WO1990011532A1 - Monitoring electric cables - Google Patents

Monitoring electric cables Download PDF

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Publication number
WO1990011532A1
WO1990011532A1 PCT/GB1990/000462 GB9000462W WO9011532A1 WO 1990011532 A1 WO1990011532 A1 WO 1990011532A1 GB 9000462 W GB9000462 W GB 9000462W WO 9011532 A1 WO9011532 A1 WO 9011532A1
Authority
WO
WIPO (PCT)
Prior art keywords
cable
leakage current
component
capacitive
fault
Prior art date
Application number
PCT/GB1990/000462
Other languages
French (fr)
Inventor
John Bottomley
Original Assignee
Raychem Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Raychem Limited filed Critical Raychem Limited
Publication of WO1990011532A1 publication Critical patent/WO1990011532A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/50—Testing of electric apparatus, lines, cables or components for short-circuits, continuity, leakage current or incorrect line connections
    • G01R31/58—Testing of lines, cables or conductors

Definitions

  • This invention relates to the monitoring of electric cables, and in particular to a method of and an arrangement for monitoring the condition, for example the continuity, of an electric cable, which may be a heater cable, such as a self- regulating heater cable.
  • Self-regulating electric heater cables for example, are available from Raychem under the trade name AUTOTRACE, ' and ' comprise a pair of spaced apart elongate metal conductors or bus bars embedded parallel with one another in a polymeric matrix whose conductivity varies with temperature such that it becomes insulating, and thus stops functioning as a heater, when it reaches a certain temperature.
  • Such heater cables, and also conventional constant wattage heater cables find application, for example, in heating fluids flowing along pipelines in the petroleum and process industries. Clearly it is of interest to ensure that the good condition, for example the continuity, of the cable is retained, and several techniques exist for monitoring this.
  • One such technique involves connecting an electrical lamp across the bus bars at the end of the cable remote from the end at which power is supplied to the heater; should a break • occur in one or the other of the bus bars at any point along its length this will be indicated by the lamp being extinguished.
  • Another technique employs a capacitor, rather than a lamp, and a detection circuit at the power supply end of the heater is arranged to indicate a fault whenever loss of the capacitance current due to the capacitor at the end of the heater cable is detected - due to a break somewhere " -. ' . along its length for example.
  • Other more sophisticated techniques exist. However, there remains a requirement for .
  • the invention is based on the realisation that analysing the capacitive and resistive components of the earth leakage current separately, useful information can be discovered about the condition of the cable.
  • a given electric cable of uniform construction especially a single phase cable which may be a heater cable, has a capacitive component of its leakage current which is a function of the length of the cable and of the voltage and frequency of the power supply.
  • the variation of leakage current with the power supply voltage and frequency is comparatively minor and can be compensated for, so that the existence and loca ⁇ tion along the length of a fault in the cable ⁇ - can be deter ⁇ mined by comparing the actual capacitive leakage current during operation with the previously measured inherent capa ⁇ citive leakage current of the complete length of the cable when in good working order - advantageously being determined for each cable individually immediately after installation on site.
  • a method of monitoring the condition of an electric cable in which leakage current flows between a live or neutral conductor of the cable and an associated component at earth potential comprising the steps of determining the magnitude of the capacitive and/or resistive component of the leakage current relative to * a predetermined magnitude of said component, and providing an ' - indication of a fault condition in the cable in the event . that said relative magnitude differs from zero.
  • a monitoring arrangement suitable for use in the method of said one aspect of the invention, and being arranged to be electrically interconnected between an electrical power supply and the electric cable, the arrange ⁇ ment comprising means arranged to measure the capacitive and/or resistive component of leakage current flowing bet ⁇ ween the live or neutral conductor of the cable and an asso ⁇ ciated component at earth potential, means arranged to compare said leakage current component with a predetermined leakage current value, and means arranged to indicate the existance of a fault condition in the event that the output of the comparison means differs from zero, or differs by a significant amount from zero.
  • the invention may include, when considering the capacitive component of leakage current, providing an indication of the location of the fault, for example by having a display gra ⁇ duated in distance from one end of the cable.
  • the capa ⁇ citive leakage current is directly proportional to the cable length - this assumes that the supply frequency is substan ⁇ tially constant (using a mains power supply for example) and that the voltage supply is substantially constant or other ⁇ wise is compensated for.
  • the electronic circuitry may be arranged to effect a certain amount of smoothing of the signal received from the cable to ensure that what is acted upon is truly representative of a fault condition.
  • smoothing can, for example, remove the effect of any slight variation in the mains supply voltage, and can also comprise a time averaging device to avoid any potentially disturbing and irrelevant transient signals that may last for a few cycles.
