PMBX No. 11

Descriptive Leaflet

3 Internal
Issue 1, Dec 1978

Private Manual Branch Exchange No. 11/1
(P.M.B.X. No. 11/1)






(a) Calling Appearances

(b) Terminations

(c) Lamp Indication and Audible Indication

(d) Through Clearing and Dialling and Non-Through Clearing

(e) Press-button Recall

(f) Automatic Ringing

(g) Automatic Hold

(h) 'Click' Engaged Test

(j) Free Line Signalling - Plus Group Busy

(k) Transmitter Cut-Out

(l) Metering

(m) Night Service

(n) 'Mains Fail' Service

(p) Alarm Lamps

(i)-(v) PMBX 11A Switchboard Position

(vi)-(viii) CTS

(q) Cord and Ring Test

(r) Position Coupling

(s) Keysender

(t) Inter-PBX Circuits


(a) Extensions

(b) Exchange Lines

(c) Cord Circuits

(d) Operator's and Miscellaneous Circuit

(e) Metering




(a) Distribution Equipment Mounting

(b) Exchange Line Equipment Mounting

(c) Signalling Units and Miscellaneous Equipment Mounting

(d) Power Equipment

(i) 50 Volt Negative Switched Supply

(ii) 50 Volt Negative Continuous Supply

(iii) Power Distribution and Alarm Arrangements

(iv) Ringing Supplies

(e) Exchange Line Relay Sets

(f) Units Signalling

(g) SSAC 13 Equipment


(a) Installation

(b) Maintenance


The switchboard PMBX 11/1A is a cord type switchboard, with lamp signalling, is used with associated equipment to provide multiple suit installations for up to 800 extensions and 80 exchange lines, or up to 40 exchange lines and 40 inter-PBX lines. The total number of exchange and inter-PBX circuits is limited to 80 per position. The switchboard, as supplied includes calling lamps and jacks for 60 extensions; and 40 exchange lines and 20 exchange lines or inter-PBX lines. Pre-wired extension units are also available to increase the face panel equipment to the desired capacity. The extension multiple jacks are fitted as required on site.

Cord circuit plug-in relay sets are fitted within the switchboard, and floor- standing units accommodate ancillary apparatus to give the facilities listed in paragraph 2. Power for the installation is normally derived from a 50 volt mains-operated rectifier additionally, secondary cells may be provided if a stand-by supply is required.

The PMBX 11/1A provides the following standard facilities:-

(a) Calling Appearances
Calling appearances for up to 800 extensions, and up to 80 ELS can be provided. Inter-PBX line appearances reduce the number of exchange line appearances which can be provided. That is, with the full 40 inter-PBX circuits, the maximum number of exchange line appearances will be 80 minus 40 = 40.

(b) Terminations for:-

(i) Exchange lines to any CB type public exchange (two versions of exchange line relay sets are available according to the type of exchange which the lines are to be terminated to) TXS, TXK1 or TXE.

(ii) Extensions, internal and external.

(iii) Private circuits.

(iv) Inter-PBX extensions.

(v) Inter-PBX private circuits.

Private circuits and inter-PBX private circuits are barred access to exchange lines, inter-PBX extensions are not allowed outgoing calls to the public exchange and exchange lines may not be connected together. Apart from these restrictions, all five of the above mentioned categories may be inter-connected by cord circuits.

(c) Lamp Indications of Calling and Supervisory Conditions
Audible Indication of Calling Conditions and, Optionally, of Clearing Conditions. A sub-unit 1A1/SA 10045 is required for the audible clear, but it is operative only to a double clear. A key is provided to disconnect the audible alarm when it is not required to sound.

(d) Through Clearing and Dialling
This is normally provided on extension to exchange connections, a double clearance signal being given when the extension clears. Optionally, cord circuits may be modified for non-through clearing with individual supervision. Individual supervision is given on other types of calls.

(e) Press-Button Operator Recall
Extension to Exchange Line calls and Extension to Private Wire calls can recall the Operator. The Operator can then transfer the call as required. It should be noted that difficulty with Recall on Extension to Extension call is usually experienced.

