BobL
28th February 2014, 08:51 PM
On making/modifying various mains AC gizmos for use around my shed, where possible I have been taking it into work for proper insulation resistance (Megger) testing. Recently the need for repeated testing has come up more often I thought it was time I got something to have permanently to hand.
The unit I purchase was dedicated Insulation Resistance Testing (IRT) meter which cost AUS$76 including delivery.
http://www.ebay.com.au/itm/Digital-Insulation-Resistance-Tester-Megger-MegOhm-Meter-100-250-500-750-1000V-/281274848828
Claimed Specs for MODEL A4431
Test Voltages : 100V/250V/500V/1000V
Output Voltage :]Test nominal Voltage of 90-120%
IR Range : 0-2000MΩ
IR MAX.Resolution 0.001MΩ
IR Accuracy : 0-200MΩ ±(3%rdg + 5 d)
IR Accuracy : 200MΩ-10GΩ ±(5%rdg+ 5 d)
IR Accuracy : 10GΩ-20GΩ ±(10%rdg+ 10 d)
ACV Range : 0-750V
ACV Accuracy: ±(2.0%rdg+ 5 d)
ACV Resolution: 1V
ACV Freq Range : 40Hz-400Hz
[Continuity test ]Range：0~200Ω，Audible Continuity signal sounds for resistance less than about 70Ω±30Ω
The 3 stated accuracies for the insulation resistance (IR) are somewhat confusing and it's because these spec relate to the 3 models of meter that cover the range 0 to 20GΩ.
The IRT meter I have cover the IR range of 0.1 to 2000MΩ.
What’s in the box.
The photos on ebay are representative of what the meter looks like.
http://www.woodworkforums.com/attachment.php?attachmentid=305828&stc=1
The meter itself comes in a tough, toy-like orange and grey, plastic case with a protective cover that clips on neatly on top or underneath when not in use.
Besides the meter itself there are 3 sets of leads and a is a lightweight zip up bag that can hold the meter and leads. The red plastic coated lead has a reasonable looking HT probe, one of the two black plastic coated probes come with a small alligator clip “add on”, while the other comes with bulldog type clip fastener. The black lead with the small alligator clip also come with neat little plastic cover to reduce the exposed area of the tip of the probe. These small features are very useful and add value to the meter.
The instructions are a one A4 page of typical chinglese that took me far longer than usual to interpret. As well as a straight out language problem, the layout of the instructions is confusing because the instructions relate to all three models of this meter. There are warnings about using the meter to measure high voltage, but nothing about using it with sensitive equipment. Given the potential danger in using this device both for equipment and humans, I would have thought they would have tried a bit harder to make it more “user friendly”, comprehensive and understandable.
Power supply
Power is provided via 6 x 1.5V AA batteries (cheap set provided inside the meter) OR a DC power socket (no adapter or adapter specification provided – I sent the store an email requesting this info and they promptly replied indicating a 12V DC, 2 or 3A supply is required). While awaiting their reply I ran it via a 9V 2A power adapter and it has performed well for about 10 hours of testing without the adapter getting even slightly warm.
Battery replacement requires 4 screws to be removed from the battery compartment cover underneath the unit, which is a PITA if it needs to be done on a regular basis. The plastic case around the batteries was slightly buckled and is the only physical aspect of the meter that makes it look on the cheap side.
I don’t know how used the batteries were prior but they only lasted for about an hour of testing before the low battery signal came on. Apparent this is normal for IRT type meters and ~50 insulation tests is about all one can expect from a set of batteries.
Using an IRT meter
If you have never used an IRT meter before I would not recommend using one without being shown how to use it, or at the very least reading up about the do’s and don’ts. The Megger website pages on its basic insulation resistance tester (http://www.megger.com/au/products/ProductDetails.php?ID=612&Description= ) has some application guides which may be useful.
The most comprehensive information I have been able to find is the Australian Standards document for In-service safety inspection and testing of electrical equipment, as it provides all the protocols for measurement as well as all the values and exceptions.
Another useful resource is the “Open electrical” wiki. http://www.openelectrical.org/wiki/index.php?title=Insulation_Resistance_Test
Operations
1) Continuity (low resistance measurements)
Perhaps not very intuitively, continuity is tested with leads connected to the pair of sockets also used for Mains AC testing. Although a numerical value of resistance is provided by the display, this meter is not really designed for accurate low resistance measurements. Only values between zero to 200Ω are displayed and values as high as about 70Ω sound audibly as continuity. The whole idea of this function is to save using another meter to perform a continuity test. Although I tested the accuracy of this function (read on) this location of the leads irritating because you often have to switch the leads between the right and left hand sockets i.e. establish continuity – then perform the IRT. I found it easier to leave the leads in the IRT sockets and use another meter to establish continuity.
