WO2007045814A1 - Method of testing the resistance of an electrical connection - Google Patents

Method of testing the resistance of an electrical connection Download PDF

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Publication number
WO2007045814A1
WO2007045814A1 PCT/GB2006/003395 GB2006003395W WO2007045814A1 WO 2007045814 A1 WO2007045814 A1 WO 2007045814A1 GB 2006003395 W GB2006003395 W GB 2006003395W WO 2007045814 A1 WO2007045814 A1 WO 2007045814A1
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WO
WIPO (PCT)
Prior art keywords
current
magnitude
capacitor
resistance
connection
Prior art date
Application number
PCT/GB2006/003395
Other languages
French (fr)
Inventor
Simon Peter Archbold
Original Assignee
Seaward Electronic Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Seaward Electronic Limited filed Critical Seaward Electronic Limited
Priority to EP06779408A priority Critical patent/EP1938114A1/en
Priority to AU2006303081A priority patent/AU2006303081A1/en
Publication of WO2007045814A1 publication Critical patent/WO2007045814A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
    • G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
    • G01R27/20—Measuring earth resistance; Measuring contact resistance, e.g. of earth connections, e.g. plates

Definitions

  • the present invention relates to a method of testing the resistance of an electrical connection, and relates particularly, but not exclusively, to a method of testing the resistance of an electrical connection by means of a portable apparatus .
  • Electrical appliances rely on protective earth connections as a means of protecting a user against electric shock.
  • a fault occurs in an electrical appliance, to prevent electrocution of a user current flows through a protective earth path to ground via the protective earth connection in a mains supply plug.
  • the resistance of the protective earth path must therefore be relatively small, typically less than 0.1 ohms, to result in a large current flow which operates a protective device such as a fuse to break the circuit and stop the flow of current from the main supply.
  • a method of testing a protective earth path is to pass a current through the protective earth path and measure the resulting voltage and current to enable the resistance to be calculated using Ohm's law. The measured value is then compared with a permissible resistance limit.
  • Most portable appliance testing instruments have a range of user selectable test currents. For example, a current of 25A is commonly used for many appliances. A lower current, such as 20OmA, is commonly used to test sensitive devices such as IT equipment which could be damaged by inappropriate connection to a high current source.
  • contact resistance which can be caused by film resistance such as surface oxidisation at junctions or interconnections within the protective earth path, or constriction resistance which is surface irregularities at the molecular level, increases the value of the calculated resistance.
  • the contact resistance can be of the same order of magnitude as the protective earth path and therefore contributes a significant error to the measurement of the resistance of the protective earth path, particularly when a low test current is used to determine the resistance of the protective earth path.
  • the present invention seeks to overcome the above disadvantage of the prior art.
  • a method of testing the resistance of an electrical connection comprising:
  • this provides the advantage that the first magnitude of current can be selected such that it is large enough to reduce film resistance and constriction resistance in the junctions of the protective earth path such that a contact resistance component of the resistance of the protective earth path is reduced.
  • the current is decreased to calculate the resistance of the connection, a more realistic reading of the resistance of the protective earth path is obtained.
  • the method comprises charging at least one capacitor and then discharging at least one said capacitor to provide the current of said first magnitude.
  • At least one said capacitor is charged by means of a DC to DC converter.
  • a testing apparatus for testing the resistance of an electrical connection comprising:
  • current supply means for providing a current of a first magnitude to a connection to be tested, wherein the magnitude of the current is decreased from said first magnitude in use.
  • the apparatus may further comprise a voltmeter.
  • the apparatus may further comprise an ammeter.
  • the current supply means comprises at least one capacitor for providing the current of said first magnitude .
  • the rating of the capacitor can be selected such that the initial discharge current is large enough to remove contact resistance, and as the capacitor continues to discharge the current will decrease to a level suitable for allowing a resistance test measurement.
  • the current supply means may comprise a current source for providing a current lower than said first magnitude.
  • the apparatus further comprises a DC to DC converter for increasing the magnitude of a supply voltage for charging at least one said capacitor.
  • This provides the advantage that a low current supply voltage can be stepped up by the DC to DC converter to provide a suitable level of voltage for charging the capacitor.
  • Using a small current supply and a DC to DC converter provides the advantage that the apparatus can have relatively small physical dimensions and relatively low cost.
  • the apparatus may further comprise switching means to isolate at least one said capacitor from the connection such that the capacitor can be charged.
  • said switching means comprises a P- channel MOSFET and an N-channel MOSFET controlled by a control signal inverter.
  • This provides the advantage of a switching means that has relatively small physical dimensions and relatively high resistance such that charge leakage from at least one said capacitor is minimised.
  • the apparatus further comprises a battery.
  • Figure 1 is a circuit diagram of a testing apparatus of an embodiment of the present invention.
  • Figure 2 is a voltage and current characteristic of a capacitor while discharging.
  • an earth continuity test apparatus comprises a high current pulse generator 2 connected to an earth continuity measuring circuit 4.
  • the earth continuity measuring circuit comprises two contacts 6 and 8 that can be connected across equipment under test 10 which represents an earth path whose resistance is to be determined.
  • the high current pulse generator 2 comprises a battery 12 connected to a DC to DC converter 14.
  • DC to DC converter 14 takes the DC output of the battery 12 and converts to a higher voltage level.
  • the DC to DC converter 14 is based on an LT3461 device which is a switched inductor device and will be known to persons skilled in the art.
  • the battery 12 is a 9V DC battery and the DC to DC converter 14 provides a 30V supply.
  • a 150 ⁇ F capacitor 16 is connected to the DC to DC converter 14 and is selectively isolated from the earth continuity measuring circuit 8 by a first switch 18 and a second switch 20.
  • First switch 18 is a P-channel MOSFET and second switch 20 is an N-channel MOSFET.
  • the MOSFETS 18 and 20 are isolated from the earth continuity measuring circuit 4 by respective diodes 24 and 26.
  • a control signal inverter 22 operates the MOSFETS 18 and 20 to selectively connect capacitor 16 to the earth continuity measuring circuit 4.
  • the earth continuity measuring circuit 4 comprises a current supply 32 for providing a test current such as 20OmA, a voltmeter 28 and an ammeter 30.
  • Current supply 32 may be variable to allow a user to select a predetermined test current, and the supplied current may be AC or DC.
  • the voltmeter 28 is used to calculate the voltage across the equipment under test 10
  • the ammeter 30 is used to calculate the current through the equipment under test 10 to enable a calculation of the resistance of the equipment under test using Ohm's law.
  • the equipment under test 10 is attached to contacts 6 and 8 which may be disposed on the ends of leads (not shown) and then the DC to DC converter 14 is used to provide a 30V output to charge capacitor 16.
  • the 30V supply provided by the DC to DC converter 14 is switched off.
  • a control signal is applied to the control signal inverter 22 which then opens MOSFET switches 18 and 20 allowing the capacitor to discharge through the earth continuity measuring circuit and therefore through the equipment under test 10.
  • a characteristic of the discharge pulse from a 150 ⁇ F capacitor charged to 30 volts through a resistance of 1 ohm is shown.
  • the upper part of the graph shows the current through the 1 ohm resistor and the lower part of the graph shows the voltage across the capacitor.
  • the initial current is approximately 6OA which decays in the space of just over 0.2ms to a current value that is negligible compared to the initial value.
  • the initial high current pulse generated is sufficient to overcome any contact resistance at interconnections in the earth path of the equipment under test.
  • current supply 32 is used to provide a test current, and the voltmeter 28 and ammeter ,30 can then be used to calculate the resistance of the equipment under test.
  • a microcontroller (not shown) is used to control the timing of discharging the capacitor and then applying the test current from current supply 32.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Measurement Of Resistance Or Impedance (AREA)

