SYNCHRONOUS CLOCKS

CLOCK SYSTEMS - CLASS B
DESCRIPTION

1. General
A Class B system uses mains driven synchronous clocks. These are dependent on the mains frequency for their accuracy, and so may be a minute or two in error by the end of a day. However, the supply companies usually correct the frequency at night, bringing synchronous clocks to the correct time, so that errors are not accumulated. The size of the system possible is unlimited, but on economic grounds (detailed below) no Class B system in the PO should have more than 20 clocks. Larger systems should be Class A. The clocks are of the non-self-starting type. This ensures that, following a short duration failure of the mains supply, the clocks will not restart and hence it will become obvious that they are showing the wrong time. In a Class B installation, the individual clocks are not necessarily synchronised one with another.

2. Size of System
Despite the lower initial cost of a Class B system over a Class A system, the maintenance of the Class B system (cleaning, lubrication and insulation testing) takes far longer since the rotor has to be dismantled each time. Also to be considered is the time necessary to re-start all the clocks after a mains interruption either due to a supply failure or to the power being switched off for other maintenance purposes. The larger the system in use, the greater is the possibility of the clocks showing different times. For these reasons a system involving more than 20 clocks should be Class A.

In sorting offices, Class A systems should be used regardless of the number of clocks involved since it is important to the operations in the building to have the correct time continuously available, with all clocks synchronised.

3. Wiring
Details of the wiring are given in B 3302.

4. Clocks
All PO Clocks with the suffix B are suitable for use in this system (e.g. Clock No. 76B), and all are interchangeable. The clocks are detailed in B 1301 and B 1361, while B 1351 has details of the synchronous clock movement.

5. 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. B 3005 gives full details.

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. (a) 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. (b) 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.

PO ENGINEERING INSTRUCTIONS
MISCELLANEOUS
TIME
B 3351
Issue 4, Sept 1970

CLASS B CLOCK SYSTEMS
Installation

1. General
This Instruction gives general information relating to the installation of synchronous clocks. For the types of standard clock available for use in public offices, see B 3004. The basis of provision is given in B 3001, and instructions for the installation of the wiring are contained in POWER, General, C 3562.

2. Installation
The following points should be considered when installing synchronous clocks:-

1. Position - Do not fit a clock above a radiator or in a position where it will be subjected to considerable heat or humidity. Avoid, if possible, a position where there would be strong daylight behind the clock.

2. Fixing - Use No. 8 wood screws for securing wall clocks and verify that they are of adequate length (not less than 1.5in.) to support the clock; drive the screws well home. If mirror plate suspension is used, the screw should be driven securely leaving 3/16in. of the screw shank protruding from the wall.

3. Suspension - Brackets for clocks suspended by horizontal op vertical suspensions should be securely fastened to the ceiling or wall and so fitted that the clock will be plumb when installed.

3. Serial numbering of clocks
If considered necessary a serial number may be allotted to each clock to simplify fault reporting. Two numbers separated by a stroke will suffice to identify any clock in the system. For example, Circuit No. 1, Clock No. 4 is expressed as 1/4. The numbers should be on the bottom of the case near to the wall, white, not more than than 0.5 in. high and may be signwritten or applied by means of adhesive tape.

4. Inspection after installation
Before leaving the clock, check that all the connections are efficient, as even momentary disconnections will stop the clock; replace the movement cover and close the access doors properly, where these are fitted. Also confirm that the clock is running by watching the minute hand for a short period. (Instructions for starting the clocks and setting the hands appear on the rear of each movement cover.

5. Records

1. Clocks installed initially - A fault card A 2538 should be prepared for each clock installation and details inserted thereon of the serial numbers of clocks installed and the date of installation.

2. Additional clocks - When additional clocks are added to existing installations, the local serial numbers and date of installation should be inserted on the relative card A 2538.

P.O. ENGINEERING DEPT.
ENGINEERING INSTRUCTIONS
MISCELLANEOUS
TIME
B 1351
Issue 3, 30.6.6 1

ELECTRICAL CLOCKS - SYNCHRONOUS TYPE
Movements

1. General
This Instruction describes the movements incorporated in synchronous clocks used by the P.O. They are all of the non-self-starting type, and are suitable for 200-250V a.c. single-phase 50 c/s supplies. Starting and correcting instructions appear on each movement cover. The manufacturer of the movement can be identified by letters stamped on the dial back plate, thus:

2. Messrs. Smith's movement (clocks with mirror plate suspension)
This type of movement is illustrated in Figs. 1 and 2, and mechanical details are shown in Figs. 3 and 4. The movement has two essential parts, the motor and the gear train.

