The present invention relates to the monitoring of voltage-limiting devices and in particular to the monitoring of devices arranged to offer circuit protection against potentially damaging high voltage and/or current levels, such as for exarnp!e surge currents.

Voltage limiting devices wiii commonly be employed to prevent unwanted voltage values arising within a circuit, or at a particular device. In one arrangement discussed further below, the voltage limiter can comprise a circuit/device protection device, such as a Surge Arrester, which is arranged to absorb surge current (energy) in order to prevent the protected circuit from experiencing high voltages. In this manner, its absorption of the surge current limits the voltage arising in other parts of the circuit. That is, the device is commonly triggered by the appearance of a trigger-voltage value at its terminals and then seeks to limit the voltage arising in the circuit by diverting the available surge current through itself.

There are many situations in which electrical circuits are required to operate in environments where voltage and current values might arise that are in excess of the desired operating range of the circuits and/or circuit devices. Such excess voltages/currents might arise internally, perhaps through failure of a circuit device or element, or externally, and can provide for such a surge in voltage/current that the circuits/devices are damaged or actually destroyed. Examples of such externally originating surges are those arising from lightning strikes on the circuit. These dangers are particularly prevalent in circuits/devices operating at least partially in external environments.

Various forms of voltage limiting devices are available and which serve to offer at least some degree of protection to sensitive circuitry and circuit elements. These devices serve to "absorb" the excess voltage/current and thereby prevent it damaging the sensitive circuitry and devices/elements. Examples of such devices comprise surge arresters such as

Transient Voltage Suppressor (TVS), i.e. so-called Transorb, devices, and also varistor devices.

For example, technical literature concerning TVS devices indicate that, unless driven to failure through becoming stressed due to, for example, a peak surge being experienced beyond its design limit, or the occurrence of frequent surges that prevents dissipation of thermal energy, they will continue to function, albeit in a degraded state, Voltage limiting devices can be sacrificial in nature and so will tend to deteriorate through use, and after absorbing a series of surges they can eventuaily fail completely, During, and of course after, such deterioration, the devices can have an undesirable influence on the operation of the circuits/devices they are employed to protect. For example, when employed within a process control loop, a deteriorating surge arrester can cause the creation of false readings.

The monitoring of the operative condition of surge arrester devices is known as a means of identifying potential deterioration. The vast majority of existing monitoring arrangements attempt to provide such monitoring through measurement of the thermal heating of the surge arrester that occurs due to the current that it conducts as it deteriorates. However, due to the reliance of such known arrangements upon the heat dissipated within a deteriorating device, the sensitivity of these known arrangements is limited and this can also serve to restrict the range of situations in which these known arrangements can be reliably employed,

It is also known to conduct some form of device monitoring by means of voltage sensing at the surge arrester device, However, the sensitivity, and aiso the accuracy, of such known devices is disadvantageously limited and, when deployed in a circuit such as a control loop, the voltage will vary with the measuring current. External power supplies are also employed for such known monitoring arrangements/devices and this can prove a commercially limiting factor, adding to the complexity of known monitoring arrangements.

The present invention seeks to provide for the monitoring of a voltage-limiting device through the provision of a monitoring arrangement, and related method, having advantages over known such arrangements and methods.

According to a first aspect of the present invention, there is provided a voltage-limiter monitor arranged to monitor current passing through the voltage-limiter by sensing current passing through a resistance arranged to be series-connected to the voltage-limiter, and wherein the monitor further comprises voltage limiting means operatively connected to the resistor to limit the voltage arising across the resistor.

The invention is advantageous in so far as, through use of current monitoring in relation to current passing through the voltage limiter, it proves possible to detect deterioration of the voltage-limiter device even when only small currents pass through the device. This is particularly beneficial in extending the range of operative conditions in which deterioration of the voltage limiter can be reliably identified. The invention can advantageously operate in situations where even a small current passing through the voltage limiter can have a detrimental effect on the circuit, and/or signalling arising within the circuit, within which the vo!tage limiter is empioyed.

