WO1992005615A1

WO1992005615A1 - Improvements in electrical zone load controllers

Info

Publication number: WO1992005615A1
Application number: PCT/GB1991/001527
Authority: WO
Inventors: Richard Sidney Rose
Original assignee: Hawker Fusegear Ltd.
Priority date: 1990-09-14
Filing date: 1991-09-06
Publication date: 1992-04-02
Also published as: AU8521491A; EP0548183A1

Abstract

A controller for controlling an electrical load comprises first processing means (19) having an output operably connectable with load switch means, a local control input operably connectable to a pair of local control conductors (14, 15), and a zone control input operably connectable to a pair of zone control conductors (34, 35, 36, 37), the first processing means (19) being arranged to apply a voltage to the zone control conductors (34, 35, 36, 37) and to sense the presence or absence of a load switch locking command applied to the zone control input and, in the absence of such a locking command, to sense the presence of a connection between the zone control conductors (34, 35, 36, 37) or the local control conductors (14, 15) to apply a load switch command to the output.

Description

Title : Improvements in Electrical Zone Load Controllers

The present invention relates to electrical load controllers. The invention is particularly applicable to controllers for electrical lighting circuits.

A typical example of an electrical circuit which may require a varied range of individual and zoned switching is an industrial, office or domestic lighting circuit*. The present invention is nor limited to such circuits, but it is particularly applicabie to them. In theory, any load can switched by the present invention provided the components are rated highly enough to take account of the expected currents.

When a relatively high voltage (eg 240 volt a.c. mains) is used as the supply to an electrical load, it is desirable to be able to control the supply by means of a less dangerous, more easily switched control circuit, preferably working at a considerably lower voltage, which is electrically isolated from the higher voltage.

It is also desirable that a control circuit be easily adapted to suit the prevailing circumstances and versatile enough to be added to or varied, as required.

Known lighting circuits switch the mains supply directly. This poses a potential danger at the switch as it requires mains cable to be run to the switches. This may make it also difficult to change or adapt a lighting circuit satis actorily without having to bury thick cable in, for example, walls. Furthermore, the switching contacts necessitate that the light switch mechanism is restricted to a certain minimum size to accommodate the mains voltage and current to be switched.

It is also desirable for portions (zones) of a lighting circuit to be controllable as a unit. For example, it may be desirable to be able to switch all the lights on one floor of a building on or off simultaneously. In such cases, it may still be desirable for local control of each light to be retained, eg by means of wall switches in each room, or for each light in the zone to be locked either on or off, individual switching being prevented.

There has now been devised an electrical load controller which enables the above requirements to be met.

According to the present invention there is provided a controller for controlling an electrical load, comprising first processing means having an output operably connectable with load switch means, a local control input operably connectable to a pair of local control conductors, and a zone control input operably connectable to a pair of zone control conductors, the first processing means being arranged to apply a voltage to the zone control conductors and to sense the presence or absence of a load switch locking command applied to the zone control input and, in the absence of such a locking command, to sense the presence of a connection between the zone control conductors or the local control conductors to apply a load switch command to the output.

The load controller according to the invention is advantageous over zone load control using conventional direct mains switching primarily in that the zone switch does not have to have the capacity to handle the potential currents for all the circuits being switched. Also, the circuit may be expanded without the currents handled by each switch becoming excessive.

In addition, groups of load controllers controlling separate circuits can be controlled by one or more zone switches the or each of which is arranged to switch on or off all loads controlled by a group of load controllers under its control.

It is also possible to control the switches in. a zone by means of a locking zone switch to apply the locking zone switch command, thereby rendering all the loads in the zone either on or off to prevent individual switching while the locking zone switch is in control. As an example, a zone switch may be used to control all load controllers controlling the lighting on a floor of an office block. By using a locking zone switch these may be locked on or off as required during certain times of the day or night.

In use, a pair of local control conductors is connected to the local control input, one or more l'ocal control switches being connected in parallel across the local control conductors.

