PAGE No. 2

January 1961

This article follows the description of the mechanical features of the Plan-Etelphone (Bulletin No. 41) and describes the facilities of some of the many new extension plan arrangements made possible by this versatile telephone.

Although the P.B.X. provides increased communication facilities for the subscriber, there are many situations where a simple
extension is inadequate and the P.B.X. too elaborate or unsuited to particular needs. Intermediate requirements over the years have therefore been met by a variety of extension plans which, in general, serve a smaller number of extensions and exchange lines than the P.B.X. Intercommunication between telephones is sometimes provided, but the primary purpose of the plan arrangements is to allow a number of extensions to receive and originate calls on one or two exchange or P.B.X. lines.

The recent introduction of the Plan-Etelphone seen at the head of the article has now made possible the improvement of this type of service. The simple mechanical features of the telephone (described, Bulletin No. 41) have considerably subscribed to the straight-forward switching functions and the logical positioning of visual signalling lamps, thus providing the ease of operation essential to any efficient telephone system.

With the plan arrangements the standard facilities can be expanded by adding extra keys and ancillary items where space is available. By use of these modules the extension plans offer various grades of service each geared to a different level of communication requirements.

In developing the extension plans certain specific classes of service were particularly in mind, but the circuits were arranged to be flexible enough to meet any demand likely to be placed on a telephone of this type and size.

Because of the diversity of plan arrangements, it is clearly outside the scope of this article to deal with all aspects of plan working. The intention, therefore, Is to describe some of the more popular types available and the variety of facilities they afford.

This is a simple extension plan consisting of a number of parallel extensions on an exchange line, with one telephone, designated the ‘main’, serving as the answering station for incoming calls. Because of its simplicity the plan provides neither secrecy nor intercommunication between instruments.

Fig 1
Simple plan arrangement with one main telephone and a maximum of eight extensions connected to one exchange line (Type 2)

As shown, only the main telephone includes pushbutton keys. Each is non-locking and up to four standard or four ‘two-in-one’ buttons (Total 8: see Bulletin 41, pp. 71/2) or any combination of the two types can be accommodated in the telephone.
An incoming exchange line call is signalled on an a.c. bell in the main telephone and from this instrument the wanted extension is called by pressing the appropriate ‘ring’ key. This action causes the d.c. bell to ring in the selected extension telephone and, upon the extension user lifting the handset, connection to the exchange line caller is completed.

From any extension outgoing calls are established in the usual manner following the lifting of the handset.

A feature of the plan is the use of exchange line potential for the ringing of extensions, dispensing with the need for any local battery.

Fig. 2
Single telephone connected to two P.B.X. extension lines (Type 6)

It is quite common to find extension plans used on P.B.X. extension lines. In the arrangement shown in Fig. 2, a single telephone with three pushbutton keys is connected to two P.B.X. extension lines, which may be either internal or external.

The first line is used for incoming and outgoing calls via the P.B.X. operator, and the second line for originating an enquiry call on the P.B.X. system.

Outgoing calls on the first line are originated by lifting the handset and established in the normal manner.

Audible indication of an incoming call is given by the a.c. bell in the telephone and the call is answered on removing the handset, no key operation being necessary.

A user, speaking over line 1, can make an enquiry call by operating the ‘enquiry’ key to the locked position, thus connecting the telephone to line 2 and applying a hold condition on line 1. On conclusion of an enquiry call, the non-locking ‘release’ key is pressed to restore the ‘enquiry’ key and cause the telephone to be reconnected to line 1. To ensure that the instrument is always returned to normal should the user fail to return to the first line, the ‘enquiry’ key is also arranged to be released when the handset is replaced.

If the P.B.X. operator wishes to trunk offer whilst the user is engaged on the enquiry line, ringing is applied over line 1 and received on a separate bell-set positioned near the telephone.

A non-locking key provides the operator-recall facility.

A variant of the above plan is Type 15 (not described), which permits an incoming call to be received and held on either line and is not restricted to P.B.X. use.

This plan uses one exchange line to serve two telephones interconnected by an external line and located, for example, in a shop and associated warehouse.

