GB2579376A

GB2579376A - Non-invasive electricity monitoring

Info

Publication number: GB2579376A
Application number: GB1819468.8A
Authority: GB
Inventors: Richard Harrison-Rowe Paul; Mcghee Andrew; Derek Martin Robert
Original assignee: Trust Power Ltd
Current assignee: Trust Power Ltd
Priority date: 2018-11-29
Filing date: 2018-11-29
Publication date: 2020-06-24
Anticipated expiration: 2038-11-29
Also published as: GB201819468D0; GB2579376B; WO2020109283A3; WO2020109283A2

Abstract

Electricity monitor 100 with a virtual ground (or earth) 160 contained within a housing 110 allowing customers to safely estimate the cost of electricity bills in the home. An opening 120 in the housing is designed to receive, e.g. open and fit around, an electricity cable or wire 130 so a conductive element 140 is in contact with the cable. The conductive element is preferably made of a conductive compressible foam (400, fig 4a) which conforms (or adjusts) to the size (or diameter) of the electricity cable inserted. The conductive element or foam may surround the cable and have a resilient metal plate (500, fig. 5). Capacitive coupling occurs between the conductive element and the cable. Voltage sensing circuitry 150 may sense a voltage between the conductive element and a virtual ground located within the housing so as to determine a phase of the sensed voltage. There may be a voltage comparator having inputs connected to the conductive element and virtual ground which can change its output state when the sensed voltage crosses zero. Real power and reactive power supplied over the cable may be calculated.

Description

NON-INVASIVE ELECTRICITY MONITORING

FIELD OF INVENTION

This invention relates to devices and methods for non-invasive electricity monitoring, particularly consumer-install devices for monitoring electricity via capacitive coupling, and methods for their use.

BACKGROUND

Electricity monitoring devices offer one way of reducing a household's energy consumption.

An electricity monitoring device estimates the amount of electricity used, allowing the consumer to keep track of their electricity usage and energy bills.

The voltage of an electricity cable can be measured non-invasively via a conductor placed adjacent to the insulation of the cable. This technique detects the voltage without requiring a direct connection between the electricity monitoring device and the cable, by a process called capacitive coupling. This voltage signal can then be used in estimating the power consumed so long as the signal is sufficiently stable and consistent.

Non-invasive electricity monitoring devices that use capacitive coupling to detect voltage already exist in the art. For example, in US 5473244 A, there is described an apparatus for performing non-contacting measurements of the voltage, current and power levels of conductive elements. The apparatus includes an arrangement of capacitive sensors coupled to the conductive element through a coupling capacitance. US 6470283 B1 describes an electric power monitoring device that senses power system voltages by capacitive coupling to one or more power system conductors. US 2010/318306 Al describes an apparatus for monitoring the voltage of electricity supplied on a cable via capacitive coupling between a sense electrode and the cable. A sense capacitor of known capacitance is included, which is electrically coupled to the sense electrode and a local ground. A secondary capacitive coupling can be formed with a related neutral wire to improve stability of the voltage waveform measurement.

It would be desirable to provide a non-invasive electricity monitoring device that provides advantages over the prior art.

SUMMARY

As described above, electricity monitoring devices that use non-invasive capacitive coupling to detect voltage exist in the art. However, these prior art electricity monitoring devices require an external connection to ground or neutral in order to detect the voltage of a cable via capacitive coupling. This external connection is used to maintain a signal stable enough for accurate voltage detection. Voltage detection using such devices includes several disadvantages. Since an external connection to ground/neutral is used to provide a stable signal reference voltage, installation of the external wiring by a qualified electrician is generally required. Additionally, suitable ground/neutral connections may not always be available at an appropriate location near the electricity meter. Other issues include external electromagnetic interference and susceptibility to inconsistencies. For example, the external wires can act as antennas for electromagnetic radiation which introduces interference that may affect the performance and accuracy of the device. Cross-over of the external cables with the mains cables can affect the detected voltage signal. As the external cables can also pick up voltage signals by capacitive coupling, they may distort the detected voltage signal, introducing inconsistencies and errors into the estimated power. In addition, the configuration of these devices can affect earth leakage protection devices as current passes through the device to ground.

In light of the above drawbacks relating to capacitively coupled electricity monitoring, it would be desirable to provide a non-invasive electricity monitoring device configured to detect voltage without requiring an external connection to ground/neutral.

According to a first aspect of the present invention, there is provided a non-invasive electricity monitoring device comprising a housing, a conductive element, and voltage sensing circuitry. The housing has an opening configured to receive an electricity cable. The conductive element is within the housing, and is configured to contact and conform to an external surface of the received cable to enable capacitive coupling between the conductive element and the cable. The voltage sensing circuitry is within the housing, and is configured to sense a voltage between the conductive element and a virtual ground located within the housing so as to determine a phase of the sensed voltage.

Since the voltage signal is referenced to a virtual ground within the housing rather than to the household ground/neutral, professional installation of the device is not required. This simplifies the setup such that the device could simply be clipped to a mains cable by an unqualified consumer. This has significant commercial advantages over the prior art, as the device could be installed and used by any unskilled person with suitable instructions.

The conductive element may comprise a conductive compressible foam. This enables the conductive compressible foam to make and maintain a consistent contact with the external surface of the electricity cable, irrespective of the diameter of the electricity cable, and irrespective of any bending of the electricity cable. Alternatively, the conductive element may comprise a resilient metal plate. This enables the resilient metal plate to make and maintain a consistent contact with the external surface of the electricity cable, irrespective of the diameter of the electricity cable.

The conductive element may surround the cable. This provides a large contact area between the conductive element and the received electricity cable, which improves the effectiveness of the capacitive coupling, resulting in a reduced likelihood of introducing errors into the determination of the phase of the sensed voltage, and consequently resulting in more accurate electricity monitoring.

The voltage sensing circuitry may comprise a voltage comparator having an output which changes state when the sensed voltage crosses zero, where a first input of the voltage comparator is coupled to the conductive element, and a second input of the voltage comparator is connected to the virtual ground. The voltage comparator enables measurement of the voltage between the electricity cable and the virtual ground. It may be calibrated to allow for any voltage phase delays introduced by the voltage sensing circuity such that the power supplied on the mains cable can be accurately estimated.

The voltage sensing circuitry may further comprise a resistor coupled between the first and second inputs of the voltage comparator so as to centre the sensed voltage around a reference voltage associated with the virtual ground. This reduces the introduction of timing errors in the voltage sensing circuitry.

The voltage sensing circuitry may further comprise two Schottky diodes coupled between the first and second inputs of the voltage comparator so as to limit an amplitude of the sensed voltage at the first input. This prevents input saturation, resultant timing distortion and damage to the voltage comparator.

The voltage sensing circuitry may further comprise an inductor configured to attenuate high frequencies and/or transients in the sensed voltage, and/or a low pass filter configured to attenuate high frequencies in the sensed voltage. This reduces noise signals and radio interference, and thereby ensures that the signal reaching voltage comparator does not introduce false crossing detections.