Similarly, in use, each load controller in a zone is connected in parallel across a pair of zone control conductors, one or more zone control switches also being connected in parallel across the zone control conductors.

According to another aspect of the invention, there is thus provided an electrical load control circuit comprising a pair of zone control conductors across which are connected in parallel at least one zone control switch, a plurality of electrical load controllers, each load controller comprising first processing means having an output operably connectable with load switch means, a zone control input to which the zone control conductors are connected and a local control input to which is connected a pair of local control conductors, at least one local control switch being connected across the local control conductors, and a locking zone switch module having second processing means operable, in response to a momentary switch command, to apply a load switch locking command via the zone control conductors to the zone control input of the first processing means of each load controller, the first processing means being arranged, in the absence of a load switch locking command, to sense the presence of a connection between the zone control conductors or the local control conductors to apply a load switch command to the output, switching of the loads being disabled when the load switch locking command is applied.

It is preferred that the processing means be arranged to apply, upon sensing connection of the zone control conductors, not only a load switch command to the output but also a switch command to the zone control conductors.

By this means, since the load controllers in a zone are connected across the zone control conductors in parallel, the switch command applied to those conductors when connection between them is sensed relays the switch command to the other load controllers in the zone and ensures that none of the load controllers under rhe command of a zone is not given sufficient time to respond to the switch command, eg due to tolerances in the components used in the various load controllers.

Preferably, the first processing means are arranged to apply to the zone control conductors a momentary switch command.

Preferably, the zone control conductors comprise a pair of on and/or off-request control conductors, the first processing means being responsive to connection therebetween to apply an off- switch and/or on-switch command to the on and/or off-request control conductors, respectively.

Preferably, mains supply is used to derive electrical power for the first processing means which may be solid state or based on a microprocessor. However, it is desirable that the mains be isolated, for example, by a transformer, from the voltage applied to the zone control conductors which is usefully significantly lower than the mains voltage being switched. In this way, the control conductors can be manufactured of light duty wire or strips of metal film if they are to handle voltages of the order of, for example, 0 to 12 volts with minimal current.

It is preferred that each load controller be provided with its own power supply, thus obviating the need for separate power supplies .

The load switch locking command applied to the zone control input of the first processing means is necessarily distinguishable from a normal zone switch command. To achieve this, the second processing means may be arranged to transmit the locking command at a different voltage to the switch command or with a different dura ion.

As a further adaptation, a master zone control switch can be connected to switch on or off all the lighting circuits from a single source. An example of this might be the master switch at the entrance to a building which can be used to shut down all rhe lighting circuits together. It will be appreciated that zone control using conventional direct mains switching would require a zone switch having the capacity to handle the potential currents for all the lighting circuits being switched. Expansion of a conventional circuit such as this would very rapidly lead to excessive currents having to be handled by a single switch.

Desirably interface means are connectable across the zone control conductors in order that ancillary equipment such as burglar alarms, smoke alarms or fire alarms or other switching means, reacring to various inputs, can usefully be connected to the circuit. This is particularly conveniently done according to the present invention when the processing means are arranged to operate at a significantly lower voltage . such as 12 volts. The interface means may be connected directly to a load controller. For some applications, however, such as the interfacing of a burglar alarm, it may be desirable for the interface to be connected to the locking zone switch module to prevent switching off of the alarm after it has been set off.

The present invention can be put into practice in various ways, specific embodiments of which will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a schematic block diagram of an electrical load control circuit according to the invention; Figures 2A and 2B are circuit diagrams of a load controller according to the invention;

Figure 3 is a flow diagram of the program used by the processor in Figure 2;

Figure 4 is a circuit diagram of an intelligent zone switch according to the invention;

Figure 5 is a flow diagram of the program used by the processor in Figure 4. Figure 6 is a perspective view of a control switch suitable for use in the circuit of Figure 2B, and Figure 7 is a circuit diagram of the switch of Figure 6.

Referring firstly to Figure 1, a control circuit according to the invention comprises a number of load controllers 10, each of which comprises a microprocessing unit which is arranged to control the switching of electrical mains power to a separate lighting circuit. Each circuit may consist of a single light source or a plurality of them. Switching is achieved by means of a bi-stable relay which responds to electrical outputs from the processors.

