3 Internal
Issue 2, Nov 1977

POWER-UNITS Nos. 50A, 51A, 52A, 53A/B, 55A, 56A, 57A, 62A, 69A, 70A, 73A & B, 77A, 78A, 86B, 88A, 100A/1, 100A/2 AND 100A/4


These power units are for converting single phase 50 Hz mains supplies in the range 220-250 volts into d.c. suitable for operation of telephone apparatus at subscribers premises. The outputs are smoothed sufficiently for their intended uses.

There is no provision for an alarm being given in the event of failure of a unit or of the mains supply.

The output voltages quoted are for nominal values of input mains supply voltages.

The units are metal cased and finished in eggshell elephant grey except for:-

(i) Some early units which are finished in battleship grey.

(ii) The Power Unit No. 70A which has a light straw finish metal cover.

(iii) The Power Unit No. 53A MK2 which has an eggshell elephant grey plastic cover.

Space and fixings for mounting a Converter, Ringing, No. 7 are provided in Power Units Nos. 50A, 51A, 52A, 55A, 56A, 57A and 62A (see Q0006 for mounting details).

Space and fixings for mounting a Transformer No. 261A are provided in Power Units Nos. 52A and 62A.

Power Unit No. 51A is shown to the right (the picture shows a setup at a PMBX No. 2/. installation where the lower box is the distribution case and the top box a Unit Auxiliary Apparatus).

Power units must not be used at installations serving emergency services, e.g. Civil Defence, police, fire stations, ambulance centres or hospitals without first inquiring whether standby battery-power to carry on full extension-to-extension service during mains failures is needed. Similarly these units must not be used to serve Police Telephone and Signal Systems or House Exchange Systems other than the House Exchange Systems Nos. 3 and 4 because,' under mains failure conditions. these systems are rendered completely inoperative. When Switchboards, AT 3796... and N 1070... are to be served by a power unit, they must be modified to Dgm N 1099 which, under mains failure conditions, makes the operators headset independent of the power-unit supply on exchange calls.

When the a.c. mains supply fails at any installations listed below served by a power unit the following conditions will obtain:-

At PMBX Installations

(a) Extension-to-exchange calls in progress will remain held.

(b) The operator can make and answer exchange calls.

(c) Extension-to-exchange calls can be made and answered using the normal night service facilities.

(d) The making of extension-to-extension calls will not be possible and any in progress will fail.

(e) Operator-to-extension or extension-to-operator calls will not be possible.

(f) Inter-switchboard circuits and number-wheel-type subscribers private meters will be inoperative.

At PABX No. 5 and No. 6 Installations
All calls in progress are lost.
Exchange lines are diverted to predetermined extensions which can then make and receive exchange calls. When the mains supply is restored any such calls in progress are maintained.

Plan-set N 625
Intercommunication facilities are lost, but exchange calls may still be made and answered at all instruments.

House Exchange Systems
On House Exchange System No. 3 calls between stations will be lost but exchange calls may be made and answered at all stations. The same conditions apply on House Exchange System No. 4 but, additionally, exchange calls may be lost at non-multiplied stations.

Amplifier and Loudspeaker No. 4
Loudspeaking Telephone No. 4 and Confraphone Amplifying facilities are lost.

House Telephone System
All facilities lost.

Key and Lamp Units No. 2A and 10A

(a) Visual signals and alarms for incoming calls will be inoperative.
(b) Exchange line or PBX extension line calls in progress will remain held.
(c) Outgoing exchange line or PBX extension line calls can be made.
(d) Private circuit calls will not be possible.

The subscriber must provide a standard 3 pin BS wall socket adjacent to the wall mounting position of the unit, supply electrical energy free of charge to the PO and provide an efficient mains earth connection. For the connection of the mains supply to any of these power units the PO will supply a suitable plug and flexible cord. If one pole of the power unit output is to be connected to a telephone signalling earth a strap must be provided at the power unit terminal block linking the telephone signalling earth to the mains protective earth. The earth conductors must not be less than 1.5 sq. mm i.e. Cord Flexible 3183Y 1.5mm from the mains plug to the power unit and Wire Earthing 9141/lW or 9142/1W for the strap and telephone signalling earth. The telephone signalling earth should be in accordance with TI A2 E1006, the preferred method being by connection to the Consumers Earthing terminal provided by the Electricity Board unless the electricity supply is by overhead distribution. The signalling earth lead must not be connected to the power supply earthing system at the socket outlet but must be independent of the Consumer's earth continuity wiring.

When Plugs, for Socket-Outlets No. 102 are used they shall be fitted with Fuses Plug for Socket Outlets No. 102/13 for the Power Unit Nos. 70A, 77A and 78A and Fuses Plug for Socket Outlets No. 102/3 for all other power units listed below.