The device may further comprise current sensing circuitry within the housing configured to sense a current flowing in the cable so as to determine a phase and an amplitude of the sensed current. For example, the current sensing circuitry may comprise a secondary winding of a current transformer that is configured to be positioned around the received cable in such a way that the cable acts as a primary winding of the current transformer. The current sensing circuitry enables measurement of the current, which may be used with the voltage measurements in the calculation of electrical power.

The device may further comprise a memory. The memory may store a voltage phase delay known to be introduced by the conductive element and the voltage sensing circuitry and/or the memory may store a current phase delay known to be introduced by the current sensing circuitry. Alternatively, the memory may store a total phase delay to account for a current phase delay introduced by the current sensing circuitry and a voltage phase delay introduced by the conductive element and the voltage sensing circuitry. Thus, either the voltage phase delay and current phase delay, or the total phase delay, are used to correctly align the sensed voltage and the sensed current. Regardless of whether the phase delays are determined individually or together, these phase delays are used to ensure the correct alignment of the sensed voltage and sensed current for accurate electricity monitoring.

The device may further comprise a microprocessor. The microprocessor may be configured to sample values of the sensed voltage and values of the sensed current. The sampling may account for the voltage phase delay and the current phase delay such that the sampled values are contemporaneous. Alternatively, the sampling may account for the total phase delay such that the sampled values are contemporaneous. This sampling automatically compensates for the phase angle between the actual voltage and the actual current.

The sampled values of the sensed voltage may be adjusted based on an estimated amplitude of a voltage supply to the cable. Estimating the amplitude saves having to measure the actual voltage amplitude on the electricity cable, which would be an invasive procedure. Estimation of the voltage amplitude is sufficient for most mains voltage supplies, which do not vary significantly.

The adjusted sampled values of the voltage may be multiplied by the sampled values of the current so as to enable calculation of the real power supplied over the cable. This enables the device to act as a consumer-installed non-invasive electricity meter which accurately estimates electricity usage.

The adjusted sampled values of the voltage may be shifted by 90° and multiplied by the sampled values of the current so as to enable calculation of the reactive power supplied over the cable.

According to a second aspect of the present invention, there is provided a method for noninvasive electricity monitoring. The method comprises the steps of (a) providing a noninvasive electricity monitoring device comprising housing containing a conductive element and voltage sensing circuitry, where the voltage sensing circuitry comprises a virtual ground, (b) receiving an electricity cable through an opening of the housing such that the conductive element contacts and conforms to an external surface of the received cable to enable capacitive coupling between the conductive element and the cable, and (c) using the voltage sensing circuitry to sense a voltage between the conductive element and the virtual ground so as to determine a phase of the sensed voltage.

The method may further comprise the step of using current sensing circuitry provided within the housing to sense a current flowing in the cable so as to determine a phase and an amplitude of the sensed current.

The method may further comprise the step of storing a voltage phase delay introduced by the conductive element and the voltage sensing circuitry and/or storing a current phase delay introduced by the current sensing circuitry. Alternatively, the method may further comprise the step of storing a total phase delay to account for a current phase delay introduced by the current sensing circuitry and a voltage phase delay introduced by the conductive element and the voltage sensing circuitry.

The method may further comprise the step of sampling values of the sensed voltage and values of the sensed current. The sampling may account for the voltage phase delay and the current phase delay such that the sampled values are contemporaneous. Alternatively, the sampling may account for the total phase delay such that the sampled values are contemporaneous.

The sampled values of the sensed voltage may be adjusted based on an estimated amplitude of a voltage supply to the cable.

The method may further comprise the step of multiplying the adjusted sampled values of the voltage by the sampled values of the current so as to enable calculation of the real power supplied over the cable.

The method may further comprise the step of shifting the adjusted sampled values of the voltage by 90° and multiplying by the sampled values of the current so as to enable calculation of the reactive power supplied over the cable.

Additional features and advantages of the invention will become apparent from the

following drawings and description.

DRAWINGS

Figure 1 schematically illustrates an exemplary non-invasive electricity monitoring device coupled to an electricity cable.

Figure 2 schematically illustrates an exemplary housing design of the non-invasive electricity monitoring device as a cross-sectional view.

Figure 3 schematically illustrates an exemplary electricity cable as a perspective view. Figures 4a schematically illustrates an exemplary cross-section of a conductive compressible foam which may be used as a conductive element of the non-invasive electricity monitoring device.

Figure 4b schematically illustrates an exemplary arrangement of the conductive compressible foam relative to an opening of the housing prior to receiving the electricity cable.

Figures 4c and 4d schematically illustrate the arrangement of Figure 4b after receiving electricity cables of different diameters.

Figure 4e schematically illustrates a perspective view of an exemplary bent electricity cable surrounded by the conductive compressible foam.

Figure 5 schematically illustrates an exemplary implementation of the non-invasive electricity monitoring device comprising a resilient metal plate as its conductive element.

Figure 6 schematically illustrates exemplary voltage sensing circuitry.

Figure 7 schematically illustrates exemplary current sensing circuitry.

Figure 8a schematically illustrates a voltage phase delay introduced by the conductive element and voltage sensing circuitry of the device.

Figure 8b schematically illustrates a current phase delay introduced by current sensing circuitry of the device.

Figure 9 schematically illustrates a flow diagram of the steps that may be included in a method of non-invasive electricity monitoring.

DETAILED DESCRIPTION

Electricity meters measure the amount of electricity used by a consumer and are usually installed in the consumer's household to monitor electricity usage for billing purposes. As electricity meters are directly connected to the household mains supply, they have direct access to the voltage and current signals and operate by continuously measuring the instantaneous voltage and current of the mains supply in order to calculate the actual amount of energy used.

Power consumed by purely resistive loads, for example kettles, electric ovens and traditional filament bulb lighting requires only the product of the RMS current and the RMS voltage. However, many household electricity loads are not purely resistive. In this case, to accurately determine the actual (real) power P consumed, the phase angle 9 between the actual voltage Vand actual current / is also required. The cosine of the phase angle El provides a power factor case. For electrical loads that are not purely resistive, the phase anglee is non-zero and hence the power factor cos° will be less than one. The electricity meter calculates the actual power P (the measure by which consumers are billed) using the product of the actual RMS current /, actual RMS voltage Vand actual power factor cose: P = 1Vcose (1) Whilst electricity meters calculate the actual power, their set up is neither safe nor practical for electricity monitoring devices intended to be installed by the consumer themselves. One way a consumer can monitor their energy usage is by using a separate non-invasive electricity monitoring device. A non-invasive electricity monitoring device estimates the amount of electricity used by the household, allowing the consumer to keep track of and potentially reduce their energy bills.

Non-invasive access to the current is relatively straightforward; the current can be sensed using a split-core current transformer clamped around the mains cable to obtain / *measured.