(f) Automatic Ringing on Calling Cords
Manual ringing is available on answering cords for call reversion purposes.

(g) Automatic Hold on Exchange Calls Originated or Answered by the Operator
This holds the exchange line when the operator restores the cord circuit 'SPEAK' key before extending the call, or before an extension has answered when the call has been extended.

(h) 'Click' Engaged Test

(j) Free Line Signalling Group and Group Busy
This is available for exchange lines and inter-PBX lines.

(k) Transmitter cut-out
A push-button switch may be provided if authorised to cut out the transmitter circuit of the operator's headset.

(l) Metering
Three types of metering are available, and can be used individually or in combination:-

(i) Metering on cord circuits using resettable meters, to indicate individual, call totals.

(ii) Metering on exchange lines, to indicate running totals.

(iii) Centralised extension metering, to indicate running totals using totals meters.

(m) Night Service
A key is provided on the Cable Turning Section to disconnect the switched 50 volt supply when the key is operated, allowing cord circuits to be used to extend selected exchange lines direct to selected extensions.

A 'Night Busy' facility is provided on the Cable Turning Section, so that customer controlled night busying may be employed when the public exchange is equipped for this service. This operates automatically when the Night Service key is operated.

(h) 'Mains Fail' Service
When the 50 volt supply fails, extension to extension calls fail, but extension to exchange calls in progress are maintained.

The operator is able to receive calls only on one line per position. The operator cannot initiate outgoing calls due to the keysender being ineffective under this condition.

To assist operating under these conditions 'Exchange Answer MF' jacks are provided to enable exchange calls to be answered by the operator, Exchange lines may be plugged through to selected extensions, as under night service working.

(p) Alarm Lamps Indications are given for the following conditions on the PMBX 11/1A Mark 1 switchboard:-

(i) 'Position Fail', indicating a failure of the negative 50 volt supply to the position, or failure of the supply to the operators circuit.

(ii) 'Exch Ans MF', indicating an incoming call on an exchange line which has been plugged through to the jack associated with the Exch Ans MF lamp.

(iii) 'Fuse Alarm 1', indicating a failure of a negative 50 volt fuse mounted within the position.

(iv) 'Fuse Alarm 2', indicating a failure of the continuous negative 50 volt supply fuse (fuse 31 within the position.)

(v) 'Fuse Alarm 3', indicating the failure of fuse 32 within the position. On the PMBX 11/1A 'Cable Turning Sections', indications are given for the following conditions.

(vi) IVF Equipt Fail', indicating the failure of a negative 50 volt within the VF equipment. The VF Oscillator failure will also give the alarm.

(vii) 'External Fuse Alarm', indicating the failure of a fuse in the Fuse Unit No.2 .... or Distribution Unit No. 3A or shelf unit 1A1/SA l0043.

(viii) 'Rect Mains Fail'. This is used at installations where a floated battery supply is provided for the 50 volt power supplies. It-indicates a failure of a rectifier output which is being used to supply the installation, and so gives advanced warning that the battery supply may fail. (There may be more than one rectifier, only one of which may have failed.)

(q) Cord and Ring Test
Cord test A, cord test B, and ring test jacks, to test cord circuits for continuity, supervision, noise and ringing.

(r) Position Coupling
The operation of the 'COUPLE' key in a position disconnects the operator's circuit and extends the connections to the adjacent left-hand position. The operator still has to use keysender associated with un-staffed position.

(s) Keysender
A keysender is provided as a standard facility.

(t) Inter-PBX Circuits
The facilities available on inter-PBX circuits are dependent on the type of signalling used, and on the type of installation at the distant end. The appropriate diagram notes for the signalling unit concerned should be consulted for details.

When requests are made for non-standard facilities, reference should be made to TI C3 A0120: "Non-standard facilities at subscribers' stations".


(a) Extensions
Extension telephones are wired to Dgm N 846R as 2-wire PBX Extensions with Recall. Lamp signalling at the switchboard is employed, and the lamp circuit is completed by a loop from the extension via auxiliary break contacts in series on the answer and multiple jacks. The lamp is extinguished when a plug is inserted into any of these jacks. Lamp No. 2/45V are used, which are satisfactory oh lines of up to 500 ohms loop resistance. On longer lines a Units Signalling SA7611 is employed to apply the calling signal to the lamp and these units are also used when duplicate calling appearances are required.

