Have you ever wonder why the cables on your appliances are safe to hold while plugging it into the electrical source? Or the electrical lines going into your house does not shock you every time you touch a metal post or every time it is wet?
This is because the conductors are properly insulated. But any leak or shortage in these electrical wires can cause shock or damage to appliances.
That is why every unit or appliances supply wires, building electrical connecting wires or electric motor cables need to be tested first for proper insulation before they are released for public use. This is where an insulation tester is being used.
Insulation testers are used to check and detect if there is a leak of current in a conductor or other electrical components. And the most commonly used Insulation tester is the Megger Tester (a brand and a company name).
For most people, once you say Megger, it is automatically an insulation tester. But we should also know that Fluke has its own Insulator Tester which is the Fluke 1507 insulation tester.
In this article, I will share how to check and verify the accuracy of your megger tester in measuring a resistance value that you may implement before using your Megger. Also, it includes the verification of the output voltage to ensure that you have a reliable reading.
Furthermore, I will also present a calibration procedure that you can implement for your in-house calibration just in case you have the necessary reference standards which I will present here. In this post, I will be using the Megger BM11D.
What is an Insulation Tester?
A multimeter can be used as insulation tester but in a limited way, mostly a continuity test and detecting the presence of a current or voltage only in normal conditions. If there is no flow of current in an insulated wire when tested, it means that the wire is safe to hold.
But what if there is a break in the insulation and a high voltage or current will pass through it? Surely, the wire will get burned or create a short circuit that can cause damage and fire. This cannot be detected by a normal multimeter. There is an accident waiting to happen.
In order to detect and test for a leakage current during a normal or overloaded condition, we will use a special instrument known as the Insulation Tester.
Functions of an Insulation tester: Principles of Operation
- The main function is to test the insulation of a set of conducting wires by measuring the resistance. The higher the resistance reading, the better. Usually in Mega Ohms reading. A lower value of resistance means that there is a short or leakage of current causing the resistance to drop.
- Used as a voltage source to test for the integrity of the conductor. An insulation tester has the capability to source out or generates a high amount of voltage. This voltage is injected into conducting wires to test if the insulation can withstand the high voltage. An insulated conductor can have a normal reading in a low voltage value but during exposure to a high voltage, the insulation can break and leakage may occur.
- Function as a continuity tester. Testing for continuity on a very long cable of wire cannot be guaranteed by a simple multimeter. Because of the high voltage generated by the insulation tester, you can measure a significant amount of resistance.
- High resistance (infinity) reading means open while a zero or very low resistance means shorted. But no need to record because most Insulation tester has an audible alarm for short or open status.
How to use and verify the accuracy of a Megger tester
Since we already know the operating principles of an insulation tester, I will show you how to verify the accuracy of your Megger Insulation Tester by using a resistor and a multimeter.
Resistance Function verification
Just directly measure a resistor with a known value. Please see the figure below. If you want to record a highly accurate result, measure first the resistor using the multimeter in order to get its actual value.
- Set the required voltage to be generated in the resistor ( 500 V)
- Connect the resistor to its probe.
- Then Press TEST button.
Voltage Function Verification
- Remove the resistor and transfer the probes on the port of the multimeter
- Set the multimeter on the required range ( Vdc)
- Set the Megger on the desired voltage range.
- Push the TEST button of the Megger to generate the voltage.
Readings should be within the specifications provided by the manufacturer.
Megger insulation Tester calibration procedure
Megger Insulation Tester is measuring resistance by generating a voltage. Using Ohms Law (V=IR), we can compute the value of the resistance. Because of this, we cannot use a direct simulation of signal like for example from a process calibrator. The process calibrator is also generating a voltage that has an effect on the measured value.
In this procedure, we will be using the Resistance Box to provide an analog resistance for the Megger. A Decade resistance box is a set of resistors arranged in such a way to provide a combined resistance reading in a decade interval packed inside a box. (Megger has its own which is called a megger calibration box)
Another reference standard is the Fluke multimeter, we need a high range and resolution multimeter capable of reading the entire range of your Megger. If you cannot reach the entire range, you can still calibrate but with a limited range that is approved by the user.
This is accomplished by comparing the resistance output of the decade box to the display of the Megger. And also the voltage display in the Megger to the voltage reading of the multimeter. This procedure can be used on all Insulation Tester that accepts an analog resistance.
- Warm-up time (UUC): At least 1 hour for proper stabilization
- Temperature: 23 +/- 5 deg C
- Humidity: 50 +/- 30%
- Measurement Data Sheet (MDS) or a Megger Test Report (check this link to learn more about Measurement Data Sheet)
Reference standard to use:
- Decade Resistance Box ( Calibration Box)
- Fluke Precision Multimeter 8846A
- Thermohygrometer ( to monitor Temperature and Humidity)
- Connecting wires
- High voltage is generated in this procedure, always observe safety.
- Check the Megger Tester for any visual defects that can affect its accuracy. Discontinue calibration if any defect is noted.
- Clean the Megger Tester with soft cloth Check if it has good batteries, replace low powered batteries.
- Power on the unit and allow stabilization of 1 hour, to be conditioned on the environmental conditions of the room.
- Prepare the measurement data sheet (MDS) and record all necessary details or information (Brand, Model, serial #, etc).
- Determine the test points to be calibrated on the Megger Tester, choose at least 5 test point covering the whole range or as per user range. Record this on the MDS.
- Using Fluke Precision Multimeter, connect the necessary test leads to measure DC Voltage
DC Voltage Verification:
- Connect positive terminal of the Megger to the positive Input HI of the Fluke multimeter and the negative terminal to the LO Return side.
- Set the Megger (Unit Under Calibration) to DC Voltage mode. Select the range of voltage to be generated.
- Press the TEST button to generate each of the required voltage setpoint values.
- Take note of the measured value on the multimeter.
- Wait for the display to stabilize then get the reading. Record readings on the MDS or Megger Test report.
- Take at least 3 trials for each set point
- Repeat number 2 to 6 until all ranges are covered.
- Remove the connection probes of the Megger that is connected on the multimeter.
- Now connect the probes to the terminal of the Decade resistance box ( same connections as with the multimeter).
- Set the resistance value of the decade resistance box to the desired value by turning the knobs. (an example is 100 Mohm)
- Press the TEST button, the Megger will inject a voltage to the resistor and generate the required resistance value.
- Wait for the display on the Megger to stabilize then get the reading. Record readings on the MDS.
- If the readings are already within limits, update the corresponding record, do labeling and sealing and issue to the owner, otherwise, do necessary repair or adjustment.
8, End of procedure
Based on manufacturers Specifications, below are the tolerance limits that can be used.
±5% of nominal test voltages (load resistance >100 MΩ)
±5% 1 MΩ to 100 GΩ at 5 kV
±5% 1 MΩ to 10 GΩ at 500 V
±20% outside these limits (above 100 kΩ)
Voltage output is 500 Vdc, allowable limits is (0.05×500 = 25 V)
Therefore, the tolerance value is from +/- 25 of 500 or (475 to 525) Vdc
Want to learn how to interpret a calibration certificate properly? Visit my other post, click here.
Understanding Resistance measurements have so many ways and Megger Insulation Tester is one of the instruments that may confuse you or intrigue you if you are new in electrical measurement. IN order to be clear in this matter, I have provided a simple explanation on the principle of its measurement. We have known that it is using an analog signal in order to generate the required resistance.
Once you have understood this principle, it is now simple to calibrate or verify an insulation tester. I have presented in this post on how to verify before use and calibrate a Megger Insulation Tester by using a reference standard.
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