is being rung.

The clear to the main exchange is given by the withdrawal of the plug from the exchange line jack or by the replacement of the receiver of any telephone connected to the exchange line.  To give the clearing signal to the P.B.X. it is necessary for the extension to "ring off".

Night Service
By using any pair of cords any extension may be connected to any exchange line for night service.  No special arrangements are necessary on the extensions, but when calling-in facilities are provided, separate " Day " and " Night " jacks are required on the exchange line, the additional jack being fitted in the ood rail below the jack field.

Calling-in Facilities
The indicators necessary for the receipt of calling-in signals should be fitted in a "Case Indicator Mounting No 1" which has a capacity for four indicators and which should be fitted on top of the switchboard framework.  The remainder of the special apparatus should be mounted locally 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 breastplate transmitter and head-gear receiver are required they should be wired to diagram C.B. 1208 (N 960)  Figures 5 and 7.

 

Switchboards W.P., D.C. No's 22 and 23

Switchboard W.P., D.C. NT No. 23 (Picture dated 1924)

Transmission Data
The allowance for the loss in the P.B.X. apparatus on a connection between an extension and a non-condensered magneto exchange is 0.5db; between an extension and a Magneto call, automatic clear exchange is 0.5 db; between an extension and a condensered Magneto or C.B. exchange is 2.0db or between an extension and any of the above types of exchange when calling-in facilities are provided is 1.2db.

Design
These switchboards are designed for fixing to a wall or other flat vertical surface.  Wiring may be led in from either side throi:1gh holes provided in the backboard.  The terminals are fixed to the backboard. The front portion of the board carries all the apparatus and is hinged on the left to give access to the interior.

Operating Features
When issued each line circuit is equipped with a jack and a 100 ohm drop indicator.  When fitted at a P.B.X. one strip of the latter (Indicator No. 1410A) should be replaced by 1000 ohm indicators (10 indicators N.T. 10 and 1 mounting 93/10A) and the exchange lines connected thereto.  When working to a condensered magneto exchange or to a C.B. exchange, two jacks are required on each exchange line for use on day and night connections respectively, the exchange line being held as long as a plug is in the former.

Double cord circuits are provided, each with a ring-off indicator and a speaking and ringing key.  A common ring-back key is fitted.

Any jacks in single mountings (N.T.12) used on private wires or exchange lines, should be fitted in the wooden rail below the jack field.

Night Service
When working to a non-condensered magneto exchange, any extension may be connected to any exchange line for night service by any pair of cords.  When working to a condensered magneto exchange or to a C.B. exchange, special jacks for such connections are provided on all exchange lines and condensers should be connected in series with all ring-off indicators.  Condensers should also be fitted in series with all ring-off indicators.  Condensers should also be fitted in the bell circuits of extensions requiring direct night service to the main exchange.

Calling-in Facilities
The indicators, necessary for the receipt of calling-in signals, are to be fitted in a "Case, Indicator Mounting, No. 2'' which has a capacity for six indicators and which should be fixed on top of the switchboard frame work.  The remainder of the special apparatus should be fitted locally in a convenient manner.

Operator's Telephone
A hand micro-telephone should usually be supplied for this purpose, but, if the traffic is sufficient to justify the use of a head-gear receiver and breast-plate transmitter, these should be supplied instead.  The operator's telephone is not included in the equipment of the switchboard.

Night Alarm
No bell or switch is included in the equipment of the switchboard.  They should be fitted locally.

Generator
No generator is included in the equipment of the switchboard.  A generator No. 20AN should be fitted to the right-hand side of the switchboard.

Cord Testing
A separate cord-testing jack, wired to Diagram C.B.S. 266 (L. 1702) Figure 2, should be included in the equipment of the switchboard.

 

 

 
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Last revised: December 30, 2018

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