PMBX No. 1A

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

Description

The BPO P.M.B.X. No. 1A is a lamp calling manual switchboard with lamp supervisories.  The switchboards are bolted together to form a suite.

The number of switchboards was dependant on traffic and extensions.  In addition to the switchboards a power supply, batteries, equipment rack and distribution frame were also supplied.

The original switchboard, "Section, Switch, P.M.B.X., No. 1A", was being superseded for by "Section, Switch, P. B. X., SA 7560", which was also used on the P.A.B.X No. 2 and No 3.

Introduced in 1940.

During the 1970's the Switchboard could be retrofitted with a press button keypad in place of the dial.  This was the Key Unit No. 4/1A and it used a Sender Unit No. 9B-1 which was located in the keyshelf.

See the Ericsson section for another description.

Click here for additional pictures

POEEJ Article from 1942

P.O. ENGINEERING DEPT.
ENGINEERING INSTRUCTIONS
TELEPHONES
P. B. X.
B 1010
Issue 3, August 1952

PRIVATE MANUAL BRANCH EXCHANGES
Details of P.M.B.X. No. 1A Installations

1. Contents

2. Scope of Instruction
This Instruction describes the floor pattern, double-cord, lamp-signalling, multiple-type switchboards used at P.M.B.X. No. 14 installations.  These switchboards are for use where more than two positions and not more than 800 extensions and a total of 160 exchange lines or private circuits, etc., are required. P.M.B.X. No. 1A installations supersede B.E.C.B. No. 9 installations, except where it is possible to extend existing B.E.C.B. No. 9 installations with equipment from stock.

The circuit arrangements enable the switchboards to be utilized in C. B. (manual) and auto, exchange areas and, with suitable auxiliary apparatus, in C.B.S. Nos. 1, 2 and 3 areas (see B 3001).  Relay-sets and other items associated with the switchboards are mounted on racks known as "Equipments, P.M.B.X., No. 1" and Equipments, P.M.B.X., No. 1/35", which are also described in this Instruction.

The original switchboard, "Section, Switch, P.M.B.X., No. 1A", referred to later in this instruction as the "o1d-type" switchboard, is being superseded for new work by "Section, Switch, P. B. X., SA 7560", referred to later as the "new-type" switchboard.  Both types of switchboard are described in this Instruction.

The "Section, Switch, P. B. X., SA 7560" is also used as the manual switchboard for standard P.A.B.X. installations.

FACILITIES

3. Lamp calling for extensions
A special lamp ("Lamp No. 2-17V") is used to eliminate the need for a line relay on extension lines not exceeding 200 ohms line loop resistance.  The lamp is connected in series with the negative feed, and glows when the extension loop is completed by the operation of the telephone gravity-switch.  Provision is made for a proportion of the extension equipments to be fitted with line relays, for extension circuits where line loop resistance exceeds the limits quoted in TRANSMISSION, Tele., B 3540.  To prevent overhearing between calling extensions, the pilot relay is shunted by a low-resistance rectifier-element. Certain rectifier-elements were incorrectly connected in the high-resistance direction, and should be reversed if this has not already been done [Works Specification S 18 (July 1941) refers].

4. Multiple
A break-jack series multiple is used for extension circuits.  The answering lamps are associated with the extension multiple jacks, but only one calling lamp may be fitted for any extension line; thus, for example, if the calling laps for extensions 0 to 19 appear in the first multiple appearance, spacing strips are fitted in subsequent appearances.

In old installations, where the cable loops into all multiple appearances, the lamp wires are looped and left uncut at appearances fitted with a spacing strip.  If it is subsequently desired to change the position at which the lamps appear, the spacing strip at the new appearance should be replaced by a lamp jack which may then be connected to the lamp cable.  The lamps should be removed from the original appearance and transferred to the new. The lamp caps should be retained at the original appearance.  Such a change should be made for a complete strip of 20 lamps, wherever possible.

In new installations, the cabling for the lamps is from the jack at the last multiple appearance, or from the cable-turning section in the case of extensions with line relay facility, to the position at which the lamps are fitted only.  If, subsequently, it is desired to change the position at which the lamps appear,
(a) the lamp cable should be cut back to the new position and the existing lamp jack replaced by a spacing strip or (b) an additional cable should be provided from the existing lamp jack to the new position.  The lamps in the existing lamp jack should be removed and the lamp caps retained. Such a change should be made for a complete strip of 20 lamps, wherever possible.

Redistribution of exchange line lamps at both old and new installations may be effected (in a manner similar to that described above for extension lamps at old installations) in complete groups of 10, instead of 20 as for extensions.

5. Cord circuits
Each pair of cords is fitted with a SPEAK and RING key, and RING-BACK and DIAL key.  The "ring" and "dial" functions are associated with the calling cord, and the "ring-back" function with the answering cord.  A supervisory lamp is associated with each cord. in addition, the "Section, Switch, P.B.X., SA 7560" is wired for two dial cords necessary for dialling over circuits to P.A.B.X.s.  Two "Cords No. 349, Blue" with "Plugs No. 316, Red", two "Pulley-weights No. 14A" and one "Key No. 212, Blue" and "Key Mounting, WL" should be provided locally where this facility is required.

