Earth connection monitoring

Abstract

The disclosure relates to a device, method and computer program for monitoring an earth connection of an electrically conductive item having a capacitance relative to the earth. The device comprises a first terminal for electrically coupling the device to the electrically conductive item; a second terminal for electrically coupling the device to earth; circuitry electrically coupled to the first and second terminals and configured to: deliver a first signal for the electrically conductive item, wherein the first signal has a first frequency; deliver a second signal for the electrically conductive item, wherein the second signal has a second frequency and wherein the first frequency is a different frequency to the second frequency; measure a first impedance associated with the first signal; measure a second impedance associated with the second signal; and determine, based on the first and second impedances and associated first and second frequencies of the first and second signals, whether the capacitance associated with the electrically conductive item is greater than a minimum capacitance value.

Description

[0001] Earth Connection Monitoring

[0002] The present disclosure relates to a device for testing a connection of an electrically conductive item, such as a tanker, to earth reference potential. In particular, although not exclusively, the disclosure relates to a device for testing whether an earth connection of a tanker having a capacitance relative to the surface of the earth is effectively connected to the earth reference potential by a resistance that is less than a maximum earth resistance value.

[0003] It has long been known that, in any organisation that stores, handles, or processes flammable liquids, gases, or powders, that the flow of these liquids, gases, or powders to or from an apparatus during transfers can lead to a buildup of electrostatic charge on surfaces. Electrostatic charge of sufficient energy levels can be a source of ignition if not properly controlled. Unfortunately, there are many industrial applications where flowing material forms a readily combustible atmosphere that can be ignited by a spark from a discharge of built-up static. Such environments are sometimes referred to as Hazardous Ex environments. An explosion caused by such a discharge could lead to substantial damage to the apparatus and the surrounding area and may result in injuries or fatalities. Therefore, equipment used in Hazardous Ex environments is required to meet stringent regulations, such as ATEX / CENELEC (European Union), NEC / CEC (North America) and IECEX (rest of the world). It is therefore imperative for such industrial applications to ensure that the apparatus involved in fluid transfers is effectively grounded. Connecting the apparatus to a low resistance electrical connection to Earth will mitigate static charge build up.

[0004] Conventional protective systems have therefore been developed that include means for effectively connecting an electrically conductive item that is liable to accumulate static electricity, such as metal equipment, a tanker, or storage container, to an earth reference potential in order to safely dissipate any static electricity build-up. For instance, existing devices will provide a connection to earth and monitor the state of the electrical connection to earth. Visual indication is provided to indicate that a connection to earth exists, and electromechanical relays are activated / deactivated to control pumps and compressors and allow the product transfer process to proceed.

[0005] A Hazardous area will often feature a verified earth point to provide the earth connection. A verified earth point is a connection point that is electrically connected to earth and has been verified by a competent person. Alternatively, the earth connection may be an earth of a mains electricity supply (line voltage), which is assumed to provide a high integrity earth connection.

[0006] However, it is sometimes necessary for a tanker to be connected to an earth connection in a remote location where mains electricity is not supplied and a verified earth is not present. These are referred to as mobile applications. In such situations, an electrical conductor may be connected to an earth rod, which is a metal rod that is driven into the earth, a stake driven into the ground, or existing metalwork connected to the earth, such as I-beams, pipework, or storage tanks. In such cases, there is a need to verify the integrity of the true earth connection and guard against misuse of non-verified earth points, and in particular to test whether the electrical conductor is reliably connected to earth reference potential by a resistance equal to or less than a predetermined value.

[0007] Existing testing and monitoring devices can detect an electrical connection to earth by utilising the capacitive properties to earth that a road tanker possesses. This is performed using an AC electrical signal generated by a relaxation oscillator, as described for example in WO 2008 / 142447. The capacitive properties of the road tanker to earth are measured for a short duration prior to product transfer. This measurement is not repeated once the initial criteria have been met. The road tanker is seen by the measurement circuit as one plate of the capacitor, the tanker's tyres, air, and the soil have dielectric properties, and earth is seen as the other plate of the capacitor. The output frequency of the relaxation oscillator in existing devices is inversely proportional to the capacitance and series resistance. The inventors have identified problems with some existing devices subject to particular operating conditions. Various aspects of the disclosure may be related to addressing such problems.

[0008] According to a first aspect of the invention, there is provided a device for monitoring an earth connection of an electrically conductive item having a capacitance relative to the earth, the device comprising: a first terminal for electrically coupling the device to the electrically conductive item; a second terminal for electrically coupling the device to earth; circuitry electrically coupled to the first and second terminals and configured to: deliver a first signal for the electrically conductive item, wherein the first signal has a first frequency; deliver a second signal for the electrically conductive item, wherein the second signal has a second frequency and wherein the first frequency is different to the second frequency; measure a first impedance associated with the first signal; measure a second impedance associated with the second signal; and determine, based on the first and second impedances and associated first and second frequencies of the first and second signals, whether (or that) the capacitance associated with the electrically conductive item is greater than a minimum capacitance value.

