A method of operation of a test meter and a test meter for measuring a resistance of an electrical insulator

Abstract

A test is performed by a test meter (1) for measuring a resistance of an electrical insulator (2), the test comprising providing a test voltage to electrical conductors separated by the electrical insulator. Without applying the test voltage, a discharge path (7) is provided in the test meter (1) between the electrical conductors to discharge a voltage held on the electrical conductors. With the voltage held on the electrical conductors discharged, and without applying the test voltage, a measurement (5) is performed of current flowing through the test meter (1). Based on the measurement of current and on an indication of an intended magnitude of the test voltage, an equivalent parallel resistance value is determined that would appear as an error in a measurement of the resistance of the electrical insulator. An indication of the determined equivalent parallel resistance value for the intended magnitude of the test voltage is displayed (10).

Description

[0001] A Method of Operation of a Test Meter and a Test Meter for Measuring a Resistance of an Electrical Insulator

[0002] Technical Field

[0003] The present invention relates generally to an improved method of operation of a test meter and to a test meter for measuring a resistance of an electrical insulator, and in particular, but not exclusively, to mitigating the effects of depolarisation current on a test.

[0004] Background

[0005] Measurements of electrical resistance of an electrical insulator, for example the insulation of windings of an electric motor or generator or the insulation of a cable, are typically performed periodically, to check the condition of the insulator, which may degrade with time. The measurement may be performed by applying a test voltage to conductors separated by the insulator and measuring the resulting current flowing in the insulator. The insulation resistance may be measured on the basis of the applied test voltage and the resulting current. When a test voltage is applied to an insulator, a current may flow that has a constant part due to the resistive properties of the insulator and also components of the current that decay with time. In particular, a polarisation current may flow due to the polarisation of electric dipoles in the dielectric material of the insulator caused by application of the test voltage. This polarisation current decays with time as the dielectric becomes polarised. This process may have a long time constant, so that current is decaying during a test. In some insulation resistance tests, the polarisation current may be allowed to settle, and then insulation resistance may be measured. In other tests, for example a polarisation index test, the current may be measured against time so that the polarisation current forms part of the test and may be taken into account to assess the condition of the insulator.

[0006] When the test voltage is removed, for example after the test, the polarised insulator may become gradually depolarised, as a depolarisation current flows in the opposite direction to the polarisation current, if the conductors separated by the insulator are connected by a low impedance. If a further insulation resistance test is carried out when the insulator is still polarised following a previous test, the depolarisation current may cause a spurious component to the current measurements for the further test, in some cases dominating the measurements of current. As a result, many test procedures stipulate that a waiting period be imposed between tests to allow a depolarisation current to dissipate, for example requiring a waiting period 4 times the length of the period for which the test voltage was applied for the previous test. Furthermore, in some cases the insulation under test may be in an unknown state of polarisation. The waiting period leads to testing becoming time-consuming, and to operators taking short cuts and risking spurious results. It would be advantageous to reliably reduce the waiting time between insulation resistance tests.

[0007] Summary

[0008] In accordance with a first aspect, there is provided a method of performing a test for measuring a resistance of an electrical insulator, the test comprising providing a test voltage to electrical conductors separated by the electrical insulator, the method being performed by the test meter and the method comprising: without applying the test voltage, providing a discharge path in the meter between the electrical conductors to discharge a voltage held on the electrical conductors; with the voltage held on the electrical conductors discharged, and without applying the test voltage, performing a measurement of current flowing through the test meter; based on the measurement of current and on an indication of an intended magnitude of the test voltage, determining an equivalent parallel resistance value that would appear as an error in a measurement of the resistance of the electrical insulator; and displaying an indication of the determined equivalent parallel resistance value for the intended magnitude of the test voltage.

[0009] Determining the equivalent parallel resistance value that would appear as an error in a measurement of the resistance of the electrical insulator based on the measurement of current and on an indication of an intended magnitude of the test voltage, and displaying an indication of the determined equivalent parallel resistance value for the intended magnitude of the test voltage gives an indication to an operator as to whether to proceed with the test according to the relative magnitude of the determined equivalent parallel resistance and the expected resistance value to be measured.

[0010] In an example, the method comprises continually updating the indication of determined equivalent parallel resistance value during the measurement of current. This feature allows an operator to monitor the change of the relative magnitude of the determined equivalent parallel resistance and the expected resistance value to be measured to identify a time when the test may proceed with an acceptable degree of error.