If dial cords for this purpose are required on a "Section, Switch, P.M.B.X., No. 1A", the sixteenth cord circuit should be rewired in accordance with the dial-cord circuit on Diagram. LD/131, and two "Cords No. 386, Blue" with "Plugs No. 316, Red", two "Pulley-weights No. 16" and one "Key No. 212, Blue" fitted in the sixteenth cord-circuit position.

In both cases, the "Labels No. 44" and key-mountings should be engraved as shown on Drawing 90053.

6. Extension to Extension calls
On extension to extension, and extension to inter-switchboard (or private circuit) connexions, the cord circuits give divided-battery feed with individual supervision.

7. Exchange to extension calls (Exchange line R/S)
On exchange to extension connexions the cord circuits, in conjunction with the exchange line relay-sets, normally give:-

1. through feed from exchange to extension

2. through clearing from extension to exchange

3. double supervision- both lamps light simultaneously when extension clears

4. through dialling from extension, and facilities for operator to have dial or keysender (a) operator recall, see par. 9

5. follow-on-call trap, see par. 10.

8. Exchange to extension calls (Cord Circuit)
On exchange to extension connexions, when authorized, the cord circuits can be arranged to give:-

1. local feed from switchboard to extension

2. local clearing from extension to switchboard

3. single supervision - when the extension clears, only that lamp associated with the cord to which the extension is connected lights

4. facilities for operator to have a dial or keysender (e) operator recall, see par. 9

5. follow-on call trap, see par. 10

9. Operator recall
(a) The operator can be recalled by depressing and releasing the gravity-switch on the extension instrument on the following types of call:-

(i) outgoing and incoming calls on lines connected to a manual exchange
(ii) Incoming calls only, on lines connected to an automatic exchange
(iii) outgoing calls routed via an auto-manual switchboard having a manual-hold facility
(iv) extension to extension and extension to inter-switchboard line.

It is essential that the gravity-switch be operated slowly, to give ample time for the release of relays.  "Flashing" by this means on calls to manual exchanges will "flash" both the P.B.X. and the main-exchange operators.

(b) Where the facility is required on outgoing calls to an automatic exchange, it can be given by fitting a press-button at the extension instrument.
A "Telephone No. 330" should be used, where possible, with a press-button wired in accordance with Diagram N430.  If a "Telephone No. 330" is not used, a "Press-button P" should be fitted and wired in a similar manner to Diagram N430.

In either case, care should be taken to ensure that the press-button is wired so that it applies an earth to that wire of the exchange line which has battery potential connected to it under "Called-subscriber Answer" conditions on an outgoing call.

(c) where local clearing has been provided (see par. 8) the operator may be recalled on all types of call by "flashing" as in (a).

(d) If difficulty Is experienced in achieving slow flashing [see (a)], "Relay-sets SA 7512" may be modified locally in accordance with works Specification S 508 (Nov. 1951) on the authority of the T. M. (Traffic Division).  See also par. 31.

10. Follow-on-call trap
When an incoming exchange call is received prior to the operator clearing the previous connexion, the supervisory lamps will flash.  The lamps are extinguished during the ringing period, and the ring is prevented from reaching the extension telephone bell.

11. Night service
This is provided by using any pal r of cords to connect a night-service exchange-line jack to any required extension jack.  Night-service keys are not required for the cord circuits which are used, but it is essential that the cord-circuit keys should be in the "normal" position when the connexions are set-up, and should then remain untouched.  If a key is accidentally operated, the supervisory lamp will glow and the connexion must be taken down and re-established, on "Sections, Switch. P.B.X., SA 7560" equipped with cord circuit relay-sets to SA 7565 Mark 1, which have been strapped to provide local clearing, separate double-ended cords must be used for setting-up the night-service connexions.

12. Private Circuits
Private circuits, Inter-switchboard private circuits, and inter-switchboard extensions appear in the exchange-line multiple field, and are provided with relay-sets appropriate to the method of signalling used, see pars. 32 to 36.

13. Miscellaneous

1. Pilot alarm. This serves all calling lamps: one lamp is provided for each position

2. Audible alarm. This is common to the suite and can be switched-off when not required

3. Engaged test. The standard "click" engaged test is provided

4. A position coupling, key is provided on each section, to connect the cord circuits to the operator's circuit of the next position on the left, when the original position is unattended

5. An emergency hand-generator key is fitted on each position

6. Monitoring facilities are provided at certain R.A.F. installations, see B3905

14. Capacity of "Section, Switch, P.M.B.X. No. 1A" and "Section, Switch, P.B.X. SA 7560"
Each section has two panels and its capacity is as follows:-

1. Exchange lines, etc.
   Jacks No. 810BN - 4 per panel (10 jacks per strip)
   Jacks, Lamp, No. 19A  - 4 per panel (10 lamps per strip)
   Lamps No. 2-24V - 40 per panel
   Caps, Lamp, No. 1-1A - 40 per panel

2. Extension lines.
   Jacks No. 520BP - 10 per panel (20 jacks per strip)
   Jacks, Lamp, No. 174 - 10 per panel (20 lamps per strip)
   Lamps, No. 2-17V - 200 per panel
   Caps, Lamp, No. 1-1A - 200 per panel
   Note - Normally, the full number of lamp jacks will not be required on any one position.  Spacing strips 0.5in. deep will be used instead.  These are supplied with the section to fill up the multiple space.

