The Importance of Measuring Insulation Resistance for Safety and Equipment Longevity
Why do we need insulation testing?
Safety
The most important reason for conducting insulation testing is to ensure public safety and personal safety. By performing insulation tests on disconnected current-carrying conductors, grounding conductors, and conductors to be grounded, the possibility of fire due to short circuits is eliminated.
Extend equipment life
Insulation testing is also important to protect and extend the service life of electrical systems and motors. Periodic maintenance testing can provide data for analysis while predicting possible system failures. In addition, when a fault occurs, insulation testing is also required to determine the cause of the fault.
Insulation test principle
Insulation testing is similar to finding leaks in water pipes. Typically, high-pressure water is injected into the pipe to look for leaks.
Pressurized water flow can make leaks easier to find. And pressure in the electrical field is voltage. When performing insulation testing, applying a relatively high DC voltage to the device under test makes the leakage areas more obvious.
The insulation resistance tester measures the leakage current when voltage is applied, and calculates the insulation resistance value through Ohm's law. Such instruments are designed to apply and control test voltages in a "non-destructive" manner. Although the voltage provided is high, the current is very limited. This avoids secondary damage to the equipment due to poor insulation and ensures the safety of operators.
Why can't I use a multimeter to measure insulation resistance?
Although a multimeter can measure resistance, it can not give an accurate indication of the working condition of an insulator. Because the multimeter uses a 9V DC power supply when making measurements, it can not obtain the high voltage required for testing.
How to measure insulation resistance？
Selection of insulation test voltage
According to the provisions of GB50150-2006 Electrical Equipment Handover Test Standard for Electrical Installation Engineering:
For electrical equipment or circuits with working voltage below 100V, use 250V test voltage;
For electrical equipment or circuits with working voltages between 100V and 500V, use 500V test voltage;
For electrical equipment or circuits with working voltages between 500V and 3000V, use 1000V test voltage;
For electrical equipment or circuits with working voltages between 3000V and 10000V, use 2500V test voltage;
For electrical equipment or circuits with working voltages above 10,000 V, use 5,000 V or 10,000 V test voltage;
Insulation resistance test steps
a. Turn off the equipment or system and keep it disconnected from all other circuits, switches, capacitors, carbon brushes, lightning arresters, and circuit breakers.
b. The system under test is completely discharged to the ground.
c. Select the appropriate test voltage.
d. Wiring. If the measured insulation resistance is large, it is recommended to use shielded wires and add a grounding wire to prevent breakdown. Test wires should be avoided from being entangled to reduce test errors.
e. Start the test, read the meter indication value after asome time(usually one minute), and record the data and the ambient temperature at this time.
f. After the test is completed, if the object under test is a capacitive device, the device should be fully discharged. Finally remove the connecting wires.
Why do I need to use shielded wires when measuring larger resistances?
When the insulation resistance to be measured is large and the measurement voltage is fixed, the current passing through the conductor is relatively small and is easily affected by the outside world. Use a shielded wire for testing. The shielded wire and the negative (-) terminal are at the same potential, which can be used to prevent surface damage due to Leakage or other unintentional leakage of current results in reduced accuracy of insulation resistance measurements. At the same time, in addition to the two test leads during the test, a grounding wire is added to prevent breakdown and ensure safety.
Instruments for measuring insulation
Insulation test tools
Insulation resistance testing is done using special testing equipment. The most commonly used test instruments are meggers or insulation resistance testers, but other types of instruments can be used to check the integrity of different insulation types.
Shake table
The hand-operated megohmmeter is called a megohmmeter. It was born in the 1950s and 1960s and is the earliest insulation resistance testing instrument. There are different specifications, such as 250V, 500V, 1000V, etc. When in use, the DC voltage is generated by shaking the handle. The pointer dial generally requires two people to operate it at the same time. One person operates the rocker and one person keeps time and records data.
Digital insulation resistance tester
Megohmmeters powered by batteries have multple adjustable test voltage levels and an electronic display to make readings more accurate. In addition, they are usually designed with safety protection functions, such as automatic discharge, leakage current monitoring, etc. The test items are also more diverse, with multimeter test functions, polarization index, medium absorption ratio, etc, making it applicable to a wider range. Compact design and simple operation, one engineer can complete all testing steps.
Leakage current clamp meter
Leakage current clamp meters can be used to measure the insulation of equipment that can not be de-energized. The magnetic fields generated by the load current cancel each other out. Any unbalanced current flow results from current leakage from the conductor to the ground or elsewhere. To measure this current, a leakage current clamp meter should be able to detect less than 0.1 mA.
Things to note when measuring insulation
1. Do not connect the insulation tester to live conductors or live equipment, and make sure to follow the manufacturer's instructions for use;
2. Use open fuses, switches, and circuit breakers to shut down the equipment under test;
3. Disconnect the branch conductor, ground conductor and other equipment connected to the equipment under test;
4. Make sure to disconnect the conductor capacitor before and after testing;
5. Some equipment may have a discharge function;
6. Check fuses, switches and circuit breakers in dead circuits for leakage current. Leakage current can cause conflicting or erroneous test readings;
7. Please do not use the insulation tester in an environment containing dangerous or explosive gases. If the insulation performance is destroyed, the instrument may generate arcs;
8. Wear insulating rubber gloves when connecting test leads.