[0009] The capacitance may be relative to a surface of the earth.

[0010] The circuitry may be further configured to calculate a capacitance value based on the first and second impedances and associated first and second frequencies of the first and second signals.

[0011] The circuitry may be further configured to determine whether the capacitance of the electrically conductive item is within a capacitance range.

[0012] The circuitry may be configured to deliver further signals with respective frequencies to the electrically conductive item. The circuitry may be configured to perform a frequency sweep in which the signal, in which the circuitry delivers signals including the first and second signals. The frequency sweep may be continuous or discontinuous in the frequency domain. In general, the circuitry may be configured to deliver a plurality of signals, each signal having a different frequency from the other signals. The circuitry may be configured to measure a plurality of impedances corresponding to the plurality of signals. The circuitry may be configured to perform the determination based on the plurality of signals and impedances.

[0013] The electrically conductive item may be a vehicle or part of a vehicle. For example, electrically conductive item may be any one of a tanker, transport vessel, or rail car. The minimum capacitance value may be a minimum characteristic capacitance of the vehicle, part of the vehicle, tanker, transport vessel, or rail car with respect to the mass of earth.

[0014] The earth connection may be isolated from a mains electricity supply.

[0015] The circuitry may be further configured to automatically or periodically repeat the first and second signal delivery, first and second impedance measurements and / or the determination of whether the capacitance associated with the electrically conductive item is greater than a minimum capacitance value.

[0016] The circuitry may be further configured to automatically or periodically repeat the first and second signal delivery, first and second impedance measurements and / or capacitance calculation.

[0017] The period between instances of performing the first and second signal delivery, first and second impedance measurements and / or the determination of whether the capacitance associated with the electrically conductive item is greater than a minimum capacitance value and / or the capacitance calculation may be any value between 10 milliseconds and 500 milliseconds.

[0018] The device may have a characterization mode and a continuity checking mode, such that: the first and second signals are delivered in the characterization mode; and the circuitry is further configured in the continuity checking mode to determine an electrical continuity across the first and second terminals.

[0019] The characterization mode may further comprise measuring the first and second impedances. The characterization mode may further comprise calculating the capacitance associated with the electrically conductive item based on the first and second impedances and associated first and second frequencies of the first and second signals.

[0020] The circuitry may be configured to intersperse the continuity checking mode and the characterization mode.

[0021] Determining the electrical continuity may comprise determining that a resistance between the first and second terminals does not exceed a maximum resistance. The determination may comprise delivering a test signal and determining the resistance based on the test signal. The resistance may be determined based on a current and voltage associated with the test signal between the first and second terminals. The test signal may be a DC signal. The resistance may be a DC resistance. The maximum resistance may be a preset value. Alternatively, the maximum resistance may be a user configurable value. In some examples, the maximum resistance may be 10 ohms.

[0022] The circuitry may be configured to measure: a first current associated with the first signal; a first voltage associated with the first signal; and the first impedance is measured based on the first current and first voltage; and a second current associated with the second signal; a second voltage associated with the second signal; and the second impedance is measured based on the second current and second voltage. The first and second currents may be peak currents of the respective first and second signals and the first and second voltages may be peak voltages of the respective first and second signals.

[0023] The peak current may be measured between the first terminal and second terminal and the peak voltage may be measured across the first terminal and second terminal.

[0024] The circuitry may be further configured to calculate a resistance associated with the earth connection. The resistance calculation may be based on the capacitance value.

[0025] The circuitry may be further configured to determine whether the resistance is less than a maximum resistance value. The maximum resistance value may be a preset value. Alternatively, the maximum resistance value may be a user configurable value. In some examples, the maximum resistance value may be 1000 ohms.

[0026] The circuitry may be configured to calculate the resistance further based on the first or second impedance and the frequency of the corresponding signal.

[0027] The resistance associated with the earth connection may be calculated using the formula:

[0028] R = Jz2- Xc2wherein Z is either the first impedance or the second impedance and Xcis the capacitive reactance associated with the frequency of the corresponding signal.

[0029] The device may further comprise an indicator configured to provide an indication of the capacitance and / or resistance.

[0030] The indicator may be configured to indicate whether the capacitance of the electrically conductive item is greater than the minimum capacitance value. The indicator may be further configured to indicate that the resistance associated with the earth connection is less than a maximum value.

[0031] The indicator may be further configured to indicate that a resistance across the first and second terminals does not exceed a maximum resistance value.

[0032] The first frequency may be between 0 and one of 5kHz, 10 kHz, 15kHz, 50 kHz, 100 kHz, 250 kHz, 500 kHz. The second frequency may be between 10- 30kHz, 20-50kHz, 50-100kHz, 100-500kHz, 700-1000kHz. However, the second frequency may always be higher than the first frequency.

[0033] The circuitry may be further configured to: deliver a third signal having a third frequency; measure a third impedance associated with the third signal; calculate a resistance associated with the tanker based on the third and fourth impedances and associated frequencies of the third and fourth signals.