I tested the continuity resistance reading of the meter against a Protek Digital Multi Meter (DMM) over a range of resistances. Graph 1 shows the deviation of the meter reading from the resistance reading obtained by the DMM. The graph shows that the meter reads ~20% too high at very low resistances but is in agreement with the DMM above 100 Ω. What I cannot show on the graph is that with the leads shorted (ie near zero resistance) the meter resistance reading was 2 Ω. Once again, this meter function is not meant to perform accurate low value resistance measurements, it’s just a continuity tester.
GRAPH 1
http://www.woodworkforums.com/attachment.php?attachmentid=305829&stc=1
2) VAC measurement.
Graph 2 shows the % deviation of the VAC reading by the meter compared to the reading by the DMM over a range from 50 to ~270v. The claimed accuracy is +/- 2% of each reading, which is represented by the length of the vertical bars at each point. As can be seen, the vertical lines cross the graph "Y axis zero line", so although the values are biased slightly on the high side they are all well within spec. The meter can read up to 750V but I could not go to more than 265V with my home setup.
GRAPH 2
http://www.woodworkforums.com/attachment.php?attachmentid=305830&stc=1
3) Insulation resistance testing (IRT)
Acceptable insulation resistance values vary depending what standards are being applied. The Australian standard for standard 240V wiring and devices (apart from a few exceptions) requires a minimum or acceptance threshold of 2MΩ. Some standards like ANSI/NEC are much higher at 25MΩ. Remember insulation resistance tests cannot be performed accurately with a regular ohmmeter as the resistance must be tested under a high potential.
IRT is performed by first attaching the necessary leads to the circuit being tested and then the “test button” is depressed. This delivers a specified DC voltage into the circuit being tested and the resulting current is measured from which a resistance determined and displayed
The Australian Standards for insulation testing require a “one minute wait” for the circuit to energize and stabilize and only then should the operator take a resistance reading. In most cases this meter reading stabilized within about 5 seconds. When the insulation resistance is >2000 MΩ the display somewhat irritatingly showed a “1” – surely these days the meter could have been programmed to display something like >2000 MΩ.
Four IRT test voltages are provided; nominally 100V for low voltage devices, 250v for 110V, 500V for 240V, and 1000V for 440 3 phase powered devices. The reason such high voltages are used for testing is that they would take into account any spikes that might occur.
To test the accuracy of the meter for insulation resistance I took the unit to work and compared it to an industrial IRT meter. We tested a number of single and 3 phase power cables and two 3 phase motors, all of which showed an insulation resistance reading above the limits of both meters ie the resistance was above 2000 MΩ.
What is more important than measuring cables and devices with resistances well above the acceptance threshold is how accurately the IRT measures resistance across the 2MΩ acceptance threshold.
At work it’s not that easy to find devices or cables with resistances near the accepted threshold as any devices that are below 2 MΩ must be repaired or disposed of. To perform this test I used a certified set of variable resistor boxes, which had an accuracy of +/- 1%.
I used the IRT meter to measure the resistance of the certified resistors starting from 0.5 MΩ up to 4 MΩ, in steps of 0.1 MΩ, which is also the resolution of the IRT meter display. I repeated this for all the IRT input voltage ranges (100, 250, 500 and 1000V) and in all cases the IRT meter reading agreed with the resistance value of the certified resistors.
Some other Testing
I tested out a range of extension cords, expander boards and electric devices in my shed and found all except one cord were > 2000 MΩ. The exception was a ~10-year-old extension cord that has suffered from exposure to welding spatter which gave a value of 7 MΩ between active and neutral (the resistance between earth and neutral, and active and earth was >2000 MΩ). These resistance values were also the same for all of the four available test voltages. This may not always be the case as resistance may decrease with an increase in the applied voltage.
For folks who do not have access to accurate testing gear one way to check if the IR meter is working correctly might be to purchase a pack of high resistance resistors and then solder these up in a chain crossing the 2 MΩ acceptance threshold. The meter could then, if required, be checked every time it is used.
Summary
Apart from the relatively minor issues mentioned above, once you get the hang of the mode of operation this IRT meter it is quick and easy to use. The most important KPI is that this IRT meter appears to be accurate for high resistance measurements (within the limit of the display) across the 2 MΩ acceptance threshold.
Would I buy it again? It’s a bit early to say as some of these cheap electronic measuring gizmos can’t handle being connected to the wrong inputs all that well. FWIW, the meter seems to have survived being set onto “Continuity testing” and then connected to 240V AC!