(b) Exchange Lines
Ringing is detected by a relay in shunt with a rectifier which normally causes the calling lamp(s) to light continuously until the call is answered. When an incoming or outgoing call is extended, two relays are connected in series with the line, one to provide supervision, and the other, differentially connected, to detect an operator recall earth from an extension. In the event of a second exchange call arriving before a previous connection is cleared down, a 'follow-on-call trap' prevents the extension bell ringing and attracts the operator's attention by dimming both cord circuit supervisory lamps at ringing periodicity.

(c) Cord Circuits
One key with 'SPEAK' and `KEYSEND' positions and two supervisory lamps are associated with each cord circuit. The operation of a 'SPEAK' key and its associated relay SK connect the operator's telephone circuit to the answering cord and to the calling cord. The circuit is arranged to allow only one SK relay to be operated at a time thus preventing interconnection between cord circuits should more than one 'SPEAK' key be operated. Interrupted ringing is automatically applied when a calling cord is inserted into an extension, private circuit, or inter-PBX line jack. Continuous ringing is connected to the answer cord via 'SPEAK' key contacts with the common 'RING ANSWER' key is operated. A capacitor type bridge provides divided battery feed for extension to extension and extension to inter-PBX line connections and gives individual lamp supervision. For extension to exchange connections the bridge is normally switched out of the circuit to give through dialling, through clearing, and double supervision. The circuit may, however, be modified by the insertion of two straps between U points to retain the bridge in circuit, to prevent through dialling and through clearing, and also to give individual supervision.

(d) Operator's and Miscellaneous Circuit
Two sub-units 1A1/SA l0046 are provided for the panel calling lamps, and sound the audible alarm in the CTS. One unit feeds each panel. Two relays, MFA and MFB, are held operated by the 50 V supply to the switchboard and release in the event of a supply failure to modify the transmitter circuit so that exchange calls may be answered using the public exchange line feed current.

A transistorised circuit SA7609 (Sub-unit SA7609) suppresses side-tone and attenuates excessively high level incoming speech by up to 4 dB. This enables the high sensitivity of the receiver in the Headset No. 1 to be fully exploited. A sub-unit 1A1/SA 10045 is provided to give an audible clearing signal. The sub-unit monitors the voltage developed across the resistors feeding earth to the supervisory lamps in each cord circuit. If both lamps are lit the voltage is sufficient to operate the circuit which connects an earth to sound the alarm buzzer.

(e) Metering
Metering Units No. 3 .... are used to detect the 50 Hz meter pulses on exchange lines for all types of metering except localised extension metering. The units provide earth pulses to control a Meter No. 150C for 'totals' metering on exchange lines, or 'trip' meters' (i.e. cord ccts metering or centralised extension metering). A Meter No. 19 may be connected to an extension to give running and individual call totals, and is operated directly by the 50 Hz pulses from the public exchange.


(a) General Construction
The face equipment, key plate, kicking panel, and plinth are coloured light French grey. The top, side, and rear panels, and shelf desk are faced with elephant grey melamine resin surface. All the panels are easily removable to give access for installation and maintenance.

Within the metal framework of the switchboard, which includes a 75 mm deep plinth, hinged mountings at the top accommodate connection strips for wiring and cabling to extension answering jacks. Stile bars are provided at the front so that jack and lamp strips may be secured by jack fasteners. Below the face panels an apron is welded to the framework to house the key and supervisory lamp mounting assembly. The shelf desk is secured to the apron and may be hinged downwards to allow the switchboard to be taken through narrow doorways. A cord rail assembly, terminal blocks for the battery and earth feeds to the lamps, and miscellaneous circuit components are located behind the apron. Towards the rear of the position a relay frame provided with jacks may accommodate up to 9 plug-in relay-sets for the operator's circuit and 15 cord circuits. A fuse panel is mounted at the bottom of the relay frame and connection strips for exchange lines, private wires, position coupling, ringing supplies, and miscellaneous circuits are mounted near the centre.

