Good, reliable lighting especially at and near junctions is vital for improving road safety. Accordingly, there is a need to monitor and replace lamps when a significant number have failed.

The lamps used in lighting heads of conventional road lights have a limited, and variable, life span so there is inevitably a trade-off between replacing failed lamps to maximise road-user safety and minimising the number of maintenance actions required in order to minimise cost and disruption. Currently, the number of failed lamps within a given stretch of motorway is determined by visual inspection at night.

Many road lights are located in positions where it is difficult, dangerous or expensive to reach those lights. For example, lights located on the central reservation of motorways are both difficult, dangerous and expensive to access, since extensive traffic management is required which can significantly disrupt normal traffic flow.

Accordingly, there is a need to minimise the maintenance required for lighting at such locations.

Furthermore, automatic detection of failure is more complicated than it might at first sight appear. Total failure of a lamp may be relatively easy to detect if the lamp then draws little current, but even this is not guaranteed, since lamps are generally packaged together with ballast, and other components. There are however other failure modes in which the lamp still draws current but does not provide adequate illumination. For example, sodium street lamps can fail in such a way that they become so-called"red burners", that is lamps that continue to glow red and never warm up to glow bright orange-yellow. Lamps may also start to flicker, which may be equally dangerous.

Thus, there remains a need for a lighting system and method that can improve lighting reliability whilst keeping maintenance costs at as low a level as possible.

According to a first aspect of the invention, there is provided a lighting head for a road light comprising a first lamp and a second lamp ; a detector for detecting failure of the first lamp and second lamp; a switching circuit for supplying electrical power to one of the lamps and for switching electrical power to the other lamp in response to a signal, directly or indirectly, from the detector indicating failure of the one lamp, and an indicator for indicating lamp failure.

In combination with a statistical analysis of lamp life the lighting head according to the invention allows a given level of lighting and service to be maintained whilst at least halving the number of access events to the lighting head required. The indicator may be a mechanical indicator, such as a visual flag that is extended or moved into a given position to indicate that the switching circuit has switched to the second lamp and accordingly that the first lamp has failed.

Alternatively, an electronic indicator may be used. These may include a radio signal, a signal sent down the power supply of the column for remote detection, an infra-red signal emitted from the lighting head or a variation in the luminosity of the lamp, for example at a non-visible frequency or a change in colour of the light of the lamp. The skilled person will readily conceive several possible indicators for signalling failure of the lamp.

According to a second aspect of the invention, there is provided a lighting control unit for a road light having a first lamp unit, a second lamp unit and a switching circuit for initially supplying alternating electrical power to the first lamp unit and for switching electrical power to the second lamp unit (or from second to first) in response to a failure signal; the lighting control unit comprising: a pair of power inputs ; a pair of power outputs; a pair of wires between the power input and the power output; and detection circuitry for detecting failure of the first lamp unit arranged to detect current flowing along the wiring pair and the voltage across the wiring pair; wherein the detector compares the current passing along the pair of wires at a predetermined time or times during the alternating voltage cycle with predetermined parameters and outputs the failure signal when the detected current deviates from the predetermined parameters.

By detecting not merely the current in the lamp on an averaged basis but with reference to the voltage cycle detecting failure in the lamp units becomes much easier and more reliable. It becomes possible to detect failure modes that draw current, as all do.

In a preferred embodiment the detector outputs the failure signal when the measured current has less than a predetermined magnitude at the voltage maxima. The detector thus measures the current substantially instantaneously at a predetermined position in the voltage cycle, namely the voltage maximum. The times the voltage maxima occur can readily be identified. Experiments were carried out with lamp units including ballast, lamp, igniter and power factor correction capacitor. In working lamp units the load was substantially resistive so that the voltage and current cycles were approximately in phase. In these experiments, failed lamp units were observed to have disrupted current signals no longer in phase with the voltage signals so that failed units had reduced current at the voltage maxima. Thus, use of the instantaneous current at the voltage maxima gave good discrimination between working and failed units.