  • the inherent leakage current components of the cable may be determined immediately subsequent to its manufacture, but advantageously is done for a completed cable, i.e. with ter ⁇ minating end fittings already attached, and preferably after its installation on site, for a more accurate measurement. This value may then be stored in a comparator circuit for use continuously, intermittently or on demand during sub ⁇ sequent operation of the cable. A reduced value of the capacitive leakage current during operation would indicate that the effective length of the cable had decreased i.e. there was break in it.
  • the fault detecting means may be arranged to be latched, so that the existence of a transient fault that has passed may be indicated.
  • the magnitude of the capacitive component of the leakage current may be measured by any suitable known means, for example a current transformer, which advantageously has an amplifier asso ⁇ ciated therewith.
  • Other components of the detecting, . measuring, comparison and indicating means may also be con ⁇ ventional and well-known to those skilled in the art.
  • Means other than a current transformer for detecting the return current indicating the condition of the ' cable include, for example sensors that detect the Hall Effect, Peltier Effect, Thompson Effect or the Magnetoresistive Effect. In these instances, however, some amplification of the detected current may be necessary. As a further alter ⁇ native, the voltage drop may be measured across an in-line resistor that conducts the monitored current signal.
  • the current that is monitored is the sum of the residual capacitive and resistive currents flowing in the cable, and the capacitive component of the current may be enhanced if a. capacitor is arranged to interconnect the line and neutral conductors with the earthed component (for example a braid of the cable) at the extreme end of the cable. Having detected the current signal that monitors the con ⁇ dition of the cable, it is then amplified and filtered so as to remove, for example, any irrelevant transient components. Subsequently, feedback circuits may be employed to enhance the monitoring.
  • One such feedback circuit may be arranged to add in a negative resistive component of current (i.e. one that is 180° out of phase) in order to cancel out the resistive component present in the monitored signal.
  • a negative capacitive component of current i.e. one that is 270° out of phase
  • An inductive component of current may be added in, as an alternative or as an addi ⁇ tion, in order to cancel the out-of-phase capacitive com ⁇ ponent.
  • Further electronic treatment of the detected monitored signal may include the use of a sine or cosine function in order to scale the magnitude of the total value of the moni ⁇ tored signal.
  • the monitored current signal may also be sampled at a par ⁇ ticular stage of its cycle, for example at exactly 90" in advance of the mains current by means of a “sample and hold” using a short gate.
  • the absolute value of the capacitive component of the current is then detected if the phase angle is exaclty 90°.
  • Improved sampling can however be achieved by allowing for the fact that the phase angle is not exactly” equal to 90°, by using a "resolver” circuit whereby the "zero crossover point" of the return signal is detected and a sample is then taken at the midpoint of two consecutive crossovers.
  • the comparison of the monitored current may be made with a predetermined magnitude that is not fixed in absolute terms.
  • the predetermined magni ⁇ tude may be that of the last reading that was sampled, or with a last average magnitude that had been calculated.
  • the use of such an "ongoing" predetermined magnitude would, for example, compensate for the (normally relatively slower) changes in the resistive component of the monitored signal without having to eliminate them completely, and would also enable detection of the (normally relatively faster) changes in the capacitive component of the monitored current asso ⁇ ciated with damage to or discontinuity in the cable.
  • the fault condition indicator a meter or indicator lamp for example, could be located remote from the heater itself, and that, for example, a display panel in a control room could have fault indicators for a plurality of cables being monitored.
  • the component at earth potential that is associated with the live and neutral conductors of the cable may conveniently be an earth conductor of the cable, and may for example be an earth braid that circumferentially encloses the other conductors.
  • the capacitive earth leakage current is within the range of 1mA to 4mA, and detection of a change of 0.1mA is quite feasible. For typical lengths of self-regulating heater cable of 30m to 150m, this results in location of a fault to within about 2m to 7m - the capacitive component of the leakage current being of the order of tens of microamps.
  • a single phase 240 V a.c, 50 Hz mains power supply 2 has its live L and neutral N outputs connected to an internal power supply 4 of a cable continuity monitoring arrange ⁇ ment 6.
  • the internal power supply 4 is arranged to deliver ⁇ 15 V d.c. to the components of the monitor 6 to be described later.
  • the earth output E of the mains supply 2 is connected to one end of the primary coil of a current transformer 8, which has an amplifier associated therewith.
  • the other end of the primary coil of the transformer 8 is connected to the earthing braid 10 of a heater cable 12.
  • the cable 12 is a self-regulating electric heater cable type 20 QTV2-CT available from Raychem under the trade name AUTOTRACE.
  • the heater cable 12 comprises two bus bar metal electrodes 14,16 that are connected to the live and neutral outputs respectively of the mains power supply 2.
