GB2559120A

GB2559120A - Method and apparatus for determining impedance of live-earth loop of an electrical power supply

Info

Publication number: GB2559120A
Application number: GB1701004.2A
Authority: GB
Inventors: Wallace James; ying Thomas; Osborne Andrew; Barker Anthony
Original assignee: SEAWARD ELECTRONICS Ltd
Current assignee: SEAWARD ELECTRONICS Ltd
Priority date: 2017-01-20
Filing date: 2017-01-20
Publication date: 2018-08-01
Anticipated expiration: 2037-01-20
Also published as: GB201701004D0; GB2559120B

Abstract

A method and apparatus 20 are disclosed for determining the impedance of a live-earth loop of an electrical power supply circuit having live, earth and neutral conductors 4, 12, 16. The method comprises: determining values of voltage between live and neutral terminals 6, 10 and between live and earth terminals 6, 14 both while a resistive load 38 is connected between the terminals and while no load is connected; applying two test current pulses of different magnitude and duration to the neutral and earth terminals 10, 14, neither pulse having sufficient magnitude and/or duration to activate a residual current device (RCD) circuit breaker 18 connected between the live and neutral terminals; determining values of voltage and current between neutral and earth terminals while the test current is applied; and determining the impedance of the live-earth loop by using the measured values. The live-earth impedance may be calculated by first obtaining impedances RL and RN for the live and neutral conductors, and RNE, the combined impedance of the neutral and earth conductors. A more accurate value of RNE may be obtained by correcting for uplift resistance, due to the presence of the RCD, and for residual voltage and/or current.

Description

(54) Title of the Invention: Method and apparatus for determining impedance of live-earth loop of an electrical power supply

Abstract Title: Determining impedance of a live-earth loop of an electrical power supply (57) A method and apparatus 20 are disclosed for determining the impedance of a live-earth loop of an electrical power supply circuit having live, earth and neutral conductors 4, 12, 16. The method comprises: determining values of voltage between live and neutral terminals 6, 10 and between live and earth terminals 6, 14 both while a resistive load 38 is connected between the terminals and while no load is connected; applying two test current pulses of different magnitude and duration to the neutral and earth terminals 10, 14, neither pulse having sufficient magnitude and/or duration to activate a residual current device (RCD) circuit breaker 18 connected between the live and neutral terminals; determining values of voltage and current between neutral and earth terminals while the test current is applied; and determining the impedance of the live-earth loop by using the measured values. The liveearth impedance may be calculated by first obtaining impedances RL and RN for the live and neutral conductors, and RNE, the combined impedance of the neutral and earth conductors. A more accurate value of RNE may be obtained by correcting for uplift resistance, due to the presence of the RCD, and for residual voltage and/or current.

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Method and Apparatus for determining impedance of live-earth loop of an electrical power supply

Technical Field

The present invention relates to a method and apparatus for determining the impedance of a live-earth loop of an electrical power supply, and relates particularly, but not exclusively, to a method and apparatus for determining the impedance of a live-earth loop of an electrical power supply containing a residual current device (RCD) circuit breaker.

Background Art

EP 1950575 A2 describes a method and apparatus for determining the impedance of live-earth loop of an electrical power supply where a residual current device (RCD) circuit breaker may be present. The live-earth loop impedance is derived from measurements of the live-neutral loop and the neutral-earth loop. The neutral-earth loop measurement described in EP 1950575 A2 uses a pulsed current having an amplitude greater than the operating current of an RCD but with a short duration such that the energy is limited and the RCD will not operate.

The method described in EP 1950575 A2 has been used successfully in a commercial product for several years. However, some users have reported what they refer to as RCD uplift where the measured earth loop impedance is greater than the expected value or greater than measurements taken using other measurement tools. In some cases, an actual earth loop of 0.5 ohms could be measured as 1.5 ohms. The applicants have determined that this RCD uplift is a result of the measurement technique used for the neutral-earth loop impedance, employing a short-duration, high-frequency pulse, which is being affected by the presence of inductance in the RCD.