[0034] The circuitry may be further configured to: deliver a third signal having a third frequency; deliver a fourth signal having a fourth frequency; wherein the third frequency is a different frequency to the fourth frequency; measure a third impedance associated with the third signal; measure a fourth impedance associated with the fourth signal; and calculate a resistance associated with the tanker based on the third and fourth impedances and associated frequencies of the third and fourth signals.

[0035] The circuitry may be further configured to calculate a capacitance associated with the tanker based on the third and fourth impedances and associated frequencies of the third and fourth signals.

[0036] The first and second signals may have frequencies in one frequency band and the third signal may have a frequency in a second frequency band. The fourth signal may have a frequency in the second frequency band. The first frequency band may be lower than the second frequency band. The first frequency band may have a centre point that is lower than a centre point of the second frequency band. The first and second signals may have frequencies which are lower than the third and fourth frequencies.

[0037] The third frequency may be between 80-120kHz, 50-150kHz, 0-200kHz. The fourth frequency may be between 120-160kHz, 100-200kHz or O-lOOOkHz.

[0038] The circuitry may be further configured to calculate the capacitance value accounting for an inherent capacitance, or internal capacitance, of the circuit. The inherent capacitance may be a stored value. The inherent capacitance may be determined, when the electrically conductive item is not coupled to the first and / or second terminals, by: delivering a first floating signal for the electrically floating terminals, wherein the first signal has a first floating frequency; delivering a second floating signal for the electrically floating terminals, wherein the second signal has a second frequency and wherein the first frequency is different to the second floating frequency; measuring a first internal impedance associated with the first floating signal; measuring a second internal impedance associated with the second floating signal.

[0039] The circuitry may be further configured to calculate the inherent capacitance value based on the first and second internal impedances and associated first and second floating frequencies of the first and second signals. The circuitry may be configured to determine, based on the first and second impedances and first and second internal impedances and associated first and second floating frequencies of the first and second signals, whether (or that) the capacitance associated with the electrically conductive item is greater than a minimum capacitance value. According to another aspect of the invention, there is provided a method for monitoring an earth connection of an electrically conductive item having a capacitance relative to the earth, the method comprising: delivering a first signal to an electrically conductive item, wherein the first signal has a first frequency; delivering a second signal to the electrically conductive item, wherein the second signal has a second frequency and wherein the first frequency is a different frequency to the second frequency; measuring a first impedance associated with the first signal; measuring a second impedance associated with the second signal; and determining, based on the first and second impedances and associated first and second frequencies of the first and second signals, whether (or that) the capacitance associated with the electrically conductive item is greater than a minimum capacitance value.

[0040] According to another aspect of the invention, there is provided a computer program product comprising instructions which, when the program is executed by one or more processors of a first device, cause the first device to carry out any method disclosed herein.

[0041] According to a further aspect of the disclosure, there is provided a method for monitoring an earth connection of an electrically conductive item having a capacitance relative to the earth, the method comprising: delivering a first signal to an electrically conductive item, wherein the first signal has a first frequency; delivering a second signal to the electrically conductive item, wherein the second signal has a second frequency and wherein the first frequency is a different frequency to the second frequency; measuring a first impedance associated with the first signal; measuring a second impedance associated with the second signal; and calculating the capacitance associated with the electrically conductive item based on the first and second impedances and associated first and second frequencies of the first and second signals. According to a further aspect of the disclosure, there is provided a computer program product comprising instructions which, when executed by one or more processors of a first device, cause the first device to perform a method for monitoring an earth connection of an electrically conductive item having a capacitance relative to the earth, the method comprising: delivering a first signal to an electrically conductive item, wherein the first signal has a first frequency; delivering a second signal to the electrically conductive item, wherein the second signal has a second frequency and wherein the first frequency is a different frequency to the second frequency; measuring a first impedance associated with the first signal; measuring a second impedance associated with the second signal; and calculating the capacitance associated with the electrically conductive item based on the first and second impedances and associated first and second frequencies of the first and second signals.

[0042] Embodiments will now be described by way of example only with reference to the accompanying drawings in which:

[0043] Figure 1 is a schematic drawing of a first embodiment of a device connected via a clamp to a tanker and connected to an earth connection.

[0044] Figure 2 is a simplified equivalent circuit diagram model of the relationship between the device, tanker, and earth connection of Figure 1.

[0045] Figure 3 is a schematic drawing of the first embodiment of a device wherein the tanker is earthed by the landing legs and / or the hose of the tanker.

[0046] Figure 4 is a simplified equivalent circuit diagram model of the relationship between the device, tanker, and ground of Figure 3.

[0047] Figure 5a illustrates a block diagram of an example device according to the present disclosure.

[0048] Figure 5b is a block diagram illustrating the steps of a method for monitoring an earth connection of an electrically conductive item having a capacitance relative to earth according to the present invention.