This non-invasive current measurement technique is well known in the art. However, as described above, non-invasive voltage measurement is more problematic. The voltage amplitude and stability reduces significantly with distance from the cable, making accurate determination of a phase of the sensed voltage more challenging. In this case, an estimate of the voltage amplitude Vesumate, and a measurement of the power factor cosemeasured can be used in determining the estimated power P * estimate: Pestimate = ImeasureaVestimateCOSOmeasurea (2) The power factor cosen,""red can vary considerably with household load and type. It is advantageous to determine this power factor to reduce errors in the power estimated by the non-invasive electricity monitoring device when compared to that of the electricity meter.

A non-invasive electricity monitoring device Figure 1 schematically illustrates an example of a non-invasive electricity monitoring device 100. The device 100 comprises a housing 110 having an opening 120 configured to receive a live electricity cable 130. The device 100 also includes a conductive element 140 and voltage sensing circuitry 150 within the housing 110. The conductive element 140 is configured to contact and conform to an external surface 135 of the received cable 130 to enable capacitive coupling between the conductive element 140 and the cable 130. The voltage sensing circuitry 150 is configured to sense a voltage between the conductive element 140 and a virtual ground 160 located within the housing 110 so as to determine a phase of the sensed voltage.

The housing 110 may be of any suitable shape or design and may be comprised of any suitable material. The opening 120 of the housing 110, which is configured to receive the electricity cable 130, may be any opening suitable for receiving the electricity cable 130.

The opening 120 may be positioned at any suitable location in the housing 110. The housing 110 may be clipped onto or clamped around the external surface 135 of the electricity cable 130. Advantageously, the housing 110 completely surrounds the opening 120.

Figure 2 illustrates an exemplary design of the device 100 in cross-section. However, any other suitable design of the housing 110 and the opening 120 of the device 100 is also possible.

In the example of Figure 2, the housing 110 comprises a hinged edge 115 that enables two portions 110a, 110b of the housing to hinge open and closed relative to one another such that the housing may easily be clipped onto or clamped around the electricity cable 130. The opening 120 is formed from two semi-circular notches, one notch in each hinged portion of the housing 110a, 110b, which meet along a line 200 when the device 100 is clamped around the electricity cable 130. When the hinge is closed, the two notches cooperate to form the opening 120. An exemplary conductive element is also shown in Figure 2 as comprising upper and lower portions 140a, 140b which contact and conform to the external surface 135 of the electricity cable 130 in use. Each of the conductive element portions 140a, 140b is made from a thin strip of conductive material which is formed into a half-pipe shape (i.e. a strip with a semi-circular cross-section). Each half-pipe section 140a, 140b is disposed around the opening 120 so as to conform to an external surface of the cable 130 when it is received within the opening 120 in use. A current transformer 210 comprising first and second split core halves 210a and 210b is also shown in Figure 2. The current transformer 210 completely surrounds the cable 130 in use. The current transformer will be described in further detail later with reference to current sensing.

Alternatively to the hinged edge 115, the housing may comprise a spring action clamp (not shown) that allows for easy opening of the device 100 for cable insertion. The spring action clamp may be any universal clamp which fits all needs. In a further alternative (not shown), the two housing portions 110a and 110b may be separable rather than being hinged.

An exemplary electricity cable is illustrated in Figure 3. The electricity cable 130 is an insulated live mains electricity cable. In the example of Figure 3, the electricity cable 130 comprises a cable core 320, with inner insulation 330 and outer insulation 340 surrounding the core 320. The core 320 is a single core conductor. For example, the core 320 may be a single large conductor, or a multi-strand single core conductor. The electricity cable has a total external surface 350 which comprises the complete outer surface of the entire length of the electricity cable 130. Only a portion of the total external surface 350 of the electricity cable 130 is received within the opening 120 of the housing 110. This received portion is -10 -the external surface 135 that is shown shaded in Figure 1. The external surface 135 of the received electricity cable 130 comprises a circumferential portion of the total external surface 350 of the electricity cable 130. This external surface 135 is located at the region where the conductive element 140 contacts and conforms to the electricity cable 130, and may also be referred to as a 'contact area 135 of the received electricity cable 130.

Returning to Figure 1, the conductive element 140 within the housing 110 of the device 100 may be any conductive element suitable to contact and conform to the external surface 135 of the received cable 130 to enable capacitive coupling between the conductive element 140 and the cable 130. The diameter of mains electricity cables can differ significantly. For example, in the UK, the mains cable diameter can vary from around 8.4 mm to around 12.5 mm for 16mm2 and 25mm2 cables up to 100 amps (as per BS6004:2000). The size of the opening 120 is advantageously sufficiently large to receive the largest diameter of the electricity cable 130 (i.e. 12.5 mm in the UK). However, it is also advantageous to ensure that the conductive element 140 is able to maintain contact with the external surface 135 of a smaller electricity cable 130 received within the opening 120. In other words, it is desirable that the device be arranged such that the conductive element 140 is able to maintain contact with the external surface 135, regardless of the diameter of the electricity cable 130. Maintaining this contact between the conductive element 140 and the external surface 135 of the electricity cable 130 increases the stability of the capacitive coupling and enables accurate detection of the phase of the sensed voltage using capacitive coupling.

The conductive element 140 is configured such that it is associated with, and is in contact with, at least the boundary of the opening 120. In order to allow for use with electricity cables having a smaller diameter than the diameter of the opening 120, the conductive element 140 may also extend into the opening 120. The conductive element 140 may then deform to surround the electricity cable 130. To provide the largest contact area 135, the conductive element 140 may completely surround a circumference of the electricity cable 130 (as in Figure 2). Alternatively, it is envisaged that the conductive element 140 may only partially surround the circumference of the electricity cable 130. For example, the two conductive element portions 140a, 140b of Figure 2 could span less than 180° each (e.g. they could span 175° each) such that there is a small gap (e.g. a 5° gap) between them on each side. In any case, it is desirable that the conductive element 140 maintains good contact with the external surface 135 of the received electricity cable 130. Maintaining a close contact between the conductive element 140 and the external surface 135 of the electricity cable 130 reduces the likelihood of introducing errors into the determination of the phase of the sensed voltage.

In one embodiment, which will be described in more detail below in relation to Figures 4a - 4e, the conductive element 140 comprises conductive compressible foam 400. The conductive compressible foam 400 is configured to contact and conform to the external surface 135 of the received cable 130 regardless of the diameter of the electricity cable 130. Thus, prior to receiving the electricity cable 130, it is advantageous for the conductive compressible foam 400 to extend into the opening 120, as described above.

In one example, the conductive compressible foam 400 may be any one of the Laird's Ecofoam conductive foam CF-500 series available from wwwlairdtech.corn. These foams provide conductivity in the X, Y and Z-axis, and may also comprise a conductive pressure sensitive adhesive (PSA) tape on one side of the foam. Figure 4a schematically illustrates the cross-section of an exemplary conductive compressible foam 400. The conductive compressible foam 400 comprises layers of metallized foam 410, adhesive layer 420, metallized fabric tape 430 and conductive PSA 440, in that order from top to bottom. Other suitable conductive compressible foams may also be used.