Incoming calls are normally answered at the intermediate telephone but may, if the intermediate is unattended, be received at the extension, both bells ringing simultaneously in this instance.

An exchange line call may be held at the intermediate and an enquiry call made to the extension. The intermediate user may then return to the exchange call or transfer it to the extension.

Outgoing exchange line calls can be made direct from the intermediate telephone. Extension-to-exchange calls, however, are switched via the intermediate, and visual indication that the line is engaged by the extension user is given by an ‘exchange line’ lamp above the dial in the intermediate telephone. Conversation between the extension and exchange parties may be made secret or non-secret from the intermediate station by appropriate terminal strapping.

The extension telephone is provided with one non-locking key for signalling the intermediate station. The intermediate telephone has four locking keys for switching, and release of any one from the locked position is effected on the depression of any other key. Besides providing a locked condition, the two inner keys are arranged to perform an additional function for use in calling the extension, either key, when overpressed, operating an auxiliary springset common to both. This facility avoids the provision of a separate ‘ring’ key.

Inbuilt transistor ringing units provide ringing current for signalling between instruments and replace the inconvenient hand generator used on earlier systems of this type.

Other features available on an optional basis are P.B.X. recall, additional parallel extensions and external bells.

It is of interest to note here, that similar facilities are included in the Plan-Etelphone, Type 7 (not described), designed for an internal line and providing d.c. signalling between instruments.

Fig. 3
Circuit arrangement of plan with one external extension and intermediate switching telephone connected to one exchange line (Type 3)

A simplified schematic of Type 3, shown in Fig. 3, serves to illustrate the operation of the system.

Extension to Intermediate
After removing the handset, the user momentarily presses the non-locking ‘ring’ key KR in the extension instrument to apply ringing current from the transistor ringing unit to the a.c. buzzer at the intermediate instrument. The attendant at the intermediate replies by pressing the ‘extension’ key and removing the handset to complete the speech path between instruments.

Extension to Exchange
Should access to the exchange line be requested by the extension user, the ‘extension-to-exchange’ key KEX is operated to switch the extension loop across the exchange line via KEX 1, KEX 2 and the S relay. Relay S operates to the exchange battery and the exchange line lamp glows steadily on the closing of contact S 1.

At this stage, the attendant may withdraw from the circuit by replacing the handset, and the extension user can proceed with the call in the usual manner. During dialling, no bell tinkling in the intermediate is possible owing to the action of contact S2, which disconnects the bell in addition to providing a low impedance speech path through capacitor Cl.

Overhearing of the call is prevented (if required) by a key contact of KEX (not shown), applying a short circuit across the intermediate telephone receiver.

Incoming Exchange Calls
Ringing current from the distant exchange operates the bell at the intermediate telephone. The attendant replies by removing the handset and pressing the ‘exchange’ key KX to connect the telephone to the line. Relay S operates as previously described, but contact Si is made ineffective by KX 4, which prevents the completion of the ‘exchange line’ lamp circuit.
To hold the line and speak to the extension, the ‘hold and extension’ key KH is pressed by the attendant to switch resistor R1 across the line and also to release the locked KX key.

By overpress action of the KH key the auxiliary springset KR is operated and the a.c. bell in the extension instrument rung.
If the exchange call is accepted by the extension user, the ‘extension-exchange’ key KEX is pressed and the KH key automatically released to complete the transfer of the exchange call to the extension. The intermediate telephone handset may then be replaced.

If the exchange line call is refused, the attendant returns to the exchange line by again operating key KX; the ‘hold and extension’ key KH restores to normal.

Intermediate to Extension (Intercom. Call)
An extension is called for purely intercommunication purposes by overpressing the ‘extension’ key (KE). By this action the auxiliary springset KR is operated and the extension instrument bell rung. Both parties are connected when the extension user lifts the handset.

Intermediate to Exchange
Connection to the exchange from the intermediate telephone is established on lifting the handset and pressing the ‘exchange’ key KX, as for an incoming call. Contact KX3 prepares the buzzer circuit to enable the extension user to signal the intermediate while the exchange call is in progress.

