Ohm my goodness!

22 August 2019

Ohm’s Law. If you are in the electrical engineering, physics, chemistry, or mathematics industry – you’ve heard the name thrown around a couple of times (or maybe thousands). At Megger, I think I hear, read, or write the words Ohm’s Law daily, maybe even multiple times per day. It’s a lot; maybe, you can relate.

So, who is this Ohm guy?

He’s clearly someone we want to know about.

Georg Ohm was born on March 16, 1789 in Erlangen, Bavaria (Germany). He was a physicist and professor of mathematics at the Jesuits’ College at Cologne in 1817 when he discovered his oh, so, famous law. Have you ever done something that you thought was out-of-this-world amazing, but no one else was excited about it? That was Ohm’s experience. His colleagues responded to his law so poorly that he had to quit his job at Cologne. That’s pretty harsh. Fortunately, things started to look up for young Georg though when he accepted a new job at the Polytechnic School of Nürnberg in 1833.

In 1841, his work was finally starting to gain recognition and Ohm won an award – the Copley Medal of the Royal Society of London. It was only a mere 14 years after he had initially published the law in his pamphlet Die galvanische Kette, mathematisch bearbeitet (The Galvanic Circuit Investigated Mathematically), but who’s counting? A year after he was awarded the medal, he was named as a foreign member of the Royal Society of London.

Then, Ohm’s fame truly peaked. The ohm (abbreviated Ω), the standard unit of electrical resistance was named after him. What a dream. Engineers, scientists, technicians, teachers, test equipment, and bloggers everywhere are using his name every single day. We can’t all have that kind of fame in our lives!


What is his law, anyways?

Well, Ohm was working in his laboratory and he discovered that the electrical current flowing through a resistance is directly proportional to the voltage applied across it. Even better, it was also inversely proportional to the resistance. At this point, he shouted at the top of his lungs out his window, "Ohm my goodness!". Just kidding, that probably didn’t happen. None of us were there though, so it's still up for debate. He had to have been excited after that discovery, and a celebration was totally in order. Okay, now back to the mathematical details. Simply put, Ohm’s law is the relationship between current, voltage, and resistance, shown below.


If you know (or measure) two of the three variable (voltage, resistance, or current), you can easily figure out the third. Hence, why an insulation resistance tester works! The instrument applies a known test voltage to excite the circuit and measures the resulting current. If your insulation was perfect, there would be no current. Fortunately, perfection is impossible to come by in the insulation world, so you’re in luck – there’s always current. Then, using Ohm’s law, the tester does some quick algebra and throws back at you the value of resistance. Depending on what tester you are using, it may look something like this.




If these were your results, you would have just measured a resistance of 673 MΩ (megohms). It’s not always going to look the same, though. You could get results in the Ω, kΩ, GΩ, or TΩ range. Historically, instruments were not this technologically savvy. Huge surprise, right? Back in the day, you actually had to READ instructions and convert units all by yourself. I wasn't there, but it sounds like it was a mess. Not anymore. Modern equipment with LCDs eliminates that hassle, saving you precious time and preventing you from making an embarrassing mistake. Testers today will choose the correct unit automatically. It’s like magic! However, you’re not completely off the hook. You still need to know what these prefixes and symbols mean.


Download insulation testing cheat sheet


Lucky for you, we have a handy graphic for you to use. Print it out, put it in your wallet, give it to a friend, or hang it in your cubicle. If you want to download it, simply click on the image below and click the download button in the top right corner. Just remember, those little ‘greater than’ (>) and ‘less than’ (<) signs are extremely important to pay attention to. If your test instrument is reading “< 10 kΩ”, you may assume that your system’s resistance is just 0. Don’t do that, please. Your results are just off the low end of the MΩ scale. All you need to do is switch to the Ω or kΩ range and test again. On the other hand, if you see a reading that is “>30 TΩ” the insulation you are testing has exceeded the maximum range of the tester.


What does this number mean, though?

I thought you’d never ask. The value (in ohms) that you read off your insulation tester can tell you all sorts of things about the overall health and condition of your insulation. If you are working with brand-new insulation, you’re looking at resistance values in the many millions of ohms range (think: giga or tera-ohms). As your equipment starts to age, you’ll start to see resistance measurements of just a few megohms. Uh-oh. Your insulation is approaching a critical status and your stress levels are likely starting to rise. If insulation resistance testing falls under your job description, listen up. This is not a one-and-done test; it’s a repeated, regularly scheduled, and always recorded measurement. As you continue to take measurements over the course of a year (or more), the plot of resistance vs. time will become an invaluable tool for assessing your insulation’s health. Since “normal” values of resistance vary from one piece of equipment to the next, it’s hard to quantify your motor’s health with just a single, stand-alone measurement. Luckily, you’re smarter than that and you’ve been building a graph of your resistance measurements over time. In that case, just look at the overall trend. Have your results suddenly taken a nosedive or are things relatively stable? A steep downward slope is never a good sign. Unless, of course, you are skiing!

If you still haven't had enough of Ohm and you're desperate for more, check out our complete guide to insulation resistance testing


 – Meredith Kenton, Digital Marketing Assistant