Commonly, an RCD operates by using a differential current transformer in the live and neutral path to detect residual currents. Any imbalance in live and neutral currents, for example due to current flowing from live to earth, causes the RCD to operate. As with any wound component, the differential current transformer will have resistance and inductance. If earth loop measurements are made using waveforms other than pure DC, the reactance due to the RCD coil inductance will be included in the measured impedance. The impedance of the RCD coil at 50Hz will be minimised by design to minimise the power dissipation within the RCD and so the influence on earth loop measurements made using an AC test waveform with a frequency close to the power frequency will be negligible. However, if the earth loop measurement waveform contains frequencies much greater than the power frequency, the impedance contribution due to the RCD coil inductance can be significant.

A measurement pulse with a duration of the order of lOus for example will contain frequencies much greater than 50Hz and the inductance of the RCD is likely to introduce significant errors in the earth loop impedance measurement. The inductance is unknown and will vary from RCD device to device, so a simple correction for the inductance value is not possible.

Summary of the Invention

According to an aspect of the present invention, there is provided a method of determining impedance of a live-earth loop of an electrical power supply circuit having live, earth and neutral conductors, the method comprising:

(i) determining values of voltage between live and neutral terminals of the circuit while a resistive load is connected between live and neutral terminals of the circuit and while no resistive load is connected between live and neutral terminals of the circuit;

(ii) determining values of voltage between live and earth terminals of the circuit while a resistive load is connected between live and neutral terminals of the circuit and while no resistive load is connected between live and neutral terminals of the circuit;

(iii) applying a test current to said neutral and earth terminals, wherein the test current is in the form of two current pulses of different magnitude and duration, neither pulse having a sufficient magnitude and/or duration to activate a residual current device (RCD) circuit breaker connected between said live and neutral terminals;

(iv) determining values of voltage and current between neutral and earth terminals of the circuit while said test current is applied; and (v) determining the impedance of a live-earth loop of the circuit by means of the values determined in steps (i), (ii) and (iv).

In a preferred embodiment, one of the current pulses is a relatively high current pulse of relatively short duration and the other current pulse is a relatively low current pulse of relatively long duration. The relatively high current pulse of relatively short duration preferably has insufficient duration to activate the RCD, while the current magnitude would otherwise activate the RCD. The relatively low current pulse of relatively long duration preferably has insufficient magnitude to activate the RCD. In a particularly preferred embodiment, the relatively high current pulse of relatively short duration comprises a pulse of 100mA for lOus. The relatively low current pulse of relatively long duration may comprise a pulse which is for example 5-10 times longer but 5-10 times lower in current magnitude than the high/short pulse. The low/long pulse may comprise a pulse of 10mA for lOOus.

In order to avoid nuisance-tripping of the RCD during testing, it is preferred that the total energy of the current pulse (calculated by (current) x time) is not greater than lOOnJ. There is a wide range of RCD types in use, and it is preferable that the method and apparatus of the present invention works with the majority of, If not all, RCD types. The upper limit of lOOnl has been found to meet this requirement, and this corresponds to the energy of the high/short pulse of 100mA for lOus. A practical lower limit for the energy of the current pulse Is preferably 10nJ, which corresponds to the energy of the low/long pulse of 10mA for lOOus. In a particularly preferred embodiment therefore, the energy of each current pulse Is from 10nJ to lOOnJ.

Preferably, the step of determining the impedance of the live-earth loop includes determining the total impedance of the earth and neutral conductors by means of the values determined in step (iv).

In a preferred embodiment, the method further comprises the steps of:

determining values of residual voltage and/or current between neutral and earth terminals of the circuit while the test current is not being applied;

subtracting the values of residual voltage and/or current from the values determined in step (iv) to provide corrected values of voltage and current between neutral and earth terminals; and determining the total impedance of the earth and neutral conductors by means of of the corrected values.

