Insulation Resistance Testing: an Often-Overlooked Predictor of Electrical Failure

It’s no secret that electrical failures rarely announce themselves. More often than not, they simmer quietly and anonymously under the surface — oftentimes inside insulation that has either begun to degrade, absorb moisture, or leak current long before a breaker trips or smoke appears. Even though insulation failure is one of the most common root causes of downtime, equipment damage, and electrical fires, insulation resistance testing is sometimes treated as an afterthought rather than a frontline diagnostic tool. That mindset needs to change.

A recent NFPA study underscores what many electrical professionals already know from experience: failures in electrical distribution equipment and lighting systems remain a leading cause of home electrical fires and fatalities. According to this report, “the item that first ignited in home fires involving electrical distribution and lighting equipment was electrical wire or cable insulation in 32% of the fires.” Insulation breakdown is frequently at the center of such failures. The good news is insulation degradation doesn’t happen instantly — and when it’s measured correctly and consistently, it can be detected long before it becomes catastrophic. But why is performing this seemingly simple task so often misunderstood?

Historically, that’s probably why one of the most frequent keyword searches on EC&M’s website continues to be “insulation resistance testing.” Similarly, some of our most popular evergreen articles like “Understanding Insulation Resistance Testing” and “Insulation Resistance Testing Simplified” continue to top the charts when it comes to most popular content. Another blockbuster performer for us showcases five of the most useful videos EC&M’s editorial team curated on how to perform a proper insulation resistance test.

In this month’s cover story, “Insulation Resistance Testing Best Practices,” longtime contributor and electrical consultant to EC&M, Randy Barnett outlines five key steps for conducting insulation resistance tests safely, accurately, and consistently. Randy is a master electrician, ICC Commercial Electrical Inspector, and NFPA-certified Electrical Safety Compliance Professional, who has worked as an electrician and technician in nuclear and coal-fired power plants, government nuclear research facilities, and in industrial maintenance and construction. He also produces our monthly Tech Talk video series, two episodes of which discuss the ins and outs of insulation resistance testing.

In Episode 2, Randy explains that the purpose of insulation is to keep electrons in the copper wire — the goal is to keep those electrons from leaking out. He offers an example. Let’s say you are running a conductor through some raceway, and you nick some insulation off inadvertently. You’ve now created a situation where, once the conductor is energized, current can flow from the copper wire through the missing or cracked insulation into the metal raceway, also known as a ground fault. Under these circumstances, excessive heat or fire could result, and someone could potentially get shocked or electrocuted. In Episode 27, he reviews the purpose and methods used for measuring the quality of electrical insulation on conductors and equipment, namely how performing a spot reading test can give you a quick evaluation of the condition of the insulation, which tells you if further testing is required. Watch both videos for the full explanation. 

The bottom line is insulation resistance testing is much more than a routine maintenance task or box to be checked at installation that’s soon forgotten. It is one of the clearest indicators of electrical system health at an electrical professional’s disposal. For electrical contractors, it supports code compliance and quality installations. For facility maintenance personnel, it provides early warning before failures disrupt operations. For everyone, it reinforces a culture that treats electrical safety as proactive rather than reactive.