3. Night-Service jacks (for exchange lines)
   Jacks No. 510BO - 1 per position (10 jacks per strip)
4. Position equipment for "Section, Switch, P.A.B.X., No. 1A"

* On early issues of the "Section, Switch, P.M.B.X., No. 14", these keys were "Keys No. 68, No. 207 and No. 68", respectively, and were fitted on the keyshelf.
** Plus one spare dial in "Mtg. Dial, Auto., No. 9, Incomplete" and "Case No. 9" fixed on the cable-turning section.

 

Position equipment for "Section, Switch, P.B.X. SA 7560"

* The switch sections are ready equipped with all keys and supervisory lamp jacks for cord circuits 1 to 15.  In addition, the cords, plugs and pulley-weights are fitted for circuits 1 to 12.
** Plus one spare dial in "Mtg., Dial, Auto., No. 9 Incomplete" and "Case No. 9" fixed on the Cable-turning Section.

15. Ultimate capacity of complete installation
Exchange lines, private circuits, and inter-switchboard lines - 160
Extensions - 800

16. Weight
Section, Switch, P.M.B.X., No. 1A, as supplied, without multiple - 241 lb. approx.
Section, Switch, P.M.B.X., No. 1A, fully equipped - 320 lb.
Section, Switch, P. B. X., SA 7560, as supplied, without multiple - 250 lb.
Section, Switch, P. B. X., SA 7560, fully-equipped - 420 lb.
Equipment, P.M.B.X., No. 1, as supplied - 280 lb.
Equipment P.M.B.X., No. 1/35, as supplied - 224 lb.
Relay-set SA 7512 - 12 lb.
Relay-set SA 7514 - 14 lb.
Relay-set SA 7515 - 9 lb.
Relay-set SA 7518 - 14 lb
Relay-set SA 7565 - 19 lb.

17. Dimensions

18. Circuit diagrams

1. Wiring.
   "Section, Switch, P.M.B.X., No. 1A" - Diagram LD 67 (Issued with Switchboard.)
   "Section, Switch, P.B.X., SA 7560" - Diagram LD 131
   Cord circuits for "Section, Switch, P.B.X., SA 7560" (Mark 1) - Diagram SAW 75650 and (Mark 2) - Diagram SAW 75651
   "Equipment, P.M.B.X., No. 1" - Diagram LD 68
   "Equipment, P.M.B.X., No. 1/35" - Diagram LD 80
   Cabling diagrams (see par. 43) - Diagram LD 71 (Issued with Switchboard)
   Cabling diagrams - Diagram LD 100
    
2. Schematic.
   "Section, Switch, P.M.B.X., No. 1A" - Cord circuit, dial, and ringing circuit - Diagram SA 7510
   "Section, Switch, P.M.B.X., No. 1A" - Operator's circuit - Diagram SA 7511
   "Section, Switch, P.M.B.X., No. 1A" - Pilot and cord-test circuit - Diagram SA 7516
   "Section, Switch, P.B.X., SA 7560" - Cord circuit relay-set (Mark 1) - Diagram SA 7565/0
   "Section, Switch, P.B.X., SA 7560" - Cord circuit relay-set (Mark 2) - Diagram SA 7565/1
   Position circuits - Diagram SA 7560
   Exchange-line circuit - Diagram SA 7512
   Extension-line circuit - Diagram SA 7513
   Private circuit, Inter-switchboard private circuit, or Inter-switchboard extension - Diagrams SA 7514, SA 7515, SA 7518 and SA 7519
    
3. Combined Wiring and Schematic.
   Supervisory flashing facilities, for generator-clearing private circuits, etc - Diagram SA 7521
   Connexion of "Units, Signalling, No. 5" - Diagram SA 7554 (only required in special cases see MISC., Den., Q 9001)

19. Drawings. (Not required for installation purposes)

1. "Section, Switch, P.M.B.X., No. 1A" Assembly - Drawing 63472 and Keyboard Assembly - Drawing 63474
2. "Section, cable turning, P.M.B.X. No. 1A" - Drawing 63473
3. "Section, Switch, P.B.X., SA 7560" - Drawing 63987
4. "Section, cable-turning, P.B.X. No. 1" - Drawing 90011
5. "End panel No. 3" - Drawing 90010
6. "End Panel No. 7" - Drawing 63966
7. "Equipment, P.M.B.X., No. 1" Assembly - Drawing 63475 and "Rack, Apparatus, No. 43" - Drawing 63476
8. "Equipment, P.M.B.X., No 1/35" - Assembly - Drawing 63485 and "Rack, Apparatus, No. 45" - Drawing 63322

20. Power supply data (see also par. 39)
Maximum permissible voltage - 30V.
Minimum permissible voltage - 20V.
Average current (at 26V.) for exchange connexion - 180ma.
Average current (at 26V.) for extension to extension connexion - 310ma.
Average current (at 26V.) for extension to inter-switchboard extension or private wire (automatic-signalling) - 530ma.
Average current (at 26V.) for extension to inter-switchboard extension or private wire (generator-signalling) - 340ma.
Average current (at 26V.) for exchange to inter-switchboard extension - 240ma.
Average current (at 26V.) for operator's circuit, when SPEAK key is operated (allowed for in above) - 108ma.

