Application Report TRUE RMS Earth resistance clamp meter from BENNING CM E1

BENNING, a manufacturer from Bocholt, has dubbed its first TRUE RMS Earth resistance clamp meter the “CM E1”.

In contrast with similar products on the market, the CM E1 is relatively affordable, at an RRP of €1199.80 plus VAT. Nevertheless, it provides some useful additional functions. Such as leakage current measurements with an impressive resolution of 0.001 mA and current measurements (produced in much the same way as with the traditional current measurement clamp meters) for which you can use it to detect currents of up to 35 A.

BENNING CM E1 scope of delivery
The BENNING CM E1 is supplied in a robust transport case. The base price includes all of the following: shoulder strap, reference resistance loop, battery, operating instructions and a test certificate from the manufacturer.

In what respects is an earth resistance clamp meter any more advantageous than the conventional earth measurement apparatus?

The task of carrying out earth measurements in electrical plants and industrial installations is not necessarily the electrical engineer’s favourite task. There is so much that depends on the condition of the plant and other external factors. And, usually in line with established measurement principles, you will have to set “ancillary earths” at specific intervals relative to each other. This may represent a problem if the surfaces available in the vicinity of the object to be measured are sealed or if they are in poor condition when it comes to tapping in the required earth rods.

In many cases, as the alternative, engineers will resort to “2-conductor reading”. This is sometimes broadly referred to as “city measurement”. But this does require a pre-existing, accessible earth connection with a known earth resistance. An adequately long electrical connection to the measurement instrument has to be set up, and an offset has to be applied in order to allow for the corresponding electrical resistance. If there is no accessible earthing connection, or not one which is reliable, then it will not even be possible to produce any reading at all.

There is often a considerable amount of work and time required when earth measurements are carried out using conventional methods!

BENNING CM E1 during the testing of a lightning conductor system
Testing a lightning conductor system using the BENNING CM E1: grip the earth conductor with the clamp, take the reading and you’re done.

The BENNING CM E1 provides a much more straightforward earth measurement method. And one which is much quicker.

Using this measurement method, you don’t have to separate the earth conductor to be measured and you don’t have to use auxiliary probes in the form of earthing rods, let alone the laborious procedure of running a cable to the measurement instrument.

Due to the way in which earth loop measurement operates, the construction of the measurement instrument itself is relatively complex. Consequently, the physical design, with particular reference to the measurement head used by the BENNING CM E1, is significantly more robust than what you find with conventional current measurement clamp meters. This is because the clamp head contains two coils instead of just one. Via a magnetic alternating field, the active coil induces a defined voltage in the conductor and this results in a current which is a function of resistance. The main measurement coil then detects the current flow, and on that basis the precise earth loop resistance is calculated and is shown as a reading on the display.

This approach achieves measurement results which are just as accurate as those which are arrived at on the basis of previous measurement processes. The whole procedure is achieved with no other ancillary equipment required and no other constraints. The only requirement is that there should be at least two earth connections: which is why this measurement method is also called “earth loop measurement”.

On the other hand, there is no theoretical upper limit, of the quantity of existing earth connections in place. In other words, the question of whether there are a further two, five or more than ten parallel earth connections is immaterial. Conversely, the measurement result obtained will be even more accurate if it is possible, and in practice it usually is possible, to have recourse to more than just one individual measurement loop.

In the rare cases where there is only one single earth connection, a straightforward solution is to make a direct connection to an existing earth electrode (to the junction box in the building, for example). Alternatively, we can set just one single auxiliary earth electrode (“earth rod”) – and this approach also sets up the required auxiliary measurement loop.

And it’s a simple enough task to determine whether or not a multiple-earth system is in place: if the earth resistance clamp display indicates “.OL” instead of any numerical figure, then the measurement value will be in excess of 1500 ohms. What this tells us is that either there is only one single earth electrode working, or that the earth connection we are looking at is defective. This will be the logical conclusion if the measurement value given is conspicuously high and if it differs significantly from acceptable values.