The load controllers are individually controlled by switches 12, which are arranged to short across pairs of wires 14 and 15 connected to the processor. The processor senses the short and initiates a preprogrammed routine to change the state of the relay from open to closed or vice versa. The invention is particularly versatile as it allows numerous switches 12 to be added across the wires 14 and 15 in parallel to each other so that any one switch can be used to change the state of the relay simply by shorting across the wires 14 and 15. The switches may be any kind providing a temporary contact, either as a result of actuation of the switch to provide momentary contact or for as long as the switch is depressed or held on.

The load controller may be considered to be analogous to a junction box, and can be thought of as an intelligent junction box. In a conventional lighting circuit, junction boxes are used to form loops in the lighting circuits, switches being connected in the loops. When the switch is opened, current flow through rhe lighting circuit is interrupted. In the present system, the load σonrroller is located in the lighting circuit and is connected to one or more control switches. In this case, however, the switches are part of a relatively low voltage circuit; the controller senses actuarion of a switch on the low- voltage circuit and switches the load on the high-voltage circuit on or off accordingly.

In addition, groups of load controllers 10 controlling separate lighting circuits can be controlled by one or more zone switches 16 the or each of which is arranged to switch on or off all lights in a group of load controllers under its control. It is also possible to control the switches in a zone by means of a locking zone switch 18 to render all the lighting in the zone either on or off to prevent individual switching while the locking zone switch 18 is in control.

The switching of a zone of light sources may also be conducted by means of a piece of ancillary equipment, such as a burglar or fire alarm, a light sensitive switch or a timer 21 which is operably connected to an interface unit 17 which performs the appropriate switching commands, for example by shorting zone control wires. The ancillary equipment may be any proprietary equipment arranged to provide an output signal to which the interface is arranged to respond, for example voltage, infrared or ultrasonic signals.

A master zone control switch 20 is connected to switch on or off all the lighting circuits from a single source. An example of this might be the master switch at the entrance to a building which can be used to shut down all the lighting circuits together.

Referring now to Figures 2A and 2B, the schematic circuit diagram of a load controller processor 19 is shown. The processor functions may be broken down into its four main constituent parts, namely power supply, individual switch control, zone switch control and load relay output.

The power supply is illustrated in Figure 2A. It derives 5 volts and 12 volts from the lighting circuit mains supply, which is 240 volts a.c. in this case. The supply comprises a srep-down transformer 22 providing 12 volts from the live neutral of the a.c. mains. This is rectified by a 4 way bridge rectifier 24 and smoorhed by means of a capacitor 26 connected across the rectifier output. The 12 volts is then supplied to a low voltage rectifier 28 which produces a stabilised 5 volt supply for the processor 19.

Turning now to Figure 2B, the individual switch control is conducted by the processor in connection with driver and sensing circuits connected to terminals PB2, PB3 and PB4. Each of the terminals PB2 are connected to driver circuits. These are essentially the same as that connected to PB3 which will now be described. The driver circuit comprises NPN transistors Tl and T2 and a PNP transistor T3. The terminal PB3 is connected, through a resistor Rl, to the base of the transistor Tl. The collector of the transistor Tl is connected to the collector of the transistor T3. The emitter of the transistor Tl is connected to ground potential. The emitter of the transistor T3 is connected to the 12 volt rail. The base of the transistor T3 is connected, through a resistor R2, to the 12 volt rail and through a resistor R3 to the collector of the transistor T2. The base of the transistor T2 is connected to the 5 volt rail through a resistor R4. The collector of the transistor Tl is also connected to one of the wires 14 and 15 across which the shorting control switches are arranged.