Figure 4b schematically illustrates the exemplary conductive compressible foam 400 configured relative to the opening 120 of the housing 110 prior to receiving the electricity cable 130. The conductive compressible foam 400 is shown shaded in Figure 4b (and in related Figures 4c -4e). The conductive compressible foam 400 is comprised of two separate portions, a top portion and a bottom portion. Each foam portion is attached to a respective portion of the housing 110, with both foam portions extending into the opening 120. This arrangement may be used in the housing design of Figure, for example.

Figures 4c and 4d schematically illustrate cross-sections of electricity cables 130a and 130b, which have different diameters and are each received in the opening 120 and surrounded by the conductive compressible foam 400. In Figure 4c, the electricity cable 130a has a relatively smaller diameter (e.g. of around 8.5 mm). The conductive compressible foam 400 is barely compressed, but maintains full contact with the external surface 135a of the cable 130a. In Figure 4d, the electricity cable 130b has a larger diameter (e.g. of around 12.5 mm). The same amount of conductive compressible foam is highly compressed, but still maintains full contact with the external surface 135b of the -12 -electricity cable 130b. Thus, Figures 4c and 4d illustrate the conductive compressible foam 400 being compressed by varying amounts after receiving the electricity cable 130a, 130b.

In addition to the differing diameters of different mains cables, it is often the case that the shape and installation points on the mains cable are not straight sections of cable. Often, the mains cables are bent or curved where it is desired to attach a non-invasive electricity monitoring device 100. The use of the conductive compressible foam 400 ensures that regardless of whether the cables are straight or curved, the foam 400 maintains contact with the external surface 135 of the electricity cable 130. For example, Figure 4e schematically illustrates a bent electricity cable 130 surrounded by the conductive compressible foam. In this case, the conductive compressible foam 400 is more compressed in some areas than in others, yet still maintains full contact with the external surface 135 of the electricity cable 130. For example, on the left hand side of Figure 4e, the top portion 140a of the foam has a depth of di and is hardly compressed, whereas the bottom portion 140b of the foam has a depth of d2 and is much more compressed. Equally, on the right hand side of Figure 4e, the top portion 140a of the foam has a depth of d3 and is considerably compressed, whereas the bottom portion 140b of the foam has a depth of d4 and is much less compressed.

In one example, the width w of the contact area 135 may be around 25 mm (see Figure 3).

However, it is advantageous for the contact area 135 to be as large as possible. A large contact area 135 improves the effectiveness the capacitive coupling between the conductive element 140 and the electricity cable 130, resulting in more accurate electricity monitoring. Increasing the contact area 135 also maximises the capacitance value of On (as described later), ensuring a high quality signal under a variety of conditions.

Figure 5 schematically illustrates an alternative embodiment, in which the conductive element 140 comprises a resilient metal plate 500. The resilient metal plate 500 is configured to contact and conform to the external surface 135 of the received cable 130 to enable capacitive coupling between the resilient metal plate 500 and the cable 130. Similar to the conductive compressible foam 400, the resilient metal plate 500 is advantageously configured to cater for different diameters of the electricity cable 130. The resilient metal plate 500 may be comprised of a thin, roughly rectangular shaped metal plate curved into an arc, such that it may accommodate the smallest diameter mains cable. The resilient metal plate 500 can also bend to form a larger diameter arc so as to accommodate larger -13 -diameter mains cables as the device 100 is clamped around the electricity cable 130 by the user. The size of the opening 120 advantageously allows for cables up to a maximum diameter of around 12.5 mm. This shaping of the resilient metal plate 500 advantageously allows for a consistent contact between plate 500 and the external surface 135 of the electricity cable 130, irrespective of the electricity cable diameter. The resilient metal plate 500 may be formed of any suitable metallic material.

Voltage sensing components Turning now to voltage sensing in the electricity cable 130, the phase of a sensed voltage of the electricity cable 130 can be determined non-invasively via the conductive element placed adjacent to the external surface 135 of the cable 130, through a process called capacitive coupling. This phase of the sensed voltage may then be used in estimating the power consumed using equation (2) above, as the phase angle (9 between the actual voltage and the actual current will be related to the phase of the sensed voltage (amongst other things). The capacitive coupling between the conductive element 140 and the electricity cable 130 is dependent on the type of cable, the cable's orientation, the thickness and properties of the cable's insulation, and how well the conductive element 140 fits to the external surface of the cable 135. As previously described, the conductive element 140 described herein is configured to contact and confirm to the external surface of the cable 135 so as to provide a consistent capacitive coupling.

Around any conductor at a voltage potential, there exists an electric field which has a potential gradient from the conductor potential to the potential of the surrounding objects. In other words, the voltage potential of the electric field diminishes with distance from the conductor and eventually reduces to the potential of the surrounding objections. As these surrounding objects are more typically at a potential closer to ground, it is possible to exploit the potential gradient to determine aspects of the sensed voltage on the conductor.

By comparing the voltage potential very close to the consumer's supply mains cable with the potential at some distance away, which is considered to be the virtual ground 160, it is possible to reliably determine a phase of the sensed voltage between the conductive element 140 and the virtual ground 160. In other words, the virtual ground 160 is at a voltage potential somewhere between the conductor potential and ground. Using this capacitive coupling technique allows the user to determine the phase of the sensed voltage -14 -without an external connection to ground. The user also does not have to make a direct contact with the mains cable.

Figure 6 schematically illustrates an example of the voltage sensing circuitry 150 used to sense, via capacitive coupling at Gin, the mains voltage Vmal" flowing in the cable 130. The voltage sensing circuitry 150 comprises the virtual ground 160. The virtual ground 160 is associated with a reference voltage \ire,. The voltage sensed by the voltage sensing circuitry 150 is the voltage between the voltage potential very close to the mains cable and the voltage potential at the reference voltage V,,,, i.e. the virtual ground 160. The virtual ground 160 does not require an external connection to the true ground; this is because the virtual ground acts as a reference voltage \ire, rather than the true ground acting as the reference voltage. As the input impedance of the voltage sensing circuitry 150 is very high, only a small amount of the electrical energy that signals the mains voltage is used, and therefore a measurable voltage signal can be sensed. Advantageously, since the actual amplitude of the mains voltage is not being measured here (only the phase is being measured), it is not important whether the potential of the virtual ground 160 inside the device 100 is the same as the true ground or not. As such, no connection external to the device 100 is required.

The exemplary voltage sensing circuitry 150 of Figure 6 also includes a voltage comparator Ul, a low pass filter, a resistor R2, two Schottky diodes D1, and an inductor L1. The inductor L1 is coupled to C. The low pass filter comprises a resistor R1 and a capacitor C1 connected in series, and is coupled to the inductor L1. Both the resistor R2 and the two Schottky diodes D1 are coupled in parallel to the capacitor C1 of the low pass filter. The capacitor C1, resistor R2 and Schottky diodes D1 are all coupled in parallel between the two inputs of the voltage comparator Ul. The virtual ground 160 is also coupled to one of the inputs of the voltage comparator Ul. The output 600 of the voltage comparator U1 may be coupled to a microprocessor and used to estimate the power supplied on the electricity cable 130.

