How to Check Insulation Resistance with a Multimeter

This site contains affiliate links to products. We may receive a commission for purchases made through these links.

Are you tired of searching the online web for the best way to check insulation resistance? Look no further because this post got you covered! 

Insulation resistance is the most typically measured parameter to determine the overall health of electrical insulation. Concerning cables, the IR differs from one cable to another depending upon the cable’s length, type of cable, and insulation thickness. 

For instance, the IR value of one meter and IR value of 100 meters of similar cable isn’t comparable in an absolute term as the later value will be 100th of the former. 

What is Insulation Resistance? 

One of the most critical parts of electrical installation and maintenance tasks is taking insulation resistance (IR) readings. That is done to confirm the insulation material’s integrity, be it cable insulation and wire, generator, or motor winding insulation. 

Remember that any electrical insulation should have the opposite qualities as the conductor. It must resist the flow of current, keeping it in the conductor. 

In its simplest form, insulation resistance is the capability of the insulation material to resist the current flow. Over time, the insulation starts to age, and that causes deterioration in the overall performance of the insulation.

Strict operating environments where the critical insulation is exposed to too many operating temperatures, chemical contamination, and moisture will hasten the deterioration stage. It is essential to understand this electrical condition of the insulation in electrical equipment to prevent any incident like a short circuit, fire, electrical shock, and so much more.  

Insulation resistance is measured between conductors, and earth, and the insulated conductors. 

Keep in mind that insulation resistance is measured by the equipment we referred to as megger. It is a high resistance meter along with a test voltage of at least 5,000 volts DC. Megger could be of the mechanical type with the hand-driven operation or digital type with electronic voltage charger and battery operation. 

The 500 V test charge is ideal for testing equipment that is rated for 440 volts AC. 

Critical Points Associated with Insulation Resistance

Circuits that depend on the high insulation resistance’s maintenance for their proper functioning must not be employed unless special precautions are taken to keep the high insulation resistance. For instance, circuits must be presented with means to check the insulation resistance.

The major propulsion circuit must be presented with ground leakage indicating devices, which will run when the insulation resistance is 100,000 ohms or less. 

The insulation resistance of every generator should be measured both in the warm and cold conditions right away before and after running at standard full load. When a cable is spliced to change a damaged area of the cable, before changing the hurt section, the insulation resistance of the wire’s remainder is measured. It is identified that the insulation’s condition is undamaged. 

Things to Keep in Mind When Testing an Insulation Resistance

Facilities using a run-to-fail maintenance tactic have added pressure to bring systems like motors back up and running as fast as possible. Each minute a motor down has an expensive impact on a business. Suddenly, troubleshooting a failed motor turns out to be an emergency.

Please take note that not all on-staff resources do preventive maintenance on their motors. Most massively depend on outside professionals to save them when motors fail. Predictive and preventive maintenance is still the preferred approach as they offer the least amount of motor downtime. However, the run-to-fail technique is still widespread these days. 

Plants often opt outside service agencies to fix failed motors in ventilation blowers, elevators, chillers, fire pimps, and printing presses, among other applications. Insulation testing of failed motors, together with a shortlist of other basic tests, will guide a service technician to identify a go/no go status for the motor. 

Here are the important things you need to consider before you try to troubleshoot a failed motor:

  • On initial inspection, it may look as if nothing is wrong with it. However, trying to restart the motor without troubleshooting the issue first could cause massive damage.
  • Find a smoke or smell for weird odors. Ensure you do a basic check of the motor.
  • Collect basic details about the motor. Gather nameplate data. Take digital multimeter measurements such as ground connections, fuse checking, and voltage.
  • Do insulation resistance tests of the load circuits and line to ground. Remember that you need to isolate any electronic controls and other devices from the circuit under test before you do any insulation resistance testing. Insulation test voltages could cause severe damage to such controls. 

How Do You Test Insulation Resistance?

  1. Take your red test lead and insert it in the positive or the live terminal. 
  2. Get your black lead and plug in the earth terminal or negative terminal in other multimeters. 
  3. Go between the live and neutral. Set your test voltage. 
  4. Take your reading and then go between the live and earth terminal. Please test it and take a reading, then get between earth and neutral test. Please test it and take a reading.
  5. Finally, go between all the conductors to the outer sheath. Take your reading. 

Final Thoughts

The significance of an insulation resistance reading depends on the component being tested. Normally, the resistance of any one component differs from one component to another and over time. Small resistance changes are typically not critical but might suggest a pattern that you should take note of.

For instance, the current passing through the element lowers as the resistance of a heating element increases, and vice versa. 

When working on a circuit board, it might be important to lift one of the resistor’s leads from the board to measure the proper resistance. Moreover, resistance measurement shown by a digital multimeter is the overall resistance through every possible path among the test lead probes. 

Caution is needed when measuring insulation resistance across an element, which is part of a circuit. Don’t forget that the component’s resistance connected in parallel along with a component being tested influences the resistance reading, normally decreasing it. Check the circuit schematic at all times for any parallel paths.

About The Author