What is the Insulation Resistance Test?
An insulation resistance test or IR test is used to calculate the overall resistance between two selected points isolated by the electrical insulation. The purpose of the test is to check the effectiveness of the electrical insulation in stopping the flow of current. The insulation resistance tests are used to measure the quality of insulation in two ways. First during the manufacturing of the products and also during the use of the product at any time.
Such tests play a crucial role in the advance detection of the expected insulation damages. This proves very helpful in avoiding the frequent mishaps and costly repair works related to electrical products. An IR tester is used to measure the insulation resistance. We will discuss these testers later.
The insulation testing setup for grounded and ungrounded components varies greatly. To test ungrounded appliances, a 2 wire ungrounded connection setup is used. This type of device includes capacitors and resistors. See the pic below to understand it.
Fig 1: Setup for ungrounded components to check insulation resistance. Here GRD stands for DUT Guard Source (Source of pic: https://www.chromausa.com/)
On the other hand, for the grounded appliances 2-wire grounded measurement is mostly used. In a grounded part, one of its two connections is attached to the ground. While in the ungrounded component none of the connection is attached to the ground. An example of a 2-wire grounded connection is the calculation of insulation resistance of cable submerged underwater. See Figure 2 to see such connections.
Fig 2: 2-Wire grounded connection (Source of pic: https://www.chromausa.com/)
How is Insulation Resistance Measured?
Measurement of Insulation resistance or IR test consists of four steps. These steps are charge, dwell, measure/calculation, and discharging.
In the first step i.e. the charging step, the voltage is increased to the required level from zero. This helps in giving the needed stabilizing time and slows down the input current intake to the DUT. After achieving the selected voltage, the voltage is allowed to decrease or maintain at the selected level. This is the second step and called the dwell.
At this moment, the measurements of the insulation resistance are taken. This is the third step called the measurement step. In the fourth and final step, the DUR voltage is dropped back to zero after taking the measurements.
Insulation resistance IR tester normally has up to four output connections. Their names are ground, shield, +, and -. These four connections are used for different types of applications. Here it is important to note that the output voltage usually ranges from 50 to up to 1000 V DC. In perfect resistance value should be higher in the range of 35 to 100
Check figures 1 and 2 to see how the DUT connections are attached. The IR tester measures the resistance and displays it. To measure the insulation resistance, the megger test performed by the megger tester is most popular, followed by the fluke 1507 insulation tester.
Important of Insulation Testing?
As we all know the color coating that surrounds a wire that protects the wire and people. However, this insulation is not forever and it starts to deteriorate over time. With deterioration, the quality of insulation also starts to decrease. If the wire is being used in a harsh climate or in certain industries, the rate of wire deterioration is also high. Therefore, it is important to check the state of insulation frequently for the safety of workers.
The insulation tests are performed to make sure the insulation resistance is in the acceptable range. There are quite a few tests currently under use to check any insulation breakage. However, the best of all and easiest to is the insulation resistance test also known as the IR test. There are certain purposes and advantages of using the IR test. Let’s have a look at a few of them.
- The IR test is used to make sure the insulation requirements are met and also to verify the right hookup.
- This test is important as a regular incident preventive maintenance job.
- The test is also important for maintaining quality control during the production of electrical appliances.
- It is also helpful for troubleshooting purposes.
Types of Insulation Resistance Tests
Generally, three types of insulation resistance tests are in use. These types include Short-time/spot reading test, Time-resistance method, and Dielectric absorption ratio. All these tests are performed using the Megger insulation tester.
This method is also known as a short time or spot reading test. In this method, you just need to connect the meter megger with the wire insulation you want to test. As the name depicts, you operate the system for a very short period of time usually less than a minute. See figure 3 to see how the resistance generally varies with time.
Figure 3: This is how the normal insulation resistance curve looks along the time for the spot reading test. The resistance is measured in Megohms (Source of pic: http://www.techni-tool.com/)
To check the resistance across the insulation, just pick a point on the curve of continuous increasing resistance value as seen in figure 3. Generally, the value is lower in the case of only 30 seconds, while the value is higher when the value is taken for 60 seconds. Remember the value of resistance may change with change in temperature and humidity.
This method has a distinct advantage over the short-time test as its value doesn’t get affected by the temperature. It also provides the right information without taking help from past tests. Its values depend on the absorption effect of the right type of insulation when compared with the insulation containing the moisture or contamination.
In the time resistance method, you need to take multiple readings at a particular time and compare them with each other. This kind of insulation testing is referred to as absorption tests. In the case of the appropriate insulation, resistance will increase continuously over the selected period of time.
On the other hand, if the insulation is contaminated and has moisture, the resistance value should be low. This means the insulation is tempered and needs repairing. See figure 4 to see the results of appropriate and contaminated/moisture ridden insulation.