In Figure 6, Vdd is the positive supply voltage to the analogue circuits of the voltage comparator Ul, and V" is the negative supply voltage to the analogue circuits of the voltage comparator Ul. \ire, is the reference voltage for the analogue circuits. This is typically close to half way between V55 and Vdd.

-15 -The voltage comparator U1 may be a high input-impedance voltage comparator integrated circuit. The output 600 from the voltage comparator U1 changes state when the sensed voltage on a first, non-inverting input (+) of the voltage comparator U1 crosses the voltage on a second, inverting input (-), which is held at Vref. The first, non-inverting (+) input of the voltage comparator U1 may be coupled to the conductive element 140, and the second, inverting (-) input of the voltage comparator U1 may be connected to the virtual ground 160 such that the voltage comparator compares the voltage between the conductive element 140 and the virtual ground 160. The output 600 of the voltage comparator U1 changes state when the sensed voltage crosses zero. Note that the sensed voltage crosses zero once a known phase delay of the voltage sensing circuitry 150 has been accounted for.

See below for details on how the known phase delay is accounted for.

On represents the capacitance between the external surface 135 of the electricity cable 130 and the conductive element 140. If the conductive element 140 does not maintain a stable contact with the external surface 135 of the electricity cable 130, On may vary significantly, depending on the cable type, diameter and installation. Any variability in Cin may introduce delays in measuring the phase of the sensed voltage, which causes a difference between the signal calibrated in manufacture and the signal sensed when installed. It is important therefore to ensure that any variability in 0. is kept to a minimum.

The resistor R2 is coupled between the inputs of the voltage comparator U1 so as to centre the sensed voltage around the reference voltage Vre, associated with the virtual ground 160. In other words, R2 ensures that the signal on the first, non-inverting input of U1 is centred around Vref, such that the sensed voltage signal remains symmetric. If the sensed voltage signal is not symmetric this may introduce timing errors to the voltage sensing circuitry 150. In one example, R2 may have a resistance of 2.2 MO. Alternatively, R2 may take any other suitable value of resistance.

The two Schottky diodes D1 are coupled between the inputs of the voltage comparator U1 so as to limit an amplitude of the sensed voltage at the first input. The Schottky diodes D1 limit the voltage swing present at the non-inverting (+) input of the voltage comparator U1 so as to prevent input saturation and any resultant timing distortion. The amplitude of the sensed voltage is dependent on each of: Gin, the actual amplitude of the mains voltage, and the proximity of any adjacent cables and other metallic items in the surrounding area. In some installations, the amplitude of the sensed voltage may exceed the supply voltage to -16 -the voltage comparator U1 (i.e., it may exceed Vdd and V"). Hence, the Schottky diodes are used to limit the amplitude of the sensed voltage input into the voltage comparator U1 to the Schottky voltage limit. For example, the Schottky limit may be 0.2 V. Any other suitable value of the Schottky voltage limit is also possible. Advantageously, the Schottky diodes D1 limit the maximum amplitude of the input into the voltage comparator Ul, preventing damage to the voltage comparator U1 and ensuring correct operation of the voltage sensing circuitry 150.

The inductor L1 is configured to attenuate high frequencies and/or transients in the sensed voltage. The inductor L1 may be a ferrite inductor and may have very high impedance at radio frequencies, but very low impedance at 50Hz mains frequency. By attenuating high frequencies and/or transients in the sensed voltage, the inductor L1 prevents radio signal interference caused by the voltage sensing circuitry 150 potentially acting as an antenna to radio signals. The inductor L1 also ensures CE compliance.

The low pass filter is configured to attenuate high frequencies in the sensed voltage. The low pass filter may be a first order low pass filter comprising the resistor R1 and the capacitor C1. The low pass filter further attenuates any high frequency content on the mains voltage signal. In one example, the resistor R1 has a resistance of 1 MD and the capacitor C1 has a capacitance of 1 nF. The low pass filter may have a -3dB point of around 10kHz to 30kHz, which can be affected by the effective capacitance of Cin. Alternatively, the resistor R1 and the capacitor C1 may take any other suitable value and, as such, the low pass filter may also take any other suitable dB point. The low pass filter reduces the amplitude of these high frequencies, ensuring that the signal reaching voltage comparator U1 does not introduce false crossing detections.

Current sensing components Returning to Figure 1, the non-invasive electricity monitoring device 100 may further comprise current sensing circuitry 170 within the housing 110. The current sensing circuitry 170 is configured to sense a current flowing in the cable 130 so as to determine a phase and an amplitude ('measured of equation (2)) of the sensed current.

Figure 7 schematically illustrates an exemplary current sensing circuitry 170. The current sensing circuitry 170 may comprise a secondary winding 700 of a current transformer. The secondary winding 700 is configured to be positioned around the received cable 130 in -17 -such a way that the cable 130 acts as a primary winding of the current transformer. The current transformer is able to sense the current flowing in the electricity cable 130 when the non-invasive electricity monitoring device 100 is clipped around the electricity cable 130. Non-invasive current sensing is relatively straightforward and well known in the art in relation to a current transformer which surrounds a single core conductor (e.g. a mains live wire). The current transformer may be any type of current transformer, for example, the current transformer may be a split-core current transformer 210 formed from the split core halves 210a and 210b shown in Figure 2. The current sensing circuitry 170 may also comprise one or more amplifier circuits 710, low pass filters 720, 730 and a microprocessor 750. In Figure 7, the secondary winding 700 of the current transformer is coupled to the one or more amplifier circuits 710. The one or more amplifier circuits 710 are coupled to the low pass filters 720 and 730. The low pass filters 720 and 730 are each coupled to the microprocessor 750. Two low pass filters 720, 730 are provided in the example of Figure 7. The first low pass filter 720 is used if the input current is in the range 0-10A, whereas the second low pass filter is used if the input current is in the range 10-100A. Thus, whilst a single low pass filter would suffice in many circumstances, two low pass filters are used to extend the hardware range.

Note that elements 720 and 730 have been described as low pass filters for simplicity. In fact, these elements are amplifier gain stages rather than just low pass filters. This provides a "dual range" input (0-10A and 10-100A) in order to increase the measurement resolution of the current. When in the 0-10A range, the microprocessor 750 will be able to measure to a resolution of circa 0.005A (=10/2000). When in the 10-100A range, the Micro will be will be able to measure to a resolution of circa 0.05A (=100/2000). The extra resolution is useful at low current sin order to measure accurately.

Further components Referring back to Figure 1, the device 100 may further comprise means for communication external to the device 100. For example, the means for communication may comprise a transceiver 192 to transmit an estimated power via Wi-Fi or another communication protocol. This may allow the estimated power data to be transmitted to an external server 194. Also, the device 100 may comprise a display 196 for displaying the estimated power. In another example, the device 100 may be coupled to an external display 198 for displaying the estimated power. Any other means of communicating the estimated power is also possible.

