PULSE CLOCKS

CLOCK SYSTEMS - CLASS A
DESCRIPTION

1. General
A Class A system is a pulsed clock system involving the use of a master clock and slave clocks. Because of the use of standby batteries, the system will continue to function for some hours after a failure of the mains electricity supply. The size of the system is almost unlimited, and it can be extended after installation. The only limitations on the size are the battery capacity and the voltage drop in the wiring. (See B 3300). The system offers a high degree of accuracy and reliability. All the slave clocks are synchronised, and can be advanced from the master clock for correction, should a system fault occur. The parts of the system and the relevant Instructions are stated below.

2. Master Clock
The master clock in use at present is the Clock No. 36. This is a pendulum clock with a beat of 1 second. The swing of the pendulum is maintained by an electro-magnet powered from the batteries. These clocks are capable of an accuracy of 8 seconds a week. Full details are given in B 1325.

Future systems will use a crystal chronometer, which has a very high order of accuracy - within one minute a year.

3. Wiring
Details of the wiring are given in B3302 and Drawing No. PE/T1/2 (This drawing should be requisitioned from PHQ-PMD/PP4/l).

4. Pulse Distribution Unit
Units, Clock Pulse, No. 1 supersede the now obsolescent Clock Units GMT 35. These transistorised Pulse Units have a constant current output capable of driving individual clock circuits of up to 9 or 19 clocks, depending on the voltage of the system. Each unit contains one Strip, Clock Pulse No. 1 and has a space for a further Strip. If this is fitted, the Unit can then drive a further circuit of 9 or 19 clocks. Details are given in B1304.

5. Clock Correction Unit
This device, shortly to be introduced, is for use on systems where Units, Clock Pulse are employed. The device, known as Advancer, Clock, No. 1 is pocket sized and can be clipped onto any pulse strip to give single pulses, or multiple pulses at the rate of one a second, at the touch of a button. This enables individual clock circuits, which have been stopped due to a circuit fault, to be advanced to the correct time, without disturbing the other clock circuits.

6. Slave Clocks
All PO Clocks with the suffix A are suitable for use in this system (e.g. Clock No. 76A), and all are interchangeable. The clocks are detailed in Bl301 and Bl321, while 51311 has details of the pulsed clock movement.

7. Special Clocks
PO Standard clocks should always be used, except that an architect may design a special face and hands for use in a public office, but a PO standard movement must be used. B3005 gives full details.

8. Power Supply
The usual power supply is a battery of lead-acid cells, either a nominal 24v or 50v, and a voltage controlled battery charger. These are housed in a Cabinet, Secondary Cell, No. D63201. For example, the battery could consist of Cells, Secondary, 21/15 charged by a Rectifier No. 93A. The cells should have sufficient capacity to ensure at least a 24 hour standby in the event of mains failure. Details are given in B 3300.

P.O. ENGINEERING DEPT. 
ENGINEERING INSTRUCTIONS
MISCELLANEOUS
TIME
B 1311
Issue 2, 20.11.58

ELECTRICAL CLOCKS - PULSE TYPE
Movements

1. General
The standard half minute pulse clocks are supplied with ratchet movements.

2. Ratchet movements
There are three varieties of ratchet movements, supplied by different contractors; they operate on the same principle but differ slightly in constructional details. Figs. 1, 2 and 3 illustrate the three types. The parts and operation of all three are as follows:-

A is the main ratchet-wheel, having 120 teeth
B the operating electromagnet
C the armature
D the armature lever
E the driving pawl, which moves the ratchet-wheel one tooth on the release of the armature
F the driving spring, which normally holds the driving pawl in engagement with the ratchet wheel, and the armature away from the electromagnet
G the back-stop lever, which prevents movement of the ratchet-wheel when the armature is attracted, as might be possible from vibration or on clocks with exposed dials - by pressure of wind on the hands
H the momentum stop, which prevents the ratchet wheel being moved more than one tooth per impulse and, with the pawl E, locks the ratchet wheel between pulses
J the stroke-limit stop, which limits the travel of the armature, and
K the non-inductive shunt, permanently connected across the electromagnet coil.

3. When electromagnet B is energized by a pulse from the Clock Unit, GMT 34 or 35, the armature C is attracted, pawl E is propelled to the right against the pressure of spring F, and drops into the next tooth on ratchet-wheel A.

4. When the pulse ceases, spring F drives the paw! E forward, and the ratchet-wheel rotates one tooth, equivalent to a half-minute on the dial. The 'minute' hand is attached to the. ratchet-wheel which also drives the 'hour' hand through a train of gears having a reduction ratio of 12 to 1.

5. Operating current
The operating current for the movement is 250mA. The resistance of the electromagnets is between 7.5 and 10 ohms and they may be connected in circuit without reference to polarity.

PO ENGINEERING INSTRUCTIONS
MISCELLANEOUS
TIME
B 3302
Issue 5, 8.4.71

ELECTRICAL CLOCKS
Wiring of Pulse and Synchronous Systems

1. General
This Instruction describes the wiring of clock systems. Drawing PE/T1/2 and Diagram GMT38 refers.

2. Pulse installations
The clocks in each circuit should be wired in series, using 1mm squared, PVC insulated wire purchased locally. The preferred wire colours are, green for earth (+ve battery) leads, and brown for -ve battery and pulse leads. Wiring should be terminated on a Clock connector, No. 96A mounted behind each clock.

1. Concealed wiring. Wiring should be concealed except in locations where surface wiring is acceptable (see POWER, Lighting, B 3010) . Clock wiring may he run in the same duct as other cables, including lighting or power cables, but it must be insulated to withstand the highest voltage that any other cable in the duct may be carrying. Where clock circuits are run in the same duct as lighting or power cables, the preferred wire colours must be used.

2. Surface wiring. This should be inconspicuous and hidden behind mouldings etc., as far as possible. Vertical drops to clocks should be buried in walls. For single wire loops 250V, PVC green or brown single wire should be used and for two wire loops twin PVC cable with the most suitable sheath colour should be provided. As twin PVC cable is only available with red and black cores, the red should be used for earth (+ve battery) leads and black for -ve battery and pulse leads.

3. Synchronous installations
The wiring of these is described in POWER, General, C 3562.

4. Proposals for flew installations
Clock wiring should be provided simultaneously with other accommodation services. The positions and types of clock, and type of wiring should be added to EL and P proposals on the building plans.

Last revised: July 05, 2023