Many users of an energy logger, power logger or power quality analyzer usually use the tool’s supplied current clamp in their application. Since the supplied clamp has a specific measurement range, it’s very well possible that it doesn’t fit your current application well.

So how do you choose the right current clamp? Here we show you how.

Let’s take the Fluke 43B single Phase Power Quality Analyzer as an example. The supplied current Clamp i400s has a selectable range of 0,5A – 40A and 0,5A – 400A, with output voltage of respectively 10mV/A and 1mV/A. So the measuring output signal is maximum 400mV.

It is a given that an AC current clamp by design is more susceptible to interference in the low part of its measurement range, and that the actual output signal is very low compared to the neighboring voltage input. Imagine your device is measuring a few mV of current clamp signal next to a 400Vac signal with high interference on top. In this situation, you can have interference on the small signal preventing an accurate clean value to be measured and a noisy waveform shown.

Choose the clamp itself and the setting of the clamp so that the expected maximum current is within the clamp range and the nominal current is well above the minimum value it can measure.

When to use the supplied current clamp

Supplied clamp: 1400s current clamp

Example 1: When you have a 400Vac system with a 32A fuse and a nominal current of 25A, use the supplied clamp (the i400s in this example) but with the setting on 0,5 to 40A.

In this way, you get the strongest signal to the instrument and lower the amount of noise or interference.

When to use a different current clamp

Fluke 15s AC current clamp

Example 2: You are measuring on the output of a current transducer that transforms the 5KA of a big system to a 0-1A output on the secondary. If you now apply the standard-issued i400s you only get a few mV out and you are on the bottom of its measurement capacity. The result will be a noisy and inaccurate waveform and value. A better choice would be a clamp that fits this range, for example the Fluke i5s. This clamp has a range of 10mA – 5A with a maximum output of 400mV/A.


Most clamps that come with energy loggers and power quality analyzer are fitted for the most common application — but always ask yourself if the clamps you have fit YOUR application.

If you want to know which power quality logger or analyzer is right for you, try our selector tool.


  1. I like that you mention how clamps have a specific measurement range so it’s important to choose one that will fit your current application. In order to do this, you’d probably want to measure the application in order to determine its size. Once you’ve done this, you’d probably want to figure out which type of clamp will work best so you can find out if its available in the size you require.

    • You’re right. When choosing the right clamp you should be aware on the size of the installation/load and the “to be expected” maximum current, and choose your clamp accordingly.

      Good starting points are the fuse rating on the branch you are measuring on, or the type plate of the load/machine you measure. Another element is the physical aspect of your measurement location. Normal clamps with a large beak sometimes don’t fit, so then flexible clamps could be an option as well. – Eric


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