[0011] In an example, the method comprises, dependent on a signal that the test should proceed to apply the test voltage, applying the test voltage to the electrical insulator and performing a measurement of resistance of the electrical insulator. Applying the test voltage dependent on a signal that the test should proceed prevents the test from preceding unless the signal is received.

[0012] In an example, the method comprises generating the signal that the test should proceed to apply the test voltage in response to an interaction of a user with the test meter. This allows an operator to indicate that the test should proceed, having seen the display of the determined equivalent parallel resistance.

[0013] In an example, the method comprises starting said test dependent on detecting a first type of interaction of the user with the test meter and generating the signal that the test should proceed to apply the test voltage in response to a second type of interaction of the user with the test meter. This feature reduces the probability that an operator starts to apply the test voltage unintentionally. For example, the first type of interaction may be a shorter press of a test button and the second type of interaction may be a longer press of a test button. The test button may be a touch sensitive screen, or an electromechanical switch, for example. In an example, the method comprises generating the signal that the test should proceed to apply the test voltage based on a comparison by the meter of the equivalent parallel resistance value with a threshold value. This feature allows the test to proceed automatically, reducing the waiting time before the test voltage is applied where appropriate, dependent on the relationship between the determined equivalent parallel resistance and the threshold value. The threshold value may be related to an expected value of insulation resistance that the test should be required to measure. The threshold value may be settable by the operator.

[0014] In an example, the method comprises determining a function relating current to time during the measurement of current; and extrapolating the determined function to predict the equivalent parallel resistance value. This feature allows an equivalent parallel resistance value to be predicted based on an expected value of current, such as depolarisation current, after the test voltage is applied.

[0015] In an example, the method comprises determining an expected waiting time for the predicted equivalent parallel resistance value to reach a given value and displaying the expected waiting time. This gives the operator prior warning as to when to start to apply the test voltage or when the test voltage would be applied automatically.

[0016] In an example, the method comprises using the predicted equivalent parallel resistance value to correct a measurement of resistance of the insulator. This allows the measurement of resistance of the insulator to be automatically corrected for the effect of the depolarisation current, for example, as extrapolated to the expected value during the measurement of resistance using the test voltage.

[0017] In an example, the method comprises displaying the polarity of the measurement of current with respect to a polarity of a current expected due to application of the test voltage. If the measurement of current produces a negative value, then this indicates that the measured current may be a depolarisation current. In an example, determining the equivalent parallel resistance value comprises using an absolute value of the measurement of current. This may simplify interpretation of the displayed result.

[0018] In an example, the method comprises inhibiting display of a resistance value measured by applying the test voltage if the polarity of the measurement of current is opposite to the polarity of a current expected by applying the test voltage. This may avoid the display of spurious results due to a dominant depolarisation current.

[0019] In an example, the indication of the intended magnitude of the test voltage is based on a position of a voltage selector switch. This provides a convenient way to determine the intended magnitude of the test voltage that is to be applied, when the test voltage is a constant voltage.

[0020] In an example, the intended magnitude of the test voltage is the starting voltage of a subsequent stepped or ramped function of voltage increasing with time. This provides a way of determining the worst case equivalent parallel resistance to display, for example preceding a polarisation index test.

[0021] In an example, the test is preceded by a series of tests comprising applying a test voltage that increases with time according to a stepped or ramped function. For example, the preceding test may be a polarisation index test.

[0022] In an example, the method comprises determining a range of values of resistance that may be measured within a given degree of error based on the determined equivalent parallel resistance value and displaying a representation of the suggested range of values of resistance. This feature provides an indication to an operator of when the test voltage should be applied, depending on the value of the resistance that is expected to be measured.

[0023] In an example, the method comprises displaying the representation of the suggested range of resistance values by blanking out parts of a scale of resistance that are expected to be measured with greater than the given degree of error. This provides an easily interpreted display. The given degree of error may be configurable by an operator of the test meter. In an example, the method comprises keeping the discharge path connected until the test voltage is applied. This feature may prevent a depolarisation current from causing a potentially dangerous voltage to build up on the conductors separated by the insulator.

[0024] In an example, the measured current without the test voltage being applied may comprise a depolarisation current sourced by the electrical insulator and / or may comprise an induced current due to noise in an electrical environment in which the insulator is tested.