Details of the switchboard assembly are shown on Drawings 93504 and 93505 for the switchboard and cable turning section respectively.

(b) Dimensions and Weight
The switchboard is 1220 mm high, 729 mm wide, and 934 mm deep. The depth is reduced to 775 mm when the shelf desk is hinged down. The unequipped switchboard, as normally supplied, weighs 136 kg (300 lbs.) but when fully equipped with relay-sets, etc, is approximately 226.79 kg (500 lbs.)

(c) Face Equipment
Two panels each provide mounting space for up to 24 12.7 mm jack or lamp strips. Each switchboard PMBX 11/1A is supplied already equipped with 3 pairs of lamp and jack strips catering for 60 extension lines in the answering position, a pair for the miscellaneous circuits and 60 pairs plus FLS strip for exchange lines, 20 of the latter may be used for inter-switchboard circuits or exchange lines.

The unequipped space is fitted in with French grey melamine faced spacing strips which may be removed to fit additional lamp and jack strips as required. Diagram EC 2112 gives details of how lamp and jack strips may be arranged to suit installation requirements. Pre-engraved opals for additional calling lamp strips are supplied in Kit No. 133/0-39.

Jack strips with 20 jacks are used for all jack positions. Unused jacks in any strip should be fitted with Pegs No. 15 Grey. The strip used for the miscellaneous circuits, has pegs fitted in the unused jack positions.

Lamp jack strips with 20 individual sockets are provided above each exchange line and extension answering jacks. The jacks positioned directly above the exchange line or inter-PBX jacks are used for calling lamps and the positions directly above the exchange line or inter-PBX calling lamps are used for free line signalling lamps. Lamp covers for 20 lamp caps are clipped into the front of each lamp jack strip. Individual lamp jacks may be inserted or withdrawn from strips by removing the lamp covers, springing open the top and bottom edges of the frame, and sliding the lamp jack in or out from the front. Opal lamp caps which are hot-stamped with the circuit designation fit into the cover over calling lamps. Light French grey dummy caps are fitted in unused positions, and green lamp caps are fitted over free line signalling lamps. A group busy lamp cap is provided for the first line in each group.

Extension Units Nos. 9 and 10 are used to add extension answering appearances. Each Unit consists of a jack strip and lamp jack strip for 20 circuits, wired to a connection strip. A lamp cover is provided with each unit, but, lamps and lamp caps have to be ordered separately. Units No. 9 are fitted in the left hand face panel (as viewed from the front), and Units No. 10 in the right hand face panel.

By using Extension Units Nos. 9 and 10, a maximum of 100 answering jacks for extension lines may be provided on each position. (Refer to Diagram EC 2112.)

Extension Units Nos. 1-8 are used to provide the extension multiple appearances. Each unit consists of a jack strip for 20 circuits pre-wired with a short length of cable for connecting to the multiple jack of the previous position.

A maximum of 400 extension multiple appearances per position is possible. Each Extension Unit is suitable for any one of five different blocks of extensions. Details are given in Specification S 1286. The extension multiple jacks are fitted above the extension answering appearances, in accordance with Diagram EC 2112.

Exchange line circuit capacity (including inter-PBX ccts) may be increased from 60 to 80 circuits by ordering and fitting 1 jack No. 520BV, 1 jack lamp No. 61/20A, and 1 cover lamp No. 8/20A, for each switchboard position. Wiring is provided for the first 60 circuits in the PMBX 11/1A Mark I switchboard. For subsequent circuits another connection strip has to be fitted in each switch- board and wired to the additional jacks and lamps.

A multiple of up to 40 exchange lines and 40 inter-PBX lines can be provided. When more than 40 exchange lines are required on each position the number of inter-PBX lines which can be provided is reduced.

'The miscellaneous jack and lamp strips are located near the middle of the right-hand face panel of the switchboard. Provision is made for a jack and lamp for restricted operations under power fail conditions for switchboard alarms and for cord circuit testing, as described in sections 2(m), 2(n), 2(p) and 2(q) respectively. The remaining 12 jack and lamp positions can be used for further alarms, or for the provision of non-standard conference facilities.

