Clamp Confidently

24 March 2020


Spring is officially here! If you’re anywhere near our office – outside of Philadelphia, Pennsylvania – it’s felt like spring for a while now though, since we didn’t really have a winter at all. We’re not complaining about the winter part though. Nonetheless, happy spring!

Now that the weather’s nice (or it’s supposed to be), there’s no excuse – it’s truly the perfect time to test your grounds. If you’ve been looking for a reason to start ground testing, here it is. Plus, you’ll get some Vitamin D while you’re out there, and we could all use a little bit more of that right now.

We’ve covered ground testing extensively in our Secrets of the Soil series, so we will most definitely not be discussing all of the available ground testing methods today.

Today, we’re talking about the clamp-on or stakeless method of ground testing because it’s simple and fast, so what’s not to love about that?


What’s the clamp-on ground testing method?

Like most everything else we talk about on here, it’s based on Ohm’s Law. Your clamp-on tester will apply a known voltage to a complete grounding circuit and measure the resulting current flow. Then, it’s going to automatically calculate the resistance for you and voila! You’re finished.

Now, before you go and throw away your traditional ground tester – wait a second. Don’t do that.

There are some rules, and a clamp-on tester can’t be used in every ground testing application.

When can I use a clamp-on ground tester?

You must be dealing with a complete electric circuit – from the start – since you’re not adding additional probes and creating your own circuit, as you would with the classic fall of potential method.

The clamp-on tester is measuring the complete resistance of the loop that the voltage is taking. If things aren’t in a true loop, then you’re not measuring the true resistance of your grounding system.

So, you can only use a clamp-on ground tester in situations where there are multiple grounds in parallel. Got it?

Sadly, many folks misuse this method by trying to use it in applications where the above is not true.

Don’t do that, okay?

You can’t use a clamp-on tester on an isolated ground, for installation checks, or for commissioning new sites. It’s also a no-go if there’s an alternate lower resistance return in the soil, like with cell towers.

Unfortunately, with this method, there’s no way to double check your results. So, if you’re going to use a clamp-on tester, you must be absolutely certain that you’re using it for the right reasons.

Don’t worry though – we’re going to tell you exactly when and where this method applies – because that’s just what we do here on the Megger blog.

What are the best applications for using a clamp-on tester?

Funny you should ask – we were just about to tell you. There are quite a few applications where a clamp-on tester is a great tool to have in your belt – or is it box?

Utility Poles/Service Entrance or Meter

The more parallel grounds in a series, the better the resistance measurement you’ll get! So, it makes sense that a series of utility poles – each with their own ground electrode – would be a pretty darn good application for this technique, right?

Hope you’re nodding your head up and down right now.

Same goes for measuring the ground electrode resistance of a service entrance or meter. There’s a little more uncertainty here, so just make sure you’re identifying the best position to make a measurement, since there’s the potential for multiple earth paths, two electrodes, or even a connection to a water pipe. Sometimes, it’s best to just clamp the electrode itself. Since you’re saving so much time with this method anyways, use a bit of that extra time to make sure you’re taking a measurement in the best possible location.

Street Lighting

It’s basically the same application as above, but this time you can clamp the cable running to each streetlight’s electrode. Simple!

Lightning Protection

If you want to be protected from lightning – as we all do – you must have effective grounds. To protect a building, grounding electrodes are usually placed at each corner of a building – to start. It can sometimes be difficult to measure the resistance, as the electrodes are often buried, but jug handles – or removable links – are the perfect place to use a clamp-on tester. Just remember that there could be other connections in the system, so make sure you’re clamping below all other connections. A visual inspection is truly key here, so you don’t get a false, yet realistic reading. No one wants to be played like that!

Street Cabinets

In case you’re not familiar with this lingo, a street cabinet is a primary cross-connection or flexibility point, which has a ground electrode installed inside. Fancy! Typically, you’re looking for a resistance below 25Ω for a street cabinet, so even if there are only two parallel earth paths in a series – you’ll know you’re doing it right, as long as you get a number below 25Ω. Right? Don’t believe us, go read our guide to clamp-on ground testing.

Telephone Pedestals

In a telephone pedestal, cable sheaths are all connected to a ground bar, which is then connected to a ground electrode. Therefore, you can take your clamp-on tester and place it around the cable that connects the ground bar to the electrode to measure the resistance. Easy as pie!

Cell Towers

Now – read carefully please. You cannot make a ground resistance measurement if the rods are linked below the soil by a ring, as they commonly are in cell towers. That being said, you can use a clamp-on tester to verify the connections beneath the soil, since the clamp-on tester will give you a reading of the resistance of the loop, rather than the soil.

There you have it - a quick guide to clamp-on testing applications. Looking for more? Download our complete guide to clamp-on ground testing or watch this video


Learn more about Megger's clamp-on ground testers