In a further preferred embodiment, the method comprises the steps of:

calculating the uplift resistance due to the presence of the RCD in the circuit by means of the values of voltage and current determined in step (iv);

subtracting the uplift resistance from the total impedance of the earth and neutral conductors to provide a corrected total impedance of the earth and neutral conductors; and determining the impedance of the live-earth loop of the circuit using the corrected total impedance ofthe earth and neutral conductors.

In a still preferred embodiment, the method comprises the steps of:

subtracting the values of residual voltage and/or current from the values determined in step (iv) to provide corrected values of voltage and current between neutral and earth terminals; and calculating the uplift resistance by means of the corrected values.

Preferably, the step of determining the impedance of said live-earth loop further comprises: determining the total impedance of the live and neutral conductors by means of the values determined in step (i);

determining the impedance of the live conductor by means of the values determined in step (ii);

determining the impedance of the neutral conductor by means of the difference between the values determined in steps (i) and (ii); and determining the impedance of the live-earth loop of the circuit by subtracting the impedance of the neutral conductor from the total impedance of the earth and neutral conductors and adding the impedance of the live conductor.

The impedance of the live conductor may include the impedance of a mains transformer in the circuit.

According to another aspect of the present invention, there is provided a measuring apparatus for determining impedance of a live-earth loop of an electrical power supply circuit having live, earth and neutral conductors, the apparatus comprising:

first measuring means for determining values of voltage between live and neutral terminals of the circuit while a resistive load is connected between live and neutral terminals of the circuit and while no resistive load is connected between live and neutral terminals of the circuit;

second measuring means for determining values of voltage between live and earth terminals of the circuit while a resistive load is connected between live and neutral terminals of the circuit and while no resistive load is connected between live and neutral terminals of the circuit;

current generating means for applying a test current to said neutral and earth terminals, wherein the test current is in the form of two current pulses of different magnitude and duration, neither pulse having a sufficient magnitude and/or duration to activate a residual current device (RCD) circuit breaker connected between said live and neutral terminals;

third measuring means for determining values of voltage and current between neutral and earth terminals of the circuit while said test current is applied; and calculating means for determining the impedance of a live-earth loop of the circuit by means of the values determined by said first, second and third measuring means.

The invention also provides a measuring apparatus for determining impedance of a live-earth loop of an electrical power supply circuit having live, earth and neutral conductors, the apparatus being configured to carry out the method described above.

In summary, and at least in preferred embodiments, the invention relates to a method and apparatus for determining impedance of a live-earth loop of an electrical power supply circuit having live, earth and neutral conductors, particularly when a residual current device (RCD) is present in the circuit. In the first stage of the method, impedances for the live (RL) and neutral (RN) conductors are obtained by known methods. In the second stage, the combined impedance of the neutral and earth conductors (RNE) is obtained. From these values, the impedance of the live-earth loop RLE can be obtained. The present invention aims to provide a corrected RNE value by employing two current pulses, one relatively short but relatively high current pulse and one relatively long but relatively low current pulse. By analysing the current waveforms and also correcting for any residual voltage/current, the uplift resistance due to the effect of the RCD can be calculated and deducted from the measured RNE value to provide a corrected RNE value for determining the live-earth loop impedance RLE.

Detailed Description of a Preferred Embodiment

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows an impedance measuring apparatus connected to a mains electrical power supply for carrying out a first phase of a method in accordance with invention;

Figure 2 shows the apparatus of Figure 1 connected to the power supply for carrying out a second phase of a method in accordance with invention;

Figure 3 shows a current pulse generating circuit used in the apparatus to carry out the second phase of the method;

Figure 4 shows a high current/short pulse and offset measurement used in the second phase of the method; and

Figure 5 shows a low current/long pulse and offset measurement also used in the second phase of the method.