One ticket recess is fitted per panel.

(d) Keyshelf
The keyshelf is initially equipped with a row of twelve cord circuit keys, each having two associated supervisory lamps and a pair of cords. The keys are designated 'SPEAK' in the forward position and `KEYSEND', in the opposite position. The pairs of cords are coloured yellow, black, green and blue, in rotation.

Provision is made for the addition of a further three sets of cord circuit equipment; the keys for cord circuits 13-15 are initially mounted below the key- shelf, the resulting holes being fitted with dummy key blanks. When extra cord circuits are required, the appropriate keys are released from below the keyshelf and secured in their working positions. Extra cords, plugs, pulley weights and supervisory lamps have to be provided.

The single common key on the left of the keyshelf is the combined 'RING ANS' and 'COUPLE' key. Space for an additional key is also provided. Of the two common key positions on the right, the one nearer the face panel is the 'EXCH ANS MF' key, and the other is a spare space for an additional key.

The operator's instrument jacks are provided in the front left hand face of the keyshelf. Access to the rear of the panel carrying the operator's jacks is obtained by the small panel under the keyshelf. A transmitter cut-out (switch No. 5A) may be fitted between the jacks when authorised.

A removable metal cover (part 1/DFA/27) with a textured PVC finish is provided on the working part of the keyshelf. The markings on the cover are obtained by hot-stamping into this PVC finish. The cord circuit keys are screwed to the keyshelf framework, the handles passing through clearance holes in the cover. The cord circuit supervisory lamp caps are a push fit into the cover.

The keyshelf cover is retained by two ball catches fitted to the key frame locating into the front edge of the plate. It can be released by applying pressure to the bar positioned centrally under the keyshelf.

Where cord circuit metering is required, the keyshelf cover (Part 1/DFA/27) is replaced by a cover (Part P/DFA/27 which has apertures provided to permit the reading and resetting of trip type meters. These meters (Meters No. 21B) are fixed to the keyshelf framework,, below the cover, and in front of the relevant cord circuit key. (NOTE: metering may not be required on all cord circuits.) Metres, Dummy No. 8 are fitted to cover the apertures of non-metered cord circuits. Transparent plastic windows (Part No. 1/DWI/87) and, if required, Ticket Clips No. 18A are fitted for those cord circuits with metering facilities.

(e) Apparatus Layout
Access to equipment mounted within the switchboard is achieved by removing the top and rear panels from the framework. Connection strips are fitted in the middle of the switchboard accessible from rear, for exchange line inter-PBX line and operator's and miscellaneous circuit terminations. Cabling access to all terminal blocks is made via entry holes in the base of the switchboard and cable turning section. Three holes are fitted on each side.

The strip-mounted plug-in relay sets for cord circuits and operator's and miscellaneous circuits are mounted one above the other in the rear of the switchboard. The relay sets are located on the framework by two guide pins, one on each vertical frame member, passing through a hole in each end of the relay-set plate. They are secured by a barrel nut at each end of the relay set screwing on to threaded studs fitted to the frame. Figure 3 shows the rear view of a partially equipped switchboard with the rear cover removed.

The allocation of relay-set positions is shown on the left hand vertical frame member. This is such that the relay sets most frequently required (i.e. the operator's and miscellaneous circuit, and the first eight cord circuits) occupy the lower position on the frame. This arrangement minimises the disturbance that is caused to a working switchboard when access to the rear of the face panel is necessary.

A maximum of 15 cord circuits, together with the operator's and miscellaneous circuit, can be accommodated internally. The exchange line circuits have to be mounted externally on a Shelf-Unit 1A1/SA l0043. A maximum of 10 circuits can be accommodated per unit. (NOTE: two ccts per relay set).

The buzzer, smaller components associated with the miscellaneous jack strip, and terminal blocks for the battery and earth connections for the face panel jacks are all mounted on the rear of the face panel rails.

A fuse panel is mounted at the bottom of the switchboard, towards the rear, and provides the distribution for the power supplies required within the switchboard and for the alarm lamps and buzzer circuits in the CTS (lst position only). This panel can accommodate 30 fuses for negative 50 volt circuits, together with two individual fuses, one of which is used for the continuous supply to cord circuit meters when these are fitted.