[0025] In accordance with second aspect, there is provided a test meter configured to perform a test for measuring a resistance of an electrical insulator, the test comprising providing a test voltage to electrical conductors separated by the electrical insulator, and the test meter comprising one or more processors configured to cause the test meter to: without applying the test voltage, provide a discharge path in the meter between the electrical conductors to discharge a voltage held on the electrical conductors; with the voltage held on the electrical conductors discharged, and without applying the test voltage, perform a measurement of current flowing through the test meter; based on the measurement of current and on an indication of an intended magnitude of the test voltage, determine an equivalent parallel resistance value that would appear as an error in a measurement of the resistance of the electrical insulator; and display an indication of the determined equivalent parallel resistance value for the intended magnitude of the test voltage.

[0026] In accordance with a third aspect, there is provided a computer program comprising instructions which, when executed by one or more processors of a test meter, cause the test meter to carry out the claimed method.

[0027] Further features and advantages of the will be apparent from the following description of exemplary embodiments, which are given by way of example only.

[0028] Brief Description of the Drawings

[0029] Figure l is a schematic diagram illustrating a test meter in an example;

[0030] Figures 2a to 2d show examples of displays on the test meter; Figure 3 is circuit diagram representing an insulator under test;

[0031] Figure 4 shows an example of components of measured current as a function of time;

[0032] Figure 5a shows polarisation current and depolarisation current as a function of time;

[0033] Figure 5b shows magnitude of polarisation current and depolarisation current as a function of time superimposed for comparison; and

[0034] Figure 6 is a flow diagram of a method in an example.

[0035] Detailed Description

[0036] By way of example, embodiments will now be described in the context of a digital meter for measuring the insulation resistance of an insulator, where the insulator is typically an insulation between windings of an electric motor or generator, or the insulation of a cable, for example a high voltage cable, or the insulation of a transformer. However, it will be understood that embodiments are not limited to these examples, and that test meters may be provided for measuring insulation properties of other types of device.

[0037] Figure 1 shows an example of a test meter 1 in an embodiment of the invention. The test meter is configured to perform a test for measuring a resistance of an electrical insulator, shown as the device under test 2. The device under test may be one or more windings of a motor or generator, for example, and comprises electrical conductors separated by the electrical insulator. The test meter is connected to the device under test via test terminals 3, 4. The test of insulation resistance comprises providing a test voltage to the electrical conductors separated by the electrical insulator, generated by the voltage generation circuit 8, measuring the voltage applied to the insulator using the voltage measurement circuit 6, and measuring the current through the insulator when the test voltage is applied using the current measurement circuit 5. The insulation resistance may be found from the measured current and voltage. However, before the test voltage is applied, any voltage present on the electrical conductors separated by the insulator, which may be held by the capacitance of the device under test from a previous test, is discharged by providing a discharge path in the meter between the electrical conductors, using the discharge switch 7. This allows currents due to the charge held by the capacitance of the device under test to dissipate, so that other currents, such as depolarisation currents, may be measured. The discharge switch 7 may be implemented by a semiconductor switch such as a series of depletion mode field effect transistors, or an electromechanical relay, or other types of switch. There may be a discharge resistance connected in series with the discharge switch 7 to provide a relative low impedance path, for example 1 kiloOhm, in the discharge path. The discharge switch may be set to conduct by default, and the discharge path may remain connected until the test voltage is applied, to prevent a depolarisation current from causing a potentially dangerous voltage to build up on the conductors separated by the insulator.

[0038] With the voltage held on the electrical conductors discharged, and without yet applying the test voltage, a measurement is performed of current flowing through the test meter, using the current measurement circuit 5. The current flowing through the test meter may be a depolarisation current, which may also be referred to as a repolarisation current or a reabsorption current, sourced by the insulator, which may still be polarised, for example due to a test voltage applied in a previous test, or some other event in which a voltage was applied across the insulator.