  • the bus bars 14,16 extend parallel with each other and are embedded in a semi-conductive polymeric matrix 18 that provides the heating medium of the cable and this is contained with an inner insulating jacket 20 around which the braid 10 extends. Finally the cable is enclosed within an outer insulating and protective jacket 22.
  • earth leakage current flowing to the cable braid 10 from either of the live or neutral conductors 14,16 passes through the primary coil of the transformer 8 of the monitor 6.
  • the signal representative of the '* earth leakage current detected by the current transformer 8 is amplified and converted to a voltage signal and then supplied to a low pass filter 24 that eliminates any high frequency components such as may arise from variations of the leakage current due to the fluctuations in the mains supply voltage.
  • the output signal from the filter 24 is then passed to a phase discriminating arrangement 26 that pro ⁇ vides two outputs, Ic representative of the capacitive com ⁇ ponent of the earth leakage current and IR representative of the resistive component of the earth leakage current.
  • This signal may be multiplexed through the subsequent com ⁇ ponents of the monitor or may be supplied to separate sets of corresponding components.
  • This signal is supplied to a full wave rectifier 28, thence to a smoothing circuit 30 and after passing through an amplifier 32 is supplied to one input of a comparator cir ⁇ cuit 34.
  • the other input of the comparator 34 receives a signal from a source 36 that is a reference voltage repre ⁇ sentative of the normal or inherent capacitive earth leakage current of the cable 12, this reference signal having been determined subsequent to installation of the heater cable (wrapped around a pipeline earring petroleum products for example) and in the absence of any fault in the cable. Any difference between the input signals is then supplied to an indicator 38, which may consist of one or more LEDs or other digital or analogue indicator.
  • a lower limit to the sensitivity of the monitor 6 may be set at a suitable part thereof, for example by arranging for the indicator 38 to respond only to signals above a threshhold level.
  • the monitoring of the present invention may be combined with a conventional residual current circuit- breaker.
  • An RCCB is arranged to operate and isolate the electric cable from the power supply typically when the earth leakage current exceeds about 30 mA, and thus would respond to leakage current of an order of magnitude higher than the monitor of the present invention.
  • Operation of the monitor of the invention in respect of capacitive leakage current can be used to determine the location along the length of the cable at which a break has occurred.
  • the indicator 38 for example may be arranged to display distance from an end of the cable.
  • Operation of the monitor of the invention in respect of resistive leakage current can be used to determine whether moisture or other liquid has entered the cable either at a termination or at a position along its length by absorption through the material of the cable or through a break in the cable outer jacket.
  • Such a state of affairs would either alter the resistance of the earth current leakage path directly by the introduction of a material of different con ⁇ ductivity, or would do so by altering the dielectric constant of a material in that path.
  • the value of the capacitive current is dependent on (and in particular increases with) its fre ⁇ quency, so that by injecting signals of different frequency (within a range, say, of several hundred KHz to several MHz) is is possible to distinguish different kinds of fault in the cable at the located fault position.
  • a further method of monitoring the condition of the cable is to inject a high frequency signal into one or both of the con ⁇ ductors, and to monitor the level of the return current, which will be predominantly capacitive.
  • This aspect of the invention relies on isolating the mains supply to the con ⁇ ductors for a period of one or several cycles while the high frequency signal is injected, and on receiving the capaciti- vely coupled return current during the same short interrup- * - tion to the mains supply.
  • a current transformer suitably '• chosen for the frequencies involved is used to detect the • - flow of the return current in the braid or earth return con ⁇ ductor of the cable.
  • an initial value for the return current, set after calibration on installation will create the set- point from which any deviation will indicate a discontinuity in the cable.
  • the changes in resistive component which have to be allowed for in the mains frequency approach discribed previously will not be an issue in this alternative method, since the return current is predominantly capacitive.
  • compensation for cable temperature changes may have to be allowed, for when interpreting the return signal strength.
  • the time interval between signal injections can be chosen to suit the application.
  • the enhanced capacitive coupling between the (earthed) braid and the other conductors of the cable means that the alternative method is particularly suitable for monitoring the condition of a branch cable, in which the return signal may be comparatively weak.
  • a time domain reflectometer would be employed to indicate a discontinuity or change in attenuation in the cable, and in different monitoring circumstances such as those of branched cables, computer aided signal or pattern comparison may be required.
  • treatment of the monitored higher frequency signal may be carried out as hereinbefore described.
  • the frequency of the injected signal needs to be significantly higher than that (usually mains frequency) normally applied to the live conductor for • normal operation of the cable, and would usually be at least- IKHz, and preferably at least lOOKHz.

Abstract

A self-regulating heater cable (12) is monitored for detection of any damage thereto or discontinuity therein. The monitoring consists in comparing the value of the capacitive component of the earth leakage current following in the cable (12) between its live or neutral conductors (14, 16) and its earthed braid (10) at any one time with a predetermined value that itself is indicative of an intact cable. The value of the capacitive component of the earth leakage current is used to identify the location of any fault along the length of the cable (12).