Among its other uses the Plan-Etelphone is well suited to secretarial service where a secretary, say, in an outer office can filter calls through the switching telephone to one principal or more.

Fig. 4
Secretarial service with one exchange line serving two principals and a secretary -(Type 1)

An example of one of the several secretarial arrangements available is the Plan-Etelphone, Type 1, given in Fig. 4. It is based on one exchange line and provides for one secretarial telephone and up to two extension telephones.
With this arrangement, outgoing exchange calls can be made direct from any instrument or obtained via the secretary’s telephone (intermediate). Secrecy from the intermediate is optional on extension-to-exchange calls.

Although answering duties for both extension users are usually performed at the intermediate telephone, an incoming exchange call can be directly received and answered at an extension telephone without the need for any intermediate switching. A manual lock-and-release key positioned next to the ‘ring’ key in each extension telephone, when switched to ‘on,’ causes the particular extension’s exchange bell to be series connected with that of the intermediate telephone; both bells respond to incoming ringing. This is a useful facility if, for example, the secretary is absent for certain periods throughout the day, or the extension is to be attended after working hours.

No key operation is necessary on direct exchange calls from an extension. Seizure of the exchange line (if free) is immediate on removing the handset, and visual indication of this action is given by an ‘exchange line’ lamp in the intermediate telephone. Providing the plan is not arranged for secrecy, the extension may call the secretary into the extension-to-exchange connection by pressing the ‘ring’ key. On operation of the buzzer in the intermediate telephone, the secretary responds by taking up the handset and listening to, or participating in the conversation as required.

An intercommunication call from an extension to the intermediate is originated on operation of the ‘ring’ key only, and established following recall by the secretary. When two extensions are fitted, the secretary is called by ‘code-calling’ - pressing the ‘ring’ key in a predetermined pattern of long and short rings. This method enables the secretary to determine the calling line and recall by pressing the appropriate ‘ring’ key and also the ‘extension’ key.

The secretary originates and receives exchange calls by operation of the ‘exchange’ key. During an exchange line call, a signal from an extension can be acknowledged or an enquiry call made by pressing the ‘extension’ key as previously described. Because the ‘exchange’ key is also operated at this stage a hold condition is applied to the exchange line while the secretary speaks to the extension. Dependent upon instructions received from the extension user, the secretary may proceed as follows:-

(a) Replace the handset to release all keys and automatically transfer the exchange call to the extension user.

(b) Repress the operated ‘exchange’ key to reconnect her telephone to the exchange line.

Fig. 5
Secretarial service, including bothway enquiry facility, with one exchange line serving a principal and secretary (Type 4)

The variation shown in Fig. 5 provides secretarial service for one secretary and one principal.

The secretary’s telephone comprises the same number of push-button keys as in the Type 1, including two with overpress action for use in sounding the extension instrument buzzer. The extension telephone has an extra key and, in common with the secretary’s telephone, incorporates an ‘exchange line’ lamp.

Intercommunication is possible between the principal and secretary, and incoming exchange call procedure is as adopted in Type 1, with the secretary normally answering and filtering exchange calls to the principal’s telephone. Exchange calls can be established direct from each instrument, but in this arrangement, because the extension is positioned between the secretary’s telephone and the exchange, the principal has switching priority. A further feature is that the principal, in addition to the secretary, can make enquiry calls.

Extra facilities can be included in this plan, for example, recall, transfer, call-back or right-of-way, and any two can be provided by overpress on a suitable button and/or extra push button.

This secretarial scheme is based on two exchange lines. Despite the additional switching involved, the circuit arrangement is such that the number of push-button keys required is only three per telephone.

Among the facilities afforded are:-

(a) Exchange line calls are signalled and filtered at the intermediate (secretary’s) telephone before being offered to the extension user (principal).

(b) In the absence of the secretary an incoming call signal is diverted to the extension.

(c) The secretary can take part in, or listen to a through conversation or be excluded at the discretion of the principal.