[0039] When the test voltage, for example from a previous test, is applied, electric dipoles in the dielectric material of the insulator may become polarised due to application of the test voltage. When the test voltage is removed, the polarised insulator may become gradually depolarised, as a depolarisation current flows in the opposite direction to the polarisation current, if the conductors separated by the insulator are connected by a low impedance. If a further insulation resistance test is carried out when the insulator is still polarised following a previous test, the depolarisation current may cause a spurious component to the current measurements for the further test, in some cases dominating the measurements of current. To account for the spurious component of current, one or more processors 9 in the test meter 1, determine an equivalent parallel resistance value that would appear as an error in a measurement of the resistance of the electrical insulator based on the measurement of current and on an indication of an intended magnitude of the test voltage. The equivalent parallel resistance value is found from the ratio of the intended (but not yet applied) test voltage to the current that is measured when the test voltage is not applied, which may be a depolarisation current and / or an induced current due to noise. The equivalent parallel resistance value according represents a hypothetical resistance value that would appear in error in parallel with a measured value of resistance of the insulator if the intended test voltage were applied. If the value of the equivalent parallel resistance is much higher than the expected resistance value of the insulator, then the error due to the high resistance appearing in parallel would be small in percentage terms. However, if the value of equivalent parallel resistance is close to or lower than the expected value of the insulation resistance that would be measured when the test voltage is applied, then it may be concluded that the error would be significant and it may be better to delay applying the test voltage until a smaller value of spurious current, and therefore a higher value of equivalent parallel resistance, is found.

[0040] The determined equivalent parallel resistance is sent by the one or more processors 9 to a display 10 on the meter, or on a user device in communication with the meter, which displays an indication of the determined equivalent parallel resistance value for the intended magnitude of the test voltage. The intended magnitude of the test voltage may be read by the processor from a voltage selector switch 11 which is used to set the voltage for the insulation resistance test. Alternatively, the intended magnitude of test voltage may be held in memory in the processor and may represent the initial test voltage to be applied in a ramped or stepped progression of increasing test voltages in an insulation resistance test. Determining the equivalent parallel resistance value that would appear as an error in a measurement of the resistance of the electrical insulator based on the measurement of current and on an indication of an intended magnitude of the test voltage, and displaying an indication of the determined equivalent parallel resistance value for the intended magnitude of the test voltage gives an indication to an operator as to whether to proceed with the test according to the relative magnitude of the determined equivalent parallel resistance and the expected resistance value to be measured. The indication of determined equivalent parallel resistance value may be continually updated during the measurement of current, so that an operator may monitor the change of the relative magnitude of the determined equivalent parallel resistance and the expected resistance value to be measured to identify a time when the test may proceed with an acceptable degree of error.

[0041] Once the equivalent parallel resistance value has been determined and displayed, the test can progress to the stage of measuring the insulation resistance by applying the test voltage, if a signal that the test should proceed to apply the test voltage is detected by the processor. The signal that the test should proceed to apply the test voltage may be generated in response to an interaction of a user with the test meter, so that an operator may indicate that the test should proceed, having seen the display of the determined equivalent parallel resistance. The interaction of the user with the test meter may be by pressing a test button 12 on the meter 1 or by any suitable interaction. The test may be initially started by a first type of interaction of the user with the test meter, for example a short press of the test button 12, and generating the signal that the test should proceed to apply the test voltage in response to a second type of interaction of the user with the test meter, for example a long press of the test button 12. The test button may be a touch sensitive screen, or an electromechanical switch, for example.

[0042] Alternatively, the signal that the test should proceed to apply the test voltage may be generated automatically by the one or more processors 9, based on a comparison of the equivalent parallel resistance value with a threshold value. The threshold value may be related to an expected value of insulation resistance that the test should be required to measure, which may be settable by the operator.

[0043] The one or more processors may determine a function relating current to time during the measurement of current and extrapolate the determined function to predict the equivalent parallel resistance value, so that an equivalent parallel resistance value to be predicted based on an expected value of current, such as depolarisation current, after the test voltage is applied. This may allow an expected waiting time to be determined, for the predicted equivalent parallel resistance value to reach a given value. The expected waiting time may be displayed, to give the operator prior warning as to when to start to apply the test voltage or when the test voltage would be applied automatically.

[0044] The predicted equivalent parallel resistance value, which may be a predicted value of depolarisation current expected to be present during a measurement of insulation resistance, may be used to correct the measurement of insulation resistance. This allows the measurement of insulation resistance to be automatically corrected for the effect of the depolarisation current, for example, as extrapolated to the expected value during the measurement of resistance using the test voltage.

[0045] In some examples, the measured current may comprise an induced current due to noise in an electrical environment in which the insulator is tested.

[0046] Figures 2a to 2d show examples of displays on the test meter. In each case, the equivalent parallel resistance value that would appear as an error in a measurement of the measurement of the electrical insulator is displayed (8.75 MOhm in Figure 2a, for example). Also, the polarity of the measurement of current with respect to a polarity of a current expected due to application of the test voltage is displayed, in these examples by indicating “nE9” (representing “neg”). If the measurement of current produces a negative value, then this indicates that the measured current may be a depolarisation current. The equivalent parallel resistance value which is displayed is an absolute (unsigned) value of resistance, using an absolute value of the measurement of current.