Abstract

An earth continuity test apparatus comprises a high current pulse generator (2) connected to an earth continuity measuring circuit (4) . The earth continuity measuring circuit comprises two contacts (6 and 8) that can be connected across equipment under test (10) which represents an earth path whose resistance is to be determined. The high current pulse generator (2) comprises a battery (12) connected to a DC to DC converter (14) . DC to DC coverter (14) provides a 30V supply to charge a capacitor (16) , which is then discharged to generate a high current pulse and remove contact resistance from equipment under test (10).

Description

Method of Testing the Resistance of an Electrical Connection
The present invention relates to a method of testing the resistance of an electrical connection, and relates particularly, but not exclusively, to a method of testing the resistance of an electrical connection by means of a portable apparatus .
Electrical appliances rely on protective earth connections as a means of protecting a user against electric shock. When a fault occurs in an electrical appliance, to prevent electrocution of a user current flows through a protective earth path to ground via the protective earth connection in a mains supply plug. The resistance of the protective earth path must therefore be relatively small, typically less than 0.1 ohms, to result in a large current flow which operates a protective device such as a fuse to break the circuit and stop the flow of current from the main supply.
It is necessary to periodically test the resistance of the protective earth path of an electrical appliance to ensure that the electrical appliance remains safe to use. A method of testing a protective earth path is to pass a current through the protective earth path and measure the resulting voltage and current to enable the resistance to be calculated using Ohm's law. The measured value is then compared with a permissible resistance limit. Most portable appliance testing instruments have a range of user selectable test currents. For example, a current of 25A is commonly used for many appliances. A lower current, such as 20OmA, is commonly used to test sensitive devices such as IT equipment which could be damaged by inappropriate connection to a high current source.
However, a problem can arise when contact resistance, which can be caused by film resistance such as surface oxidisation at junctions or interconnections within the protective earth path, or constriction resistance which is surface irregularities at the molecular level, increases the value of the calculated resistance. The contact resistance can be of the same order of magnitude as the protective earth path and therefore contributes a significant error to the measurement of the resistance of the protective earth path, particularly when a low test current is used to determine the resistance of the protective earth path.
The present invention seeks to overcome the above disadvantage of the prior art.
According to an aspect of the present invention, there is provided a method of testing the resistance of an electrical connection, the method comprising:
passing a current having a first magnitude through the connection, wherein the magnitude of the current is subsequently decreased; and
measuring the voltage across the connection during flow of current at a second magnitude in order to calculate the resistance of the connection, wherein the current having said first magnitude reduces the contact resistance of the connection and said second magnitude is lower than said first magnitude .
By passing a current having a first magnitude through the connection and then reducing the magnitude of the current, this provides the advantage that the first magnitude of current can be selected such that it is large enough to reduce film resistance and constriction resistance in the junctions of the protective earth path such that a contact resistance component of the resistance of the protective earth path is reduced. When the current is decreased to calculate the resistance of the connection, a more realistic reading of the resistance of the protective earth path is obtained.
In a preferred embodiment, the method comprises charging at least one capacitor and then discharging at least one said capacitor to provide the current of said first magnitude.
This provides the advantage that when the fully charged capacitor begins to discharge, the initial current will be large such that a high current pulse will flow to reduce contact resistance. A lower current can then be applied to enable a measurement of the resistance of the protective earth path to be made with a lower contact resistance component.
In a preferred embodiment, at least one said capacitor is charged by means of a DC to DC converter.
This provides the advantage that a low current power supply can be stepped up by the DC to DC converter for use in charging the capacitor. Using a small current supply is advantageous because a small current supply can have small physical dimensions allowing portability as well as having relatively low cost.
According to a further aspect of the present invention, there is provided a testing apparatus for testing the resistance of an electrical connection, the apparatus comprising:
current supply means for providing a current of a first magnitude to a connection to be tested, wherein the magnitude of the current is decreased from said first magnitude in use.
This provides the advantage that the current of a first magnitude can be selected so that it is large enough to reduce any contact resistance. A lower current can then be applied to enable a measurement of the resistance of the protective earth path to be made with a lower contact resistance component.
The apparatus may further comprise a voltmeter.
The apparatus may further comprise an ammeter.
In a preferred embodiment, the current supply means comprises at least one capacitor for providing the current of said first magnitude .