By removing the top panel of the switchboard, access can be gained to the connection strips of the extension answering field, these strips connection are mounted in the top of the switchboard, they can be hinged upwards to facilitate access for installation and maintenance purposes.

The Extension Answer Jack Cabling is supported by lacing them to the lugs welded to the switchboard frame, and by side bearers at the side of the switch-board frame which extends down to the access aperture at the bottom of the switchboard.

(f) Cable Turning Section (CTS)

(i) General Construction
The face equipment, key plate, and plinth are coloured light French grey. The top, side and rear panels, and shelf desk are faced with elephant grey melamine resin surface. All the panels are easily removable as on the switchboard positions to give access for installation and maintenance. The Cable Turning Section comes complete with two side panels one of which will be used as the side panel of the switchboard position furthest away from the Cable Turning Section as the positions are not supplied with side panels.

The Cabling Turning Section includes a 75 mm deep plinth. Connection strips for the cables to the alarm lamps, buzzer circuit and night service keys are mounted on the rear of the fuse panel above the key plate.

Details of the Cable Turning Section assembly are shown on Drawing 93505.

(ii) Dimensions and Weight
The Cable Turning Section is 1220 mm high, 432 mm wide and 934 mm deep. The Cable Turning Section as supplied, weighs 68.18kg (150 lbs.).

(iii) Face Equipment
face equipment of the Cable Turning Section is equipped with the standard alarm facilities provided on manufacture.

On the face panel there is a mounting plate equipped with three alarm lamps and two keys. These cater for the 'EXTERNAL FUSE ALARM', 'RECTIFIER MAINS FAIL', and `IVF EQUIPMENT FAIL' alarm lamps and the 'NIGHT SERVICE' and 'ALARM CUT OFF' keys.

The 'Alarm Cut Off' key silences the audible alarm. The function of the other alarm lamps and key has already been described in the Facilities Section, para 2(m), (n) and (p) (vi-viii).

Two spare lamp positions and three spare key positions are provided with dummies, and may be utilised for authorised non-standard facilities.

(iv) Incoming Cable Supports
A framework fitted with Cable Support Pins is provided in the CTS to support the incoming cables. On manufacture the cable pins are fitted in the framework to suit an installation growing from left to right, i.e. with the CTS on the left hand side of the suite of positions. However when the CTS is fitted on the right hand side, to suit a right to left growth, the cable pins can be moved over to corresponding positions on the frame-work. The CTS has been designed to cater for an ultimate cable capacity for an installation with 800 extension lines. Cable entry is usually from floor level, but exceptionally, a hole may be cut in the top to allow for vertical cable entry from above.

When a separate apparatus room is provided the auxiliary equipment associated with the PMBX 11/1A may be mounted on a wall, or a rack provided locally except for the exchange line relay sets which must be mounted in a shelf unit 1A1/SA l0043. If a separate apparatus room has not been allocated and the equipment is on public view in say, an office, then specially designed equipment mounting units are used to mount the equipment. This avoids the necessity of mounting the equipment on a wall as well as presenting a neat appearance.

(a) Distribution Equipment Mounting
Mounting D 9344 provides fixings for jacks test and for strips connection. All extension, exchange and inter-PBX lines are taken through a jacks test for maintenance purposes.

The mountings are positioned above a plinth (Drawing D 93445) and may be stacked vertically or arranged side by side.

Typical arrangements of plinths and mountings are shown in Drawing EC 2116, and further details are given in Specification S 1286.

Where a separate apparatus room is provided and batteries are fitted then Frames Distribution No. 5B are used.

(b) Exchange Line Equipment Mounting
Shelf-Unit 1A1/SA 10043 provides the mounting for up to 10 exchange line circuits. These shelf-units are designed to be stacked in a similar manner to the Distribution Equipment mountings using the same type plinth unit as the base of each stack, and may be stacked vertically or arranged side by side.

Typical arrangements of plinths and mountings are shown in Drawing EC 2116 and further details are given in Specification S 1286.