[0049] Figure 5c illustrates a simplified block diagram of an example embodiment of circuitry of a device according to the present invention. Figure 6a depicts a simplified RC circuit diagram representing the capacitance of an electrically conductive item and a resistance of an earth connection to the ground in series.

[0050] Figure 6b depicts the resulting capacitive reactance and resistance in series when an AC signal is applied across the circuit of Figure 5a.

[0051] Figure 6c illustrates the phase relationship between resistance and capacitive reactance.

[0052] Embodiments of the present invention relate to a device that can enable an operator to continuously monitor an earth connection of an electrically conductive item having a capacitance relative to the earth. The device according to the invention is of particular utility in relation to the dissipation of electrostatic charge from an electrically conductive item in environments in which a build-up of static electricity is undesirable, and in particular to the monitoring of the connection to earth in mobile applications in which a verified earth point or a mains supply earth point is not available.

[0053] Various difficulties that the inventors have identified with conventional testing devices will now be described with reference to Figures 1 to 4.

[0054] Figure 1 illustrates a typical arrangement of a conventional testing and monitoring device for testing an earth connection of a tanker at a location without a verified earth connection. The conventional testing and monitoring device 10 is connected to a tanker 20 via a clamp 22. The device 10 is also connected to ground 50 via a stake or earthing rod 30. In this way, the device can provide a connection between the tanker 20 and ground 50 that can be verified.

[0055] The device 10 uses an AC electrical signal generated by a relaxation oscillator to determine whether the tanker 20 has a capacitance relative to earth that is within a range that is typical of a tanker, and further determines whether a resistance of the earth connection provide by the earthing rod 30 is less than or equal to a maximum resistance value. The capacitance 40 of the road tanker to earth and resistance between the earth connection and earth potential are verified for a short duration prior to product transfer.

[0056] Figure 2 is a simplified equivalent circuit diagram model of the arrangement in Figure 1 showing a relationship between a device 10', a tanker 20', and ground 50'. The device 10' is connected to the ground 50 via a resistance 30' associated with the grounding rod 30 of Figure 1. The device 10' is also coupled to the ground 50' via the tanker 20'. The equivalent circuit diagram includes components due to the tanker 20', including a capacitance 40' and a resistance 42' between the tanker 20' and ground 50'. This resistance 42' is in parallel with the tanker capacitance 40'. The resistance 42' is associated with the tanker tyres. Since the insulting properties of the tyres effectively electrically isolate the tanker 20' from the ground 50', this parallel resistance 42' is considered large enough to be discounted in simplified models.

[0057] It has been found by experiment that typical road tankers define capacitances with an adjacent surface of the earth of between InF (nano Farads) and 3pF (micro Farads).

[0058] Integrity of the connection to earth is confirmed if the resistance to earth is less than 1000 ohms. This resistance value is defined by international standards (IEC 60079-32-l)and the National Fire Protection Association, NFPA, code 77. By using a relaxation oscillator to verify that the values of capacitance and resistance are within a range consistent with the above-defined value ranges before commencing fluid transfer, existing devices are able to confirm that the device is connected to a tanker and able to confirm earth integrity in many circumstances.

[0059] Once these initial criteria have been verified, such devices will monitor the DC electrical connection from the electrically conductive item to the earth and if this resistance is below a specified resistance then the associated equipment will "go permissive", i.e. permit transfer of material. Resistance testing is usually conducted using a two-wire measurement. The specified resistance may be 10 ohms. This resistance value is defined by the National Fire Protection Association, NFPA, code 77.

[0060] Figure 3 illustrates the arrangement described previously with regard to Figure 1 during fluid transfer. The device 10 is in a permissive state and the landing legs 24 and / or hose 26 of the tanker 20 have been deployed to facilitate product transfer.

[0061] Figure 4 is a simplified equivalent circuit diagram model of the arrangement in Figure 3. The equivalent circuit of Figure 4 differs from that described previously with respect to Figure 2 in that shorting path 24' / 26' provides a path for current to flow between the connector of the device 10' attached to the tanker 20' and ground 50'. The shorting path 24' / 26' represents the electrical path provided by the landing legs 24 and / or hose 26 between ground 50 and the tanker 20 I device 10 in Figure 3.

[0062] When using some conventional earth testing devices such as the device 10 of Figures 1 to 4, the initial capacitance and resistance verification obtained prior to product transfer cannot be repeated once the initial criteria have been met because the lowering of the landing legs 24 and / or hose 26 to facilitate product transfer effectively shorts the capacitance of the tanker 20 with respect to the ground 50. In such conditions, a relaxation oscillator circuit becomes ineffective for capacitance measurements.