21. Exchange and extension line limits
For transmission and signalling resistance limits see TRANSMISSION, Telephones, B 3540.

A "Relay No. 625" and "Resistor, Coil, No. 12 - 200 ohms" will be required for the calling equip-merit of extension circuits when the loop line resistance necessitates the provision of a line relay (see pars. 3 and 28).  These extensions should be grouped within the first, fourth, seventh and tenth strips of the extension multiple in panels 0 and 1. (See Diagram LD 71 and B1011).

DESCRIPTION OF SECTIONS

22. "Sections, Switch, P.M.B.X., No. 1A"
The sections are of the 2 panel type, arranged for lining-up and fixing by means of five 3/8in. B.S.W. x 2.5in. bolts (with nuts and washers), obtained locally, per section.  They form a continuous switchboard, with a cable-turning section secured by wood screws at each end.  Each section is equipped with 12 cord circuits (with wiring for 16), an operator's circuit, and miscellaneous circuits.  The lamp jacks, multiple jacks and multiple wiring are provided during installation.

23. "Section, Switch, P.3 .X., SA 7560"
The sections are of the 2-panel type arranged for lining-up and fixing by means of six 1/4in. B.S.W. bolts (with nuts and washers), included with the section, per section.  They form a continuous switchboard with a cable-turning section bolted on at each end.

Each section is equipped with an operator's telephone circuit, other position circuits and battery-supply fuses.  It is wired for two dial cord circuits and 15 connecting cord circuits. The cord circuit relays are on plug-in 2000-type mountings in sets of three circuits per mounting.  The lamp jacks, multiple jacks, multiple wiring and Cord-circuit relay-sets are provided during
installation.

24. Face equipment
A "home" position is not provided, but calling lamps are fitted in the multiple field, and the multiple repeats after the fourth panel.  The extent of the multiple number of positions, and the location of the lamp jacks in the panels is determined for any particular installation by the development forecast and traffic considerations.  Each panel has capacity for 40 exchange lines and 200 extensions and the ultimate capacity of an installation is, therefore, 160 exchange lines, etc. and 800 extensions.
The arrangement of jacks and lamps is as follows:-

1. Exchange lines. From the pilot rail, the first eight spaces are reserved for exchange line
   jacks and lamps, in strips of ten. Each lamp jack is fitted immediately above Its respective multiple jack.
   Only one calling lamp (exceptionally) per exchange line may be used. At other appearances, a spacing strip should be fitted.

2. Inter-switchboard lines and private circuits are fitted in the exchange line multiple.

3. Extension jacks and lamp jacks are in strips of 20; the lamp jacks and spacing strips are arranged In a manner similar to that described for the exchange lines, see (a) only one calling lamp per extension line may be used, and at other appearances a spacing strip should be fitted.

4. Night-service jacks. Space is available, at the top of the multiple field in the left-hand panel of each position, for fitting, when required, a "Jack No. 510BO" for exchange-line night-service.  Not more than one strip of night-service jacks should be fitted on each position.

5. Hand generator, position coupling, and dial emergency keys. A "Key No. 279" for bringing the hand generator into use if power ringing fails, and a "Key No. 308" for coupling the cord circuits to the operators circuit of the next position on the left when the original position is unattended, are accommodated on a "Key-mounting, PD" in the right-hand panel at the top of the multiple field.  A "Key No. 279", for bringing the dial into circuit during failure of a mechanical or electrical keysender, should be fitted on this mounting when required.

6. Cord-test jack is fitted in the pilot rail of the left-hand panel on "Section, Switch, P.M.B.X., No. 1A" and in the top of the left-hand panel on "Section, Switch, P.B.X. SA 7560".

7. Position pilot lamp. This is fitted in the pilot rail on the right-hand panel.

8. Fuse-alarm lamp. This is fitted in the pilot rail on the left-hand panel on "Section, Switch, P.B.X., SA 7560" only.

25. Keyboard
From the pilot rail to the front of keyboard, the equipment is arranged as follows:-

1. Answering cords are "Cords, Switchboard, No. 386" on "Section, Switch, P.M.B.X., No. 1A" and "Cords, Switchboard, No. 349" on "Section, Switch, P.B.X., SA 7560".  The cord colour scheme in each case is black, yellow, green and slate-grey.  Each cord is fitted with a "Plug No. 316, Red".