In particular, the use of a resistance of reasonable value in series with the limiting device is required for current sensing, but brings with it problems and limitations insofar as it will prevent the limiting device from performing its function. Such problems and limitations of current sensing are advantageously overcome by the present invention by allowing the use of such reasonable value resistance, but without affecting the operation of the limiting device, nor exposing the circuit devices to potentially damaging voltage levels.

Yet further, the invention proves advantageous in so far as, through use of the voltage limiting means operativeiy connected to limit the voltage arising across the resistance, a resistance of high value can be employed. This proves particularly beneficial for use in relation to smali current values and, for example, to ensure that signai-to-noise ratio levels remain adequate for the required level of detection.

Advantageously, the voltage limiting means can comprise switching means, in particular, said switching means can comprise a diode switch means in parallel with the resistor.

In one particular example, the voltage limiting means can comprise a back-to-back diode pair connected in parallel with the resistance.

Such a circuit arrangement provides for a particularly simple and reliable solution to problems faced by the present invention.

As appreciated, the voltage-limiter monitor can be arranged to monitor operation of any appropriate voltage limiting-device such as, but not limited to, surge arresters such as for example a Transient Voltage Suppressor.

Yet further, the monitor can comprise sensing circuitry arranged to sense the current through the series connected resistance. The sensing circuitry can advantageously include an operational amplifier of high input impedance, or other complex integrated circuits..

As a further advantageous feature of the present invention, the monitor can be arranged to receive power from the circuitry within which the voltage-limiter is employed.

Advantageously therefore, a monitor of the present invention can be arranged io scavenge its required power from the circuit whereby a separate external power source is not required. in such an example, voltage dropping devices can be employed within the circuit being protected to provide for a voltage difference required for powering the sensing circuitry. As one example, the voltage dropping devices can comprise at least two series connected diode devices. It should however be appreciated that any appropriate number of such devices can be employed as required as well as other voltage drop means.

Further, through appropriate choice and selection of such voltage dropping deyices, the sensor can be arranged to scavenge its required power without having any deirimentai effect on the operation of the circuit employing the voltage-limiting device.

In particular, the current path providing for the voltage drop can be advantageousiy provided from within the current loop without stray leakage.

Of course the resistance can take any appropriate form and in a particular embodiment comprises a resistor advantageously of appropriately high value for the monitoring of small currents.

As a further feature, indicator means can be included within the sensor so as to provide a visual indication as to when a current through the voltage limiter, serving to indicate deterioration of the limiter means, is sensed, The indicator can comprise any appropriate arrangement/ device to provide any appropriate signal such as, but not limited to, visual, audio, electrical, radio etc.

Further additional, or alternative, indicator means can be provided to indicate eventual failure of the voltage limiter.

In preferred embodiments, such indicator means can comprise one or more visual indicator means such as, for example, light emitting diode devices.

According to another aspect of the present invention there is provided a method of monitoring a vo!tage-limiter operatively connected within a circuit and including the step of sensing current passing through the voitage-!imiter by sensing current passing through a resistance series-connected to the voltage limiter, and further including the step of limiting the voltage arising across the resistance upon that voltage exceeding a threshold value.

As above, and, through use of current monitoring in relation to current passing through the voitage-limiter by way of the method of the present invention, it proves possible to detect deterioration of the voltage limiter device even when it only passes small currents as it starts to, and possibly continues to, deteriorate.

Also, through use within the method of the invention of the voltage limiting means operatively connected to limit the voltage arising across the resistance, a resistance of high value can be employed within the method that proves particularly beneficial for use in relation to small current values and to ensure that signal-to-noise ratio levels remain adequate for the required level of detection.

Advantageously, the voltage across the resistance can be limited by switching means. In particular, said switching means can comprise a diode switch means in parailei with the resistance.

As will be appreciated, the voltage-iimiter monitoring can be arranged to monitor operation of any appropriate voltage limiting device such as, but not limited to, surge arresters such as example a TVS.

The method can include the step of powering the monitor by way of power from the circuitry within which the voltage !imiter is employed.

Further, the method can include the step of scavenging its required operational power such that a separate external power source is not required.

The method can then include the step of dropping a voltage within the circuit employing the voltage-iimiter to provide for the required power.