Description

MONITORING ELECTRIC CABLES
This invention relates to the monitoring of electric cables, and in particular to a method of and an arrangement for monitoring the condition, for example the continuity, of an electric cable, which may be a heater cable, such as a self- regulating heater cable.
Self-regulating electric heater cables, for example, are available from Raychem under the trade name AUTOTRACE,' and' comprise a pair of spaced apart elongate metal conductors or bus bars embedded parallel with one another in a polymeric matrix whose conductivity varies with temperature such that it becomes insulating, and thus stops functioning as a heater, when it reaches a certain temperature. Such heater cables, and also conventional constant wattage heater cables, find application, for example, in heating fluids flowing along pipelines in the petroleum and process industries. Clearly it is of interest to ensure that the good condition, for example the continuity, of the cable is retained, and several techniques exist for monitoring this. One such technique involves connecting an electrical lamp across the bus bars at the end of the cable remote from the end at which power is supplied to the heater; should a break • occur in one or the other of the bus bars at any point along its length this will be indicated by the lamp being extinguished. Another technique employs a capacitor, rather than a lamp, and a detection circuit at the power supply end of the heater is arranged to indicate a fault whenever loss of the capacitance current due to the capacitor at the end of the heater cable is detected - due to a break somewhere "-.' . along its length for example. Other more sophisticated techniques exist. However, there remains a requirement for. a cable condition monitoring technique that is simple, reliable, inexpensive, and preferably one that can also allow the location of the fault along the cable to be iden¬ tified. UK Patent No. 587137 and US Patent No. 3 700 966 disclose measurement of capacitive currents to check the integrity of the insulation of an electric cable, however neither of those specifications indicates how location of the fault along the length of the cable may be determined.
The invention is based on the realisation that analysing the capacitive and resistive components of the earth leakage current separately, useful information can be discovered about the condition of the cable. For example, a given electric cable of uniform construction, especially a single phase cable which may be a heater cable, has a capacitive component of its leakage current which is a function of the length of the cable and of the voltage and frequency of the power supply. The variation of leakage current with the power supply voltage and frequency is comparatively minor and can be compensated for, so that the existence and loca¬ tion along the length of a fault in the cable ■- can be deter¬ mined by comparing the actual capacitive leakage current during operation with the previously measured inherent capa¬ citive leakage current of the complete length of the cable when in good working order - advantageously being determined for each cable individually immediately after installation on site.
Thus, in accordance with one aspect of the present inven¬ tion, there is provided a method of monitoring the condition of an electric cable in which leakage current flows between a live or neutral conductor of the cable and an associated component at earth potential, the method comprising the steps of determining the magnitude of the capacitive and/or resistive component of the leakage current relative to* a predetermined magnitude of said component, and providing an'- indication of a fault condition in the cable in the event . that said relative magnitude differs from zero.
In accordance with another aspect of the present invention, there is provided a monitoring arrangement suitable for use in the method of said one aspect of the invention, and being arranged to be electrically interconnected between an electrical power supply and the electric cable, the arrange¬ ment comprising means arranged to measure the capacitive and/or resistive component of leakage current flowing bet¬ ween the live or neutral conductor of the cable and an asso¬ ciated component at earth potential, means arranged to compare said leakage current component with a predetermined leakage current value, and means arranged to indicate the existance of a fault condition in the event that the output of the comparison means differs from zero, or differs by a significant amount from zero.
The invention may include, when considering the capacitive component of leakage current, providing an indication of the location of the fault, for example by having a display gra¬ duated in distance from one end of the cable. In this --■ respect it should be noted that for a given cable, the capa¬ citive leakage current is directly proportional to the cable length - this assumes that the supply frequency is substan¬ tially constant (using a mains power supply for example) and that the voltage supply is substantially constant or other¬ wise is compensated for.
The electronic circuitry may be arranged to effect a certain amount of smoothing of the signal received from the cable to ensure that what is acted upon is truly representative of a fault condition. Such smoothing can, for example, remove the effect of any slight variation in the mains supply voltage, and can also comprise a time averaging device to avoid any potentially disturbing and irrelevant transient signals that may last for a few cycles.
The inherent leakage current components of the cable may be determined immediately subsequent to its manufacture, but advantageously is done for a completed cable, i.e. with ter¬ minating end fittings already attached, and preferably after its installation on site, for a more accurate measurement. This value may then be stored in a comparator circuit for use continuously, intermittently or on demand during sub¬ sequent operation of the cable. A reduced value of the capacitive leakage current during operation would indicate that the effective length of the cable had decreased i.e. there was break in it.
