5 Things You Need to Know About Low Resistance Testing
What is low resistance testing?
Typically, low resistance in the electrical testing industry is referring to any resistance values below 1 ohm.
Now, if this is a new concept to you, you may be thinking that 1 ohm is already low. How can we possibly go lower? Trust us, there is a whole world of values below 1 ohm.
To measure low resistance, you’ll need an instrument with very high accuracy and a high enough test current. These are non-negotiable requirements. Usually, for test current, anything greater than 100 mA to 600 A will suffice.
How does low resistance testing work?
Well, your basic low resistance ohmmeter is a four-terminal tester with current and potential terminals. If you’re familiar with low resistance testing already, you’ll know that it uses Kelvin Bridge technology.
The current probes will typically go across the ends of the test item, so you can inject a test current through the item under test, while the potential probes are connected across. Check out the image below for a better visual. In this example, we’re looking at the voltage drop across a joint in cables, since that can often be a potential problem area. Using the parameters of current and voltage, the instrument can then calculate the resistance – thanks to Ohm’s Law.
How should the probes be set-up?
You’re in luck, we’ve got a rule for that! For maximum precision for entering results in a database or if you’re dealing with a third party, the potential and current probes should be spaced 1.5 times the circumference of the test item.
If you’re just trying to find a problem or run a go/no-go test, keep in mind that you might not necessarily need to follow this rule. But, it’s always a great suggestion.
What parameters will affect low resistance measurements?
Really great question. There are a lot of factors that can make low resistance measurements challenging, including: temperature, noise ratio and induced currents, thermal EMF, contamination, and hotspot effects. Don’t worry though, we’re going to tell you how you can eliminate each of these influencing factors.
Let’s start with temperature. Temperature has a pretty big effect on resistance. Generally, as the temperature increases, so does the resistance – and vice versa. So, if you’re making reports or keeping a database, you’ll need to adjust to a common temperature. Generally, good equipment and software will have the formula for temperature correction built in, but if not – you can do the math yourself. How fun? We’ve included the formula below, just in case you’re interested. By the way, if you don’t correct for temperature, you could have huge discrepancies in data, just days apart, simply because the weather changed. Therefore, you might assume there is a problem, when there’s actually not. That’s never good.
Next up, noise. With low resistance measurements, noise is a particularly big factor, especially if you’re testing out in the field. If you increase your test current though, you can typically reduce noise, so that’s always a good option. A good tester will usually tell you if noise is affecting your results, too.
When you connect two different conducting materials – like in a circuit breaker, you may be dealing with thermal EMF. For instance, if you connect copper and nickel, you could be looking at up to a 400% error. That’s a big deal. Fortunately, like noise, if you have the ability to increase the current, you can significantly decrease this potential for error too.
Contamination is another factor to deal with, since the test surfaces you’re dealing with are always at risk of contamination when in-use or just sitting around. A practical example is circuit breaker contacts, since you want almost no resistance across contacts. However, when left open for a while, oxides can develop. Like the above factors, increasing the current can improve your results by literally breaking through the contamination.
And finally, hotspots. Some test items, which could even be in relatively good shape, have spots where some of the metal has worn away, creating breaks. A continuity test won’t reveal an issue, but while running an application, you’ll start to see problems. Fortunately, by doing a low resistance test, you can spot these hotspots.
What are the benefits of low resistance testing?
One of the benefits is that you can focus on the goals of your specific testing needs. With go/no go testing, you can troubleshoot or pass/fail equipment, which allows you to save time. Whereas, if your goal is condition monitoring, you’ll be using your low resistance measurements to create a historical database. There’s also like testing which allows you to ensure that ‘like’ elements of a system are of similar resistance.
Likewise, you can identify unacceptable increases in resistance, which can prevent equipment damage and increase efficiency. Since a fault will always take the path of lowest resistance, your low resistance testing can also be used for lightning protection of your assets and buildings. Finally, low resistance testing can drive a predictive maintenance program for your plant, since you can spot the increasing resistance and correct it before it becomes a problem.