A voltage sensing circuit 30 is used to enable the processor 19 to detect the prevailing load switch status from the difference in the voltages applied to the wires 14 and 15 in response to outputs of the terminals PB2 and PB3. The voltage sensing circuit comprises a voltage divider resisror pair R5 and R6 connected between the wire 14 and ground, and a transient suppressing capacitor Cl connected between the terminal PB4, to which the sensed voltage signal is applied, and ground. The terminal PB4 is also connected between the two resistors R5 and R6 from which the sensed voltage is derived.

The driver circuit associated with the terminal PB2 is the same as that for PB3 except that the voltage sensing circuit 30 is omitted. As the processor 19 is programmed to produce voltages of 12 or 0 volts, by means of driver circuits, oppositely on the two wires 14 and 15, only the indication on one wire is needed to be sensed. Of course, it is equally possible to arrange for both to be sensed to ensure the correct voltages are produced on each wire.

Assuming the presence of transmitting a high signal on terminal PB3, the collector-emitter path of the transistor Tl is conducting. Also the collector-emitter path of the transistor T3 is conducting, establishing a low signal or substantially 0 volts on the wire 14. At the same time, the processor 19 is programmed to produce the opposite voltage on the wire 15. Similarly, a low signal on the terminal PB3 will cause a high voltage on the wire 14 and vice versa in relation to the terminal PB3.

To change the state of a light or lights controlled by the load controller, one of the switches 12 is depressed to create a short across the wires 14 and 15. When the wire 14 is at a higher voltage than the wire 15, this causes the voltage on the terminal PB4 to rise to 12 volts and thus indicate a command to switch the light or lights controlled by the load controller either on or off.

When the voltage at the terminal PB3 is low, the transistor Tl is non-conducting, the transistor T2 is conducting, the transisror T3 is also conducting and supplies current to the ourput on the wire 14. The transistor T3 thus provides a translation of the 5 volt output from the processor 19 into a stronger 12 volt signal on the wires 14 or 15.

When a short is connected across the wires 14 and 15, by means of one of the switches 12, (when the voltage on the wire 14 is 12 volts and that on the wire 15 is at ground potential) the resistor divider chain R5 and R6 of"the voltage sensing circuit creates a logic high voltage at the connection to the terminal PB4.

Thus, the relatively high and relatively low voltages on the wires 14 and 15 can be switched between the wires to coincide with the status of the relay control of the lighting circuit. The use of a higher voltage on the wire 14 and a lower voltage on the wire 15 can arbitrarily be chosen to denote the open or closed state of the load controller relay and vice versa. Any two voltages on the wires 14 and 15 may be adopted with the use of suitable driver circuitry. For example + or - 12 volts or zero and 12 volts.

Light emitting diodes (LED's) Dl and D2 are connected between the wires 14 and 15 in opposite directions for conduction. Thus, when the high voltage is applied to the wire 14, the LED D2 will be illuminated to denote that the load controller relay is, for example, closed and the lighting is on. Similarly, when zero volts are applied to the wire 14 the LED Dl will be illuminated to denote that the relay is open and the lighting circuit controlled by the load controller in question is disconnected from the supply. Of course, only one status LED need be used simply to indicate when the lighting is either on or off in accordance with the voltages applied to the terminals PB2 and PB3.

A switch unit suitable for use in the circuit of Figure 2B is shown in Figures 6 and 7. The unit comprises a plastics housing 61 having a momentary action push-button switch 62. Adjacεnr to the push-button swirch 62 is an aper ure in which is visible a light-emitting diode 63.

As shown in Figure 7, rhe swirch unit also comprises a screw rype connecror 64 for connection to the associated control conductors, in this case rwo-core light-duty cable, a resistor 65 to limir the current to rhe lighr emitting diode 63 and an ordinary diode 66 connected in the opposite sense 'to the lighr emitting diode 63 so as to protecr rhe larter during the time that the voir ge is applied in such a sense that the light emitting diode 63 does not conduct.

The load controllers 10 are also connected for control by zone switches 16. This requires the use of four wires 34, 35, 36, and 37 and one of a pair of request switches 38 and 40 for each shutdown and/or switch-off.