The fault detecting means may be arranged to be latched, so that the existence of a transient fault that has passed may be indicated.
The magnitude of the capacitive component of the leakage current, identifiable since its phase angle leads that of the mains supply voltage by approximately 90°, may be measured by any suitable known means, for example a current transformer, which advantageously has an amplifier asso¬ ciated therewith. Other components of the detecting, . measuring, comparison and indicating means may also be con¬ ventional and well-known to those skilled in the art.
Means other than a current transformer for detecting the return current indicating the condition of the' cable include, for example sensors that detect the Hall Effect, Peltier Effect, Thompson Effect or the Magnetoresistive Effect. In these instances, however, some amplification of the detected current may be necessary. As a further alter¬ native, the voltage drop may be measured across an in-line resistor that conducts the monitored current signal.
The current that is monitored is the sum of the residual capacitive and resistive currents flowing in the cable, and the capacitive component of the current may be enhanced if a. capacitor is arranged to interconnect the line and neutral conductors with the earthed component (for example a braid of the cable) at the extreme end of the cable. Having detected the current signal that monitors the con¬ dition of the cable, it is then amplified and filtered so as to remove, for example, any irrelevant transient components. Subsequently, feedback circuits may be employed to enhance the monitoring. One such feedback circuit may be arranged to add in a negative resistive component of current (i.e. one that is 180° out of phase) in order to cancel out the resistive component present in the monitored signal. In this way, and changes in the monitored resistive component do not affect measurement of the capacitive component. Alternatively, a negative capacitive component of current (i.e. one that is 270° out of phase) may be added in in order to cancel out the capacitive component present in the monitored signal. In this way any changes in the monitored capacitive component will not affect measurement of the resistive component, which is then used as the indication of the condition of the cable. An inductive component of current may be added in, as an alternative or as an addi¬ tion, in order to cancel the out-of-phase capacitive com¬ ponent.
Further electronic treatment of the detected monitored signal may include the use of a sine or cosine function in order to scale the magnitude of the total value of the moni¬ tored signal.
The monitored current signal may also be sampled at a par¬ ticular stage of its cycle, for example at exactly 90" in advance of the mains current by means of a "sample and hold" using a short gate. The absolute value of the capacitive component of the current is then detected if the phase angle is exaclty 90°. Improved sampling can however be achieved by allowing for the fact that the phase angle is not exactly" equal to 90°, by using a "resolver" circuit whereby the "zero crossover point" of the return signal is detected and a sample is then taken at the midpoint of two consecutive crossovers. It is also envisaged that the comparison of the monitored current may be made with a predetermined magnitude that is not fixed in absolute terms. Thus, the predetermined magni¬ tude may be that of the last reading that was sampled, or with a last average magnitude that had been calculated. The use of such an "ongoing" predetermined magnitude would, for example, compensate for the (normally relatively slower) changes in the resistive component of the monitored signal without having to eliminate them completely, and would also enable detection of the (normally relatively faster) changes in the capacitive component of the monitored current asso¬ ciated with damage to or discontinuity in the cable.
It is envisaged that the fault condition indicator, a meter or indicator lamp for example, could be located remote from the heater itself, and that, for example, a display panel in a control room could have fault indicators for a plurality of cables being monitored.
The component at earth potential that is associated with the live and neutral conductors of the cable may conveniently be an earth conductor of the cable, and may for example be an earth braid that circumferentially encloses the other conductors.
Typically, the capacitive earth leakage current is within the range of 1mA to 4mA, and detection of a change of 0.1mA is quite feasible. For typical lengths of self-regulating heater cable of 30m to 150m, this results in location of a fault to within about 2m to 7m - the capacitive component of the leakage current being of the order of tens of microamps.
Although the invention is applicable to electric cables in general, of uniform construction such that the capacitance per unit length is substantially uniform throughout its length, it is particularly useful with self- egulating heater cables, for example those available from Raychem under it CHEMELEX and AUTOTRACE trade names. Method of and apparatus for monitoring an electric cable, each in accordance with the present invention, will now be described, by way of example, with reference to the accom¬ panying drawing which shows schematically a monitoring cir¬ cuit interconnected between a power supply and an electric heater cable.
A single phase 240 V a.c, 50 Hz mains power supply 2 has its live L and neutral N outputs connected to an internal power supply 4 of a cable continuity monitoring arrange¬ ment 6. The internal power supply 4 is arranged to deliver ± 15 V d.c. to the components of the monitor 6 to be described later. The earth output E of the mains supply 2 is connected to one end of the primary coil of a current transformer 8, which has an amplifier associated therewith. The other end of the primary coil of the transformer 8 is connected to the earthing braid 10 of a heater cable 12. The cable 12 is a self-regulating electric heater cable type 20 QTV2-CT available from Raychem under the trade name AUTOTRACE. The heater cable 12 comprises two bus bar metal electrodes 14,16 that are connected to the live and neutral outputs respectively of the mains power supply 2. The bus bars 14,16 extend parallel with each other and are embedded in a semi-conductive polymeric matrix 18 that provides the heating medium of the cable and this is contained with an inner insulating jacket 20 around which the braid 10 extends. Finally the cable is enclosed within an outer insulating and protective jacket 22.