[0063] As such, the inventors have identified problems with some existing devices that can cause a safety issue under certain operating conditions. For example, if, after the initial measurements have taken place, the earthing rod 30 becomes dislodged from the ground 50 and hence is no longer connected to electrical earth, the device 10 cannot detect this, and material transfer may continue to proceed without any electrostatic charge protection. Another problem is the potential to "trick" the device to remain in a permissive state. This may occur when the device has indicated a permissive state and then the clamp 22 is disconnected from the tanker 20 and subsequently reconnected to a different electrically conductive item, such as a metal fence in the vicinity of the tanker 20. In this case, material transfer may commence upon reconnection of the clamp without the initial measurements being conducted by the device, i.e. with no confirmation as to whether the clamp was reconnected to an electrically conductive item with a capacitance consistent with that of a road tanker. This "tricking" of the devices is unfortunately common and poses a serious safety hazard.

[0064] To overcome the above-described problems, there is required a device which can monitor the capacitance of the electrically conductive item as well as the resistance between an earth connection and earth reference potential during product transfer in conditions such as those described above.

[0065] Figure 5a shows a schematic representation of a device 100 for monitoring an earth connection of an electrically conductive item 200 having a capacitance 400 relative to the earth 400. In practice, the device 100 may be substituted for the device in the arrangement described previously with respect to figures 1 to 4. However, in this example, the device 100 is configured to operate in a different way to the previous example in order to verify the capacitance of the conductive item 200 with respect to earth 400 and a resistance of the device's earth connection 300.

[0066] The device 100 comprises a housing 110 with a first terminal 120 and a second terminal 140. The first terminal 120 is connected to the electrically conductive item 200 and the second terminal 140 is connected to earth by an earth connection 300. In this example, the earth connection 300 is a metal earthing rod which is driven into the ground 400.

[0067] In preferred embodiments, the first terminal 120 of the device 100 is removably connected to the electrically conductive item 200 via a removably connectable electrical connector 220 such as a clamp. The point on the electrically conductive item 200 to which the electrical connector 220 is connected is an earthing point of the electrically conductive item 200. In the case that the electrically conductive item 200 is a tanker, the earth point is generally a part of the tanker's 220 chassis as described with respect to the arrangement of Figures 1 to 4. An earthing clamp providing the electrical connector 220 may comprise a pair of spring-loaded jaws which are engaged with the earth connection or the electrically conductive item. Application of the earthing clamp by means of spring-loaded jaws has the advantage that the earthing clamp may be positioned easily using just one hand, even where the user is wearing protective gloves or other clothing. Other forms of application may, however, be used where appropriate, e.g. clamping by threaded bolts or the like. In presently preferred embodiments, the tips of the jaws of the earthing clamp carry at least two separate electrical contacts by which the electrical connection is made between the earthing clamp and the earth connection or electrically conductive item to which it is applied. Most preferably, the earthing clamp is provided with three contacts, one of these contacts being provided on one of the earthing clamp jaws and the other two contacts being provided on the other jaw. When the earthing clamp is closed, the two contacts on one jaw preferably lie to either side of the single contact on the other jaw, the three contacts preferably lying substantially in a common plane. Most preferably, two of these three contacts make electrical contact with the earth connection and are connected to the circuit, the third contact being of insulating material.

[0068] The housing 110 of the device 100 further comprises circuitry (not shown) that is electrically coupled to the first and second terminals. The circuitry is configured to perform functionality as will be described below with reference to Figure 5b.

[0069] Figure 5b is a block diagram illustrating a method 400 for monitoring an earth connection of an electrically conductive item having a capacitance relative to earth according to the present invention. The method may be performed by hardware or software, or a combination of the two.

[0070] The method 400 comprises delivering 402 a first signal to an electrically conductive item. The first signal has a first frequency. The method further comprises delivering 404 a second signal to the electrically conductive item. The second signal has a second frequency. The first frequency is a different frequency to the second frequency. The first signal may be delivered 402 before, after or at the same time as that the second signal is delivered 404. The method further comprises measuring 406 a first impedance associated with the first signal and measuring 408 a second impedance associated with the second signal. The first impedance may be measured 406 at the same time that the first signal is delivered 402. The second impedance may be measured 408 at the same time that the second signal is delivered 404. As such, the first signal may be measured 406 before, after or at the same time that the second signal is measured 408.

[0071] The method further comprises determining 410, based on the first and second impedances and associated first and second frequencies of the first and second signals, whether (or that) the capacitance associated with the electrically conductive item is greater than a minimum capacitance value.

[0072] The minimum capacitance value may be between InF (nano Farads) and 3pF (micro Farads).

[0073] The device 100 discussed in relation to Figure 5a which performs the functionality described in Figure 5b can be used in place of the device 10 described with reference to Figures 1-4 in order to address at least some of the above-described difficulties with conventional devices identified by the inventors.