[0047] The processor may determine a range of values of resistance that may be measured within a given degree of error based on the determined equivalent parallel resistance value and display a representation of the suggested range of values of resistance. This feature provides an indication to an operator of when the test voltage should be applied, depending on the value of the resistance that is expected to be measured. In Figures 2a to 2d, the representation of the suggested range of resistance values is displayed by blanking out parts of a scale of resistance that are expected to be measured with greater than the given degree of error. For example, in Figure 2a only 100k and IM are shown, since higher values of resistance would have an unacceptable degree of error. Also, the bars under the scale show the unavailable resistance ranges.

[0048] Not shown in these examples, the method comprises inhibiting display of a resistance value measured by applying the test voltage if the polarity of the measurement of current is opposite to the polarity of a current expected by applying the test voltage. This may avoid the display of spurious results due to a dominant depolarisation current.

[0049] Figure 3 is circuit diagram representing an insulator under test. The insulator under test may be one of a wide range of insulators used in industry, for example an insulator for high voltage (HV) and low voltage (LV), including cables, bushings, transformers, motors, generators, etc. These insulation systems exhibit specific behaviours during insulation resistance (IR) testing, but the underlying principles are similar in all cases, due to the physics behind the behaviour of dielectric materials exposed to voltage stress. The IR testing is typically performed under DC excitation voltage, but it can also be performed with a uni-polar stepped or ramped voltage.

[0050] Insulation systems typically comprise at least two conductive (metallic) electrodes separated by some insulation. Such a system exhibits certain amount of capacitance (Cx in Figure 3) which must be charged to the required DC voltage so that the information of interest can be extracted from the following measurement. This means that in the initial charging phase the capacitive charging current is typically dominating over other components of current, and this large current persists for as long as the applied HV DC voltage is changing, but it decays quickly to zero (or at least practically negligible values) as soon as the voltage is stable.

[0051] The main information of interest during IR testing is the value of the Rx resistance (Figure 3), which would be typically practically constant as long as the applied voltage is constant. This is the value of insulation resistance that is to be measured in an ideal case.

[0052] However, there is an additional component of current called polarisation current, which may also be referred to as absorption current. This component is caused by the polarizability of the dielectric dipoles inside the dielectric material. Once the electric field is applied across the dielectric the dipoles begin to respond, trying to align to the field or redistribute the local trapped charges. These processes can take extremely long time, tens of minutes and even hours. In the simplified equivalent circuit of Figure 3, they are represented as the branch Ra / Ca. Their real behaviour is more complex, and more parallel branches (Ra n / Ca n) might need to be added to model such behaviour so that the appropriate rate of change of current can be represented. These parallel branches might all have different time constants, but in a general case the resulting time constant is typically much longer than the decay of the capacitive current.

[0053] So the total current measured by the IR tester is typically changing from some higher value, decaying gradually to some asymptote, which may or may not be reached during the designated test time. For simple IR or DAR (dielectric absorption ratio) testing only 60 sec test time might be used and thus mostly the steady IR value is not reached. For more complex diagnostics methods such as the Polarization Index the test time might be 10 min or more, but even then it is not guaranteed that the steady IR value is going to be reached. At the end of the test the dielectric material is polarised, and if at the end of the test the voltage is quickly discharged from Cx then the long-constants branches take equally long time to depolarize as they took to polarise, even though the voltage across the insulation is short-circuited to be zero.

[0054] Figure 4 shows an example of components of measured current as a function of time after the start of an insulation resistance test. The current charging the capacitance of the conductors separated by the insulator 24 is initially high but falls quickly. The leakage current due to the resistance of the insulator 22 ramps up quickly to a steady state. It can be seen that the polarisation current 23, also referred to as absorption current, gradually decays over a longer time period, so that the total current 21 asymptotically approaches the leakage current.

[0055] Figure 5a shows polarisation current 25 and depolarisation current 26 as a function of time and Figure 5b shows magnitude of polarisation current 27 and depolarisation current 28 as a function of time superimposed for comparison.

