This invention relates to improvements in the operation of switches in electrical power distribution networks, eg switchgear in electrical power distribution substations or pole-mounted switches, and in particular to apparatus with which motor-driven operation of such switches may be overridden to permit manual operation.

The distribution of electrical power within the United Kingdom involves distribution networks operating at a number of voltages. Power is distributed from power stations at very high voltages, eg 40OkV, via overhead power lines. Further distribution then takes place through networks operating at intermediate voltages, typically 33kV or 11 kV, before the electricity is finally supplied to consumers at normal "mains" voltages of 240V (single phase) or 415V (three phase).

Intermediate voltage networks generally include substations that house switchgear to permit circuits connected to the substation to be switched ON or OFF. Such circuits may need to be switched OFF, for instance, to permit maintenance operations to be carried out or to isolate faults.

Switches may also be installed at other points in the distribution network, eg above ground on poles that support overhead power cables.

In the past, in order to switch such circuits from OFF to ON, or vice versa, an operator had to visit the substation and manually operate the switch, ie physically move the switch from one condition to the other. Clearly, in such a case, it may not have been possible to switch the circuit as quickly as might be desired, eg in the case of a fault condition having developed where the connections within the network may need to be reconfigured before the power can safely be restored.

Switching of the circuit back to its original state would similarly require a return visit by the operator, again leading to delay.

More recently, systems have been developed that permit automated switching, the operations being controlled from a remote control centre. Typically, such systems comprise electric motors capable of applying high torque to a switch shaft, thereby causing it to move from ON to OFF, or vice versa. Even with such an arrangement, however, it may from time to time be necessary or desirable to override the automatic operation of the switch, eg in the event of malfunction or loss of power to the motor or a breakdown in the communication link to the remote control centre.

Certain known systems for automated operation of such switches do permit a manual override. However, such systems suffer from certain disadvantages. For instance, it may be necessary to demount the motor assembly from the switch in order to operate the switch manually, and to remount it subsequently. Clearly, such operations are laborious and time-consuming. In addition, after manual operation of the switch, the condition of the switch may be no longer unambiguously evident at the remote control centre.

There thus exists a need for apparatus that permits automated and remote operation of a switch in an electrical power distribution network, but which also permits the switch to be operated manually, simply and easily, without the need for dismantling of the apparatus. There also exists a need for such apparatus in which the condition of the switch may be monitored remotely, irrespective of whether, and how often, the switch is actuated manually or automatically.

There has now been devised apparatus that addresses these objectives and which overcomes or substantially mitigates the above-mentioned and/or other disadvantages of the prior art.

Thus, according to a first aspect of the invention, there is provided a drive assembly for a switch in an electrical power distribution network, the assembly comprising a switch shaft mechanically coupled to a motor-driven actuating

member by a clutch mechanism, whereby the switch shaft may be decoupled from the actuating member so as to permit manual operation of the switch.

The drive assembly according to the invention is advantageous primarily in that in normal operation the switch is operated under control of the motor, but the clutch mechanism enables the switch shaft to be rapidly and easily decoupled from the motor, thereby permitting manual operation of the switch, without the need to dismantle the assembly. The drive assembly may form part of the original switch equipment, or may be supplied as a retrofit to existing, manually operable switchgear.

By "clutch mechanism" is meant any mechanism by actuation of which the actuating member is decoupled from the switch shaft. The clutch mechanism is most preferably a mechanical clutch assembly. Thus, the switch shaft and the actuating member are preferably formed with cooperating formations that, when engaged, couple the switch shaft and actuating member together and cause rotation of the actuating member to rotate the switch shaft. In a preferred embodiment, the switch shaft is provided with one or more projections that locate in a corresponding recess or recesses in the actuating member. Most preferably, resilient means, eg a compression spring, are provided to bias the switch shaft and the actuating member into the coupled condition. Thus, after manual operation, remote actuation of the switch recouples the switch shaft and actuating member.

Preferably, the actuating member comprises a wheel or disc mounted about the switch shaft for rotation about the axis of the switch shaft. Alternatively, the actuating member may consist of only a part, eg a segment, of such a disc or wheel. Most conveniently, the actuating member has a toothed rim that is engaged by a gear, most preferably a worm gear, that is driven by the motor.