[0074] The device may be used before a transfer process starts to determine that a capacitance associated with the electrically conductive item relative to earth is greater than a minimum capacitance value as required by the applicable safety standards and verify that a low resistance path to earth (a "true earth") exists. Measurements may be stored for maintenance and diagnostic purposes. Advantageously, the device according to some embodiments is able to distinguish between the capacitance of the electrically conductive item and the resistance to earth. In this way, the device may continuously monitor the capacitance of the electrically conductive item and / or the resistance to earth. Further, advantageously, the device according to some embodiments is able to do so using a low amount of power. The device may provide functionality for the capacitance check to be bypassed in some use-cases. Further, unlike prior art devices, once product transfer has begun in a configuration such as that described with respect to figures 3 and 4, the device described with reference to figures 5a and 5b may detect if the earth rod or stake becomes dislodged from the ground and go into a non-permissive state preventing material transfer without electrostatic charge protection. The device may also detect if the clamp is disconnected from the tanker and subsequently reconnected to a different electrically conductive item, such as a metal fence in the vicinity of the tanker. Therefore, the device cannot be "tricked" into staying in a permissive state by the operator during operation in inappropriate conditions.

[0075] Further aspects of operation of the device may be better understood with reference to the example embodiment of Figure 5c and Figures 6a to 6c.

[0076] Figure 5c illustrates a simplified block diagram of an example embodiment of circuitry 500 of a device according to the present invention.

[0077] The circuitry 500 comprises a sinewave generator 520, a processor 580 (or microcontroller), a current detector 540 and a voltage detector 560.

[0078] The sine wave generator 520 is configured to generate signals for application to a conductive object. The current detector and voltage detector and configured to measure signals at an earth connection. The output frequency of the sinewave generator 520 may be configurable. The microcontroller 580 may control the sine wave generator to set the first and second frequencies. This allows for several test frequencies to be selected which allows multiple measurements to be taken across a frequency spectrum.

[0079] The processor 580 is configured to determine first and second impedance values from respective first and second voltage and current measurements performed by current detector 540 and voltage detector 560. The processor 580 is also configured to determine the capacitance associated with the tanker truck and / or resistance of the earth connection. The microcontroller 580 may further perform or cause to be performed any of the functionality discussed with respect to Figure 5b above.

[0080] In this way, the circuitry may monitor whether the resistance is below a maximum resistance. The maximum resistance may be 10 ohms. If the DC resistance is measured to be below a maximum resistance, the system may go permissive (or remain permissive, as the case may be).

[0081] Further detailed example schemes for determining a resistance and capacitance based on multiple impedance readings are discussed below with reference to Figures 6a to 6c.

[0082] Figure 6a is a simplified equivalent circuit diagram model of a capacitance of a conductive object and a resistance of an earth connection such as that described previously with reference to Figure 5a to 5c. The simplified equivalent circuit diagram is an RC circuit comprises a capacitance 620 and a resistance 610 in series, representing the capacitance of a conductive object and a resistance of the earth connection, respectively.

[0083] Figure 6b depicts the circuit of Figure 6a when an AC signal is applied across the circuit of Figure 6a. The resulting impedance Z 600 is a series combination of the resistance R 610' and a capacitive reactance Xc620'.

[0084] Figure 6c illustrates a relationship between resistance R 610', capacitive reactance Xc620', and impedance Z 600. The relationship may be described mathematically using Pythagoras Theorem, as shown by Equation 1 :

[0085] Z = JXC2+ R2(1)

[0086] Equation 2 defines a relationship between capacitive reactance, applied frequency, and capacitance: Therefore, due to the dependence of capacitive reactance on applied circuit frequency, the separate application of two different frequencies across an R.C circuit will result in two different impedances being observed. For example, if a first frequency Fi and a second frequency F2were applied to the circuit, two different corresponding impedances, Zi and Z2, would be observed, as shown by Equations 3 and 4:

[0087] Zi = Jxcl2+ R2(3)

[0088] Since the magnitude of a resistance is not dependent on the frequency of an applied AC signal, to the inventors have devised to use substitution to factor out the resistive element, leaving only the reactance, and use the resulting formula to solve for the capacitance. To achieve this, at least two frequencies are applied, and corresponding impedance measurements are taken for each.

[0089] By way of example, a substitution method using two applied frequencies is detailed below. However, it is contemplated that more than two frequencies may be used to calculate the values of capacitance of an electrically conductive item and / or resistance to earth whilst still falling within the scope of the appended claims. For example, although the dielectric constant of the capacitance may be assumed to be constant, in practice it may have a degree of variation with the applied frequency. Multiple measurement may be made in order to calculate the dielectric constant or its dependence with frequency in order to obtain more accurate values.

[0090] By subtracting the second impedance from the first impedance, i.e. by subtracting equation 4 from equation 3, the resistive R. component may be cancelled out in order to obtain an equation defining the difference between the impedances in terms of their respective capacitive reactance components, as shown by Equations 5 and 6:

[0091] Z±2- Z22= XC12- XC22(6)

[0092] By substituting in Equation 2, Equation 6 can be rewritten in terms of the applied frequencies Fi and F2and the capacitance, which is assumed to stay constant. Equations 7 and 8 show the steps of this substitution:

[0093] Using the Maths Identity ) - Q) = Equation 8 can be rearranged to solve for capacitance C.