-18 -The device 100 may further comprise a memory 180. The memory 180 may be any nonvolatile memory suitable for storing data and/or computer programs. The memory 180 may be coupled to one or more of the voltage sensing circuitry 150 and the current sensing circuitry 170. The memory 180 may be comprised within the housing 110 of the device 100.

Alternatively, a remote memory (not shown) may be provided that is accessible to the device 100 by means of the transceiver 192, for example.

The memory 180 may store a value of a known voltage phase delay introduced by the conductive element 140 and the voltage sensing circuitry 150. This voltage phase delay is caused by the capacitive input and the low pass filter of the voltage sensing circuitry 150. The voltage phase delay may be measured and stored in the memory 180 during the manufacturing process. As a result of this voltage phase delay, the output 600 of the voltage comparator U1 does not change state at exactly the same time as the actual mains voltage crosses zero, even when the actual mains voltage Vand actual mains current / are exactly in phase (i.e., when e = 0°). Figure 8a illustrates an example of the voltage phase delay introduced by the conductive element 140 and the voltage sensing circuitry 150. The solid line represents the actual mains voltage V./ns and the dashed line represents the output 600 from the voltage comparator Ul. The voltage phase delay, represented by the arrow VPD, is the delay between the zero-crossing points of the actual mains voltage on the cable 130 and the sensed voltage as measured by the voltage sensing circuitry 150. Based on the known value of the voltage phase delay stored in the memory 180, it is possible to calibrate the output 600 of the voltage comparator U1 to account for the delay introduced by the conductive element 140 and the voltage sensing circuitry 150.

The memory 180 may further store a value of a known current phase delay introduced by the current sensing circuitry 170. This current phase delay is caused by the current transformer, amplifier circuits 710 and low pass filters 720, 730 of the current sensing circuitry 170. The current phase delay may be measured and stored in the memory 180 during the manufacturing process. As a result of this current phase delay through the current sensing circuitry 170, the measured current does not change state at exactly the same time as the actual mains current crosses zero, even when the actual mains voltage V and actual mains current /are exactly in phase (i.e., when 6 = 0°). Figure 8b illustrates an example of the current phase delay introduced by the current sensing circuitry 170. The solid line represents the actual mains current /mains and the dashed line represents the -19 -measured current / . measured. The current phase delay, represented by the arrow IPD, is the delay between the zero-crossing points of the actual mains current on the cable 130 and the sensed current as measured by the current sensing circuitry 170. Based on the known value of the current phase delay stored in the memory 180, it is possible to calibrate the measured current /,""",d to account for the delay introduced by the current sensing circuitry 170.

As a result, the voltage phase delay and current phase delay may be used to align the sensed voltage and current signals with the actual voltage and current signals so as to calculate the phase angle B. Consider a phase angle e between the actual mains voltage and the actual mains current.

In other words, the phase angle e is the difference between when the actual voltage waveform crosses zero, and the corresponding point at which the actual current waveform crosses zero: B = Actual V transition point -Actual I transition point = (Sensed V transition point -VPD) -(Sensed I transition point -IPD) = (Sensed V transition point -Sensed I transition point) -(VPD -IPD) = (Sensed V transition point -Sensed I transition point) -TPD where VPD is the voltage phase delay, IPD is the current phase delay, and TPD is a total phase delay which accounts for both the voltage phase delay and the current phase delay. Therefore, alternatively, the memory 180 may store a value of the total phase delay which accounts for both the current phase delay introduced by the current sensing circuitry 170 and the voltage phase delay introduced by the conductive element 140 and the voltage sensing circuitry 150.

In one example, the total phase delay may be measured in microprocessor clock 'ticks'. If there are 32768 clock ticks per second, each clock tick represents 30.5 ps. At a 50Hz mains frequency, each mains cycle will be 655.36 clock ticks, so each clock tick represents 0.55°.

By knowing the actual mains voltage V, actual mains current / and actual phase angle e, any delays in both the sensed voltage and the sensed current can be calculated.

-20 -Advantageously, in manufacture, the actual mains voltage Vand actual mains current / may be artificially synthesised so that the phase angle B between them can be very accurately set. The total phase delay may then be determined relative to this phase angle 9 between the synthesised mains voltage Vand current I. For example, the synthesised mains voltage and current may have a phase angle of 0.7° (see e in the above equations).

The timing of transitions between the output 600 from the voltage comparator U1 and the measured current / * measured may be 126 clock ticks, or 69.3° (see "Sensed V transition pointSensed/transition point" in the above equations). As such, the absolute difference between the synthesised phase angle and the sensed phase angle is then 69.3° -0.7°1= 59.6°. This is the total phase delay (see TPD in the above equations). Thus, the value of the total phase delay may be established during manufacturing testing and stored in the memory 180. The device 100 can then compensate for this total phase delay in use.

Thus, to compensate for the voltage and current phase delays due to the sensing circuitry, the appropriate phase delays are stored in the memory 180 during manufacturing testing and can subsequently be used to correct the timings of the sensed voltage and the sensed current by assessing the relative transition timings of the output 600 from the voltage comparator U1 and/or the measured current / * measured. Regardless of whether the phase delays are determined individually or together, these phase delays are used to ensure the correct alignment of the sensed voltage and sensed current for accurate electricity monitoring.

As described above, these phase delays are caused by the sensing circuitry of device 100 only. Other factors which could affect the sensed current and voltage (e.g., near-field and far-field radiation caused by other nearby mains cables) are considered to be negligible.

Returning to Figure 1, the device 100 may further comprise a microprocessor 190. The microprocessor 190 may be coupled to one or more of the voltage sensing circuitry 150, the current sensing circuitry 170 and the memory 180. The microprocessor 190 may be any data processing unit suitable for processing data received from the voltage sensing circuitry and/or the current sensing circuitry 170. For example, the microprocessor 190 may be configured to receive and process the output 600 from the voltage comparator U1 and the measured current!measured. Thus, the microprocessor 190 may comprise the microprocessor 750 of the current sensing circuitry 170. The microprocessor 190 may be configured to execute one or more computer programs (which may be stored in the memory 180) to -21 -process the received data. The microprocessor 190 may comprise one or more data processing units operating separately, in parallel or in combination with each other. The microprocessor 190, in carrying out data processing operations, may store data and/or read data from the memory 180. The microprocessor 190 may be comprised within the housing 110 of the device 100. Alternatively, a remote microprocessor (not shown) may be provided that is accessible to the device 100 by means of the transceiver 192, for example.

Data sampling The microprocessor 190 may be configured to sample values of the sensed voltage and values of the sensed current. The sampling may account for (i.e. compensate for) the voltage phase delay and the current phase delay such that the sampled values are contemporaneous. Alternatively, the sampling may account for (i.e. compensate for) the total phase delay such that the sampled values are contemporaneous. When we say that a sampled value of the current is contemporaneous with a sampled value of the voltage, this means that the two values correspond to the same instant in time in terms of the actual voltage and current waveforms carried by the electricity cable 130.

