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Loop Impedance Measurements for Earth Grid Commissioning and Assessment

The loop impedance meter can, for certain installations and under accurately controlled conditions, be utilised to conduct a series of tests at individual components within an earth grid to calculate the combined grid resistance. Where a grid resistance value is sufficient information for the assessment, and for infrastructure installed in dangerous or highly trafficked areas, this can be a highly effective and beneficial testing method.

Earth grid resistance is a critical design aspect of distribution and transmission infrastructure. It impacts the protection system and overall safety of the asset during fault conditions and lightning strikes. Resistance to earth can be measured with a variety of testing instruments and methods. Discussed in this article is the effective use of the loop impedance meter, also known as the clamp or tong meter, to perform this measurement and discussion around the use of the meter beyond its typical use case.

The loop impedance meter is a handheld earthing testing instrument with a voltage and current coil, forming a clamp-like device (pictured in Figure 1 below). The meter is ‘clamped’ around a single earthing connection. It uses high frequency magnetic induction to induce a voltage on the earthing system and measures the resultant induced current. Through application of ohms law it calculates and displays the impedance of this ‘loop’.

Figure 1

The optimal use case of this meter is in instances where the return circuit, i.e. impedances in the loop other than the earth grid under test, are close to negligible (~0Ω). This usually occurs when the return path includes a significant number of parallel return circuits. The most common example of this in the distribution and transmission field, is the testing of a single down conductor and local earth of a sub-transmission pole with an overhead earth wire. In this arrangement, the loop impedance meter induces current in the single pole’s local earth grid, and the current returns through the adjacent pole’s earths and OHEW connections as pictured in Figure 2. Given the numerous low impedance parallel return paths, these are assumed to be negligible, and the impedance of the loop is approximately equal to the earth grid resistance of the local pole earth under test. For these arrangements the meter is highly effective and with very little effort gives a fairly accurate reading of the local grid resistance.

Figure 2

The device, however, does not perform accurately for many use cases other than described above. For earth grid arrangements where there is no return path (e.g., single lightning mast with local electrode) the device cannot form a loop and hence cannot give a measurement. Similarly, for more complex grids where there are multiple bonds between the structure and the buried earth grid components, a loop will be formed internally within the grid and the reading will give a short circuit value.

However, methodology exists for using this meter for arrangements and tests beyond the ideal cases, i.e. for scenarios where the return path impedance cannot be assumed as negligible (but are not open or short circuited).

Assessment of multi-component earth grids with the loop impedance meter – Simultaneous Equation Methodology

Common earth grid arrangements with multiple electrodes (as pictured in Figure 3), when installed in an optimal ‘spur’ arrangement, can have the individual electrodes tested with a loop impedance meter. The resultant resistance is the resistance of the electrode in addition to the return path – which is the parallel resistance to earth of the other electrodes, and cannot be assumed as negligible.

Figure 3

This value by itself does not provide enough information to determine the earth grid resistance of the combined system, however, the test can be repeated at each electrode giving multiple readings. Together these readings can be used to create a system of simultaneous equations and solved to give individual electrode values and the combined earth grid resistance.

This is an exciting prospect, as it seems to allow for earth grid resistance measurement with the use of a single, handheld device for more complex arrangements – far easier than standard practice of establishing a remote grid and performing a current injection test. However, this methodology introduces two unique sources of error, MER and Missing Return Paths.

1. MER (between components): Measuring individual components of a local earth grid by inducing current in a loop within the grid itself – causes significant Mutual Earth Resistance (MER) error. Unlike resistance in a typical electric circuit, resistance to earth is not merely a set electrical property of a physical component, but rather a complex interaction between the buried component and the ‘earth’ (adjacent soil). The value used to describe earth resistance is the theoretical drop in voltage due to a current (think V/I = R) between the component and ‘remote earth’ i.e. an infinite distance away where the soil voltage has dropped to 0V. Where buried earth grids are not sufficiently far away from each and a current is injected between them, the soil voltages interact, causing a reduction effect on each other – known as mutual earth resistance (see Figure 4). In some instances, this is the correct and desired effect, however for the loop impedance test proposed within the local earth grid – this effect is an artifact of the test arrangement and causes MER error. Methods for accounting for MER are possible ranging from simple error correction formulas to software modelling – and can help account for the impact.

Figure 4

Figure 5 shows an example where this methodology is not effective. The HV switchroom pictured has several of the above-mentioned issues that prevents accurate measurement with the loop impedance meter. These include non-measurable return paths such as the bare copper grading ring and concrete footings as well as looped connections to the grid that will read as a short circuit. Additionally, commissioning for this HV switchroom is likely to require more information than its local grid resistance, namely step and touch voltage measurements that can only be achieved through a more thorough and traditional earthing test, i.e., a current injection test (CIT).

Figure 5

Whereas Figure 6 shows an example of an earth grid installation where the loop impedance meter can effectively measure the grid performance. The fuel tank LPS system pictured consists of four electrodes and a shipping container on insulative footings ensuring the electrodes are the only path to earth. MER impacts are also low due to the spacing between electrodes and low soil resistivity of the area. For the commissioning of the tank LPS, running cables (for a CIT) is dangerous as it is located adjacent to a haul road, and to prove compliance touch and step voltages are not required – only a 10Ω resistance to earth.

Figure 6 

Conclusion: Earth grid resistance is a critical value that requires assessment and commissioning testing for distribution and transmission infrastructure. Beyond the typical use cases the loop impedance meter can, for certain installation and under accurately controlled conditions, be utilised to conduct a series of tests at individual components within an earth grid to calculate the combined grid resistance.

By Ben McMurray – Electrical Engineer, Safearth

For more information on Safearth’s products and services, visit their website at au.safearth.com