(c) Signalling-Units and Miscellaneous Equipment Mounting
Mounting D 93443 provides the mounting for signalling units, metering equipment and other auxiliary equipment where a separate apparatus room is not provided. As for the exchange line and distribution equipment the units are mounted on a Plinth D 93445 and may be stacked vertically or arranged side by side.

Typical arrangements of plinths and mountings are shown in Drawing EC 2116 and further details are given in Specification S 1286.

Where a separate apparatus room is provided, which will invariably be the case where stand-by batteries are fitted, the signalling units and other miscellaneous equipment may be mounted on the wall, using wooden battens where necessary.

(d) Power Equipment

(i) 50 Volt Negative Switched Supply
The PMBX 11/1A is operated from a nominal 50 v negative supply. It is designed, however, to function within the limits of 45 v to 55 V. The power is derived from one or more mains operated rectifiers depending on the current required for the particular installation (refer to specification S 1286). The 50 volt distribution is via a Distribution Unit No. 3 at installations which do not have standby batteries or via two or more Fuse Units No. 2 .... when batteries are provided.

(ii) 50 Volt Negative Continuous Supply
Certain equipment which may be provided at PMBX 11/1A installations require power during night service. For example, metering equipment and SSAC 13 equipment. A 50 volt negative supply for this is obtained from a small power unit, fitted locally when required in the Distribution Unit 3A, or obtained from the main supply via separate Fuse Units No. 2 .... when standby batteries are provided. The supply to these fuse units is not disconnected when the Night Service key on the CTS has been operated.

(iii) Power Distribution and Alarm Arrangements
The negative 50 volts and positive (earth) leads are taken from the rectifiers) to a Distribution Unit No. 3A or where stand-by batteries are fitted, to Fuse Units No. 2 .... From there, individual fused outlets are cabled to the switchboard positions, signalling unit(s), etc. The cables to each position are terminated on an alarm type fuse mounting, within the switchboard itself, serving cord circuit relay sets, operator's and miscellaneous circuits relay set, etc. An associated fuse alarm lamp is provided at each position. The alarm circuit from the Distribution Unit or Fuse Unit is connected to an alarm lamp on the Cable Turning Section. (iv) Ringing Supplies The interrupted and continuous ringing supply on an installation with standby power is obtained from a Converter Ringing No. 11A or a Converter Ringing No. 12A. Converter Ringers are required as follows:-

2-6 positions, provide two Converters Ringing No. 11A.

7-9 positions, provide two Converters Ringing No. 11A and one Converter Ringing 12A. 10-12 positions, provide two Converters Ringing No. 11A and two Converters Ringing 12A.

Ringing distribution should be arranged in accordance with diagram EC 2113 fig 13.

The interrupted and continuous ringing supply on an installation without standby power is obtained from a Converter Ringing No. 6 with an associated Ringing Interrupter 1A. Both these items are incorporated within the Distribution Unit No. 3A.

(e) Exchange Line Circuits
The exchange line circuit relay sets are mounted in a Shelf-Unit 1A1/SA 10043 which can accommodate 5 jack in relay sets. One relay-set caters for 2 circuits. For details of the Shelf-Unit see (b).

(f) Signalling Unit Circuits
A range of signalling units numbered SA 7612 to SA 7615, are used for inter-PBX circuits to the PMBX 11/1A switchboard. These units are suitable for wall mounting and are a standard width of 305 mm, with a 185 mm projection from the wall. Where a separate apparatus room is not provided signalling units are normally accommodated in Mountings D 93443 which cater for 2 to 4 circuits each depending on the type of unit (see para (c)).

All inter-PBX signalling systems are catered for by these signalling units, with the exception of SSAC 13 (see below).

Further details, and references to EC diagrams, are given in Specification S 1286.

A signalling unit SA 7611 is available, to provide long extension circuits for lines over 500 ohms resistance, this unit can be used for duplicate answering facilities.

(g) SSAC 13 Equipment
The terminating equipment at PMBX 11/1A installations for the 1 VF SSAC 13 system is Equipment Signalling No. 25/2A. This comprises an open type rack which stands on the floor. Each rack caters for two inter-PBX SSAC 13 circuits. Racks are supplied complete with shelves, shelf-wiring and jacks, on which the appropriate relay sets are mounted.