When 3 pin/5 Amp plugs are used, the socket outlet must be protected at a distribution board by a fuse, or similar protective apparatus, rated at not more than 5 Amp. When only a 15 Amp 3 pin socket-outlet is available an adapter should be purchased locally and fitted to permit the use of a single Plug, for Socket Outlet No. 102 fitted with the appropriate fuse.

All power units are to be connected to the mains socket outlet plug with Cord Flexible 3183y 1.5mm except the Power Units Nos. 53A and B which, as neither pole of their outputs is to be earthed, are connected with Cord Flexible 3183Y 0.75mm.

Power Unit No. 53B is intended for wall mounting, using Screws, for Wood........ Size 6.

Power Units Nos. 53A, 73A, 73B, 86B and 88A are intended for wall-mounting, using Screws, for Wood....... Size 8.

Power Units Nos. 50A, 51A, 52A, 55A, 56A, 57A, 62A, 77A and 100A/.. are intended for wall mounting, using Screws for Wood .... Size 10 and are fitted with brackets for this purpose.

Power Units Nos. 69A and 70A are fitted inside the PABX cabinets.

Power Units Nos. 50A, 51A, 52A, 62A and 100A/.. conform to the general appearance and mounting arrangements of the auxiliary apparatus unit used with Switchboards, PMBX No. 2/... and PMBX No. 4/1A. All units are mounted one above the other in the same vertical wall assembly, the power unit being placed near the bottom and immediately above the connection box to ensure adequate ventilation. The connection box, power unit and auxiliary units are fixed together (using ORA nuts and bolts provided with the connection box and the auxiliary unit) to form a complete assembly which should be attached to the wall with as many fixing points as are necessary.

Power Unit No. 50A complete with Converter Ringing No. 7 (fitted to the front left)

Power Units No. 77A is similar in appearance to Power Units Nos. 50A, 51A, 52A, 62A and 100A/.. but should be mounted clear of other apparatus to allow a free circulation of air.

Power Unit No. 78A is intended to stand on the floor.

A guide to the selection of the appropriate power unit for a particular installation is given below.

The Power Units Nos. 100A/.. contains an integral ringing converter which will ring up to four series connected Bells No. 59A in series with a 2PF capacitor.

The Power Units Nos. 53A and B are electrically interchangeable but the latter is physically smaller.

Power Unit No. 53A

PSU Uses

Apparatus   Power Unit No.
Autodial No. 101A   PU 88A
Planset N625   PU 53A or 53B
HES No. 3   PU 100A/1
HES No. 4   PU 100A/2
HTS   PU 53A or 53B
Key & Lamp 2A and 10A   PU 52A
LST No. 4   PU 86B
PABX No. 5   PU 69A
PABX No. 6   PU 70A
PMBX No. 2/2   PU 100A/1
PMBX No. 2/3   PU 100A/2
PMBX No. 2/4 (without aux apps)   PU 100A/2
PMBX No. 2/4 (with aux apps)   PU 100A/4
PMBX No. 3/5   PU 100A/4
PMBX No. 4 (up to 4 amps)   PU 62A
PMBX No. 4 (up to 8 amps)   PU 77A
PMBX No. 4 (up to 12 amps)   PU 78A
SWITCHBOARD 10 + 30   PU 56A
SWITCHBOARD 10 + 50   PU 57A


Additional Information

Power Unit Number Output Current Output Voltage (DC) Diagram
PU 43A 2A 50v N639
PU 50A 0.5A 50v N635
PU 51A 1A 50v N636
PU 52A 2A 50v N637
PU 53A 0.25A 10v N631
PU 55A 0.3A 25v N667
PU 56A 1A 25v N668
PU 57A 2A 25v N669
PU 62A 4A 50v N670
PU 69A 4A 50v N672
PU 70A 5A 50v N673
PU 73A 1A 12v N674
PU 77A 8A 50v N684
PU 78A 12A 50v N685
PU 86A 0.275A 50v N689
PU 88A 1A 18v N688
PU 100A/1 1A 50v N700
PU 100A/2 2A 50v N700
PU 100A/4 4A 50v N700

Power Units 100A/.. are equivalent to Power Units 51A, 52A and 62A except the 100A/.. has a Ringing Convertor factory fitted.



When working on power units always switch the mains off and isolate the supply (pull out the plug).  Even with the input isolated, capacitors within the unit will take a time to discharge - do not work on the unit for at least 5 minutes.

On most modern GPO power units, the smoothing capacitor had a resistor wired across it to discharge it when the mains is switched off.  Whilst preventing possible electric shock to those working on the PSU the resistor will remain hot for a long period of time, thus providing the possibility of burns.


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