[0056] An example of operation in an embodiment of the invention is as follows. A first IR test is completed, and the voltage from Cx is discharged by short circuiting with the internal relay and discharge resistors inside the IR tester, for at least a few seconds so that measurable voltage across the HV+ and HV- terminals (as shown in Figure 3) is zero. For example, this first test could have been applied on motor windings, between Phase A and grounded (Phase B + Phase C + stator core). The operator just quickly re-connects the leads to the new positions, so that full depolarisation is not achieved.

[0057] After this test is completed, the operator wishes to test between Phase B and grounded (Phase A + Phase C + stator core). Clearly, the insulation between Phase A and Phase B is obviously pre-polarised and will attempt to inject certain amount of current into the tester, due to the depolarisation process, because before the IR test is commenced, the discharge resistors and the relay provide the path for such current to flow and for the IR tester to measure it.

[0058] Therefore, this current can be measured even before the next test is started. The IR tester is set to a specific nominal test voltage Vnom for the next test, and thus the instrument may predict how the calculation is going to be affected. So the IR calculation is performed as:

[0059] (1) R(reACT) = Vnom / abs (I)

[0060] This is the information to be used for displaying on the main numeric display as well as on the arc scale, or other representation, as shown in Figures 2a to 2d.

[0061] In Figure 2a, the display is shown a relatively short time after the previous tests. The return current could be relatively high, and thus the provisionally calculated resistance (as per equation (1)) is relatively low. If the next IR test is started in this configuration, then any values which would be higher than this number would be false, because the negative component of the current due to the re-absorption effects could be higher than the real measured current during the test. The information about the current being negative is further emphasized for the operator (“neg” in bottom right of the display).

[0062] Visually, this is indicated by the fact that the range values above the arc are suppressed and thus the operator should be wary for commencing the next test. The segments of the arc can indicate the amount of range that is blocked out by the standing current. For example, if the expected value to be measured in the next test should be above 50 GQ (100 nA), then this test would be completely falsified by present state of polarisation of the insulation.

[0063] Instead, is it better to wait until the current decays to a smaller value, and thus the error of resistance diminishes to a larger value (larger IR means smaller error, because the current dictates the errors). This is shown in Figure 2b, which represents the display after some additional waiting time compared to Figure 2a. The current decreased significantly, by over 4 orders of magnitude, and thus a value of 50 G becomes measurable, even though some errors might be still present. It is up to the operator to decide if this range is acceptable, and the next test can start.

[0064] However, Figure 2c shows a case in which the next test is to be run at a reduced nominal test voltage (500 V). This means that the same current level as from Figure 2b now represents much lower resistance, and thus the 50 GQ cannot be performed with any degree of accuracy.

[0065] Conversely, in Figure 2d the next test is to be run at a higher nominal test voltage. This means that the same “standing’ current will have proportionally smaller effect, and thus the projected IR value reflects that.

[0066] It is clear that the longer the waiting time and the higher the difference in levels for the nominal voltage of the next test, then the smaller will be the influence of the standing, re-absorption or de-polarisation currents.

[0067] In another embodiment the calculation for the resistance error is configurable by the operator, and may be set to a different percentage, rather than to the numbers as strictly calculated by equation (1). So for example, the blocked out range could be assessed under the criterion of 10 % error. This means that if for instance the provisionally calculated value of equivalent parallel resistance was to indicate 100 GQ, then this would mean a 10% error for the measured value of 10 GQ, because the current due to 100 GQ would be 10 % of the current at 10 GQ. Therefore, the “block out” range could be shown such that only maximum of 10 GQ would be indicated as available, because anything else would be affected by too-large an error.

[0068] Figure 6 is a flow diagram of a method in an example according to steps S6.1 to S6.4.

[0069] The test meter may be a stand-alone device including the one or more processors 9 and the display 10. However, the test meter may comprise more than one physical part, for example, the functions of the test meter may be provided by a measurement unit connected to the device under test and a user device connected to the test unit, for example by a wireless link, which may provide the display 10 and may also provide one or more of the processors 9. In other examples, at least some of the processing to provide the claimed method may be provided by a remote processing, for example, cloud processing, provided by a server to which the measurement unit is connected.

[0070] There may be provided non-transitory memory which holds executable computer code, which, when run by the one or more processors 9 causes the test meter to carry out the claimed method.