If required, Covers D 92417 can be supplied, finished in elephant grey, to encase Equipment Signalling No. 25/2A. With these covers, the overall dimensions of the SSAC 13 equipment are 1626 mm high, 540 mm wide and 474 mm deep.

Further information on the SSAC 13 equipment is included in Diagram N 764, as well as in Specification S 1286.


(a) Installation Information for PMBX 11/1A installation is given in Specification S 1286. A list of diagrams required for installation purposes is included in the specification.

(b) Maintenance Maintenance information is given in TI E5 D1411 "Switch- board PMBX No. 11/1A Maintenance". A full list of diagrams relevant to the switchboard and its associated apparatus is given together with maintenance instructions. (Circuit Diagram Folder No. 14 is available for PMBX 11 installations and should be requisitioned as indicated in TI C2 D0010.


This Circular describes the switchboard PMBX 11, which is now nationally available, and details Marketing Policy, Tariff, Sales and Engineering procedures.

The PMBX 11 is an improved multipled version of the PMBX 4. It has been designed primarily to ease the engineering difficulties that are encountered when the PMBX 4 is supplied as a multipled multi-position suite.

The PMBX 11 has a capacity of 400 extensions; this is 200 extensions more than the maximum capacity of the multipled PMBX 4. The switchboard sections are similar in appearance to the PMBX 4 although a cable turning section is always provided on the PMBX 11 and this accommodates the common alarm keys and lamps. An SC Keysender No. 1 is supplied as standard on all PMBX 11 positions.

To improve access in the PMBX 11 switchboard, much of the equipment is accommodated in separate equipment units that are designed to be acceptable alongside office furniture. These units each measure 370mm high, 838mm wide, and 353m deep and may be stacked, placed side-by-side against a wall, or they may be free-standing as best meets the available accommodation. A typical 3 position PMBX 11 will probably require 8 equipment units which may for example, be provided in 2 stacks each of 4 units. A power unit to match the equipment units has also been designed and supplies of this will be available at the turn of the year.

From the issue of this Circular, PMBX 11 is available nationally and it should be offered to all customers requiring a multipled PMBX installation of up to 100 extensions maximum capacity. It should be noted, however, that any PMBX 11 installations that axe required to be brought into service before the purpose-designed power unit mentioned at Paragraph 2 above is available must be supplied with either of two existing units which have maximum outputs of 12 and 25 Amps. These arrangements will meet the requirements of typical 2 and 3 position installations and some 4 position installations but precise power requirements must be assessed. For details see Appendix C. This Circular will be re-issued and a Descriptive Leaflet made available when supplies of the new power unit are available.

Subject to the availability of power supplies and planning periods (see TI B4 E0001) all new requirements for multipled PMBX's should be met as follows:

PMBX 11 - up to 400 extensions

PMBX 1A - up to 800 extensions

PMBX 1B - up to 1,200 extensions

PMBX 4 should normally be supplied only for single-position and 2 position non-multipled installations.

Subject to the required service dates opportunity should be taken wherever possible to re-negotiate outstanding orders for multipled PMBX 4 installations that have not been met.

The maintenance of PMBX 11 is similar to the multipled PMBX 4. The major differences are the housing of the Exchange Line Relay Sets away from the switchboard which gives easier accessibility for cord siting, and the larger extension multiple.

Maintenance instructions are contained in TI E5 D1411.

Face layouts are shown in Drawing EC 2112. Jack strips with 20 jacks are used for all circuits and lamp jack strips accommodate 20 lamps; circuit designations being hot stamped on the opal lamp caps.

A maximum of 100 extension answering jacks be provided on each PMBX 11 position and any extension multiple number may be associated with any Jack.

Exchange lines may be multipled to all switchboard positions or to alternate positions by removing lamps as required. If more than 30 exchange lines are to be provided both odd and even jack positions should be asked from the onset. If 30 or less exchange lines are neared the even numbered jack positions only may be used to improve operating access to the exchange line jacks.


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Last revised: April 27, 2023