The wires 34 and 35 are associated with the off-request zone switch 38. The wires 36 and 37 are associated with the on- request zone switch 40. Each of the zone switches 38 and 40 is connected between the 12 volt supply and ground potential. The load conrroller processor 19 must be able to distinguish between an ordinary zone switch "on" or "off" requesr and the same commands from a locking zone switch, as the latter are intended to disable further lighting control until rhe locking zone request is removed by a further connection between the appropriates wires. To enable this to be done, the ordinary zone and locking zone switches 16 and 18 use 12 volrs signalling to differentiate a zone command from a locking zone command respecrively. It should be noted thar the locking zone switch is arranged to transmit 12 volt zone switching signals continually only after the locking zone switch 16 has been held connecring the appropriate wires for two seconds or more.

Dealing firstly with an off request from a 12 volt ordinary zone switch 16, this is provided by depressing the ordinary zone switch 16 to short between the 12 volt supply rail and the input line to a 12 volts detection terminal PA2 on the processor 19. The terminal PA2 on the processor is connected between a resistive divider comprising resistors R7 and R8 which are connected to the zero volts rail and, through a resistor R9 and a diode D4 to the collector of a PNP transistor T . The emitter of rhe transistor T4 is connecred to the 12 volt supply. The base of the transistor T4 has a resistor RIO which is also connected to the 12 volt supply rail through a further resistor Rll and to the collector of a transistor T5. The base of the transistor T5 is connected to a terminal PB6 of the processor 19 through a resistor R12, used to produce a 12 volt extended pulse output.

The 12 volt off-request is detected through the voltage divider pair of resistors R7 and R8 at the terminal PA2 of the processor 19.

Similarly, the 12 volt off-request associated with a locking zone switch is detected at the terminal PAO which is connected to the junction between a resistor R13, serially connected with the 5 volt supply rail, and a diode D5 which is connected to the resistor R9 on the wire 35.

The zone input on-request circuitry connected to terminals PAl, PA3, and PB7 is similar to that described in relation to the off- request and shall not be described in detail again.

It will be appreciated that a zone switch may be arranged for only off- or on-requests. This is done simply by connecting a pair of zone control wires 36 and 37 or 38 and 39 to the appropriate connections of the zone switch. For cosmetic reasons it is advantageous to use a single activating button and the facility for only the pair of connections required. This is illustrated in the zone switch 16' in Figure 1.

Re erring to Figure 3 which is the flow diagram of the procedure which the processor 19 is programmed to execute, after conventional initialising of the processor, including clearing- down registers, a set of registers are polled which record the last zone status signals, ie 12 volts momentary or continuous detecred at PAO and PA2 respectively, which indicate whether the last zone command was a locking zone on or off request signal or nor.

On rhe assumption that the new signal is an individual switch command specific to the load controller, the processor interrogates the results of the poll of the zone switch command history register. In the event that a locking zone command is indicated, the zone registers are cleared to allow for the following loops in the programme to increment the registers from zero.

The processor is pre-programmed to determine whether or not a further register, which increments on each cycle of the loop until the relay command has been energised sufficiently long enough to ensure its successful actuation, has been incremented sufficiently for the relay actuation command to be removed. In the initial cycle, the register will be clear. Thus, at the next cycle a flag indicating whether a locked zone command prevails is read. If no locking command is present the sensor signal at PB4 is read to see whether it is a local switch being depressed. After this the loop individual switch counter is incremented. After this, the processor checks to see whether the command has already been processed.

The problem of contact bounce is overcome in the processor by allowing the programme to execute a number of loops until sufficient time has elapsed for the switched signal to have settled down. A suitable time for this is around 100ms. Once the debounced cycling is satisfied, the relay is actuated and a relay counter incremented on each cycle to allow sufficient time for energisation current to be applied to the relay. For each cycle of the loop, an 8ms counter is started. When each cycle reaches the energisation of the relay, interrogation of the counter takes place. Only when the remaining period of the 8ms is completed by the counter will the loop be allowed to embark on another cycle. By this means, the time for each loop is held to a constant regardless of the path taken.

