GB2416594A

GB2416594A - Electrical resistance measuring apparatus

Info

Publication number: GB2416594A
Application number: GB0416461A
Authority: GB
Inventors: David Robert Harris
Original assignee: KEW TECHNIK Ltd
Current assignee: KEW TECHNIK Ltd
Priority date: 2004-07-23
Filing date: 2004-07-23
Publication date: 2006-02-01
Also published as: GB0416461D0

Abstract

An apparatus to indicate whether a measured resistance 2 is above or below a preset threshold comprises: means such as a high voltage inverter 1 to generate an electrical potential; means such as potential divider 4,5 shunt 3 and analogue to digital converter 6 to measure the potential and the current; means such as digital divider 7 to divide voltage by current; and means such as comparator 8a to generate signals B,G indicating whether the resistance value is above or below a predetermined value. Signals B,G may be indicate by displays 9a,9b and may be used to indicate whether the voltage or current is less than respective predetermined values. The apparatus may be used by an electrician to measure the insulation resistance between live and neutral conductors in a circuit in a building and may be housed in a housing having contacts that may be inserted into an electrical socket.

Description

241 6594 Title: Electrical Resistance Measuring Apparatus The present

invention relates to an apparatus to measure whether the resistance of an electrical item under test is greater than or less than a predetermined value.

When a new wiring circuit has been installed in a building, it is normally a requirement that an electrician checks that there are no short circuits or open circuits, and that good insulation, i.e. high resistance, exists between positive and negative and earth wires. A common method of measuring this is to apply a voltage across the circuit of 500V and then measure the actual voltage and actual current flowing. From this, the actual resistance ofthe insulation can also be calculated by the formula V/I = R. The electrician then needs to decide whether the resistance is above or below a predetermined value usually of the order of 2MQ.

Testers currently used to take these measurements have a number of disadvantages. One disadvantage is that it talces time for the tester to measure the actual voltage and current since it has to scan through all the possible voltage and current values from zero to the actual voltage or current value. This is wasted time for the electrician and it is also a drain on the battery life of the tester. Also the electrician has to interpret the results and decide by calculation whether the insulation value is too high or low. This can lead to human errors.

In reality the exact value of voltage or current flowing to obtain an actual resistance value is not critical, all that is critical is to find out if the resistance value is above or below about 2MQ.

The invention seeks to provide an improved tester to solve these difficulties by measuring whether the resistance of an electrical item under test is greater than or less than a predetermined value.

Apparatus to measure whether the resistance of an electrical item under test is greater than or les than a predetermined value comprising: a) mean to generate an electrical potential across an item of electrical equipment b) means to measure the voltage assuming it is within a voltage range across the electrical item c) means to measure the current assuming it is within a current range across the electrical item d) means to create a measured resistance value by dividing the measured voltage by the measured current e) means to compare the measured resistance value with a predetermined resistance value, and e) means to generate a first signal if the measured resistance value is greater than the predetermined resistance value, and mean to generate a second signal if the measured resistance value is less than the predetermined resistance value.

Preferably the apparatus further includes means to compare the measured voltage with a predetermined voltage value and to generate said second signal if the measured voltage is less than the predetermined voltage value.

Preferably the apparatus further includes means to compare the measured current with a predetermined current value and to generate said first signal if the measured current is less than the predetermined current value.

Preferably the apparatus includes a display to indicate the generation of said first or second signal.

Preferably the means to measure the voltage and current includes an analogue to digital converter to convert analogue measurements to digital values. Preferably the means to create a measured resistance value includes a digital divider.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a schematic block diagram, and Figure 2 shows a flow diagram.

Referring to Figures 1 and 2 there is shown an apparatus to measure whether the resistance of an electrical item under test is greater than or les than a predetermined value.

The apparatus has a high voltage inverter 1 to generate an electrical potential across an item of electrical equipment 2. Item 2 in this example is an electrical circuit and the resistance being measured is the resistance of insulation between positive and negative and earth wires. Inverter I typically delivers 500volts. It is expected that the resistance of the insulation will be greater than 2MQ, the value expected in wiring cables. In theory therefore (on the basis that V/1 = R), an expected current of 25OIlA would be expected.

A current shunt 3 and analogue to digital converter 6 are used to measure the current and are set expecting to read a current value within a current range of about 250,uA.

A potential divider 4,5 and analogue to digital converter 6 are used to measure the voltage and are set expecting to read a voltage value with a voltage range of about 500V. s

A digital divider 7 measures a resistance value by dividing the measured voltage by the measured current as provided by the analogue to digital converter 6.