2. Calling cords, as in (a).

3. Answering-cord supervisory 1ainbs giving positive supervision, i.e. glowing when operator's attention is required.

4. Calling-cord supervisory lamps, as in (c).

5. DIAL and RING-BACK key.  This is a "Key No. 293" on "Section, Switch, P.M.B.X., No. 1A", and a "Key No. 298" on "Section, Switch, P.B.X., 54 7560".  (DIAL position locking, RING-BACK position nor-locking)

6. SPEAK and RING key.  This is a "Key No. 308" on "Section, Switch, P.M.B.X., No. 1A", and a "Key No. 295" on "Section, Switch, P.B.X, 54 7560" (SPEAK position locking, RING position non-locking.).
7. The key-handle colours correspond to the colours Lf the cord concerned.

On the right-hand side of the keyshelf - in line with the supervisory lamps - space has been reserved (and wiring provided) for fitting a dial, when required.  So that the keyshelf may be opened fully without the dial fouling the plugshelf, the dial mounting should be located with its rear edge 1/4in. from the hinge.

The hand generator ("Generator No. 26A") is fitted under the keyshelf on the right-hand side of the section, and is earthed on the "ring" side for operation of relay RR in "Relay-sets SA 7514" and "Relay-sets SA 7518".  This earth connexion should be provided locally if the existing earth is obtained from the power-ringing circuit; otherwise, correct operation will not be possible when the power ringing fails [works Specn. S18 (July 1941) refers].

Provision is made in the cord-circuit wiring to permit a "Keysender No. 5..." to be fitted.  If an electrically-operated keysender is required, it will be necessary to cut away part of the keyboard to fit the digit-key strip.  With either type of keysender, an additional relay should be fitted locally in the section. ("Relay No. 7321" in old type sections, or Relay No. 10299" in new type sections).

26. Layout of apparatus at rear of "Section, Switch, P.M.B.X., No. 1A"
The cord-circuit relays - SK, SKR, AS, CS, LA and LC - are 3000-type relays, fitted on "Mountings No. F151/18AH" (3 cord circuits per mounting) and are arranged as follows, commencing at the top:-

1. (i) Cord circuit No. 16 (wiring only)
   (ii) Apparatus for operator's cord-test and audible-alarm circuits, with space for key-sender relay, when required.
2. Cord circuits Nos. 13 to 15 (wiring only, With space for "mounting, No. F151/I8AH")
3. Cord circuits Nos. 10 to 12
4. Cord circuits Nos. 7 to 9
5. Cord circuits Nos. 4 to 6
6. Cord circuits Nos. 1 to 3

A "Strip, Connexion, No. 60" for connecting battery and earth feeds, also position-coupling key wiring and power ringing to cord circuits, is fitted below the relay mountings.  The capacitors for the cord circuits, operator's and dial circuits are fitted above the jack-field, at the top of the section.
When additional cord circuits, i.e. more than 12 per position, are required, apparatus as shown on Diagram SA 7510 should be requisitioned for each additional cord circuit and should be fitted locally.  A "Mounting No. F151/I8AH" will also be required to mount the relays for cord circuits Nos. 13 to 15. Additional cord circuits should only be fitted where those existing are found to be insufficient to carry the traffic.

27. Layout of apparatus at rear of "Section, Switch, P.M., SA 7560"
At the bottom are the mountings for the plug-in 2000-type relay-sets, each of which contains relays, capacitors, etc., for 3 cord circuits.

Above this is a "Mounting No. F151/18AH" which carries the relays, capacitors, etc., for the operator's telephone, pilot, cord-test, fuse-alarm and dial-cord circuits and space for the keysender relay.

At the top of the section, above the jack-field, are the "Fuse mountings Nos. 1031C and 631B" for the battery-supply fuses for the section and a "Strip, Connexion, No. 128C" for terminating the position-coupling key, power-ringing, etc., wiring. A battery jack is provided adjacent to the fuse-mountings.

ASSOCIATED APPARATUS

28 "Equipment P.M.B.X. No. 1"
Comprises a "Rack, Apparatus, No. 3", on which the following apparatus is mounted:-

1. Connexion strips, fitted at the top of the rack, for terminating:-
   (i) cables to main frame
   (ii) cables to switchboard extension multiple and lamp jacks
   (iii) cables to switchboard exchange-i inc multiple and lamp jacks
   (iv) wiring to "Jacks No. 61F" for relay-sets.
    
2. "Fuse-mounting No. AA".  With capacity for 96 "Fuses No. 35/..." mounted in two rows, each containing 48 fuses. The fuses are allocated as follows:-
   10 fuses for each of five positions *
   15 fuses for relay-sets
   12 fuses for extension sleeve resistors of 20 extensions
   1 fuse for audible alarm
   1 fuse for 20 line relays for long extensions
   1 fuse for position battery jacks *
   1 fuse for rack battery jacks
   15 spares for miscellaneous requirements, 10 of these to be used for a sixth position when the number of exchange lines does not justify an additional "Equipment, P.M.B.X. No. 1", see par. 30.
   * When used with "Sections, Switch, P.B.X. SA 7560", these fuses are not used, because the position fuses are mounted on the section.
    