[0094] In a further example, in addition to this calculated capacitance, the determined capacitance can be further adjusted to include a component that accounts for an internal capacitance of the system. The internal capacitance may be a preset or stored value, or a calculated value, that accounts for an inherent capacitance within the circuit. The internal capacitance may be determined by using, for example, the equations 1 to 8 set out above in the situation where there is no external load connected to the earthing clamp. The internal capacitance may be determined, when the electrically conductive item is not coupled to the first and / or second terminals, by: delivering a first floating signal for the electrically floating terminals, wherein the first signal has a first floating frequency; delivering a second floating signal for the electrically floating terminals, wherein the second signal has a second frequency and wherein the first frequency is different to the second floating frequency; measuring a first internal impedance associated with the first floating signal; measuring a second internal impedance associated with the second floating signal. The circuitry may be further configured to calculate the internal capacitance value based on the first and second internal impedances and associated first and second floating frequencies of the first and second signals.

[0095] Impedance may be calculated using ohms law:

[0096] Z = V / /

[0097] (9) Resistance may be determined using Pythagoras' Theorem, as shown in Equation 10 below and in Figure 6c:

[0098] R = Jz2- Xc2(10)

[0099] The values of the first and second frequencies Fi and F2 may be chosen according to a predetermined method. In general, the first and second frequencies are selected to be suitable for use with predetermined C and R values. The first and second frequencies may be determined by assuming a center point of capacitance value and resistance value, i.e. InF and lkQ, and determining the frequency at which the impedance of the RC circuit would be perfectly balanced (i.e. 2kQ total impedance) with the aforementioned parameters. The applied frequencies may be chosen such that the determined frequency lies centrally between the two applied frequencies.

[0100] The substitution assumes that capacitance does not change over frequency. However, in practice, dielectric constant of a capacitor may change over frequency. Therefore, the assumption that capacitance does not change over frequency is of greater accuracy when the values of the frequencies applied are close together such that any change in capacitance is negligible. In some examples, a first and second frequency may be within 5 Hz of each other. In some examples, a first and second frequency may be within 10Hz of each other. In some examples, a first and second frequency may be within 20Hz of each other. In some examples, a first and second frequency may be within 50Hz of each other. In some examples, a first and second frequency may be within 100Hz of each other. The second frequency may be no more than 50% or 100% greater than the first frequency, in some examples.

[0101] Advantageously, the application of only two frequencies may minimise the time associated with measurement and calculations.

[0102] The inventors have further found that precision of the calculated capacitance values may be improved by ensuring that the capacitive reactance is the dominant component of impedance when the AC signal is applied. This is achieved by ensuring that the two frequencies applied are at a low frequency band. In this way, the dynamic range of an ADC of the may be optimized for the signals from the voltage and current detectors during determination of the capacitance.

[0103] However, the inventors have discovered that, in order to improve the precision of the calculated resistance, a further two frequencies at a high frequency band may be applied in order to make the resistance the dominant component of the impedance. In this way, the dynamic range of an ADC may be optimized for the signals from the voltage and current detectors during determination of the resistance.

[0104] The devices described with reference to Figures 5a to 5c may have a characterization mode and a continuity checking mode. In characterization mode, the first and second signals are delivered and the first and second impedances are measured. In continuity checking mode an electrical continuity across the first and second terminals may be determined.

[0105] The characterization mode may further comprise determining that the capacitance of the electrically conductive item is greater than a minimum capacitance value. The characterization mode may further comprise calculating the capacitance associated with the electrically conductive item based on the first and second impedances and associated first and second frequencies of the first and second signals. The characterization mode may further comprise calculating the resistance of the earth connection based on the capacitance value. In the continuity checking mode, determining the electrical continuity may comprise determining that a resistance between the first and second terminals does not exceed a maximum resistance. That is, in continuity mode, the device preferably monitors, either continually or intermittently, whether the resistance between the first and second electrical contacts of a removably connectable electrical connector is less than a maximum resistance. The maximum resistance is preferably 10 ohms, as discussed above. The determination may comprise delivering a test signal and determining the resistance based on the test signal. The test signal may be a constant current. The resistance may be determined based on the constant current and a voltage drop across the clamp to the ground connection. The resistance determination may be completed by a microcontroller unit. The resistance may be a DC resistance. The maximum resistance may be 10 ohms.

[0106] The device may initialise in the characterization mode. The device may switch to the continuity checking mode after the device has verified that a capacitance associated with the electrically conductive item is greater than a minimum capacitance value. The device may switch to the continuity checking mode after the device has verified that a capacitance associated with the electrically conductive item is greater than a minimum capacitance value and the resistance of the earth connection is less than a maximum resistance value. The device may automatically and repeatedly (for example periodically) switch between the characterization mode and the continuity checking mode. In this way, the device may continuously monitor the capacitance of the electrically conductive item and / or the resistance to earth.