It is particularly preferred that the switch shaft should protrude from the drive assembly, the distal end of the switch shaft being formed for engagement with a

suitable lever or handle by which the switch may be manually operated. In such a case, the switch shaft is preferably decoupled from the actuating member by depression of the distal end of the switch shaft, most preferably against the action of a compression spring mounted about the switch shaft.

The drive assembly most preferably includes means by which the angular position of the switch shaft, and hence the condition of the switch (OFF or ON) can be monitored. The drive assembly therefore preferably includes sensors that are responsive to the angular position of the switch shaft. Such sensors may take various forms. However, the sensors are preferably of the electrical induction type, which generate a local electrical field and are sensitive to changes in that field due to the presence of electrically conducting material within that field. Preferably, at least one such sensor is positioned sufficiently close to the switch shaft to be sensitive to rotation of the switch shaft. Conveniently, at least one electrically conducting projection is mounted on the switch shaft, such that rotation of the switch shaft causes the projection to move into and out of registration with the sensor. In a particularly preferred arrangement, two pairs of sensors and projections are provided, at a relative angular separation corresponding to the angular separation between the positions of the switch shaft when the switch is in the ON and OFF positions. Most preferably, the two sensors are aligned, and the two projections are mounted on the switch shaft with such an angular separation.

According to another aspect of the invention, there is provided an electrical power distribution network including a switch and drive assembly for the switch, the drive assembly being as described above.

As described above, the switch may be part of an electrical power distribution substation. This is the most common form of switch to which the present invention is applicable, and so according to one specific aspect of the invention there is provided an electrical substation including a switch and a drive assembly for the switch, the drive assembly being as described above.

The invention will now be described in greater detail, by way of illustration only, with reference to the accompanying drawings, in which

Figure 1 is a schematic side view of a switch drive assembly according to the invention, with the switch drive assembly in a normal, motor-driven operating condition;

Figure 2 is a sectional view on the line M-Il in Figure 1 ,;

Figure 3 is a fragmentary view of a clutch mechanism forming part of the switch drive assembly, in a manually overridden operating condition; and

Figure 4 is a sectional view on the line IV-IV in Figure 3.

Referring first to Figure 1 , a switch drive assembly according to the invention is generally designated 1. The assembly 1 forms part of an 11 kV electrical power distribution substation, either as part of the original substation equipment or being retrofitted as a modification to an existing substation.

The assembly 1 comprises a housing 2 divided by an internal partition 4 into two compartments 6,8. The housing 2 is fixed to the outer casing 10 of a switch assembly, through which casing 10 extends a switch drive shaft 12. The switch assembly and switch shaft 12 may be of generally conventional form, save that the end of the switch shaft 12 is formed for engagement with the switch drive assembly 1 , specifically with a drive shaft 14 of the switch drive assembly 1 , as described in more detail below.

The right-hand (as viewed in Figure 1 ) compartment 8 of the housing 2 accommodates an electric motor 16. The motor 16 may be of a type conventionally used in similar applications, ie to operate a switch of an electrical power distribution substation, and has a shaft 18 (see Figure 2) upon which is mounted a worm gear 20. The worm gear 20 engages the toothed rim of a main

drive wheel 22 that is mounted about a shaft 24. One end of the shaft 24 is formed for engagement with the drive shaft 14 such that rotation of shaft 24 causes rotation of the drive shaft 14. For instance, and as illustrated, the end of the shaft 24 may be formed as a projection 23 of non-circular (eg hexagonal or square) cross-section, and the end of drive shaft 14 may be formed with a socket 13 of similar internal cross-section within which the end of the shaft 24 is received.

The shaft 24 extends through the left-hand (as viewed in Figure 1 ) compartment 6, and terminates, externally of the housing 2, in an end portion that is adapted for engagement with a suitable lever for manual operation of the switch. For instance, and as illustrated, the end portion may take the form of a socket 25 of non-circular (eg hexagonal or square) cross-section into which a corresponding part of a lever may be inserted.

Within the left-hand compartment 6, a compression spring 26 is mounted about the shaft 24 and is captivated between the partition 4 and a spring shroud 28 that is fixedly mounted on the shaft 24. The spring shroud 28 has the form of a cylinder that is open at the end closest to the partition 4, such that the spring 26 acts between the partition 4 and the internal face of the other end of the spring shroud 28.