In embodiments of the invention the detector is implemented to include: a voltage detector for providing a voltage signal representing the voltage across the pair of wires; a current detector for providing a current signal representing the current along the pair of wires; a phase shifter for shifting the phase of the detected voltage signal by substantially 80-100°, a comparator for detecting the phase-shifted voltage signal and outputting a clock signal, and a latch.

The latch may be a latched comparator having a signal input connected to the output of the current detector and latched by the clock signal output by the comparator; and logic for detecting whether the output of the latched comparator falls below the predetermined voltage and outputting a lamp fail signal when it does so.

Conveniently, the lighting control unit also includes a switch for switching the road light on and off arranged between the power input and the detection circuitry. The switch may be remote controlled, for example, or alternatively an optical sensor for switching on the power during darkness. Alternatively, the switch may be omitted. In

this latter case, the road light may be controlled by switching on the power to the light remotely of the light when illumination is required.

The switch for switching the road light on and off could alternatively be arranged between the detection circuitry and the power output. In this case additional circuitry that detected the status of power to the lamp control gear would connect to the logic and delay generator. This connection would inhibit the delay generator and lamp fail flag until power was applied to the lamp control gear.

Further lamp control gear may be arranged between the detection circuitry and the power output for controlling the connected lamp unit.

A delay generator may be provided for disabling the detector for a predetermined period after the lighting head is switched on to allow the lamp to warm up before detecting failure. This allows the detector to ignore current fluctuations in the initial period after power to the lamp unit is applied.

The invention also relates to a road light comprising: a lighting control unit according to any preceding claim; a first lamp unit; a second lamp unit ; a switching circuit for initially supplying alternating electrical power to one lamp unit and for switching electrical power to the other lamp unit in response to a failure signal output by the detector; and an indicator for indicating failure of the lamp unit in response to the failure signal.

The first and second lamp units may each include a lamp and a ballast.

The road light may include a substantially vertical pole and a lighting head at the top of the pole, wherein the first and second lamp units are mounted at the lighting head and the lighting control unit is mounted for access on the pole.

Further, the invention relates to a method of managing road lights, comprising: providing a plurality of road lights with lighting heads having first and second lamps; providing initially electrical power to the first lamps; detecting failure of the first lamp

and in response to the detected failure switching to the second lamp and indicating the failure of the first lamp with an indicator; and monitoring the indicators to determine when to replace the lamps.

In particular, the method may include waiting until a predetermined number of first lamps have failed; possibly a significant fraction such as over 50%, and then replacing? all the failed lamps in one go. In this way, fewer accesses to the lighting heads may be required, minimising disruption and expense.

Alternatively, the method may include replacing all units when the predetermined number of lamps have failed.

The method may in particular include automatically monitoring the failure of the first and second lamps. In this way, visual inspection of the indicators may not be required.

Alternatively, where the indicator is simply a manual flag, these may be reviewed during the day rather than at night which can minimise inspection costs.

The method may further include detecting the current-voltage phase relationship between the voltage applied to the first lamp unit and the current through the first lamp unit and outputs the switch signal when the current-voltage phase relationship deviates from predetermined parameters. (Also applies to the second lamp.) For a better understanding of the invention, specific embodiments will now be described,. purely by way of example, with reference to accompanying drawings, in which: Figure 1 shows a motorway lighting system; Figure 2 is a systematic drawing of the lighting heads of the system of Figure 1; Figure 3 shows the circuit diagram of the switching circuitry ; Figure 4 is a circuit diagram of the detection circuit Figure 5 shows waveforms in the detection circuit when the circuit is connected to a working lamp; and

Figure 6 shows waveforms in the detection circuit when the circuit is connected to a failed lamp.

Referring to Figure 1, a motorway 1 has a plurality of road lighting columns 3 extending up the central reservation, each road lighting column 3 having in this example two lighting heads 5. The road lights may also be arranged on other roads and locations as appropriate.