When a zone on- or off-request is detected, the zone count is incremented every cycle to establish^*that the request is held for a predetermined minimum period and that contact bounce is eradicated. If the zone request is still present at the end of the zone count, the zone request flag is interrogated to establish whether the command has already been processed. If not, the locking flag is read to establish if the system is to be locked or not. In the absence of a set locking flag, the zone request register is read to establish whether the zone request has been maintained for a sufficiently long period. In the event that it is, the relay is actuated, the sense of the voltages on the terminals is changed, if the state of the relay has to be changed, in order to comply with the zone request. If the relay is already switched according to the command, the on- or off- request will have no effect.

When a switch command from a zone switch is effected, the processor will swap the voltages applied to the wires 14 and 15 in order that the correct status is indicated on any LED's used on the individual switches 12. Additionally, an extended 12 volt pulse is applied to the on- or off request line as appropriate.

Due to tolerances in the components used the load controllers under the control of a zone switch may react at different speeds.

Thus, not all load controllers may be actuated in response to an initial zone request. The extended 12 volt pulse is applied over the appropriate request lines which are connected in parallel to the same lines of the other load controllers within the zone. As a result, each load controller relays the on- or off-request of a zone switch to the remaining load controllers to ensure that none of the load controllers under the command of a zone is not given sufficient time to respond to the request.

As with the individual switches, the relay is actuared for the period taken to execute a count incremented on every cycle of the loop. When the count is complete, the relay actuation command and the zone pulse are cleared.

Referring to Figure 4, the locking zone switch uses a processor 19' which is the same as the processor 19, but is programmed differently. The individual switch circuitry of terminals PB2, PB3 and PB4 is omitted.

By depressing the on- or off-request zone switches the locking zone switch operates as a normal zone switch unless it is held for a period of greater than two seconds. In this latter case, the processor 19' is programmed to apply a continuous 12 volt signal on either of the lines 35' and 36' as appropriate which is detected by the load controllers as previously described. If the switch is held on for less than 2 seconds a 12 volt signal is applied on the lines 35 and 36 and a normal zone on- or off- request is processed as before. When a continuous signal is detected, the load controller is prevented from further actuation of the associated relay until the locking request is removed by a further depression of the request switch for greater than 2 seconds which the processor is programmed to process as a release of the locking command and restoration of the load controllers to normal zone and individual switch control.

The flow diagram of the programmed locking zone switch is shown in Figure 5. In this program the processor interrogates the terminals PAO and PA2 to see if a zone switch has been actuated. In the event that it has and as part of the program loop, the zone count register is incremented. When the count reaches 2 seconds the locked status of the locking zone switch is determined. If the locking zone switch is already locked, an opposite command cannot be executed until the previous locking command has been rescinded. As described above, to do this the same locking zone switch is actuated for greater than 2 seconds which causes rhe processor to remove the continuous locking command from the on or off request line to which it is applied. The lock command on the on- or off-request line is then applied either to remove or impose a 12 volt signal. Again, a time delay is used to equalise the routes to 8ms, as before, before a new cycle is executed.

The processing means may be preprogrammed and therefore not require modification in order to control further circuits or for expansion in association with further load controllers. Thus, the invention is particularly versatile as only light gauge wiring is needed.

By arranging for the processor to output switchable dissimilar voltages on the individual switch control lines, it is possible for the switch status of the relay to be indicated- by means of simple light emitting diodes connected across the lines. As the individual switches are connected across the individual switch control lines in parallel, numerous such switches may be added without having to modify either the processor programme or the wiring other than to extend the control wires as necessary.

Similarly, the zone switches can be connected in parallel for siting at convenient locations. It will be noted that four wires are required to differentiate between the on- and off-requests. However, a locking or normal zone switch can be arranged to provide only an off- or on-request, if desired. Up to about 100 junction boxes may be connected in parallel with each other using the 4 control wire arrangement.

It will also be appreciated that other processor than the present one, which is an Arizona Microchip model No. 16C54, can be used. Indeed, the circuitry of the processor and driver circuits may be implemented in hardware or by means of a custom built integrated circuit.