FIG. 1. MESSRS. SMITH'S MOVEMENT FRONT VIEW	FIG. 2. MESSRS. SMITH'S MOVEMENT REAR VIEW

3. Motor
The motor consists of a rotor R (Fig. 3) and a stator (Fig. 4).

(a) Stator (Fig. 4)
This consists of two channel-section iron rings T, each having 15 projections on the inner periphery. The coil C is enclosed between these rings, leaving an air-gap between the interleaved projections of the two halves, which form alternate poles when the coil is energized. A stator with 30 poles is therefore obtained.

(b) Rotor R (Fig. 3)
This consists of a 6-pole spider-shaped cobalt-steel magnet M, held between two brass disks and mounted on a steel spindle. The poles of the magnet are bifurcated, to obtain correct positioning with the stator poles. The spindle of the rotor also carries a steel worm A and a pinion wheel D. The speed of the rotor is 200 rev/mm.

The rotor bearings B1 and B2 (Fig. 3) are made of laminated, bakeized fabric, interleaved with paper which is saturated with lubricant. The pivots of the rotor are of hardened polished steel, one being spring-loaded to prevent excessive end-float on the rotor.

FIG. 3. MESSRS. SMITH'S MOVEMENT DETAILS	FIG. 4. MESSRS. SMITH'S MOVEMENT STATOR AND COIL

4. Gear train
The gear train consists of a steel worm A and a gear wheel E (Fig. 3), a steel worm F, and the gear wheels G and H (Fig. 1). To reduce the noise, the first wheel E in the gear train is of synthetic-resin-bonded paper.

5. Operation
When the a.c. supply is connected to the stator coil, a pulsating field is set up in the stator, which reacts on the rotor. Due to the poles of the stator and rotor being in magnetic balance, the rotor will not rotate until the starting rod K is pressed (Fig. 1). The starting rod, which is also used for setting the hands, has a press-up-and-release action, spring S restoring the rod on release.

The action of pressing the rod K causes the shoulder L - attached to the rod K - to push the lever U upwards (Fig. 1). When this lever reaches the stop N (Fig. 1), the shoulder L travels past the lever U, thus allowing the lever to fall back. Associated with the lever U is a pawl P (Fig. 3). As the lever U falls back, the pawl P engages momentarily with the wheel D (Fig. 1) - attached to the rotor - and gives the rotor a 'flick'. The rotor then rotates and keeps in step with the stator flux, i.e. in synchronism with the frequency of the a.c. mains. An even rotation is maintained by the disk BD (Fig. 3), which acts as a flywheel.

6. The motion from the rotor is transferred to the hands by means of the gear train A, E, F, G, H and through a friction clutch. (The purpose of the friction clutch is to allow adjustment of the hands without interfering with the running of the movement). As the a.c. mains frequency is controlled, the clock keeps correct time.

7. To correct the clock, the rod K (see Fig. 1) is pressed upwards: as far as possible. The wheel W - associated with the rod K - engages with the wheel Q which gears with the minute spindle via the wheel Y. The hands may be set to any desired position by turning the rod K. The action of pressing this rod upwards also starts the rotor, as indicated in par. 5.

8. Messrs. Gent's movement (clock with mirror plate suspension)
The general appearance of this type of movement is illustrated in Figs. 5 and 6, and details are given in Fig. 7. The movement has two essential parts, the motor and the gear train.

9. The motor consists of a stator and a rotor R (Fig. 7).

(a) Stator (Fig. 7)
This consists of a laminated iron core L, on which is wound the energizing coil C. The core has extended pole-pieces 0, which are toothed and shaped to cover an extended arc and to obtain correct position with the rotor poles.

(b) Rotor R (Fig. 7)
This consists of a circular piece of soft iron Z, which has 30 poles and is held between two brass disks, mounted on a steel spindle, the periphery of the rotor being castellated, to form the poles. The steel spindle also carries a steel worm A, the start wheel D, and the flywheel F. The speed of the rotor is 200 rev/mm.