Referring to Figure 1, a mains power supply circuit has a live terminal 6 connected to a live conductor 4 having impedance RL, a mains transformer 8, a neutral terminal 10 connected to a neutral conductor 12 having impedance RN, and an earth terminal 14 connected to an earth conductor 16 having impedance RE. A residual current device (RCD) circuit breaker 18 is connected between the live conductor 4 and the neutral conductor 12.

An impedance measuring apparatus 20 embodying the present invention has a housing 22 and includes a voltage-measuring device 24, a current generator 26, a processor 28, and a display 30, which may be any suitable display device familiar to persons skilled in the art, such as a liquid crystal display (LCD).

In order to determine the impedance of the live-earth loop of the power supply, for example in order to determine whether the correctly rated fuses are installed in the power supply circuit, the impedance measuring apparatus 20 is firstly connected by means of leads 32 and 34 between the live and neutral terminals 6 and 10 of the power supply circuit. The voltage drop between the live and neutral terminals 6 and 10 is then determined for unloaded and loaded conditions, i.e. with switch 36 closed, allowing current to pass through a load resistor 38, or with the switch 36 open.

The difference between the loaded and unloaded measurements of VI, the voltage between the live and neutral terminals 6 and 10, indicates the voltage drop through the resistance RL of the live conductor 4, the mains transformer 8, and the resistance RN of the neutral conductor 12. A total value can therefore be obtained for RLN using:

RLN = ^V1™loaded -^loaded ‘load

The voltage between the live and earth terminals 6 and 14 is shown in Figure 1 as V2. This is also measured when the load resistor 38 is connected between the live and neutral terminals 6 and 10 and disconnected. The difference between the measured values of V2 for unloaded and loaded conditions (across the live and neutral terminals) indicates the voltage drop through the resistance RL of the live conductor 4 and the mains transformer 8 and a value can be obtained for RL (incorporating the mains transformer 8 resistance).

Further, by subtracting the difference in the measured values of V2 when the resistor 38 is connected and disconnected from the difference between the loaded and unloaded measurements of VI, the resistance RN of the neutral conductor 12 can be determined.

Referring now to Figure 2, in a second stage of the process of the present invention, the leads 32 and 34 of the apparatus 20 are connected between the neutral and earth terminals 10 and 14, and a test current is generated by current generator 26 and passed between the neutral and earth terminals 10 and 14. The test current is in the form of a series of current pulses as described below.

By measuring the test current and the voltage across the neutral-earth loop, the combined resistance RNE of the neutral conductor 12 RN and the earth conductor 16 RE can be determined by the apparatus 20. RNE is corrected for any uplift resistance due to the presence of an RCD as described below, and the loop impedance of the live-earth loop is then determined from:

Loop Impedance RLE = RNE (corrected) - RN + RL

The current generator 26 in accordance with the invention is shown in more detail in figure 3.

The current generator produces two pulses, one high current short duration pulse (nominally 100mA for lOus) and one low current long duration pulse (nominally 10mA for lOOus). RCDs with a rated tripping current of 30mA are widely used for supplementary protection. Therefore the low/long pulse is too low in magnitude to trip typical RCDs. Limiting the duration of the high/short pulse limits the energy to a point where the RCD cannot 'see' the pulse.

Although it is possible to use batteries to generate the current pulses required for the neutral earth measurement, this is inefficient and can lead to a significantly reduced battery life. The arrangement in figure 3 enables the generation of a high current test pulse and a low current test pulse which uses the mains voltage as a source. This involves charging capacitors Cl and C2 on one half cycle and using the charge in capacitors Cl and C2 to create the test currents on the inverse half cycle.

When the current test is required, a solid-state relay SW1 is closed on every negative half mains cycle. This enables capacitor Cl or C2 to charge to 22V on negative mains half cycles. The capacitor to be charged is selected by means of solid state relays SW3 and SW4. An internal voltage monitor (not shown) is used to confirm that capacitor Cl or C2 is fully charged. The charge in capacitors Cl and C2 is limited by zener diodes D2 and D3. Relay SW1 is constantly switched throughout the duration of the test to maintain the charge in capacitors Cl or C2, and when the capacitor is fully charged the test can be performed.

