With so many different technologies and clamp meter models available, choosing the right product for the job can be a bit of a minefield. Many clamp meters seem to come with too many functions you don’t need and poor performance from the ones that you do.

Martindale make it easy by focusing on the key functions and features needed for the most common applications and delivering on performance, reliability and accuracy in these. areas.

It’s not the number of functions, but being able to measure what is really needed for the application and doing it well, whether it’s low current AC, DC for photovoltaic and battery systems, or leakage current measurement to troubleshoot RCD tripping.

With the CM Series you get the right tool every time.

To simplify the choices there are 4 main categories:


  • 1) Basic electrical testers for electricians with current measurement capability like the Martindale ET4 and ET5
  • 2) Pocket sized, high performance AC and AC/DC clamp meters, with high accuracy measurement over a wide current range for electrical contractors, maintenance teams and installers in residential, commercial and industrial applications. (CM51 to CM79)
  • 3) High current large jaw, CAT IV safety rated clamp meters for use on bus bars in industrial and utility applications (CM82 to CM87)
  • 4) Light weight flexible current meters enabling access to difficult to reach and large conductors where conventional clamps won’t go (CM95 to CM100).


Before we go into more detail about the different categories of clamp meters and what they do, some basic dos and don’ts of current measurement that are always worth covering.


  • For all general purpose applications excluding leakage current, it’s important only to clamp around a single current carrying conductor which may mean splitting out the conductor for the rest of the cable – basic explanation.
  • A useful tip for low currents, multiplying the number of turns of the current carrying cable within the jaw will multiply the reading by the number of turns.
  • Always try and keep the conductor in a central position in the jaw for the most accurate readings, with good quality clamp meters the positional sensitivity is much less of a problem.
  • Watch out for current carrying conductors outside the clamp meter jaw, but next to it, the magnetic field from current flowing in the external conductor can cause reading errors. Once again, not a big problem if you are using a high quality clamp meter.
  • Always make sure your clamp has the right safety category for the area of the installation you are working in.
  • Use a True RMS current clamp meter to ensure accurate and repeatable measurements, particularly when troubleshooting overloads.


True RMS is becoming even more important with the increased use of modern low energy lighting systems and power supplies which generate harmonics and cause distorted current waveforms. Basic clamp meters without True RMS capability are calibrated for the measurement of sine waves only and will give large errors on distorted current waveforms. Errors will depend on the levels of harmonics and can lead to readings 50% lower than the actual value. The size of the errors will vary depending on the level of harmonics at the time, making comparative measurements impossible.

The first category for basic electrical testers where current measurement is an additional feature includes current forks with multimeter functionality such as the ET4 and ET5. These are often referred to as open jaw current meters.

Most current forks will only measure AC current and are not suitable for accurate low current measurements.

The Martindale ET and ET5 open testers measure both AC and DC current without breaking into the circuit and include additional functionality for capacitance and temperature on the ET5. Both models have a non-contact voltage indicator built in to the jaw and a dual display for current and voltage.

For more accurate current measurement and improved resolution, its necessary to select a conventional closed jaw clamp meter like the new Martindale pocket sized CM Series.

For residential and commercial installations and maintenance and repair applications, Martindale offer a comprehensive range of compact, easy to use clamp meters with current ranges from 30 Amps through to 600A. This starts with the CM51 current only AC clamp meter through to the CM79 AC/DC True RMS clamp with essential multimeter functionality and Min/Max recording.

All of the pocket sized clamps enable good access where space is limited and most have backlit displays for easy viewing under all conditions. All models measure down to a resolution of 10mA with the capability to accurately measure from less than 0.25A up to 600A for the CM57 and CM79. A convenient thumbwheel and auto ranging makes one handed operation straight forward.

For more specialist high current applications there is the CM82 to CM87. These have large jaws, with a CAT IV safety rating for supply side applications and are suitable for use on bus bars in industrial and utility applications with current ranges up to 2000A for AC and DC applications.

The forth category is lightweight flexible current meters, such as the CM95 and CM100, enabling access to difficult to reach and large conductors where conventional clamps just can’t go.

The smaller flex of the CM95 will resolve down to 1mA whereas the large diameter flex of the CM100 will measure up 3000A. In contrast to conventional CTs, they cannot be overloaded.

In contrast to conventional current transformers (CTs) the flex cannot be damaged by high overload currents.

To help choose the right product in your category of clamp meter, here is the essential check list:



  • Minimum and maximum current levels.
  • Safety Installation Category Rating for your application.
  • Resolution and accuracy to accurately measure the minimum level of current.
  • Jaw size for access and maximum conductor size.
  • AC only current or AC and DC Current.
  • Ease of use, including features such as auto ranging, backlight and thumbwheel for function selection.
  • Any Additional functionality including AC/DC Voltage, Resistance and Min/Max recording.
  • True RMS measurement.