3. Accommodation for "Mountings No. F57/10AV".  Space is provided for mountings No. F157/10AV", which should be requisitioned (as required) to accommodate the "Relays No. 625" for long extension 1ines, see par. 3.  Wiring is provided from the relay mounting space to the connexion strip at the top of the rack.
    
4. "Mountings, P 62391" containing 240 "Resistors, Coil, No. 12 - 250 ohms" are provided for the extension sleeve connexions.  The resistors are mounted five per spindle and are wired to the connexion strips, fitted at the top of the rack. (An additional "Mounting D62391" should be provided to accommodate the "Resistors, Coil, No. 12 - 200 ohms" which are required (in series with the relay contact and lamp) for long extensions.  Wiring is provided from this mounting space to the relay connexion strips at the top of the rack, see (c).
    
5. (e) "Jacks No. 61F".  Three rows of five "Jacks No. 61F" are provided for 15 relay-sets, which can be either "Relay-sets, SA 7512" (two exchange line circuits per relay-set) or "Relay-sets, SA 75111, SA 7515 or SA 7518" (one private circuit, inter-switchboard private circuit, or inter-switchboard extension per relay-set).  The Jacks No. 61F are wired to the connexion strips fitted on the top of the rack.

29. "Equipment, P.M.B.X., No. 1/35"
This comprises a "Rack, Apparatus, No. 115", on which the following apparatus is mounted:-

1. Connexion strips, fitted at the top of the rack, for terminating:-
   (i) cables to main franc
   (ii) cables to exchange-line multiple and lamp jacks (for private circuits, etc.)
   (iii) wiring from "Jacks No. 61F".
    
2. "Jacks No. 61F".  Seven rows of five "Jacks No. 61F" are provided for 35 "Relay-sets, SA 75111, SA 7.515 or SA 7518" (one circuit per relay-set).
    
3. Fuse mountings.  Three "Fuse-mountings No. 1031C" and one "Fuse-mounting No. 631B" are mounted on the left-hand side of the front to accommodate one fuse per relay-set.

30. To ascertain the number of "Equipments, P.M.B.X., No. 1 and 1/35" required per installation, compute the number of "Equipments, P.M.B.X. No. 1" required, on the basis of one to every 30 exchange lines.  If the number of spare relay-set positions after leaking due allowance for development of exchange lines (two circuits per relay-set) as shown in the relative traffic data] is not capable of accommodating the private circuits, etc. (one circuit per relay-set), then one "Equipment, P.M.B.X. No. 1/35" should be provided for each group of up to 35 private circuits, etc.

Where it is necessary to provide "Equipments, P.M.B.X., No. 1/35", all relay-sets associated with private circuits, etc. should be fitted on these equipments, before utilizing for these relay-sets any spare exchange line relay-set positions available on the "Equipment, P.M.B.X., No. 1".

31. "Relay-sets, SA 7512A"
These relay-sets are used for terminating exchange lines.  They are designed to jack into the "Jacks No. 61F" fitted on "Equipments, P.M.B.X., No. 1".  The rely-set accommodates two exchange-line circuits, each circuit employing five 3000-type relays, i.e. relays AC, LG, L, LR and D, the functions of which are as follows:-

Relay AC - Calling relay; operates to incoming ringing from the main exchange
Relay L - Line supervisory relay: operated while circuit is engaged
Relay LR - Relief relay on L Relay LG. Line guard relay
Relay D - Operator recall relay. Is differentially connected and operates when recall button on extension is depressed

Resistor YA in the exchange line circuit dims the supervisory lamps. its value was formerly 1500 ohms, but is now reduced to 500 ohms to give an adequate engaged-test "Click".  Any 1500-ohm resistors which remain should be changed to 500 ohms [works Specification S 18 (July 1941) refers].

Early versions of the relay-set will only respond to slow flashing from an extension; removal of the extension telephone loop releases relay L which causes a short-circuit on relay LR, which releases slowly.  A contact of relay LR lights the supervisory lamps. on later versions of the relay-set, the contact of relay L lights the supervisory lamp directly and so permits more rapid flashing (see par. 9).

32. Relay-sets for private circuits, etc
Three types of relay-set - "Relay-sets, SA 7514, SA 7515 and SA 7518" exist for terminating private circuits, inter-switchboard private circuits, and Inter-switchboard extensions.  The relay-sets, which are described in pars. 33 to 37, are wired so that by strapping appropriate U-points on the "Jack No. 61F", the following methods of signalling may be used, see B3102.

The line-resistance limits for these relay-sets are given in the diagrams quoted.

Automatic signalling should be adopted in preference to generator signalling wherever line conditions permit.

Exchange prohibition is provided on each relay-set by inserting a strap between u-points 23 and 30.