During a positive half cycle of the mains, relay SW1 is open and one of switch SW3 and SW4 is closed depending on the required pulse amplitude. FET SW2 is enabled. The duration for which FET SW2 is enabled is controlled by the microprocessor 28 and the duration of the test pulse can therefore be controlled. Resistor R3 is chosen such that the peak amplitude of the test current delivered by capacitor C2 is approximately 10% of that delivered by capacitor Cl. This allows the duration of the test current pulse delivered by capacitor C2 to be extended, by means of FET SW2, without the risk of tripping any RCD in the circuit under test.

The capacitor Cl or C2 discharges through diode D4, the resistance RN of the neutral conductor 12, the resistance RE of the earth conductor, resistor R2 and the FET SW2. During the test pulse both V3 and V4 are measured. V3 measures the voltage drop directly across the neutral and earth resistance RN and RE while V4 measures the drop across the current measurement shunt R2 and therefore the current flowing can be calculated (I = V4/R2).

Voltage can be present on the neutral conductor 12 due to loads on the circuit under test. Diode D4 is used to isolate the test circuit from the neutral and relay SW1 is used to bypass diode D4 on negative half cycles so that the capacitor can be charged.

Directly after the current pulse is applied, any residual voltage present between the neutral and earth conductors is measured and will be subtracted from the value of V3 measured when the test pulse is applied.

Measurements are made on successive positive going mains half cycles and the measurement sequence is:

1. High current/short pulse measurements (V3 and V4)

2. V3 offset (residual voltage) and V4 offset (residual current) measurement

3. Low current/long pulse measurements (V3 and V4)

4. V3 offset (residual voltage) and V4 offset (residual current) measurement

Measurement steps 1 and 2 in the sequence are shown in Figure 4. Voltage samples are obtained by measuring voltage V3. Current samples are obtained by measuring voltage V4.

Short pulse measurement • Start Short pulse after T_s from positive zero crossing.

• Start voltage and current samplings after T_Ss • Sample a times before the end of the short pulse (i.e. V_SPo, Ispo, , V_SPa-i, Ispa-i) • All samples are raw ADC output values

Offset measurement • Start voltage and current samplings after (7_S + Tss) from positive zero crossing • Sample a times (i.e. V_oso, loso, -, V_Osa-i, losa-i) • All samples are raw ADC output values

Short pulse voltage V_s and short pulse current l_s values are calculated as follows:

a-1 ^Vs = ~ ^x JOsPfc - ^vos_k) k=o k = - ^x - Ios_k) k=o

Measurement steps 3 and 4 in the sequence are shown in Figure 5. Again, voltage samples are obtained by measuring voltage V3 and current samples are obtained by measuring voltage V4.

Long pulse measurement • Start Long pulse after T_L from positive zero crossing.

• Start voltage and current samplings after T_LS • Sample b times before the end of the long pulse (i.e. V_LP0, l_LP0,..., V_Lpb-i, kpb-i) • All samples are raw ADC output values

Offset measurement • Start voltage and current samplings after (T_L + T_LS) from positive zero crossing • Sample b times (i.e. V_oso, loso, -, V_osb-i, losb-i) • All samples are raw ADC output values

Long pulse voltage V_L and long pulse current l_L values are calculated as follows:

b-l ^VL ⁼~j~ ^X ^(YLPk ~ ^VOS_k) k=o i ^bl ^II=b^x Σ ^^LPk ~^Ios^ k=o

The sampled data is then analysed as follows to determine any error, referred to below as RCD uplift, introduced by the inductance of an RCD present in the circuit under test.