Thus, earth leakage current flowing to the cable braid 10 from either of the live or neutral conductors 14,16 passes through the primary coil of the transformer 8 of the monitor 6. Within the monitor 6, the signal representative of the '* earth leakage current detected by the current transformer 8 is amplified and converted to a voltage signal and then supplied to a low pass filter 24 that eliminates any high frequency components such as may arise from variations of the leakage current due to the fluctuations in the mains supply voltage. The output signal from the filter 24 is then passed to a phase discriminating arrangement 26 that pro¬ vides two outputs, Ic representative of the capacitive com¬ ponent of the earth leakage current and IR representative of the resistive component of the earth leakage current. These two signals may be multiplexed through the subsequent com¬ ponents of the monitor or may be supplied to separate sets of corresponding components. For convenience reference herein will be made only to the capacitive component I . This signal is supplied to a full wave rectifier 28, thence to a smoothing circuit 30 and after passing through an amplifier 32 is supplied to one input of a comparator cir¬ cuit 34. The other input of the comparator 34 receives a signal from a source 36 that is a reference voltage repre¬ sentative of the normal or inherent capacitive earth leakage current of the cable 12, this reference signal having been determined subsequent to installation of the heater cable (wrapped around a pipeline earring petroleum products for example) and in the absence of any fault in the cable. Any difference between the input signals is then supplied to an indicator 38, which may consist of one or more LEDs or other digital or analogue indicator.
It will be appreciated that a lower limit to the sensitivity of the monitor 6 may be set at a suitable part thereof, for example by arranging for the indicator 38 to respond only to signals above a threshhold level.
It is envisaged that the monitoring of the present invention may be combined with a conventional residual current circuit- breaker. An RCCB is arranged to operate and isolate the electric cable from the power supply typically when the earth leakage current exceeds about 30 mA, and thus would respond to leakage current of an order of magnitude higher than the monitor of the present invention. Operation of the monitor of the invention in respect of capacitive leakage current can be used to determine the location along the length of the cable at which a break has occurred. To this end, the indicator 38 for example may be arranged to display distance from an end of the cable.' Operation of the monitor of the invention in respect of resistive leakage current can be used to determine whether moisture or other liquid has entered the cable either at a termination or at a position along its length by absorption through the material of the cable or through a break in the cable outer jacket. Such a state of affairs would either alter the resistance of the earth current leakage path directly by the introduction of a material of different con¬ ductivity, or would do so by altering the dielectric constant of a material in that path.
It has been found that the value of the capacitive current is dependent on (and in particular increases with) its fre¬ quency, so that by injecting signals of different frequency (within a range, say, of several hundred KHz to several MHz) is is possible to distinguish different kinds of fault in the cable at the located fault position.
Thus a further method of monitoring the condition of the cable, in accordance with the present incention, is to inject a high frequency signal into one or both of the con¬ ductors, and to monitor the level of the return current, which will be predominantly capacitive. This aspect of the invention relies on isolating the mains supply to the con¬ ductors for a period of one or several cycles while the high frequency signal is injected, and on receiving the capaciti- vely coupled return current during the same short interrup- *- tion to the mains supply. A current transformer, suitably '• chosen for the frequencies involved is used to detect the •- flow of the return current in the braid or earth return con¬ ductor of the cable. In similar manner to the method pre- viously discussed, an initial value for the return current, set after calibration on installation, will create the set- point from which any deviation will indicate a discontinuity in the cable. The changes in resistive component which have to be allowed for in the mains frequency approach discribed previously will not be an issue in this alternative method, since the return current is predominantly capacitive. Depending on the cable characteristics, compensation for cable temperature changes may have to be allowed, for when interpreting the return signal strength. The time interval between signal injections can be chosen to suit the application.
The enhanced capacitive coupling between the (earthed) braid and the other conductors of the cable (for example the bus bar conductors of a braided self-regulating heater cable), means that the alternative method is particularly suitable for monitoring the condition of a branch cable, in which the return signal may be comparatively weak. Typically, a time domain reflectometer would be employed to indicate a discontinuity or change in attenuation in the cable, and in different monitoring circumstances such as those of branched cables, computer aided signal or pattern comparison may be required.
It will be understood that treatment of the monitored higher frequency signal may be carried out as hereinbefore described.