[0071] The above embodiments are to be understood as illustrative examples of the invention. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

Claims1. A method of performing a test for measuring a resistance of an electrical insulator, the test comprising providing a test voltage to electrical conductors separated by the electrical insulator, the method being performed by a test meter and the method comprising: without applying the test voltage, providing a discharge path in the meter between the electrical conductors to discharge a voltage held on the electrical conductors; with the voltage held on the electrical conductors discharged, and without applying the test voltage, performing a measurement of current flowing through the test meter; based on the measurement of current and on an indication of an intended magnitude of the test voltage, determining an equivalent parallel resistance value that would appear as an error in a measurement of the resistance of the electrical insulator; and displaying an indication of the determined equivalent parallel resistance value for the intended magnitude of the test voltage.

2. A method according to claim 1, comprising continually updating the indication of determined equivalent parallel resistance value during the measurement of current.

3. A method according to any preceding claim, comprising, dependent on a signal that the test should proceed to apply the test voltage, applying the test voltage to the electrical insulator and performing a measurement of resistance of the electrical insulator.

4. A method according to claim 3, comprising generating the signal that the test should proceed to apply the test voltage in response to an interaction of a user with the test meter.

5. A method according to claim 4, comprising: starting said test dependent on detecting a first type of interaction of the user with the test meter; and generating the signal that the test should proceed to apply the test voltage in response to a second type of interaction of the user with the test meter.

6. A method according to claim 5, wherein the first type of interaction is a shorter press of a test button and the second type of interaction is a longer press of a test button.

7. A method according to claim 3, comprising generating the signal that the test should proceed to apply the test voltage based on a comparison by the meter of the equivalent parallel resistance value with a threshold value.

8. A method according to any preceding claim, comprising: determining a function relating current to time during the measurement of current; and extrapolating the determined function to predict an equivalent parallel resistance value.

9. A method according to claim 8, comprising determining an expected waiting time for the predicted equivalent parallel resistance value to reach a given value; and displaying the expected waiting time.

10. A method according to claim 8 or claim 9, comprising using the predicted equivalent parallel resistance value to correct a measurement of resistance of the insulator.1911. A method according to any preceding claim, comprising displaying the polarity of the measurement of current with respect to a polarity of a current expected due to application of the test voltage.

12. A method according to any preceding claim, wherein determining the equivalent parallel resistance value comprises using an absolute value of the measurement of current.

13. A method according to any preceding claim, comprising inhibiting display of a resistance value measured by applying the test voltage if the polarity of the measurement of current is opposite to the polarity of a current expected by applying the test voltage.

14. A method according to any preceding claim, wherein the indication of the intended magnitude of the test voltage is based on a position of a voltage selector switch.

15. A method according to any one of claims 1-13, wherein the intended magnitude of the test voltage is the starting voltage of a subsequent stepped or ramped function of voltage increasing with time.

16. A method according to claim 15, wherein said test is preceded by a series of tests comprising applying a test voltage that increases with time according to a stepped or ramped function.

17. A method according to any preceding claim, comprising: determining a range of values of resistance that may be measured within a given degree of error based on the determined equivalent parallel resistance value; and displaying a representation of the suggested range of values of resistance.2018. A method according to claim 17, comprising displaying the representation of the suggested range of resistance values by blanking out parts of a scale of resistance that are expected to be measured with greater than the given degree of error.

19. A method according to claim 17 or claim 18, wherein the given degree of error is configurable by an operator of the test meter.

20. A method according to any preceding claim, comprising keeping the discharge path connected until the test voltage is applied.

21. A method according to any preceding claim, wherein the measured current of said measurement of current comprises a depolarisation current sourced by the electrical insulator.

22. A method according to any preceding claim, wherein the measured current of said measurement of current comprises an induced current due to noise in an electrical environment in which the insulator is tested.

23. A test meter configured to perform a test for measuring a resistance of an electrical insulator, the test comprising providing a test voltage to electrical conductors separated by the electrical insulator, and the test meter comprising one or more processors configured to cause the test meter to: without applying the test voltage, providing a discharge path in the meter between the electrical conductors to discharge a voltage held on the electrical conductors; with the voltage held on the electrical conductors discharged, and without applying the test voltage, performing a measurement of current flowing through the test meter; based on the measurement of current and on an indication of an intended magnitude of the test voltage, determining an equivalent parallel resistance value21 that would appear as an error in a measurement of the resistance of the electrical insulator; and displaying an indication of the determined equivalent parallel resistance value for the intended magnitude of the test voltage.

24. A computer program comprising instructions which, when executed by one or more processors, cause the test meter to carry out the method of claim 1.

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