This provides the advantage that when at least one said capacitor begins to discharge, the rating of the capacitor can be selected such that the initial discharge current is large enough to remove contact resistance, and as the capacitor continues to discharge the current will decrease to a level suitable for allowing a resistance test measurement.
The current supply means may comprise a current source for providing a current lower than said first magnitude.
In a preferred embodiment, the apparatus further comprises a DC to DC converter for increasing the magnitude of a supply voltage for charging at least one said capacitor.
This provides the advantage that a low current supply voltage can be stepped up by the DC to DC converter to provide a suitable level of voltage for charging the capacitor. Using a small current supply and a DC to DC converter provides the advantage that the apparatus can have relatively small physical dimensions and relatively low cost.
The apparatus may further comprise switching means to isolate at least one said capacitor from the connection such that the capacitor can be charged. In a preferred embodiment, said switching means comprises a P- channel MOSFET and an N-channel MOSFET controlled by a control signal inverter.
This provides the advantage of a switching means that has relatively small physical dimensions and relatively high resistance such that charge leakage from at least one said capacitor is minimised.
In a preferred embodiment, the apparatus further comprises a battery.
A preferred embodiment of the invention will now be described, by way of example only, and not in any limitative sense, with reference to the accompanying drawings in which :-
Figure 1 is a circuit diagram of a testing apparatus of an embodiment of the present invention; and
Figure 2 is a voltage and current characteristic of a capacitor while discharging.
Referring to Figure 1, an earth continuity test apparatus comprises a high current pulse generator 2 connected to an earth continuity measuring circuit 4. The earth continuity measuring circuit comprises two contacts 6 and 8 that can be connected across equipment under test 10 which represents an earth path whose resistance is to be determined.
The high current pulse generator 2 comprises a battery 12 connected to a DC to DC converter 14. DC to DC converter 14 takes the DC output of the battery 12 and converts to a higher voltage level. The DC to DC converter 14 is based on an LT3461 device which is a switched inductor device and will be known to persons skilled in the art. In practice, the battery 12 is a 9V DC battery and the DC to DC converter 14 provides a 30V supply. A 150μF capacitor 16 is connected to the DC to DC converter 14 and is selectively isolated from the earth continuity measuring circuit 8 by a first switch 18 and a second switch 20. First switch 18 is a P-channel MOSFET and second switch 20 is an N-channel MOSFET. The MOSFETS 18 and 20 are isolated from the earth continuity measuring circuit 4 by respective diodes 24 and 26. A control signal inverter 22 operates the MOSFETS 18 and 20 to selectively connect capacitor 16 to the earth continuity measuring circuit 4.
The earth continuity measuring circuit 4 comprises a current supply 32 for providing a test current such as 20OmA, a voltmeter 28 and an ammeter 30. Current supply 32 may be variable to allow a user to select a predetermined test current, and the supplied current may be AC or DC. The voltmeter 28 is used to calculate the voltage across the equipment under test 10, and the ammeter 30 is used to calculate the current through the equipment under test 10 to enable a calculation of the resistance of the equipment under test using Ohm's law.
It will be appreciated by persons skilled in the art that the apparatus of Figure 1 is portable such that it can be operated by an Engineer to test the resistance of the earth path of electric appliances.
In order to test the resistance of the earth path of an electrical appliance, the equipment under test 10 is attached to contacts 6 and 8 which may be disposed on the ends of leads (not shown) and then the DC to DC converter 14 is used to provide a 30V output to charge capacitor 16. When the capacitor 16 is fully charged, the 30V supply provided by the DC to DC converter 14 is switched off. A control signal is applied to the control signal inverter 22 which then opens MOSFET switches 18 and 20 allowing the capacitor to discharge through the earth continuity measuring circuit and therefore through the equipment under test 10.
Referring to Figure 2, a characteristic of the discharge pulse from a 150μF capacitor charged to 30 volts through a resistance of 1 ohm is shown. The upper part of the graph shows the current through the 1 ohm resistor and the lower part of the graph shows the voltage across the capacitor. It can be seen that the initial current is approximately 6OA which decays in the space of just over 0.2ms to a current value that is negligible compared to the initial value. The initial high current pulse generated is sufficient to overcome any contact resistance at interconnections in the earth path of the equipment under test. After discharge of the capacitor, current supply 32 is used to provide a test current, and the voltmeter 28 and ammeter ,30 can then be used to calculate the resistance of the equipment under test. A microcontroller (not shown) is used to control the timing of discharging the capacitor and then applying the test current from current supply 32.
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 alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims. In particular, it will be appreciated by persons skilled in the art that using a capacitor to provide an initial large current pulse is only one way of achieving the objective of the present invention. The skilled person would realise that other sources of high current pulses could be used.