Claims

1. A controller for conrrolling an electrical load, comprising firsr processing means having an output operably connectable with load switch means, a local control input operably connectable ro a pair of local control conductors, and a zone control input operably connectable to a pair of zone conrrol conductors, the firsr processing means being arranged to apply a voltage to the zone control conductors and to sense the presence or absence of a load switch locking command applied to the zone control inpur and, in the absence of such a locking command, to sense the presence of a connection between the zone control conductors or the local control conductors to apply a load switch command to the output.

2. A load controller as claimed in Claim 1, wherein, in use, a pair of local control conductors is connected to the local control input, one or more local control switches being connected in parallel across the local control conductors.

3. A load controller as claimed in Claim 1 or Claim 2, wherein, in use, each load controller in a zone is connected in parallel across a pair of zone control conductors, one or more zone control switches also being connected in parallel across the zone control conductors.

4. A load controller as claimed in any preceding claim, wherein the first processing means is arranged to apply, upon sensing connection of the zone control conductors, not only a load switch command to the output but also a switch command to the zone conrrol conductors.

5. A load controller as claimed in Claim 4, wherein the first processing means are arranged to apply to the zone control conductors a momentary switch command.

6. A load controller as claimed in any preceding claim, wherein the zone control input is operably connectable to a pair of on and/or off-request conrrol conductors, the first processing means being responsive to connection therebetween to apply an off- switch and/or on-switch command to the on and/or off-requesr control conductors, respectively.

7. A load controller as claimed in any preceding claim, wherein mains supply is used to derive electrical power for the firsr processing means and the mains is^* isolated from the voltage applied to the zone control conductors which is lower than the mains voltage.

8. An electrical load control circuit comprising a pair of zone control conductors across which are connected in parallel at least one zone conrrol switch, a plurality of electrical load controllers, each load controller comprising first processing means having an output operably connectable with load switch means, a zone control input to which the zone control conductors are connected and a local control input to which is connected a pair of local control conductors, at least one local control switch being connected across the local control conductors, and a locking zone switch module having second processing means operable, in response to a momentary switch command, to apply a load switch locking command via the zone control conductors to the zone control input of the first processing means of each load controller, the first processing means being arranged, in the absence of a load switch locking command, to sense the presence of a connection between the zone control conductors or the local control conductors to apply a load switch command to the output, switching of the loads being disabled when the load switch locking command is applied.

9. An electrical load control circuit as claimed in Claim 8, wherein the second processing means is arranged to transmit the load switch locking command at a different voltage to the normal zone switch command or with a different duration.

10. An electrical load control circuit as claimed in Claim 8 or Claim 9, further comprising a master zone control switch connected to switch on or off all the loads controlled by the circuit from a single source.

11. An electrical load control circuit as claimed in any one of Claims 8 to 10, further comprising interface means connected across the zone control conductors in order that ancillary equipment such as burglar alarms, smoke alarms or fire alarms or other switching means, reacting to various inputs, can be connected to the circuit.

12. An electrical load control circuit as claimed in any one of Claims 8 to 11, which a lighting control circuit.

13. An elecrrical load control circuit as claimed in any one of Claims 8 to 12, wherein the first processing means is arranged to apply a voltage to the local control conductors and to reverse the polarity of that voltage in response to a connection being established between the local control conductors.

14. An electrical load control circuit as claimed in Claim 13, wherein the local control switch comprises control switch means connected across the local control conductors and a semi¬ conductor light source connecred across the local control conductors in parallel with the control switch means.

15. An electrical load control circuit as claimed in Claim 14, wherein a pair of semi-conductor light sources is connected in opposite directions across the local control conductors.

16. An electrical load control circuit as claimed in Claim 14 or Claim 15, wherein the semi-conductor light source is housed in the same housing as the control switch means, so as to provide an indication of the status of the load controlled by the circui .

17. An electrical load control circuit as claimed in any one of Claims 14 to 16, wherein the control switch means are operable to establish a momentary connection between the local control conductors.