Claims

CLAIMS1. A device for monitoring an earth connection of an electrically conductive item having a capacitance relative to the earth, the device comprising: a first terminal for electrically coupling the device to the electrically conductive item; a second terminal for electrically coupling the device to earth; circuitry electrically coupled to the first and second terminals and configured to: deliver a first signal for the electrically conductive item, wherein the first signal has a first frequency; deliver a second signal for the electrically conductive item, wherein the second signal has a second frequency and wherein the first frequency is a different frequency to the second frequency; measure a first impedance associated with the first signal; measure a second impedance associated with the second signal; and determine, based on the first and second impedances and associated first and second frequencies of the first and second signals, whether the capacitance associated with the electrically conductive item is greater than a minimum capacitance value.

2. The device of claim 1, wherein the circuitry is further configured to calculate a capacitance value based on the first and second impedances and associated first and second frequencies of the first and second signals.

3. The device of claim 2, wherein the electrically conductive item is any one of a tanker, transport vessel, or rail car, and the minimum capacitance value is a minimum characteristic capacitance of said tanker, transport vessel, or rail car.

4. The device of claims 1-3, wherein the earth connection is isolated from a mains electricity supply.

5. The device of any of claims 1-4, wherein the circuitry is further configured to automatically repeat the first and second signal delivery, first and second impedance measurements and capacitance calculation.

6. The device of any of claim 1-4, wherein the circuitry is configured to perform periodically the first and second signal delivery, the first and second impedance measurements and the determination of whether the capacitance associated with the electrically conductive item is greater than a minimum capacitance value.

7. The device of any preceding claim, wherein the device has a characterization mode and a continuity checking mode, such that: the first and second signals are delivered and the first and second impedances are measured in the characterization mode; and the circuitry is further configured in the continuity checking mode to determine an electrical continuity across the first and second terminals.

8. The device of claim 7, wherein the circuitry is configured to intersperse the continuity checking mode and the characterization mode.

9. The device of claim 7 or claim 8 wherein determining the electrical continuity comprises determining that a resistance between the first and second terminals does not exceed a maximum resistance.

10. The device of claim 9 wherein the maximum resistance is 10 ohms.

11. The device of any preceding claim wherein the circuitry is configured to measure: a first current associated with the first signal; a first voltage associated with the first signal; and the first impedance based on the first current and first voltage; and a second current associated with the second signal; a second voltage associated with the second signal; andthe second impedance based on the second current and second voltage.

12. The device of claim 11, wherein the first and second currents are peak currents of the respective first and second signals and the first and second voltages are peak voltages of the respective first and second signals.

13. The device of claim 12, wherein the peak current is measured between the first terminal and second terminal and the peak voltage is measured across the first terminal and second terminal.

14. The device of any preceding claim, wherein the circuitry is further configured to calculate a resistance associated with the earth connection based on the capacitance value.

15. The device of claim 14, wherein the circuitry is further configured to determine whether the resistance is less than a maximum value.

16. The device of any of claims 14 to 15, wherein the circuitry is configured to calculate the resistance further based on the first or second impedance and the frequency of the corresponding signal.

17. The device of any of claims 14 to 16 wherein the device further comprises an indicator configured to provide an indication of the capacitance and / or resistance.

18. The device of claim 17, wherein the indicator is configured to indicate whether the capacitance of the electrically conductive item is greater than the minimum capacitance value.

19. The device of claim 17 or claim 18, wherein the indicator is further configured to indicate that the resistance associated with the earth connection is less than a maximum value.

20. The device of claim 19, wherein the indicator is further configured to indicate that a resistance across the first and second terminals does not exceed a maximum resistance value.

21. The device of any preceding claim, wherein the circuitry is further configured to: deliver a third signal having a third frequency; measure a third impedance associated with the third signal; calculate a resistance associated with the tanker based on the determined capacitance and the third impedance and associated frequency of the third signal.

22. The device of claim 21, wherein the circuitry is further configured to: deliver a fourth signal having a fourth frequency; wherein the third frequency is a different frequency to the fourth frequency; measure a fourth impedance associated with the fourth signal; and calculate a resistance associated with the tanker based on the third and fourth impedances and associated frequencies of the third and fourth signals.

23. The device of claim 22, wherein the first and second signals have frequencies in one frequency band and the third and fourth signals have frequencies in a second frequency band, wherein the first frequency band is lower than the second frequency band.

24. A method for monitoring an earth connection of an electrically conductive item having a capacitance relative to the earth, the method comprising: delivering a first signal to an electrically conductive item, wherein the first signal has a first frequency; delivering a second signal to the electrically conductive item, wherein the second signal has a second frequency and wherein the first frequency is a different frequency to the second frequency;measuring a first impedance associated with the first signal; measuring a second impedance associated with the second signal; and determining, based on the first and second impedances and associated first and second frequencies of the first and second signals, whether the capacitance associated with the electrically conductive item is greater than a minimum capacitance value.

25. A computer program product comprising instructions which, when the program is executed by one or more processors of a first device, cause the first device to carry out the method of claim 24.

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