Referring to Figure 2, each lighting head 5 includes a first lamp unit 7 and a second lamp unit 9 arranged in an outer housing 11 with a transparent region 13. Each unit 7, 9 may include for example a lamp, ballast and igniter, and may further include a power factor correction capacitor so that the unit presents a substantially resistive load for increased power efficiency. Such lamp units 7,9 are known in the art and accordingly will not be described further.

Electrical power is supplied to the lighting head 5 through electrical connections 15 which are connected into an electronic circuit 17 in the lighting head 5 having a switching circuit 19 for switching the power into the first unit 7 or the second unit 9. A detector 21 detects the failure of the first lamp 7 and an indicating flag 23 is pivotally mounted on the lighting head moving between a first position 25, shown with full lines, and a second position 27 shown with dashed lines. The flag could alternatively be an in/out button or coloured slider inside a viewing window.

In use, generally during the hours of darkness, electrical power is supplied to the lighting head 5 when light is required. Initially, the switching circuit 19 directs the electrical power into the first light lamp 7 which illuminates and provides motorway lighting.

The detection circuit 21 monitors the power supplies to the lamp unit 7, for example by monitoring the current passing through the lamp. When the detector detects a failure condition in the first unit 7, it causes the switching circuit 19 to switch over to supply electrical power to the reserve or second unit 9. At the same time, it supplies a signal to move the indicator flag 23 from the first position 25 to the second position 27 to

indicate a failure condition. The position of the flag 27 can be visually inspected from the ground.

In the embodiment described above using a mechanical indicator, it is not necessary to supply electrical power to the lighting head or even to the lighting column for the indicator to be visible. Accordingly, the invention will work even in systems whereby the lighting is controlled centrally by supplying power to the street lights when light is required, and supplying no power when light is not required. An electronic circuit outputting a signal to indicate that the first lamp has failed will also work with this system if it outputs the signal when the fault is detected i. e. when electrical power is available.

In other systems, electrical power is also available when the lamp units are switched to off, for example, some lighting systems respond automatically to low lighting conditions by switching on. In such systems, a simple mechanical arrangement such as the arrangement of the embodiment of Figure 2 may be replaced by an electronic circuit outputting a signal to indicate that the first lamp has failed. An example of such an arrangement is shown schematically in Figure 3, in which like components are given like reference numerals.

In the arrangement of Figure 3, the switching circuit 19 includes switch 20 that switches electrical power to the first unit 7 when lighting is required. When the detector 21 detects that the first unit 7 has failed, it sends a signal back to the switching circuit 19 which switches power over to the reserve unit 9 and also sends a signal to the indication circuit 29 which is a radio transmitter transmitting a failure signal on an antenna 31.

In a particularly preferred implementation, indication circuit 29 is a cellular telephone transmitter, for example a GSM transmitter. This is arranged to transmit a signal, for example a suitable text or data message, to a central server 33 which may, for example, log the signals, to determine which lamp has failed, and output maintenance orders as and when required.

The detector 21 is shown in more detail in Figure 4. Power inputs 41 connected to an alternating current power supply are connected to power outputs 43, which are in turn connected to the lamp units, through optical switch 45, detection circuitry 47 and along pair of wires 49. Current transformer 53 is in series with one of the wires and outputs a current signal corresponding to the alternating current along the wires. Voltage transformer 55 is placed across the pair of wires 49 and outputs a voltage signal corresponding to the alternating voltage across the wire 49.

The optical switch could alternatively be sited after the detection circuitry.

The phase shift circuit 57, here implemented with a resistor 59 and capacitor 61, delays the phase of the voltage signal output by voltage transformer 55 by substantially 90°.

The phase shifted voltage signal is then fed to the input of comparator 63 which detects the zero crossings of the phase shifted voltage signal and outputs a clock signal which is fed in turn to the latch input 66 of a latched comparator 65 which has signal input 67 connected to the signal from current transformer 53. The comparator 65 detects whether the instantaneous magnitude of the current at the rising or falling edge of the clock signal exceeds a preset value or not.