Detecting RCD uplift:

• RCD uplift is detected if (V_LFmnt-V_LEnd) > Uplift_Limit, where • v_LFront = - χ ’Z_k=o(y_SPk — v_OSk) • ^LEnd = ~ ^x T,k=b-y/sp_k ~ V_OSk) • 0 < x < b, typically x=b/2 • 0 < y <b, typically y=b/2 • The value of the Uplift_Limit is set based on the ADC resolution and Signal to Noise ratio. In preferred embodiments, the value was derived from empirical data collected by capturing the pulse response of a wide variety RCD makes and models. The data was analysed and used to derive a suitable value.

Calculating Uplift Resistance (R_adj) if RCD uplift is detected:

• Vadj = UpliftJactor x (V_L - V_LEnd)/V_LEnd • 0 < Upliftjactor < 1, typically Upliftjactor = 0.5 • Radj ~ M_eX Vadj/^L • M_l is the long pulse calibration factor, which is a scale factor determined during the calibration of the voltage and current measurement functions when measuring a known resistance value.

Calculating RNE:

• If no RCD uplift is detected:

o Rne = M_s x — + C_s where Is o M_s and C_s are the short pulse calibration factors, which are scale factors determined during the calibration of the voltage and current measurement functions when measuring a known resistance value.

• If RCD uplift is detected:

° ^— Ms ^X I 0_s — R_adj ¹S

The live-earth loop impedance is then determined from:

Loop Impedance RLE = RNE (corrected) - RN + RL

It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only, and not in any limitative sense, and that various alternations and modifications are possible without departure from the scope of the invention as defined by the appended claims.

Claims

1. A method of determining impedance of a live-earth loop of an electrical power supply circuit having live, earth and neutral conductors, the method comprising:

2. The method of claim 1, wherein one of the current pulses is a relatively high current pulse of relatively short duration and the other current pulse is a relatively low current pulse of relatively long duration.

3. The method of claim 2, wherein the relatively high current pulse of relatively short duration has insufficient duration to activate the RCD.

4. The method of claim 2 or 3, wherein the relatively low current pulse of relatively long duration has insufficient magnitude to activate the RCD.

5. The method of claim 2, 3 or 4, wherein the relatively high current pulse of relatively short duration comprises a pulse of 100mA for lOus.

6. The method of any of claims 2 to 5, wherein the relatively low current pulse of relatively long duration comprises a pulse of 10mA for lOOus.

7. The method of any preceding daim, wherein the energy of each current pulse is from lOnJ to WOnJ.

8. The method of any preceding claim, wherein the step of determining the impedance of the live-earth loop includes determining the total impedance of the earth and neutral conductors by means of the values determined in step (iv).

9. The method of claim 8, further comprising:

10. The method of claim 8 or 9, further comprising:

subtracting the uplift resistance from the total impedance of the earth and neutral conductors to provide a corrected total impedance of the earth and neutral conductors; and determining the impedance of the live-earth loop of the circuit using the corrected total impedance of the earth and neutral conductors.

11. The method of claim 10, further comprising:

12. The method of any of claims 8 to 11, wherein the step of determining the impedance of said live-earth loop further comprises:

determining the total impedance of the live and neutral conductors by means of the values determined in step (i);

13. The method of claim 12, wherein the impedance of the live conductor includes the impedance of a mains transformer in the circuit.

14. A measuring apparatus for determining impedance of a live-earth loop of an electrical power supply circuit having live, earth and neutral conductors, the apparatus comprising:

15. The measuring apparatus of claim 14, wherein one of the current pulses is a relatively high current pulse of relatively short duration and the other current pulse is a relatively low current pulse of relatively long duration.

5

16. The measuring apparatus of claim 15, wherein the relatively high current pulse of relatively short duration has insufficient duration to activate the RCD.

17. The measuring apparatus of claim 15 or 16, wherein the relatively low current pulse of relatively long duration has insufficient magnitude to activate the RCD.

.0

18. A measuring apparatus for determining impedance of a live-earth loop of an electrical power supply circuit having live, earth and neutral conductors, the apparatus being configured to carry out the method of any of claims 1 to 13.

Application No: GB 1701004.2