A first comparator 8A compares the measured resistance value with a predetermined resistance value of 2MQ. Comparator 8A then generates a first signal if the measured resistance value is greater than the predetermined resistance value which is displayed as a "pass" or >2MQ by display 9A, or generates a second signal if the measured resistance value is less than the predetermined resistance value which is displayed as a "fail" or <2MQ bydisplay9B.

A second comparator 8b compares the measured voltage as provided by the analogue to digital converter 6 with a predetermined minimum voltage value and generates the second signal if the measured voltage is less than the predetermined voltage value which is displayed as a "fail" or <2MQ by display 9B.

A third comparator 8C compares the measured current with a predetermined current value and to generate the first signal if the measured current is less than the predetermined current value which is displayed as a "pass" or >2MQ by display 9A.

It is envisaged that the apparatus shown in Figure I would be housed in a housing with three contacts positive negative and earth extending therefrom so that the housing could be inserted into an electrical socket.

As shown in Figure 2, in use an electrician could insert the apparatus into an electrical socket and press a test button. The inverter I would then apply a 500V potential across the positive and negative wires in the circuit. After a short period of time, the voltage would first be measured and compared by comparator 8b with a predetermined minimum voltage value. If it was less than this value, this would indicate short circuit or other similar fault immediately displaying a "fail" in display 9b. The current would then be measured and compared by comparator 8c with a predetermined maximum value. If it was less than this, would indicate that the resistance of the insulation was well in excess of 2MQ e.g. 200MQ displaying a "pass" in display 9b. Thus the test for the actual resistance becomes avoidable. Comparator 8C may be an optional feature.

Assuming the voltage test and current test did not indicate a "fail" or "pass" respectively, the digital divider would then measure the resistance and compare it with the predetermined resistance value of 2MQ. Comparator 8A then generates a first signal if the measured resistance value is greater than the predetermined resistance value which is displayed as a "pass" or >2MQ by display 9A, or generates a second signal if the measured resistance value is less than the predetermined resistance value which is displayed as a "fail" or <2MQ by display 9B.

The invention described provides a quick method of determine whether the insulation in a circuit is satisfactory. Because the apparatus works on the principle of comparing voltage, current and resistance and comparing these values with predetermined values, the necessity for accurate measurements is avoided which reduces the measurement time.

Also the voltage and current are measured with the apparatus assuming to measure a voltage or current value within a voltage or current range which also reduces the measurement time. This has savings in both battery life and electricians time.

Also the electrician does not have to interpret the results and decide by calculation whether the insulation value is too high or low. The apparatus indicates whether it is satisfactory or not, thus reducing human errors.

Although the apparatus is described as being useful for determining the resistance of the insulation in wiring, the principal could be used to measure the resistance of other electrical items.

The invention may take a form different to that specifically described above.

further modifications will be apparent to those skilled in the art without departing from the scope of the present invention.

Claims

1. An apparatus to measure whether the resistance of an electrical item under test is greater than or less than a predetermined value comprising: a) means to generate an electrical potential across an item of electrical equipment b) means to measure the voltage assuming it is within a voltage range across the electrical item c) means to measure the current assuming it is within a current range across the electrical item d) means to create a measured resistance value by dividing the measured voltage by the measured current e) means to compare the measured resistance value with a predetermined resistance value, and f) means to generate a first signal if the measured resistance value is greater than the predetermined resistance value, and mean to generate a second signal if the measured resistance value is less than the predetermined resistance value.

2. An apparatus according to claim 1, wherein the apparatus further includes means to compare the measured voltage with a predetermined voltage value and to generate said second signal if the measured voltage is less than the predetermined voltage value.

3. An apparatus according to claim 1 or 2, wherein the apparatus further includes means to compare the measured current with a predetermined current value and to generate said first signal if the measured current is less than the predetermined current value.

4. An apparatus according to claim 1, 2 or 3, wherein the apparatus includes a display to indicate the generation of said first or second signal.

5. An apparatus according to any preceding claim, wherein the means to measure the voltage and current includes an analogue to digital converter to convert analogue measurements to digital values.

6. An apparatus according to any preceding claim, wherein the means to create a measured resistance value includes a digital divider.

7. An apparatus to measure whether the resistance of an electrical item under test is greater than or less than a predetermined value substantially as hereinbefore described with reference to and as shown in the accompanying drawings.