The value of having and IRT in a home workshop is not only to uncover actual problems like rogue extension cords and defective motors etc, but to monitor the degradation of these cords or devices over time so that potential problems can be determined before they bite an operator.
The unit I purchase was dedicated Insulation Resistance Testing (IRT) meter which cost AUS$76 including delivery.
http://www.ebay.com.au/itm/Digital-Insulation-Resistance-Tester-Megger-MegOhm-Meter-100-250-500-750-1000V-/281274848828
Claimed Specs for MODEL A4431
Test Voltages : 100V/250V/500V/1000V
Output Voltage :]Test nominal Voltage of 90-120%
IR Range : 0-2000MΩ
IR MAX.Resolution 0.001MΩ
IR Accuracy : 0-200MΩ ±(3%rdg + 5 d)
IR Accuracy : 200MΩ-10GΩ ±(5%rdg+ 5 d)
IR Accuracy : 10GΩ-20GΩ ±(10%rdg+ 10 d)
ACV Range : 0-750V
ACV Accuracy: ±(2.0%rdg+ 5 d)
ACV Resolution: 1V
ACV Freq Range : 40Hz-400Hz
[Continuity test ]Range：0~200Ω，Audible Continuity signal sounds for resistance less than about 70Ω±30Ω
The 3 stated accuracies for the insulation resistance (IR) are somewhat confusing and it's because these spec relate to the 3 models of meter that cover the range 0 to 20GΩ.
The IRT meter I have cover the IR range of 0.1 to 2000MΩ.
What’s in the box.
The photos on ebay are representative of what the meter looks like.
http://www.woodworkforums.com/attachment.php?attachmentid=305828&stc=1
The meter itself comes in a tough, toy-like orange and grey, plastic case with a protective cover that clips on neatly on top or underneath when not in use.
Besides the meter itself there are 3 sets of leads and a is a lightweight zip up bag that can hold the meter and leads. The red plastic coated lead has a reasonable looking HT probe, one of the two black plastic coated probes come with a small alligator clip “add on”, while the other comes with bulldog type clip fastener. The black lead with the small alligator clip also come with neat little plastic cover to reduce the exposed area of the tip of the probe. These small features are very useful and add value to the meter.
The instructions are a one A4 page of typical chinglese that took me far longer than usual to interpret. As well as a straight out language problem, the layout of the instructions is confusing because the instructions relate to all three models of this meter. There are warnings about using the meter to measure high voltage, but nothing about using it with sensitive equipment. Given the potential danger in using this device both for equipment and humans, I would have thought they would have tried a bit harder to make it more “user friendly”, comprehensive and understandable.
Power supply
Power is provided via 6 x 1.5V AA batteries (cheap set provided inside the meter) OR a DC power socket (no adapter or adapter specification provided – I sent the store an email requesting this info and they promptly replied indicating a 12V DC, 2 or 3A supply is required). While awaiting their reply I ran it via a 9V 2A power adapter and it has performed well for about 10 hours of testing without the adapter getting even slightly warm.
Battery replacement requires 4 screws to be removed from the battery compartment cover underneath the unit, which is a PITA if it needs to be done on a regular basis. The plastic case around the batteries was slightly buckled and is the only physical aspect of the meter that makes it look on the cheap side.
I don’t know how used the batteries were prior but they only lasted for about an hour of testing before the low battery signal came on. Apparent this is normal for IRT type meters and ~50 insulation tests is about all one can expect from a set of batteries.
Using an IRT meter
If you have never used an IRT meter before I would not recommend using one without being shown how to use it, or at the very least reading up about the do’s and don’ts. The Megger website pages on its basic insulation resistance tester (http://www.megger.com/au/products/ProductDetails.php?ID=612&Description= ) has some application guides which may be useful.
The most comprehensive information I have been able to find is the Australian Standards document for In-service safety inspection and testing of electrical equipment, as it provides all the protocols for measurement as well as all the values and exceptions.
Another useful resource is the “Open electrical” wiki. http://www.openelectrical.org/wiki/index.php?title=Insulation_Resistance_Test
Operations
1) Continuity (low resistance measurements)
Perhaps not very intuitively, continuity is tested with leads connected to the pair of sockets also used for Mains AC testing. Although a numerical value of resistance is provided by the display, this meter is not really designed for accurate low resistance measurements. Only values between zero to 200Ω are displayed and values as high as about 70Ω sound audibly as continuity. The whole idea of this function is to save using another meter to perform a continuity test. Although I tested the accuracy of this function (read on) this location of the leads irritating because you often have to switch the leads between the right and left hand sockets i.e. establish continuity – then perform the IRT. I found it easier to leave the leads in the IRT sockets and use another meter to establish continuity.