Earth loop resistance measurement makes it considerably more straightforward to carry out troubleshooting for defective earth electrodes. In this instance, specifically, we can rule out the possibility of receiving the misleading impression of an intact earth system due to the presence of parallel earth connections. Any earth electrode which is defective or which exceeds the limit values will be recognised as “defective”.

Opened measurement head of BENNING CM E1
BENNING CM E1 measurement head with two separate measurement coils and a substantial clamp spacing of 38 mm.
BENNING CM E1 display with measurement value memory and data logger
Integrated measurement value memory and data logger for up to 116 readings.
Diagram of conventional earth measurement system using auxiliary earths.
Diagram of conventional earth measurement system using auxiliary earths.
Alternative circuit diagram for earth loop measurement using an earth resistance clamp.
Alternative circuit diagram for earth loop measurement using an earth resistance clamp.

It’s almost impossible to get incorrect readings with the BENNING CM E1.

This is achieved firstly because of the employed measurement principle, but secondly, because the measurement clamp automatically self-calibrates every time it is switched on.
Additionally, the CM E1 will recognise any incorrectly completed surface contact made with the measurement clamp. A warning message is also issued if there are any external disruptive signals which could affect the reading. And the operator can always consult the reference resistance loop to check up on the correct function and level of measurement accuracy.

For the measurement of leakage currents and load currents, a total of 6 measurement ranges – from 0.300 mA to 35.00 A is available, and the most appropriate measurement range is selected automatically. Here, resolution will range from 0.001 mA to 10 mA; and this level of resolution is unlikely to be exceeded even by specialised current clamps.
The task of working in relatively inaccessible, difficult environments, is made easier by the fact of the hold function and a programmable threshold for an acoustic alarm for earth loop resistance.

BENNING CM E1 during the routine checking of the measurement accuracy through using the reference resistance loop (included as standard)
Routine checking of measurement accuracy using the reference resistance loop (included as standard).


BENNING’s new TRUE RMS CM E1 earth resistance clamp meter makes earth measurements a straightforward and rapid task. Readings can be taken with no risk to the test engineer and to the plant in place, because the earth electrodes to be tested do not need to be separated. Consequently electrical safety and plant operation remain assured at all times.

In city areas in particular, the problems that often used to occur when setting the required auxiliary earth electrodes are now a thing of the past. Now, you don’t have to lug around a range of earthing rods, rolls of cable, connecting lines and tools for hammering rods in, so just this compact measurement clamp will be adequate in earth systems which are predominantly multiple-grounded.

And where your systems are topographically extended, such as in solar power sites for example, you can dispense with the task of repeatedly re-siting individual auxiliary earths. That speeds up the whole exercise of taking earth readings. Workload and the time required are greatly reduced when the BENNING CM E1 is utilised, and for that reason the cost of investment should be paid off relatively quickly for most professional operators.

BENNING CM E1 during the measuring of the total leakage current in a sub-distribution system
Measuring the total leakage current in a sub-distribution system.

In most cases it should not be necessary, thanks to the BENNING CM E1’s high resolution and accuracy, to go to the expense of using a separate earth leakage current clamp. And in many cases there will be no need to use a separate AC current measurement clamp, since the CM E1’s load current measurement function extends to cover the measurement of AC load currents up to 35 A. The fact of routine application of the TRUE RMS measurement procedure enables precise measurement of the value of currents – even those which are not sinusoidal.

It’s a welcome benefit that this earth resistance clamp meter is user-friendly, almost intuitive. You don’t have to keep looking things up in the operating instructions. With this instrument, there are no complicated menu chains to cope with and no functions that can only be activated with outlandish key combinations.

Users do not have to pay extra for a manufacturer’s test certificate; this is already included in the base price and is supplied together with the product.

Further Information

contact: Tobias Enck
telephone: +49 2871 93 447

author and photos: Robert Braun

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