It will be appreciated that the frequency of the injected signal needs to be significantly higher than that (usually mains frequency) normally applied to the live conductor for • normal operation of the cable, and would usually be at least- IKHz, and preferably at least lOOKHz.

Claims

CLAIMS :
1. A method of monitoring the condition of an electric cable in which leakage current flows between a live or neutral conductor of the cable and an associated component at earth potential, the method comprising the steps of determining the magnitude of the capacitive and/or resistive component of the leakage current relative to a predetermined magnitude of said component, and providing an indication of the existence and location of a fault condition in the cable in the event that said relative magnitude differs from zero.
2. A method according to Claim 1, wherein the capacitive component of the leakage current is measured, and wherein the location along the length of the cable at which the fault exists is indicated.
3. A method according to Claim 1 or Claim 2,' comprising the step of initially determining the inherent capacitive and/or resistive components of leakage current of the cable thereby to establish said predetermined magnitude, and moni¬ toring the value of the leakage current components relative thereto during subsequent operation of the cable.
4. A method according to any preceding claim, comprising the step of applying to the cable a signal at a frequency that is significantly above that of the normal signal applied to the live conductor, and detecting a signal of the higher frequency in the component of the leakage current, thereby to enhance the monitoring of the condition of the cable.
5. A method according to any preceding claim, wherein in •. the event of a fault condition, whether temporary or per- - manent, an indicator is arranged to indicate that a fault has occurred.
6. A method according to any preceding claim, wherein the electric cable comprises a heater cable, preferably a self- regulating heater cable.
7. A monitoring arrangement suitable for use in the method of any preceding claim, and being arranged to be electri¬ cally interconnected between an electrical power supply and the electric cable, the arrangement comprising means arranged to measure the capacitive and/or resistive com¬ ponent of leakage current flowing between the live or neutral conductor of the cable and an associated component at earth potential, means arranged to compare said leakage current component with a predetermined leakage current value, and means arranged to indicate the existence of a fault condition in the event that the output of the com¬ parison means differ from zero.
8. An arrangement according to Claim 7, wherein the measuring means comprises a current transformer.
9. An arrangement according to Claim 7,or 8, wherein the indicating means, until reset, is arranged to continue to indicate that a fault condition exists subsequent to the passing of the fault condition.
10. An arrangement according to any one of Claims 7 to 9, wherein the indicating means is arranged to provide an indi¬ cation-,-in the event of a fault condition, "at a location remote from the electric cable.
11. An arrangement according to any one of Claims 7 to 10, wherein said associated component comprises an earthing conductor of the cable.
12. An arrangement according to any one of Claims 7 to 11 and an electric cable, preferably a heater cable, electri¬ cally connected thereto.
13. An arrangement according to any one of Claims 7 to 12, and an earth leakage current breaker that is arranged to interrupt supply of electrical power to the" cVble in the event that the total earth leakage current exceeds a prede¬ termined value.
* * * * * * * * * *
PCT/GB1990/000462 1989-03-28 1990-03-28 Monitoring electric cables WO1990011532A1 (en)

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US6084207A (en) * 1998-01-19 2000-07-04 Msx, Inc. Method and apparatus for using direct current to detect ground faults in a shielded heater wire
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GB2358934A (en) * 2000-02-05 2001-08-08 Smiths Group Plc Cable Testing
WO2007022087A3 (en) * 2005-08-15 2007-05-18 Univ Denver Testing procedure for evaluating diffusion and leakage currents in insulators
WO2008019446A1 (en) * 2006-08-18 2008-02-21 Aurora Energy Pty Ltd Method and apparatus for detecting a fault in a supply line
EP3379263A1 (en) * 2017-03-24 2018-09-26 Rosemount Aerospace Inc. Probe heater remaining useful life determination
EP3379265A1 (en) * 2017-03-24 2018-09-26 Rosemount Aerospace Inc. Probe heater remaining useful life determination
CN108710016A (en) * 2018-03-01 2018-10-26 华南理工大学 A kind of computational methods of the single-core cable distributed electrical capacitance current of the golden cloth containing insulation