Fig. 6
Circuit arrangement of secretarial service with two exchange lines serving a principal and secretary (Type 8)


Extension to Exchange

Following the lifting of the extension handset the locking ‘exchange’ key KB is pressed to switch the extension directly to the filter line. Should the principal not require his conversation to be overheard by his secretary, the locking ‘exchange secret’ key KC is operated when the call is established, to release key KB and isolate the intermediate telephone circuit at contact KC4. The call proceeds as normal and, on completion, the handset is replaced to release the operated key.

Extension to Intermediate
To call the intermediate, the principal operates the non-locking ‘ring’ key KA, and battery is extended via contact KA1 to operate the buzzer in the intermediate instrument.

The secretary replies by lifting the handset and operating the locking ‘extension’ key KB to cause the speech path between instruments to be completed via key contacts KB 1, KB2 (intermediate telephone) and other series connected contacts. Replacement of the intermediate instrument handset at the conclusion of the call restores the operated key KB to normal.

Exchange Call (Secretary’s Line)
The exchange line is seized by simply removing the handset.

Intermediate to Extension
To call the extension, the ‘extension’ key KB is overpressed, after removing the handset, to operate an associated springset unit KBC1. If the principal is not engaged on the filter line, the buzzer sounds in the extension instrument and both parties can converse upon the principal lifting his handset.

Should the principal be engaged, the ‘exchange’ key KB will be operated or the ‘exchange secret’ key KC. The action of either KB3 or KC3 therefore prevents the operation of the buzzer in the principal’s instrument. The buzzer in the secretary’s telephone, however, is sounded via contact KB3 or KC3 operated, to give warning that the extension is engaged.

Enquiry Call
To answer an incoming exchange call on the filter line, the locking ‘answer filter’ key KD is pressed by the secretary to connect her telephone across the line via key contacts KD2, KB 1, instrument loop, KB2, KD 1 and key contact KC4 in the extension telephone.

The incoming call may be held on operation of the ‘extension’ key KB to the locking position, to cause a holding loop to be switched across the line via KB3 (x). The principal may then be called by overpressing key KB as previously described, key contacts KB 1 and 2 preventing the exchange party from overhearing the conversation.

On conclusion of the enquiry call the secretary may perform any of the following operations:-

(a) Replace the handset to release keys KB and KD, after the principal has accepted the call and pressed his ‘exchange’ key KB or KC.

(b) Repress the ‘answer filter’ key KD to release key KB and connect her telephone to the exchange line as instructed by the principal.

Secretary Absent
Before leaving the instrument telephone unattended, the ‘absent’ key KA is pressed and locked. Key contact KA1 applies a short circuit across the bell in the intermediate instrument, and exchange ringing is diverted to the bell in the principal’s telephone.

Cabling requirements obviously vary with the type of plan, the facilities provided and the disposition of the telephones. Indication of the number of conductors between an extension telephone and the intermediate (or main) station is given below for the plan arrangements in the order described.

Plan-Etelphone, Type 2 - 4 wires

Plan-Etelphone, Type 6 - none

Plan-Etelphone, Type 3 - 2 wires

Plan-Etelphone, Type 1 - 6 wires (1 Extension)

Plan-Etelphone, Type 1 - 7 wires (2 Extensions)

Plan-Etelphone, Type 4 - 10 wires

Plan-Etelphone, Type 8 - 8 wires

An internal extension is usually located within the same building as the intermediate (or main) telephone. Cable length between the two instruments is governed by a loop resistance of 40 ohms which allows, for example, 2500 ft. of 1-pair 20 lb/mile cable to be used, if necessary.

External Extension Resistance
An external extension is connected by a 2-wire line, which can be up to 1000 ohms loop resistance from the main exchange; the intermediate being situated anywhere between the extension and the exchange.

Excluding plan arrangements requiring no separate local battery for operation (e.g., types 2 and 6), the various plans are arranged to work from a 6V d.c. supply. This may be conveniently taken from a dry battery or, dependent on the type of plan, a power feed or power unit.

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Last revised: October 15, 2019