The monitor can also be powered form any appropriate power source, which can comprise a separate power source such as for example its own battery supply. Further, the method can include the step of providing a, preferably visual, indication as to when current through the voltage-iimiter device, serving to indicate deterioration of the device, is identified.

In addition, or alternatively, the method can include providing a, preferably visual, indication of failure of the voltage-iimiter.

As will therefore be appreciated, the present invention can comprise a method of voltage limiting within a circuit and including monitoring steps such as outlined above.

The method can be provided in particular in relation to control circuits such as process control loops and emergency shutdown systems.

Also, the invention allows for ready detection of deterioration of the effective "loop value" through the appearance of the error current in the limiting device, and also provides for an indication of the general operational status "health" of the failing device. Such an indication can advantageously be employed in a predictive manner serving to indicate when failure might occur. As will therefore be appreciated, a monitor arrangement, and related method, such as that of the present invention can be advantageously arranged such that the voltage- limiting means serves to bypass the resistance in the event of a surge at the voltage iimiter. Such bypassing assists in allowing the voltage limiter to perform its function, and prevents a potentially damaging voltage developing across the resistor.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 comprises a schematic illustration of a simplified circuit arrangement according to an embodiment of the present invention;

Fig. 2 comprises a schematic illustration of a simplified circuit arrangement according to a second embodiment of the present invention and including the scavenging of power for the monitoring from the circuit and control device; and

Fig. 3 comprises a schematic illustration of a simplified circuit arrangement according to another embodiment of the present invention and including the scavenging of power for the monitoring from the circuit and control device.

As noted below, the operation of the embodiments of the invention illustrated in Figs.1 and 2 is in the context of a voltage-limiting device in the form of a surge arrester such as employed within 4-20mA analogue input and output loops of a process control circuit arrangement. Such arrangements can allow for the provision of battery power for digital input/output signalling. However, it should be appreciated that this is merely illustrative of one of many examples of the present invention, and the concept behind the present invention can be employed for other configurations and indeed in a wide variety of other circuit protection scenarios.

The illustrated embodiment of use with the 4~2GmA loops is however particularly illustrative of the advantageous sensitivity and accuracy offered by the present invention. Within such an example, the monitoring device can be arranged to indicate an error current flowing in the voltage-limiting device, once such current exceeds a threshold value. For such loops, such a threshold value, i.e. alarm point for the monitoring, could be anywhere down to for example 100uA. Such a -level can advantageously be chosen to represent approximately 1% of the operating range of the loop. Such a percentage value of the operating range commonly corresponds to the tolerance of the op control devices. A preferred value is lOOuA. Further, given that detection of deterioration will in such circumstances occur at an error current value of 100uA, thermal analysis will of course not be readily available since an error current of such value is unlikely to dissipate more than 25mW.

Monitoring by sensing the current value through the failing device advantageously does not suffer such limitations. Turning first to Fig. 1 , there is illustrated a control loop 10 arranged for the exchange of control/data signals between a field device 12 and control device 14 operating in an environment where surge currents/voltages might arise in the loop 10 due for example to a lightning strike. The loop 10 serves to interface the control device 14 to the field device 12 and such control device 14 can commonly comprise a programmable logic controller (PLC), a distributed control system (DCS) or an emergency shutdown (ESD) system. in order to protect against such surge currents, a surge arrester, in this example in the form of a Transient Voltage Suppressor {TVS) 16, is employed to absorb the energy in any such surge in the loop 10 and thus prevent the surge appearing at, and Hke!y damaging or destroying, the field device 12, or a control device 14. The voltage iimiter 18 and its related monitor can therefore be located in proximity to the field device 12 or the control device 14, or if required both, of the illustrated embodiment.

However, and particularly within the context of the present invention, it is appreciated that voltage-limiting devices such as the TVS 18 can be sacrificial devices which will deteriorate over time, prior to eventual failure, after absorbing one or multiple surges. As a result of such deterioration and partial failure, the TVS 16 will be prone to the conduction of small error currents. The existence of such currents can have a detrimental effect on the required operation of the loop 10 so that their existence should ideally be identified.