Also mounted on the shaft 24, within the left-hand compartment 6, are two sensor rings 30,32, the operation of which is described more fully below. Each sensor ring 30,32 may be fixed to the shaft 24 by means of one or more grub screws or the like (not shown) that occupy threaded bores in the sensor rings 30,32. Each sensor ring 30,32 carries a metal lug 31 ,33 that cooperates with a corresponding sensor 34,36. The sensors 34,36 are of the electrical induction type that generate a localised electrical field in the vicinity of the sensor, and are sensitive to modifications of that electrical field caused by the presence of electrically conducting material within that field. The sensor rings 30,32 are positioned on the shaft 24 such that when the switch is in the OFF position, the metallic lug 31 of a

first of the sensor rings 30 is disposed adjacent its corresponding sensor 34, while the metallic lug 33 of the other one of the sensor rings 32 is displaced from its corresponding sensor 36. This is the condition shown in Figure 1. If the shaft 24 is rotated, in order to bring the switch to the ON condition, the lug 33 of the second sensor ring 32 is brought into proximity with the corresponding sensor 36, while the other lug 31 is rotated away from the corresponding sensor 34.

As described above, the main drive wheel 22 is mounted about the shaft 24. However, the main drive wheel 22 is not fixedly mounted on the shaft 24. Instead, the shaft 24 passes through a central bore of the main drive wheel 2, the main drive wheel 22 being coupled to the shaft 24 by means of a coupling disc 38 that is fixed to the shaft 24. The coupling disc 38 carries a pair of lugs 40 that locate in corresponding bores 41 in the main drive wheel 22.

In the normal operating condition of the unit 1 , in which operation of the switch is controlled by remote actuation of the electric motor 16, if it is desired to move the switch from an open to a closed position (or vice versa), the motor is turned on. This rotates the motor shaft 18 and the worm gear 20. Rotation of the worm gear 20 causes rotation of the main drive wheel 22. Since the main drive wheel 22 is mechanically coupled, via engagement of the lugs 40 in the bores 41 of the coupling disc 38, to the shaft 24, rotation of the main drive wheel 22 is transmitted to the shaft 24, and hence to the switch shaft 12. The switch drive assembly 1 may be connected to a remote control centre by any suitable means, eg by cabling or by a wireless connection.

As described above, rotation of the shaft 24 causes one of the metallic lugs 31 ,33 to be displaced away from its corresponding sensor 34,36 and the other one of the lugs 31 ,33 to be brought into registration with the corresponding sensor 34,36. Operation of the motor 16 is therefore continued until the sensors 34,36 indicate that the desired change of condition (ON to OFF, or OFF to ON) has occurred. To return the switch to the previous condition, the motor 16 is again operated, but in the opposite sense, until the sensors 34,36 indicate that the desired condition has

been reached. The outputs of the sensors 34,36 are preferably transmitted to the remote control centre, thereby providing a remote operator with an indication of the condition of the switch.

If it is desired to override the motor-driven operation of the switch, and to operate the switch manually, a suitable lever is engaged with the socket at the external end of the shaft 24. By depressing the shaft 24 inwards, against the action of the spring 26, the lugs 40 are displaced from the bores 41 in the main drive wheel 22, hence decoupling the shaft 24 from the drive wheel 22. The shaft 24 can then be rotated manually, using the lever. The extent to which the shaft 24 can be depressed inwardly is limited by the depth of the socket 13 at the end of the switch shaft 12.

When the pressure on the shaft 24 is released, the action of the spring 26 returns the shaft 24 towards the position shown in Figure 1 , but with the lugs 40 (which are no longer aligned with the bores 41 ) bearing against the surface of the main drive wheel 22 (see Figures 3 and 4).

Again, rotation of the shaft 24 may be continued until the sensors 34,36 indicate that the desired condition has been reached. Alternatively, for instance if the power supply to the unit 1 has failed and the sensors 34,36 are not operational, manual rotation of the shaft may be continued simply until some other indication of switching is noticed (eg a change of state of the circuit controlled by the switch, or simply an audible noise that occurs on switching).

Irrespective of whether power is supplied to the sensors, however, it will be noted that at all times the positions of the sensor rings 30,32 correlate with the condition of the switch, irrespective of whether the switch is actuated by the motor 16 or manually.

Motor-driven operation of the switch can be restored by operation of the motor 16, which causes the main drive wheel 22 to rotate. Once the bores 41 come into

registration with the lugs 40, the action of the spring 26 causes the lugs 40 to relocate within the bores 41 , once again coupling the shaft 24 to the main drive wheel 22.