Claims

1. A method of testing the resistance of an electrical connection, the method comprising:
passing a current having a first magnitude through the connection, wherein the magnitude of the current is subsequently decreased; and
measuring the voltage across the connection during flow of current at a second magnitude in order to calculate the resistance of the connection, wherein the current having said first magnitude reduces the contact resistance of the connection and said second magnitude is lower than said first magnitude .
2. A method according to claim 1, comprising charging at least one capacitor and then discharging at least one said capacitor to provide the current of said first magnitude.
3. A method according to claim 2, wherein at least one said capacitor is charged by means of a DC to DC converter.
4. A testing apparatus for testing the resistance of an electrical connection, the apparatus comprising:
current supply means for providing a current of a first magnitude to a connection to be tested, wherein the magnitude of the current is decreased from said first magnitude in use.
5. An apparatus according to claim 4, further comprising a voltmeter .
6. An apparatus according to claim 4 or 5, further comprising an ammeter.
7. An apparatus according to any one of claims 4 to 6, wherein the current supply means comprises at least one capacitor for providing the current of said first magnitude.
8. An apparatus according to any one of claims 4 to 7, wherein the current supply means comprises a current source for providing a current lower than said first magnitude.
9. An apparatus according to claim 7 or 8, further comprising a DC to DC converter for increasing the magnitude of a supply voltage for charging at least one said capacitor.
10. An apparatus according to any one of claims 7 to 9, further comprising switching means to isolate at least one said capacitor from the connection such that the capacitor can be charged.
11. An apparatus according to claim 10, wherein said switching means comprises a P-channel MOSFET and an N-channel MOSFET controlled by a control signal inverter.
12. An apparatus according to any one of claims 4 to 11, further comprising a battery.
13. An apparatus substantially as hereinbefore described with reference to the accompanying drawings .
PCT/GB2006/003395 2005-10-19 2006-09-14 Method of testing the resistance of an electrical connection WO2007045814A1 (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
EP06779408A EP1938114A1 (en) 2005-10-19 2006-09-14 Method of testing the resistance of an electrical connection
AU2006303081A AU2006303081A1 (en) 2005-10-19 2006-09-14 Method of testing the resistance of an electrical connection