When the TVS 18 starts to leak it will steal current from the loop 10 but the effect that such leakage error could have within the loop may not be noticed instantly due to compensation within the process closed loop. When considering different possible control devices, such as for example, but not limited to those mentioned above, a DCS user will typically receive an alarm that the loop control point has shifted, however the plant within which the field device is present will continue to operate and typically an investigation will then have to be initiated at the plant to identify the perceived problem.

A PLC user however will typically not notice such errors until the loop stops working, or has deteriorated to a malfunction level.

For analogue input loops, the error is likely to be more quickly noticed and, for example for a DCS user, an out-of-range alarm may occur. More problematic however is use in relation to an ESD whereby a plant-shutdown procedure is likely to be initiated. Such shutdown could prove catastrophic and/or costly such that the use of diagnostics as offered by the present invention can prove particularly attractive. In order to provide for such diagnostic sensing, the illustrated embodiment of the invention of Fig. 1 provides for a monitoring/sensing arrangement 18 serving to determine the, likely rather small, error current flowing in the TVS 16 once it starts to fail. To achieve this in a particularly advantageous and simpie manner, the sensing arrangement 18 of the invention employs a resistance such as a resistor 20 series-connected with the TVS 16, so that as error current flows in the TVS 16, it will also flow through the resistor 20.

However, as should be appreciated, in view of the required sensitivity and accuracy related to the small error current values, and having particular regard to related signal-to-noise ratios, a high value is to be selected for the resistor 20. In turn however, such a high value can prove problematic when a surge occurs in so far as the surge current will pass through the series-connected resistor 20 and TVS 18. Due to the high resistance value, a large voltage wili develop across the resistor 20 which may not only damage the said resistor and/or the remainder of the sensing arrangement connected to the resistor, but much of the energy associated with the surge will then simpiy be passed-on to the field device 12, rather than being absorbed in the TVS 18.

The choice of a small value resistor however is not a viable alternative since, although it wili allow for required absorption of the surge energy within the TVS 18, and develop little voltage across its own terminals (and so not damage the remainder of the sensing arrangement), the small resistance value will be wholly inappropriate for the initial stages of determining an error current within the deteriorating TVS 18.

The present invention advantageously allows for use of a high-value resistor 20, through employment of a voltage limiting switch arrangement 22, 24 across the terminals of the resistor 20 and so operative in parallel therewith. In the illustrated embodiment of Fig. 1 , this voltage limiting switch arrangement comprises a pair of back-to-back diodes 22, 24. As described below, the diode pair 22, 24 is arranged to turn on as a surge voltage develops across the resistor 20 and once turned on (forward biased) the vast majority of the surge current will be diverted via one of the diodes 22 or 24 and into the TVS 18 wherein the energy associated with the surge can be absorbed as required rather than passed-on to the field device. The diode pair 22, 24 serves to limit the voltage appearing across the resistor 20 to that forward bias voltage (0.7V to 1.2V) and so also to prevent damage to the remainder of the sensing arrangement. As will be appreciated the diode 22 is arranged to operate when the surge current is in reverse direction compared to that operating through the diode 20.

Thus, by means of the monitoring arrangement of the present invention, appropriate protection against, for example, potential surges can be provided for the loop 10, while, even during non-surge (i.e. normal operation) the sensing arrangement is sensitive enough to detect small error currents through the voitage iimiter and thus provide an eariy indication of deterioration of the TVS 16.

As illustrated, the error current flowing in the resistor 20 is sensed by means of a current sensor 26 preferably employing a high input impedance op amp. As described further below, the sensing arrangement serves to provide an indication of deterioration once the error current in the TVS 16 exceeds the threshold value of, for example, 100uA.

Turning now to Fig. 2, there is illustrated a second embodiment of the present invention including features for scavenging the required input power for the sensing arrangement from the loop itself, and particularly from the control device.... In this embodiment the sensing is provided at the upper part of the circuit as illustrated.

This second embodiment is again illustrated in relation to a control loop 30 providing for a control interface from a control device 32 to a field device 34, and which loop 30 includes a surge arrester such as, but not limited to, a TVS 38 which as with the embodiment of Fig. 1 is connected to a sensing arrangement comprising a resistor 38 series-connected to the TVS 38, a back-to-back diode pair 40, 42 connected in parallel to the resistor 38 and serving to limit the voltage arising across the resistor 38 and so deliver the fuli surge current to the TVS 36, and a current sensor 44 arranged to determine the error current flowing through the TVS 36 and thus the resistor 38. The sensing and surge protection operation of this further embodiment follows that described in relation to the previous embodiments.