The output from latched comparator 65 is fed to logic 69 which checks once per cycle if the output of the latched comparator has fallen below the preset value. If it falls below the preset value for a number of successive clock cycles the lamp is deemed to have failed and the circuit outputs a failure signal on line 71. The failure signal is output until the lamp has been turned off and on again, when the process restarts.

Delay generator 73 is used to provide a delay of 10 to 30 minutes before the circuit starts to check the output of the latched comparator. This allows the lamp to warm up.

Further circuitry may be provided to reset the logic when the clock starts indicating that the power has been provided to the lamp. The logic may be arranged to retain information about previous states, if required, in particular to output a failure signal when the first lamp unit has failed even before the end of the delay caused by the delay generator, if required in the particular application.

Experiments were carried out using a lamp unit, in the example a Philips SOX plus 135W low pressure sodium lamp with a BSX 135 H96 ballast, SX74 igniter and 6.811F capacitor. The results are illustrated in Figures 5 and 6.

The circuit of Figure 4 measures the instantaneous current at the point where the waveform of the applied voltage is maximum, i. e. at the rising or falling edges of the square wave 81. In fact, comparator 65 is latched on the rising edges of square wave 81, once per cycle.

Figure 5 shows results taken using a working lamp unit. Curve 81 is the clock signal output from comparator 63 and curve 83 is the current signal output by current transformer 53. The results for curve 81 are shown as 2V/division and for curve 83 as 0. 5V/division. The calibration of the current waveform is 1. 93A/V so that the indicated waveform gives a peak current of about 0.96A.

The square wave (curve 81) is in quadrature with the input voltage and so the rising or falling edges correspond to maxima of the input voltage. The current waveform 83 illustrated in Figure 5 shows low distortion. Further, the current waveform 83 shows a good power factor-it should be remembered that the voltage signal 81 is delayed by phase shift circuit 57 by about 90°.

In contrast, Figure 6 shows the results with a broken lamp. The current (curve 85) is reduced somewhat, has a poor waveform and lags the voltage by around 90°.

The magnitude of the current at the rising edges of the phase-shifted voltage signal 81 can clearly be seen to be very different in Figures 5 and 6. In Figure 5, for a working lamp, the current at the zero-crossing signal 81 has a magnitude of around its maximum value of 0.96A. In contrast, in Figure 6, the current is low on the rising edges of the zero-crossing clock signal 81. Thus Figures 5 and 6 illustrate how by measuring the current not over the complete cycle but instantaneously at a predetermined time within the cycle a good discrimination between working and non-working lamps can be obtained.

The skilled person will readily conceive alternative indication and transmission systems that may be used. For example, an indication signal radio may be transmitted on the AC power lines 15, on an alternative maintenance line, a separate indication lamp, or any other number of other possibilities that will readily occur to the skilled person.

The switching circuit 19 may be wholly electronic, using transistors, or may use relays.

Other current and voltage detector arrangements may be used instead of the current and voltage transformers to supply voltages tracking the current and voltage. The skilled person will also be aware of other circuit components that may be used to detect the current at predetermined times in the voltage cycle, and these may be used if required.

Other forms of latch circuit including triggered data capture circuitry may replace the latched comparator. The logic circuit may be arranged to output failure signals as required. A memory may be included to retain the state of the system, times, and measured parameters, if required.

Various alternative arrangements and technologies could be used to perform the functions described for the detector.

For example, a micro controller or microprocessor based system may be used to replace the logic, clock, latch and delay generator. Additionally, by the use of analogue to digital converters on the outputs from the current and voltage transformers, the phase shifter and comparator functions can also be performed by a micro controller or microprocessor based system.

The micro controller or microprocessor based system referred to above could be replaced with a gate array or other custom logic circuit.

Alternative methods could be used to analyse the signals from the current and voltage detectors. These methods could employ synchronous rectification or spectral analysis of the signals using digital techniques or analogue hardware. Although these methods still rely on detecting the instantaneous current-voltage relationship, measurements may be made at several points in time during the alternating voltage cycle and could include for example the instantaneous voltage at the current maxima.