I tested the continuity resistance reading of the meter against a Protek Digital Multi Meter (DMM) over a range of resistances. Graph 1 shows the deviation of the meter reading from the resistance reading obtained by the DMM. The graph shows that the meter reads ~20% too high at very low resistances but is in agreement with the DMM above 100 Ω. What I cannot show on the graph is that with the leads shorted (ie near zero resistance) the meter resistance reading was 2 Ω. Once again, this meter function is not meant to perform accurate low value resistance measurements, it’s just a continuity tester.
GRAPH 1
http://www.woodworkforums.com/attachment.php?attachmentid=305829&stc=1
2) VAC measurement.
Graph 2 shows the % deviation of the VAC reading by the meter compared to the reading by the DMM over a range from 50 to ~270v. The claimed accuracy is +/- 2% of each reading, which is represented by the length of the vertical bars at each point. As can be seen, the vertical lines cross the graph "Y axis zero line", so although the values are biased slightly on the high side they are all well within spec. The meter can read up to 750V but I could not go to more than 265V with my home setup.
GRAPH 2
http://www.woodworkforums.com/attachment.php?attachmentid=305830&stc=1
3) Insulation resistance testing (IRT)
Acceptable insulation resistance values vary depending what standards are being applied. The Australian standard for standard 240V wiring and devices (apart from a few exceptions) requires a minimum or acceptance threshold of 2MΩ. Some standards like ANSI/NEC are much higher at 25MΩ. Remember insulation resistance tests cannot be performed accurately with a regular ohmmeter as the resistance must be tested under a high potential.
IRT is performed by first attaching the necessary leads to the circuit being tested and then the “test button” is depressed. This delivers a specified DC voltage into the circuit being tested and the resulting current is measured from which a resistance determined and displayed
The Australian Standards for insulation testing require a “one minute wait” for the circuit to energize and stabilize and only then should the operator take a resistance reading. In most cases this meter reading stabilized within about 5 seconds. When the insulation resistance is >2000 MΩ the display somewhat irritatingly showed a “1” – surely these days the meter could have been programmed to display something like >2000 MΩ.
Four IRT test voltages are provided; nominally 100V for low voltage devices, 250v for 110V, 500V for 240V, and 1000V for 440 3 phase powered devices. The reason such high voltages are used for testing is that they would take into account any spikes that might occur.
To test the accuracy of the meter for insulation resistance I took the unit to work and compared it to an industrial IRT meter. We tested a number of single and 3 phase power cables and two 3 phase motors, all of which showed an insulation resistance reading above the limits of both meters ie the resistance was above 2000 MΩ.
What is more important than measuring cables and devices with resistances well above the acceptance threshold is how accurately the IRT measures resistance across the 2MΩ acceptance threshold.
At work it’s not that easy to find devices or cables with resistances near the accepted threshold as any devices that are below 2 MΩ must be repaired or disposed of. To perform this test I used a certified set of variable resistor boxes, which had an accuracy of +/- 1%.
I used the IRT meter to measure the resistance of the certified resistors starting from 0.5 MΩ up to 4 MΩ, in steps of 0.1 MΩ, which is also the resolution of the IRT meter display. I repeated this for all the IRT input voltage ranges (100, 250, 500 and 1000V) and in all cases the IRT meter reading agreed with the resistance value of the certified resistors.
Some other Testing
I tested out a range of extension cords, expander boards and electric devices in my shed and found all except one cord were > 2000 MΩ. The exception was a ~10-year-old extension cord that has suffered from exposure to welding spatter which gave a value of 7 MΩ between active and neutral (the resistance between earth and neutral, and active and earth was >2000 MΩ). These resistance values were also the same for all of the four available test voltages. This may not always be the case as resistance may decrease with an increase in the applied voltage.
For folks who do not have access to accurate testing gear one way to check if the IR meter is working correctly might be to purchase a pack of high resistance resistors and then solder these up in a chain crossing the 2 MΩ acceptance threshold. The meter could then, if required, be checked every time it is used.
Summary
Apart from the relatively minor issues mentioned above, once you get the hang of the mode of operation this IRT meter it is quick and easy to use. The most important KPI is that this IRT meter appears to be accurate for high resistance measurements (within the limit of the display) across the 2 MΩ acceptance threshold.
Would I buy it again? It’s a bit early to say as some of these cheap electronic measuring gizmos can’t handle being connected to the wrong inputs all that well. FWIW, the meter seems to have survived being set onto “Continuity testing” and then connected to 240V AC!
The value of having and IRT in a home workshop is not only to uncover actual problems like rogue extension cords and defective motors etc, but to monitor the degradation of these cords or devices over time so that potential problems can be determined before they bite an operator.