However, and as illustrated, additional circuit elements are included by means of which the sensing arrangement obtains power, effectively through scavenging, from the loop 30 and particularly from the control device 32 side of the loop 30.

These further elements comprise a voltage dropping arrangement 46 including series- connected diodes 48, 50 serving to provide for voltage dropping to create a potential difference for powering the monitor and connected in the loop 30. A further diode 52 is included to provide for protection and to complete the scavenging circuit while allowing any surge current through, irrespective of its polarity. This configuration of diodes has no undesired effect on the surge-arresting operation.

Indicating means 54 allows for the user to monitor the state of the TVS 38. As noted, such indicating means can comprise any appropriate component whether providing an audible, visual, radio, electrical signal indication of the monitoring or Indeed any combination thereof. In one specific example, the indicating means can comprise and LED arranged either to turn-on, turn-off or vary its output, in order to provide the required indication of the status/result of the monitoring.

Turning now to Fig. 3, there is illustrated a further embodiment of the present invention again including features for scavenging the required input power for the sensing arrangement from the loop itself and particularl from the control device side. However, as compared with the second embodiment illustrated in Fig.2, the embodiment of Fig.3 employs sensing within the lower part of the circuit as illustrated.

This further embodiment is again illustrated in relation to a control loop 80 providing for a control interface from a control device 62 to a field device 84, and which loop 80 includes a surge arrester such as, but not limited to, a TVS 66 which as with the pervious embodiments is connected to a sensing arrangement comprising a resistor 88 series-connected to the TVS 68, a back-to-back diode pair 70, 72 connected in parallel to the resistor 68 and serving to limit the voltage arising across the resistor 88 and so deliver the full surge current to the TVS 88, and a current sensor 74 arranged to determine the error current flowing through the TVS 88 and thus the resistor 88. The sensing and surge protection operation of this further embodiment follows that described in relation to the previous embodiments.

However, and as illustrated, additional circuit elements are included by means of which the sensing arrangement obtains power, effectively through scavenging, from the loop 80 and particularly from the control device 82 of the loop 80.

These further elements comprise a voltage-dropping arrangement 76 including series- connected diodes 78, 80 and 82 serving to provide for voltage dropping to create a potential difference for powering the monitor and connected in the power supply line from the voltage dropping arrangement 76 to the current sensor 74. Further diodes 84 and 88 are included to provide for backward surge protection and so provide surge protection irrespective of polarity. This configuration of diodes has no undesired effect on the surge-arresting operation.

Indicating means 88 allows for the user to monitor the state of the TVS 66, As noted, such indicating means can comprise any appropriate component whether providing an audible, visual, radio, electrical signal indication of the monitoring or indeed any combination thereof, in one specific example, the indicating means can comprise and LED arranged either to turn-on, turn-off or vary its output, in order to provide the required indication of the status/result of the monitoring. However, it should be appreciated that as one possible alternative, an external power supply can be provided to power the control/sense/indication circuitry of the monitor.

Such design considerations can prove particuiarly advantageous in the scavenging of power from the loop for the diagnostic circuitry of the invention, and without affecting the accuracy of the control loop.

As will therefore be appreciated, the present invention provides for a practical solution for protecting circuitry in employing a voltage-limiting device, and in particular in a manner detecting small error currents when the voltage-limiting device has partially failed, and in a manner that will not prevent, for example, a surge arresting from absorbing significant energy when, for example, a lightning strike occurs.

The present invention can therefore serve to provide for an indication of deterioration in the voltage-limiting device, and subsequently also complete failure of the device. Such indication can be provided by an appropriate means such as, but not limited to, electrical signal, audio, visual, radio, and relay means. One particular example comprises one or more LED indicators. Respective devices can be provided for each required different indication, i.e. deterioration or failure, or a common device can be employed and arranged to provide different output patterns responsive to the state, i.e. deterioration or failure, of the device.