US10180449B2 (en) 2017-03-24 2019-01-15 Rosemount Aerospace Inc. Probe heater remaining useful life determination
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US10564203B2 (en) 2017-03-24 2020-02-18 Rosemount Aerospace Inc. Probe heater remaining useful life determination
EP3667330A1 (en) * 2018-12-14 2020-06-17 Rosemount Aerospace Inc. Electric arc detection for probe heater phm and prediction of remaining useful life
US10914777B2 (en) 2017-03-24 2021-02-09 Rosemount Aerospace Inc. Probe heater remaining useful life determination
US11060992B2 (en) 2017-03-24 2021-07-13 Rosemount Aerospace Inc. Probe heater remaining useful life determination
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EP4067910A1 (en) * 2021-03-30 2022-10-05 Rosemount Aerospace Inc. Predicting failure and/or estimating remaining useful life of an air-data-probe heater
EP4067909A1 (en) * 2021-03-30 2022-10-05 Rosemount Aerospace Inc. Predicting failure and/or estimating remaining useful life of an air-data-probe heater
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AU724559B2 (en) * 1997-05-14 2000-09-28 Canon Kabushiki Kaisha Photovoltaic power generation apparatus
WO1998058269A1 (en) * 1997-06-17 1998-12-23 Siemens Aktiengesellschaft Method and device for monitoring a cable
DE19725611A1 (en) * 1997-06-17 1999-01-28 Siemens Ag Monitoring method and monitoring device for a cable
DE19725611C2 (en) * 1997-06-17 2001-03-08 Siemens Ag Monitoring method and monitoring device for a cable
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US6218647B1 (en) * 1998-01-19 2001-04-17 Msx, Inc. Method and apparatus for using direct current to detect ground faults in a shielded heater wire
GB2358934A (en) * 2000-02-05 2001-08-08 Smiths Group Plc Cable Testing
GB2358934B (en) * 2000-02-05 2003-11-19 Smiths Group Plc Cable testing
WO2007022087A3 (en) * 2005-08-15 2007-05-18 Univ Denver Testing procedure for evaluating diffusion and leakage currents in insulators
WO2008019446A1 (en) * 2006-08-18 2008-02-21 Aurora Energy Pty Ltd Method and apparatus for detecting a fault in a supply line
AU2007283998B2 (en) * 2006-08-18 2010-12-02 Aurora Energy Pty Ltd Method and apparatus for detecting a fault in a supply line
US10151785B2 (en) 2017-03-24 2018-12-11 Rosemount Aerospace Inc. Probe heater remaining useful life determination
EP3627157A1 (en) * 2017-03-24 2020-03-25 Rosemount Aerospace Inc. Probe heater remaining useful life determination
US11060992B2 (en) 2017-03-24 2021-07-13 Rosemount Aerospace Inc. Probe heater remaining useful life determination
EP3379263A1 (en) * 2017-03-24 2018-09-26 Rosemount Aerospace Inc. Probe heater remaining useful life determination
US10180449B2 (en) 2017-03-24 2019-01-15 Rosemount Aerospace Inc. Probe heater remaining useful life determination
US10197517B2 (en) 2017-03-24 2019-02-05 Rosemount Aerospace, Inc. Probe heater remaining useful life determination
US10914777B2 (en) 2017-03-24 2021-02-09 Rosemount Aerospace Inc. Probe heater remaining useful life determination
US10564203B2 (en) 2017-03-24 2020-02-18 Rosemount Aerospace Inc. Probe heater remaining useful life determination
EP3614152A1 (en) * 2017-03-24 2020-02-26 Rosemount Aerospace Inc. Probe heater remaining useful life determination
EP3379265A1 (en) * 2017-03-24 2018-09-26 Rosemount Aerospace Inc. Probe heater remaining useful life determination
US10895592B2 (en) 2017-03-24 2021-01-19 Rosemount Aerospace Inc. Probe heater remaining useful life determination
CN108710016B (en) * 2018-03-01 2019-10-18 华南理工大学 A kind of calculation method of the single-core cable distributed electrical capacitance current containing the golden cloth that insulate
CN108710016A (en) * 2018-03-01 2018-10-26 华南理工大学 A kind of computational methods of the single-core cable distributed electrical capacitance current of the golden cloth containing insulation
EP3667330A1 (en) * 2018-12-14 2020-06-17 Rosemount Aerospace Inc. Electric arc detection for probe heater phm and prediction of remaining useful life
US10962580B2 (en) 2018-12-14 2021-03-30 Rosemount Aerospace Inc. Electric arc detection for probe heater PHM and prediction of remaining useful life
US11061080B2 (en) 2018-12-14 2021-07-13 Rosemount Aerospace Inc. Real time operational leakage current measurement for probe heater PHM and prediction of remaining useful life
US11630140B2 (en) 2020-04-22 2023-04-18 Rosemount Aerospace Inc. Prognostic health monitoring for heater
EP4067910A1 (en) * 2021-03-30 2022-10-05 Rosemount Aerospace Inc. Predicting failure and/or estimating remaining useful life of an air-data-probe heater
EP4067909A1 (en) * 2021-03-30 2022-10-05 Rosemount Aerospace Inc. Predicting failure and/or estimating remaining useful life of an air-data-probe heater
US11762040B2 (en) 2021-03-30 2023-09-19 Rosemount Aerospace Inc. Predicting failure and/or estimating remaining useful life of an air-data-probe heater

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