33. "Relay-sets, SA 7514"
These relay-sets are now superseded by "Relay-sets, SA 7515 and SA 7518". They should still be used in accordance with par. 37 for terminating new private circuits, inter-switchboard private circuits, and inter-switchboard extensions, out should not be requisitioned from the Supplies
Dept. (see para. 37).  Each relay-set accommodates one circuit, employing seven 3000-type relays, i.e. LL, LR, LD, RR, S, SR, the functions of which arc as follows:-

Relay L - Line relay, for generator signalling inward
Relay LL - Line and supervisory relay, for automatic signalling inward
Relay LR - Relief for relays Land LL
Relay LD - Relief for relays L and LL
Relay RR - Ringing relay for generator calling outward
Relay S - Sleeve and exchange-prohibition relay
Relay SP - Relief for relay S.

34. "Relay-sets, SA 7515" (Diagram SA 7515)
These should normally be used for private circuits, inter-switchboard private circuits, and inter-switchboard extensions, when automatic signalling (Methods A and B, par. 32) is possible.  See also sub-par. 37 (a).

The relay-sets include equipment for generator calling outward, if required, but are cheaper to produce than "Relay-sets, SA 7514 end SA 7518", which include (in addition) equipment for generator calling and clearing inward.  Each relay-set accommodates one circuit, employing five 3000-type relays, the functions of which are as follows:-

Relay LL - Line relay for automatic signalling
Relay LR - Control relay for automatic signalling
Relay RR - Ringing relay for generator calling outward when required by distant equipment
Relay S - Sleeve and exchange prohibition relay
Relay SR - Relief for relay S.

35. "Relay-sets, SA 7518" (Diagram SA 7518)
These should be used [subject to sub-para. 37 (a)) for private circuits and inter-switchboard private circuits, when generator-signalling (Method D, par. 32) is necessary, or when "loop-generator" signalling inward "loop" signalling outward (Method C) is necessary for special purposes. T hey may also be used for any circuits of the above-mentioned types on which generator signalling is necessary for special reasons, such as arrangements for switching to alternative terminations.  Each "Relay-set, SA 7518" accommodates one circuit, employing seven 3000-type relays, the functions of which are similar to those in "Relay-set, SA 7514", see par. 33.  The relay-set is inter-changeable with "Relay-set, SA 7514" but, when used for generator signalling, an additional strap is required on the "Jack No. 61F".  Certain improvements in design have been incorporated for the following purposes:-

1. Lessening the likelihood of false clears and false calling signals during generator calling and clearing, respectively
   and

2. Facilitating the addition of supervisory-flashing facilities, for generator clearing circuits, see B 3103 and B 3210.

36. "Relay-set, SA 7518" (Modified to Diagram SA 7519)
Where balanced battery (G/DC) signalling is required by line conditions on a private circuit or inter-switchboard private circuit, terminated at a P.M.B.X. No. 1A installation, a "Relay-set SA 7518" should be modified locally to Diagram SA 7519 [as described in works Specn. S 526 (July 1951)].

37. Use of "Relay-sets, SA 7514"

1. Before a "Relay-set, SA 7518" (par. 35) is requisitioned for a new circuit, an attempt should be made to meet the requirement by transferring a "Relay-set, SA 7514" from a suitable existing circuit, and replacing it by a "Relay-set SA 7515" (see par. 34). When this is done, the straps on the "Jack No. 61F" of the existing circuit will require rearrangement, as shown in Diagram 54 7515.

2. Existing "Relay-sets, SA 7514" should be replaced by "Relay-sets, SA 7515" only when actually required for transfer to new circuits, as described in (a), or when faulty.

38. Distribution frame
The incoming lines are terminated on a main distribution frame.  "Frame, M.D. 0/240" or one or more "Frames, M.D. 0/480 may be used.  Protection should be provided in accordance with T.I. III Part 2 and PROTECTION, General, B 1001.

The internal side of the frame is cabled to the "Equipments, P.M.B.X., No. 1" and "Equipments, P.M.B.X., No. 1/35", the cables terminating on the connexion strips at the top of the equipments.  The multiple cables from the switchboard terminate on connexion strips at the top of the "Equipments, P.M.B.X., No. 1", and/or the "Equipments, P.M.B.X., No. 1/35".

39. Power plant
A power installation of secondary cells will be required, nominally 24 volts,

1. Where 200/250V, single phase, 50 c/s. A.C. mains are available, a single-battery floating scheme with "Rectifier No. 38" (see Diagram N 638) is suitable, see also POWER, Gen., S 1320 and S 3320. When a "Rectifier No 38" is used, a "Lamp, Alarm-indicating, No. 4" should be fitted in the first cable-turning section and connected as shown in Diagram N 2304.

2. Where D.C. mains only are available, a double-battery scheme is normally required, and is installed in accordance with T.I. XXIV.

3. The size of power plant is computed from the traffic data and the information given in par. 20.

40. Power distribution
For installations of such a size that the rating of the main battery fuse does not exceed 40 amp., the cables to the "Equipment, P.A.B.X., No. 1" and the "Sections, Switch, P.B.X., SA 7560" should be taken direct from the main battery fuse.