The rotor bearings B1 and B2 (Fig. 7) are made of synthetic-resin-bonded paper. Each bearing consists of two pieces, SO constructed that a lubrication cavity is formed. The bearing B2 also acts as a pivot to take the thrust of the rotor. The bearing B1 is spring-loaded, to prevent excessive end-float of the rotor.

FIG. 5. MESSRS. GENT'S MOVEMENT FRONT VIEW	FIG. 6. MESSRS. GENT'S MOVEMENT REAR VIEW	FIG. 7. MESSRS. GENT'S MOVEMENT DETAILS

10. Gear train (Fig. 7)
The gear train, by which the motion of rotor is transferred to the hands, consists of a steel worm A, the wheel G, the steel worm H, the wheel J, the worm M and the wheel N. To reduce the noise the wheel G is made of synthetic-resin-bonded paper.

11. Operation
When the a.c. supply is connected to the stator coil, a pulsating field is set up in the stator, which reacts on the rotor. Due to the poles of the stator and rotor being in magnetic balance, the rotor will not rotate until the starting rod K is pressed. The starting rod, which is also used for setting the hands, has a press-up-and-release action, spring S restoring the rod on release.

The action of pressing the rod K prepares the pawl P for engaging the start wheel D. On the release of the rod K, the pawl P engages with the start wheel D and gives the rotor a 'flick', causing it to rotate. The rotor then keeps in step with the stator flux, i.e. in synchronism with the frequency of the a.c. mains. An even rotation is maintained by the flywheel F, to which the rotor is frictionally fastened. The motion from the rotor is transferred to the hands by the gear train A, G, H, J, M and N. As the frequency of the a.c. mains is controlled, the clock keeps correct time.

12. To correct the clock, the rod K is pressed upwards as far as possible. When this is done, a wheel (not shown in the illustrations) associated with the rod K engages with the wheel N, which is connected to the 'minute' spindle. The hands may be set to any desired position by turning the rod K. When released, the rod K is restored to its normal position by the spring S. At the same time, the rotor is started as indicated in par. 11.

FIG. 8. MESSRS. ENGLISH CLOCK SYSTEMS MOVEMENT DETAILS	FIG. 9. MESSRS. GENT'S MOVEMENT FOR CLOCKS WITH CONCEALED HINGE SUSPENSION (TYPICAL)

13. Messrs. English Clock Systems movement (clocks with concealed hinge suspension)
The motor consists of a stator and a rotor R (Fig. 8). The stator consists of a laminated iron core L, around which is wound the energizing coil. L is extended, toothed and shaped to cover an extended arc and to obtain correct position with the rotor poles.

The rotor consists of a soft iron disk with a castellated periphery which forms the rotor poles and acts as part of the starting device. The rotor is mounted on a steel spindle which carries a steel worm A and the flywheel F.  When the a.c. supply is switched on, a pulsating field is set up in the stator which reacts on the rotor. As the poles of the stator and the rotor are in magnetic balance the rotor remains stationary until the starting device is operated.

14. Starting clocks with concealed hinge suspension
The movements of these clocks differ in the method of starting and correcting from the movements of Clocks Nos. 4B, 50B, 52B, 56B and 58B (all of which have mirror plate suspension and 'press-up-and-release' starting and correction).

(a) Messrs. Gent's movement
In the movement shown in Fig. 9 the clock is started by rotating the starting knob S a quarter of a turn in the direction of the arrow. This moves the starting pawl SP (to a position shown dotted) to engage the starting ratchet D. When the knob is released smartly, SP restores and spins rotor Z and its flywheel F to start the clock. Correction is by means of a knob not shown in Fig. 9, but the position of the knob is clearly marked on the movement cover.

(b) Messrs. English Clock Systems Movement
Fig. 8 shows the method of starting by means of a lever and correcting by means of a knob. When the rotor back plate P is in position, the starting pin SP rests in the recess C. When the starting lever SL is operated, SP engages with the castellations R of the rotor. Then when SL is released smartly, the rotor rotates, and SP frees itself from the rotor, by resting in the recess C. The clock correcting knob HS is shown in position, but this must be removed before removing or replacing the movement cover. The motion of the rotor is transferred to the hands in a similar manner to Messrs. Smith's movement as described in par. 4, i.e. steel worm A. gear wheel E, steel worm F and gear wheels G and H.

Last revised: July 05, 2023