Fig 4: Trends of different insulations in Time-Resistance Method (Source of pic: http://www.techni-tool.com/)
Dielectric Absorption Ratio
This test shows that good insulation demonstrates a slow increase in IR when the test voltage is applied to it. After connecting the connection appropriately, the test voltage is applied and the values of IR are noted at two separate times. This time is usually 30 seconds and 1 minute. However, it can be 60 seconds and 10 minutes depending on the electrical appliances.
In order to calculate the value of IR, the 2nd value is divided by the 1st value. For example, the value of resistance at 10 minutes is divided by 1 minute to calculate the value. This ratio is known as the polarization index (PI). We find the state of insulation resistance with the help of PI. See the table below in this regard.
Table 1: Insulation condition with regard to PI. The higher the PI value, the better is the condition of the insulation. (source: https://www.ecmweb.com)
Insulation Resistance Testers
Without a quality IR tester, the true value of insulation resistance can never be found. Here, it is important to note that a particular IR tester can’t be good for all types of industries and applications. There are certain factors that determine which type of resistance testers should be used. Some of these factors include:
- What type of equipment is being tested?
- The place and industry type where the IR tester needs to be used.
- The magnitude of the voltage required for the testing.
- Other factors include the experience of the person conducting the tests and the standard of safety at the place where tests will be conducted.
Best IR Testers
There are dozens of IR testers in the market. Every tester has some pros and cons. However, some of the testers are better than the others. Let’s have a look at some of these testers.
Victor VC60B+ insulation tester megger is among the best available IR testers in the market. This tester provides accurate results and comes with a 9V powerful battery. Using this, one can test resistance at various voltages including 250, 500, and 1000V. The best part of this tester is the great range of testing including as low as 0.1 to as high as 2000 Megaohm.
Figure 5: The Megger Megohm Meter. (Source of Pic: Amazon.com)
Victor VC60B+ insulation tester can operate at a wide range of temperature and humidity. The meter megger comes with a wide, easy to use and understand LCD display screen. The insulation tester megger is best to use in telecom, mechanical and electrical wires, and appliances.
The next IR tester on the list is Fluke 1507 insulation tester. The is a lightweight, easy to handle tester. It works on AA Alkaline batteries and is literally a noiseless tester. Like the megger meter, it can test resistance at various voltages including 50, 100, 500, and 1000V. It comes with a great insulation testing range, starting from as low as 0.1 Megaohms to as high as 10 Gigaohms.
Figure 6: A Fluke 1507 Insulation Tester (Source of Pic: Amazon.com)
The best thing about Fluke 1507 insulation tester is its automatic calculations of PI and dielectric absorption ratio. It also powers off automatically when not under use for a long time. It is best for the professional and technologically advanced industries.
Designed by the REED, the R5600 insulation tester comes with strong and double molded plastic which makes it very durable. You can test resistance at various voltage levels including 250, 500, and 1000V. It can measure resistance up to 2000 megaohms. It is equipped with a conventional dual display screen. This makes its measurement easy to understand and read.
R5600 insulation tester is best for indoor testing of electrical appliances, motors, and wires.
Figure 7: A R5600 Insulation Tester (Source of Pic: Amazon.com)
Safety Precautions for Insulation Resistance (IR) Testing
During insulation resistance testing, it is necessary to take some precautionary steps to ensure the safety of the workers. Let’s have a look at some of the important safety precautions given below.
- Never perform insulation tests on energized electrical appliance or wire. Always perform the test when it is turned off.
- Keep away from holding or touching the test meter as much as you can.
- Close down equipment and disconnect all the connected switches. Discharge the terminals only into the earth.
- In order to see if the circuit is dead, use the three points testing method. These points are: check and test the available live circuit, after it tests the selected circuit, and test again the first live circuit to see if the IR meter is working well.
- Protective equipment is necessary if you are testing the live circuit. Protective equipment is also necessary if you working in a possible inflammable environment.
- Use insulated gloves while testing and connecting the switches and leads for tests.
The Final Words
Insulation resistance testing is important to check the condition of insulation over a wire or electrical appliances. It also plays important role in incident prevention related to electricity and checking quality control during the production of electrical appliances. There are three types of IR tests including the Short-Time Test, Time-Resistance Method, and Dielectric Absorption Ratio. These methods are used for separate applications.
The quality and reliability of the IR tester are very important to make sure the IR measurements are 100% reliable. To select an appropriate IR tester, keep important factors in mind. Some of these factors include what type of equipment is being tested, place and industry type, and voltage requirements for the IR testing. Last but not the least, always follow the above-given precautions before performing the IR testing.