Where the installation is of such a size that the main battery fuse will be rated at more than o amp., a distribution-fuse panel, consisting of a panel or frame made up locally and upon which are fitted "Fuse-mountings No. 120A" with "Fuses No. 22/ 40", should be fitted.

Each group of four "Sections, Switch, P.B.X., SA 7560" and each pair of "Equipments, P.M.B.X., No. 1. or No. 1/35" should be fed from a separate fuse.  The feed to the switchboard sections should preferably be taken to the first section of the group of four and looped in and out of the succeeding sections.  Alternatively, the feed may be taken to the last section of the group and separate feeds tee-off to the preceding sections.

The rack feeds should be treated similarly.

Typical power distribution arrangements for a small and large P.M.B.X. No. 1A installation are given in Diagram EC 2025.

INSTALLATION

41. Accommodation and layout of equipment

1. Switchboards.  A good working space should be allowed for in the rear of the switchboards: a minimum of three feet is recommended. The accommodation should provide for the switchboard being extended to the estimated ultimate number of positions.

2. Cable-turning sections.  Two cable-turning sections are required at an installation, one at each end, to close the ends of the suite of switchboard sections.
   "Section, Cable-turning, P.M.B.X., No. 1" should be used with a "section, Switch, P.M.B.X., No. 1A".  "Section, Cable-turning, P.B.X., No. 1" should be used with a "Section, Switch, P.B.X., SA 7560".
   Two cable-turning sections may be used together at the non-growing end of a suite with a large multiple, i.e. more than 400 extensions ultimate, to allow sufficient space for the cables to be turned and the jacks on the first panel to be removed for maintenance purposes.

3. Angle sections.  When necessary, the switchboard sections can be placed at an angle to one another, but must not be cut in any vey. Angle sections should be constructed locally to match the switchboard, and the angles should never be greater than a quarter of a right angle, i.e. 22 degrees.  The backs of the angle sections should be fitted with removable panels.

4. "Equipments, P.M.B.X., No. 1"and "Equipments, P.M.B.X., No. 1/35".  It is desirable that these should be placed as near as possible to the switchboard so as to economize in cabling.

5. M.D.F. and Power plant.  The disposition of the distribution frame and the power plant should ensure that cable runs are short.

6. Batteries.  It is an advantage to have the secondary cells in a separate room.  The appropriate size of battery cable required to give the maximum permissible potential drop between the battery and P.B.X. busbars is detailed in POWER, General, S 3320.

The cable between the distribution fuse panel and the section busbars or rack busbars will normally be of 40 amp. rating ("cable, E.L., 250V, 0.0225 sq. in.").

42. Cable racking
Cable racking will be required between the M.D.F. and "Equipments, P.M.B.X., No. 1 and 1/35", and between the latter and the switchboard. This racking should be constructed locally to meet individual requirements.

43. Cabling
The cabling requirements are indicated on the relative diagrams as follows-

1. Main frame to "Equipment, P.M.B.X., No. 1 or No. 1/35" - Diagram LD 71

2. Switchboards to "Equipment, P.M.B.X., No. 1 or No. 1/35" - Diagram LD 71

3. Extension multiple and lamp cables - Diagram LD 100

44. Mixing of "Sections, Switch, P.M.B.X., No, 1A and "Sections, Switch, P.B.X. SA 7560" in one suite
Where possible, new installations should use "Sections, Switch, P.B.X., SA 7560" (new.-type sections). Recovered "Sections, switch, P.M.B.X., No. 1A", (old-type sections) should not be used at new installations but used for extending existing installations already equipped with old-type sections. old- and new-type sections may be mixed when the supply position makes it necessary to do so; they have the same profile at the front.

Where an old-type and a new-type section are adjacent, a gap will exist at the back between the sides of the lift-out panels due to the difference in depth of the two types of section. This gap should be filled by attaching a wooden strip to the edge of the lift-out panel of the old-type section. It will be necessary to drill holes in the old-type section to line up with the bolting-together holes in the new-type section.

45. Labelling
The following labels are required:-

46. Records
Pockets for holding a set of diagrams and the cards for the "Red Label"  relays, are provided in the cable-turning sections.  The relevant diagrams (see par. 18) should be requisitioned and the relay cards obtained - in accordance with E.I. Automatic B 5144 - for the relays mentioned below (when fitted) and the diagrams and cards should be placed in the pocket in one of the cable-turning sections, for use by the maintenance staff.

Relay cards for:-
Relay No. 7333 - relay D in "Relay-set SA 7512" - See pars. 9 and 31.
Relay No. 7334 - relay L in "Relay-set SA 7512" - See pars. 9 and 31.
Relay No. 6910 - relay L in "Relay-set SA 7512" - See pars. 9 and 31.
Relay No. 5075 - relay LL in "Relay-set SA 7514 and SA 7518".
Relay No. 7696 - relay LL in "Relay-set SA 7515".

47. Testing-out
On completion of the installation, the correct functioning of the various circuits should be checked, by applying the routine tests detailed In TESTS & INSPECTIONS, Routine, S5101, S5107 and S5108

Last revised: July 26, 2023