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0521197.4 2005-10-19
GB0521197A GB0521197D0 (en) 2005-10-19 2005-10-19 Method of testing the resistance of an electrical connection

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WO2007045814A1 true WO2007045814A1 (en) 2007-04-26

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AU (1) AU2006303081A1 (en)
GB (1) GB0521197D0 (en)
WO (1) WO2007045814A1 (en)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102009050279A1 (en) 2009-10-21 2011-05-05 Metrawatt International Gmbh Method for testing resistance of electrical ground connection of electrical load, involves producing cleaning power by discharge of capacitor, and charging capacitor by testing current generator producing testing current
CN103383417A (en) * 2013-07-24 2013-11-06 中达电通股份有限公司 Insulation monitoring method for switching type direct current system
WO2021237977A1 (en) * 2020-05-26 2021-12-02 捍防（苏州）实业有限公司 Apparatus and method for monitoring resistance between null line and ground line

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1100171A (en) 1965-09-20 1968-01-24 Ml Aviation Co Ltd Improvements relating to earth line continuity testers
EP0295800A2 (en) 1987-05-27 1988-12-21 Robin Electronics Limited Circuit testing
GB2268811A (en) 1992-06-20 1994-01-19 Jekyll Electronic Technology L Calibrator for loop impedence tester
GB2277598A (en) 1993-04-20 1994-11-02 Jekyll Electronic Technology L Portable appliance testers
DE4332649A1 (en) 1993-09-27 1995-03-30 Metrawatt Gmbh Gossen Method for safety testing of an electrical device, in particular for measuring the earth-conductor resistance
EP0763745A2 (en) 1995-09-18 1997-03-19 Lem Norma GmbH Method and device for testing electrical apparatus with or without protective earth
US6208146B1 (en) 1999-02-08 2001-03-27 General Motors Corporation Method and apparatus for measuring contact resistance for spot welding simulations

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB1100171A (en) 1965-09-20 1968-01-24 Ml Aviation Co Ltd Improvements relating to earth line continuity testers
EP0295800A2 (en) 1987-05-27 1988-12-21 Robin Electronics Limited Circuit testing
GB2268811A (en) 1992-06-20 1994-01-19 Jekyll Electronic Technology L Calibrator for loop impedence tester
GB2277598A (en) 1993-04-20 1994-11-02 Jekyll Electronic Technology L Portable appliance testers
DE4332649A1 (en) 1993-09-27 1995-03-30 Metrawatt Gmbh Gossen Method for safety testing of an electrical device, in particular for measuring the earth-conductor resistance
EP0763745A2 (en) 1995-09-18 1997-03-19 Lem Norma GmbH Method and device for testing electrical apparatus with or without protective earth
US6208146B1 (en) 1999-02-08 2001-03-27 General Motors Corporation Method and apparatus for measuring contact resistance for spot welding simulations

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102009050279A1 (en) 2009-10-21 2011-05-05 Metrawatt International Gmbh Method for testing resistance of electrical ground connection of electrical load, involves producing cleaning power by discharge of capacitor, and charging capacitor by testing current generator producing testing current
DE102009050279B4 (en) * 2009-10-21 2017-12-14 Metrawatt International Gmbh Apparatus and method for checking the resistance of electrical ground connections
CN103383417A (en) * 2013-07-24 2013-11-06 中达电通股份有限公司 Insulation monitoring method for switching type direct current system
WO2021237977A1 (en) * 2020-05-26 2021-12-02 捍防（苏州）实业有限公司 Apparatus and method for monitoring resistance between null line and ground line

Also Published As

Publication number Publication date
EP1938114A1 (en) 2008-07-02
AU2006303081A1 (en) 2007-04-26
GB0521197D0 (en) 2005-11-23

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