Imagine insulation resistance testing as a means of verifying whether the insulation within our switchboards is still functioning. It prevents electricity from spilling where it is not needed. To verify this, electricians test it using an instrument called a megohmmeter, or “megger.” What it does is apply a high voltage to the insulation and measures the amount of current that leaks through. If the insulation is healthy, the resistance reading will be strong. If it’s weak, then there could be an issue.

How Do We Prepare for This Testing?

Before we begin testing, we have to ensure that everything is safe. That involves turning off and isolating the switchboard so no power’s flowing and locking it out to avoid someone else switching it back on accidentally. We also need to drain any capacitors and remove sensitive electronics as the test voltage will burn them out. Oh, and take a try at testing in dry conditions since moisture will interfere with our readings and make things seem worse than they are.

What Happens During the Test?

Under testing, we subject various sections of the switchboard to a DC voltage—often 500 or 1000 volts. We take readings of resistance between live conductors and earth, between phases, and phase to neutral. We maintain the voltage for around a minute and observe how the readings fluctuate. At times we record the values at 30 seconds, and at times at one minute. This gives us a more accurate estimate of the condition of the insulation.

How Do We Know if the Insulation Is Good or Bad?

Generally speaking, if our reading is above 1 megaohm, we’re probably in the clear. But if it’s below that, it’s a red flag. Keep in mind that temperature affects our readings—a warmer day can make resistance drop—so always consider environmental factors. What’s really useful is keeping an eye on how readings change over time. If the resistance is dropping steadily, that’s our cue to look deeper.

There are also a couple of snazzy measurements such as the Polarization Index (PI) that compare 10 minute and 1 minute readings. A high PI indicates our insulation is in good shape, but a low PI can indicate wetness or old insulation.

What Problems Can This Test Identify?

The most common culprits that cause low insulation resistance are moisture getting into the switchboard, dirt and dust contamination, physical damage like cracking or abrasion, and overheating. For example, we’ve seen factories where rodents chewed on cables inside the switchboard, causing the insulation to fail the test. Catching that early saved them from a nasty shock or worse.

Staying Safe While Testing

Safety is a serious consideration here. We double-check that the switchboard is de-energized before starting every time. We wear personal protection gear such as insulated gloves and safety glasses, and observe safe work practices. Proper grounding of our test equipment is important too. If we do all this, we can prevent ugly accidents while performing this critical test.

How Often Should We Test?

It makes sense to check insulation resistance prior to putting any new switchboard into commission, and afterward on a regular basis every 6 to 12 months—if the switchboard happens to be in a hard environment such as a dusty factory or a waterlogged coastal building. And whenever there is a repair or an upgrade, we ensure to check again to ascertain all is well.