Whilst the currently sensing circuitry 170 is able to sense the actual current amplitude on the cable 130, the device 100 does not measure the actual voltage amplitude of the mains supply, as this is difficult to do accurately using a non-invasive monitoring technique.

Instead, an estimated amplitude of the voltage supply to the cable 130 is used by the device 100 in calculating the estimated power consumption.

The amplitude of the mains voltage will depend primarily on the mains supply voltage itself.

For example, in the UK, 240V is an exemplary standard mains supply voltage, and the mains voltage amplitude typically varies by circa +/-5 volts of 240 V. However, the amplitude of the mains supply voltage may vary significantly (e.g. from 176 V to 276 V) from one electricity supply to another. In addition, the amplitude of the actual voltage of the cable 130 is dependent (to a much lesser degree) upon the particular construction of the mains cable itself and the environment in the user's home. For example, other electrical cables close to the cable 130 to which the device 100 is fitted to may affect the amplitude of the actual voltage. However, these are relatively small effects, and it is not possible to account for such variations during the manufacturing process of the device 100. Instead, it would be necessary to calibrate the voltage amplitude after installation in the user's home, which is not practical. Therefore, such effects are considered to be negligible here.

-22 -Instead, the sampled values of the sensed voltage may be adjusted based on an estimated amplitude of the voltage supply to the cable 130 to provide adjusted sampled values of the sensed voltage. In one example, the estimated voltage may be assumed to be a sinusoidal waveform with an RMS amplitude of 240 V such that the estimated amplitude of the voltage supply to the cable 130 is 240 V RMS. The 240V value is used to set the amplitude of the estimated voltage waveform, and the phase of the sensed voltage (adjusted by the voltage phase delay) is used to set the phase of the estimated voltage waveform. The estimated amplitude of the voltage supply to the cable 130 (e.g. 240V RMS) may be pre-stored in the memory 180 during manufacture based on the standard mains supply voltage where the device 100 will be used (e.g. 240V for the UK). In one embodiment, it is possible to remotely configure the estimated amplitude of the voltage supply to the cable 130 based on other factors (e.g. information from the device owner's electricity supplier).

As mentioned above, the mains voltage amplitude varies by circa +/-5 volts of 240 V in the UK, which provides an accuracy error of circa +/-2 % if the amplitude is assumed to be exactly 240V. This accuracy error may be reduced by remotely configuring the amplitude of the actual mains voltage in order to set the estimated amplitude of the voltage supply to the cable 130 closer to the actual amplitude of the mains voltage (e.g. based on information from the device owner's electricity supplier, as mentioned above). This further improves the accuracy of the device 100.

Once the sampled values of the sensed voltage have been adjusted based on the estimated amplitude of the voltage supply to the cable 130, the now adjusted sampled values V, of the voltage may be multiplied by the contemporaneously sampled values of the current so as to enable calculation of the real power Pear supplied over the cable 130. In other words, having accounted for phase delays by means of the sampling, the 240V-adjusted values of the sensed voltage are multiplied with the sampled values of the sensed current to enable accurate calculation of the real power: P"at = (3) Here n represents the number of samples taken at sample points i. By taking account of the phase delays as discussed above, the sampling points of the adjusted voltage are at exactly the same time as the sampling points of the sensed current. In one embodiment, the sampling points of the adjusted voltage have been shifted forward in time by the known -23 -voltage phase delay, and the sampling points of the sensed current have been shifted forward in time by the known current phase delay such that the sampling points are contemporaneous in terms of the electricity flowing in the cable 130. In an alternative embodiment, the sampling points of the adjusted voltage have been shifted forward in time by the known total phase delay relative to the sampling points of the sensed current such that the sampling points are contemporaneous in terms of the electricity flowing in the cable 130.

Similarly, the adjusted sampled values V of the voltage may be shifted by 90° and multiplied by the sampled values /, of the current so as to enable calculation of the reactive power P -reactive supplied over the cable 130: v,n (I 1+90°)- (4) Pr eacttve 71 It is not necessary to know the phase angle between the actual voltage and the actual current in order to calculate the power because the phase angle is automatically compensated for when aligning the adjusted voltage samples with the sensed current samples. Instead, only the phase delays inside the device 100 are needed to correctly align the samples and allow calculation of the power. The real power Preal (measured in Watts) is then estimated by multiplying together the sampled values V of the adjusted voltage and the sampled values /, of the sensed current. The reactive power P reactive (measured in Vars) is estimated by shifting the adjusted sampled values V of the voltage by a further 90° and then multiplying these shifted values together with the sampled values of the sensed current.

In an example where the electrical load is purely resistive, the actual voltage and actual current will be aligned in phase. In this case, the real power will always be positive, and the reactive power will always sum to zero. In other words, when the phase angle elis 0°, the real power is at a maximum and the reactive power is at a minimum. In an example where the electrical load is purely inductive, the actual voltage and actual current will be 90° out of phase. In this case, the real power will sum to zero, and the reactive power will always be positive. In other words, when the phase angle e is 90°, the real power is at a minimum and the reactive power is at a maximum.

-24 -The sensed voltage and the sensed current may be sampled at any number of points during a period to provide the sampled values. For example, the sensed voltage and sensed current may be sampled at a certain number of times per second. The sensed voltage and current may also be sampled in terms of voltage phase degrees. An increased sampling frequency will increase the accuracy of the real and reactive power calculations as set out in equations (3) and (4). If the sampling frequency is too low, the accuracy of the device 100 will be affected adversely. Therefore, a sampling frequency of at least 48 samples per second is preferred. However, the sampling frequency also affects the battery life of the device 100, so a balance needs to be struck.

Method of non-invasive electricity monitoring Figure 9 illustrates a flow diagram of the steps that may be included in a method 900 of non-invasive electricity monitoring using a non-invasive electricity monitoring device (e.g. the non-invasive electricity monitoring device 100 shown in Figure 1). All of the above description relating to the device 100, including various alternative embodiments, is equally applicable to the method 900.

Step 910 of the method 900 comprises providing a non-invasive electricity monitoring device (e.g. the device 100) comprising a housing (e.g. the housing 110) containing a conductive element (e.g. the conductive element 140) and voltage sensing circuitry (e.g. the voltage sensing circuitry 150), where the voltage sensing circuitry comprises a virtual ground (e.g. the virtual ground 160).

After step 910, step 920 comprises receiving an electricity cable (e.g. the electricity cable 130) through an opening (e.g. the opening 120) of the housing such that the conductive element contacts and conforms to an external surface (e.g. the external surface 135) of the received cable to enable capacitive coupling between the conductive element and the cable.

After step 920, step 930 comprises using the voltage sensing circuitry to sense a voltage between the conductive element and the virtual ground so as to determine a phase of the sensed voltage.

-25 -Step 940 is an optional step comprising sensing a current flowing in the cable using current sensing circuitry (e.g. the current sensing circuitry 170) provided within the housing so as to determine a phase and an amplitude of the sensed current.

In one example, the method 900 may comprise storing a voltage phase delay known to be introduced by the conductive element and the voltage sensing circuitry. The method 900 may further comprise storing a current phase delay known to be introduced by the current sensing circuitry. These phase delays may be used to align the voltage and current signals. In particular, in this example, a sampling and processing step 950 may comprise sampling values of the sensed voltage and values of the sensed current, where the sampling accounts for the voltage phase delay and the current phase delay such that the sampled values are contemporaneous.

In another example, the method 900 may comprise storing a total phase delay to account for both the current phase delay introduced by the current sensing circuitry and the voltage phase delay introduced by the conductive element and the voltage sensing circuitry. In this example, the sampling and processing step 950 may comprise sampling values of the sensed voltage and values of the sensed current, where the sampling accounts for the total phase delay such that the sampled values are contemporaneous.

As described above, the sampled values of the sensed voltage may be adjusted based on an estimated amplitude of a voltage supply to the cable. The sampling and processing step 950 may further comprise multiplying the adjusted sampled values of the voltage by the sampled values of the current so as to enable calculation of the real power supplied over the cable. Additionally or alternatively, the sampling and processing step 950 may further comprise shifting the adjusted sampled values of the voltage by 90° and multiplying by the sampled values of the current so as to enable calculation of the reactive power supplied over the cable.

The method 900 may further comprise communicating the estimated real and/or reactive power external to the device and/or displaying the estimated real and/or reactive power.

Claims

-26 -CLAIMS: 1. A non-invasive electricity monitoring device comprising: a housing having an opening configured to receive an electricity cable; a conductive element within the housing, wherein the conductive element is configured to contact and conform to an external surface of the received cable to enable capacitive coupling between the conductive element and the cable; and voltage sensing circuitry within the housing configured to sense a voltage between the conductive element and a virtual ground located within the housing so as to determine a phase of the sensed voltage.
2. The device of claim 1, wherein the conductive element comprises a conductive compressible foam.
3. The device of claim 1, wherein the conductive element comprises a resilient metal plate.
4. The device of any preceding claim, wherein the conductive element surrounds the cable. 20
5. The device of any preceding claim, wherein the voltage sensing circuitry comprises a voltage comparator having an output which changes state when the sensed voltage crosses zero, a first input of the voltage comparator being coupled to the conductive element, and a second input of the voltage comparator being connected to the virtual ground.
6. The device of claim 5, wherein the voltage sensing circuitry further comprises a resistor coupled between the first and second inputs of the voltage comparator so as to centre the sensed voltage around a reference voltage associated with the virtual ground.
7. The device of claim 5 or claim 6, wherein the voltage sensing circuitry further comprises two Schottky diodes coupled between the first and second inputs of the voltage comparator so as to limit an amplitude of the sensed voltage at the first input.-27 -
8. The device of any preceding claim, wherein the voltage sensing circuitry further comprises one or more of: an inductor configured to attenuate high frequencies and/or transients in the sensed voltage; and a low pass filter configured to attenuate high frequencies in the sensed voltage.
9. The device of any preceding claim, further comprising current sensing circuitry within the housing configured to sense a current flowing in the cable so as to determine a phase and an amplitude of the sensed current; and optionally, wherein the current sensing circuitry comprises a secondary winding of a current transformer, the secondary winding configured to be positioned around the received cable in such a way that the cable acts as a primary winding of the current transformer.
10. The device of any preceding claim, further comprising a memory.
11. The device of claim 10, wherein the memory stores a voltage phase delay introduced by the conductive element and the voltage sensing circuitry.
12. The device of claim 10 or claim 11 when dependent on claim 9, wherein the memory stores a current phase delay introduced by the current sensing circuitry.
13. The device of claim 10 when dependent on claim 9, wherein the memory stores a total phase delay to account for a current phase delay introduced by the current sensing circuitry and a voltage phase delay introduced by the conductive element and the voltage sensing circuitry.
14. The device of any one of claims 10-13, further comprising a microprocessor.
15. The device of claim 14 when dependent on claims 11 and 12, wherein the microprocessor is configured to sample values of the sensed voltage and values of the sensed current, wherein the sampling accounts for the voltage phase delay and the current phase delay such that the sampled values are contemporaneous.
16. The device of claim 14 when dependent on claim 13, wherein the microprocessor is configured to sample values of the sensed voltage and values of the sensed current, -28 -wherein the sampling accounts for the total phase delay such that the sampled values are contemporaneous.
17. The device of claim 15 or claim 16, wherein the sampled values of the sensed voltage are adjusted based on an estimated amplitude of a voltage supply to the cable.
18. The device of claim 17, wherein the adjusted sampled values of the voltage are multiplied by the sampled values of the current so as to enable calculation of the real power supplied over the cable.
19. The device of claim 17 or claim 18, wherein the adjusted sampled values of the voltage are shifted by 90° and multiplied by the sampled values of the current so as to enable calculation of the reactive power supplied over the cable.
20. A method for non-invasive electricity monitoring, comprising the steps of: providing a non-invasive electricity monitoring device comprising a housing containing a conductive element and voltage sensing circuitry, wherein the voltage sensing circuitry comprises a virtual ground; receiving an electricity cable through an opening of the housing such that the conductive element contacts and conforms to an external surface of the received cable to enable capacitive coupling between the conductive element and the cable; and using the voltage sensing circuitry to sense a voltage between the conductive element and the virtual ground so as to determine a phase of the sensed voltage.
21. The method of claim 21, further comprising the step of: using current sensing circuitry provided within the housing to sense a current flowing in the cable so as to determine a phase and an amplitude of the sensed current.
22. The method of claim 20 or claim 21, further comprising the step of: storing a voltage phase delay introduced by the conductive element and the voltage sensing circuitry.
23. The method of claim 21, or claim 22 when dependent on claim 21, further comprising the step of: storing a current phase delay introduced by the current sensing circuitry.-29 -
24. The method of claim 23, further comprising the step of: sampling values of the sensed voltage and values of the sensed current, wherein the sampling accounts for the voltage phase delay and the current phase delay such that the sampled values are contemporaneous.
25. The method of claim 21, further comprising the step of: storing a total phase delay to account for a current phase delay introduced by the current sensing circuitry and a voltage phase delay introduced by the conductive element and the voltage sensing circuitry.
26. The method of claim 25, further comprising the step of: sampling values of the sensed voltage and values of the sensed current, wherein the sampling accounts for the total phase delay such that the sampled values are 15 contemporaneous.
27. The method of claim 24 or claim 26, wherein the sampled values of the sensed voltage are adjusted based on an estimated amplitude of a voltage supply to the cable.
28. The method of any one of claims 24, 26 or 27, further comprising the step of: multiplying the adjusted sampled values of the voltage by the sampled values of the current so as to enable calculation of the real power supplied over the cable.
29. The method of any one of claims 24 or 26-28, further comprising the step of: shifting the adjusted sampled values of the voltage by 90° and multiplying by the sampled values of the current so as to enable calculation of the reactive power supplied over the cable.