# How to measure Earthing resistance in your Home

Ground system design

Simple earthing systems consist of a single ground electrode driven into the ground. The use of a single ground electrode is the most common form of grounding. Complex grounding systems consist of multiple ground rods, connected, mesh or grid networks, ground plates, and ground loops.

These systems are typically installed at power generating substations, central offices, and cellphone tower sites. Complex networks dramatically increase the amount of contact with the surrounding earth and lower ground resistances.

Soil resistivity measurement

Soil resistivity is necessary when determining the design of the grounding system for new installations (green field applications) to meet your ground resistance requirements. Ideally, you would find a location with the lowest possible resistance. Poor soil conditions can be overcome with more elaborate grounding systems. The soil composition, moisture content and temperature all impact soil resistivity. Soil is rarely homogenous and its resistivity will vary geographically and at different depths. Moisture content changes seasonally, varies according to the nature of the sublayers of earth and the depth of the permanent water table. It is recommended that the ground rods be placed as deep as possible into the earth as soil and water are generally more stable at deeper strata.

Calculating soil resistivity

The measuring procedure described here uses the Wenner method and uses the formula:

ρ = 2 π A R

where:

ρ = the average soil resistivity to depth A in: ohm-cm.

π = 3,1416.

A = the distance between the electrodes in cm.

R = the measured resistance value in ohm from the test instrument.

Measuring soil resistance

To test soil resistivity, connect the ground tester as shown in Fig. 1. Four earth ground stakes are positioned in the soil in a straight line, equidistant from one another. The distance between earth ground stakes should be at least three times greater than the stake depth. The Fluke1625 earth ground tester generates a known current through the two outer ground stakes and the drop in voltage potential is measured between the two inner ground stakes. The tester automatically calculates the soil resistance using Ohm’s Law (V=IR).

Fig. 1: Test current paths in the stakeless method.

Additional measurements, where the stake’s axes are turned 90°, are always recommended because measurement results are often distorted and invalidated by underground metal, underground aquifers etc.

A profile is produced that can determine a suitable ground resistance system by changing the depth and distance several times. Soil resistivity measurements are often corrupted by the existence of ground currents and their harmonics.

Fall-of-potential measurement

The fall-of-potential test method is used to measure the ability of an earth ground system or an individual electrode to dissipate energy from a site. The earth electrode of interest must be disconnected. The tester is then connected to the earth electrode. Then, two earth stakes are placed in the soil in a direct line – away from the earth electrode, for the 3-pole fall of potential test. Spacing of 20 m is normally sufficient.

Placing the stakes

It is essential that the probe be placed outside the sphere of influence of the ground electrode under test and the auxiliary earth to achieve the highest degree of accuracy when performing a 3-pole ground resistance test or the effective areas of resistance will overlap and invalidate any measurements.

Table 2 is a guide for setting the probe (inner stake) and auxiliary ground (outer stake). Reposition the inner stake (probe) 1 m in either direction and take a fresh measurement to test the accuracy of the results and to ensure that the ground stakes are outside the spheres of influence. If there is a significant change in the reading (30%), you should increase the distance between the ground rod under test, the inner stake (probe) and the outer stake (auxiliary ground) until the measured values remain fairly constant when repositioning the inner stake (probe).

Stakeless measurement

The Fluke 1625 earth ground tester can measure earth ground loop resistances for multi grounded systems using only current clamps. This test technique eliminates the dangerous step of disconnecting parallel grounds, as well as the process of finding suitable locations for auxiliary ground stakes.

You can also perform earth ground tests in places you have not considered before: inside buildings, on power pylons or anywhere you don’t have access to soil.

With this test method, two clamps are placed around the earth ground rod or the connecting cable and each is connected to the tester (see Fig. 2). Earth ground stakes are not used at all. A known voltage is induced by one clamp, and the current is measured using the second clamp. The tester automatically determines the ground loop resistance at this ground rod. If there is only one path to ground, the stakeless method will not provide an acceptable value and the fall-of-potential test method must be used.The earth ground tester works on the principle that in parallel/multi-grounded systems, the net resistance of all ground paths will be extremely low compared to any single path (the one under test). So, the net resistance of all theparallel return path resistances is effectively zero. Stakeless measurement only measures individual ground rod resistances in parallel to earth grounding systems. If the ground system is not parallel to earth, you will either have an open circuit or be measuring ground loop resistance.

Fig. 2: Setup for the stakeless method.

Ground impedance measurements

When attempting to calculate possible shortcircuit currents in power plants and other highvoltage/current situations, determining the complex grounding impedance is important since the impedance will be made up of inductive and capacitive elements. Because inductivity and resistivity are known in most cases, actual impedance can be determined using a complex computation.

Since impedance is frequency dependent, the Fluke 1625 uses a 55 Hz signal for this calculation to be as close to voltage operating frequency as possible. This ensures that the measurement is close to the value at the true operating frequency. Power utility technicians testing high voltage transmission lines are interested in two things.The ground resistance in case of a lightning strike and the impedance of the entire system in case of a short circuit on a specific point in the line. Short circuit in this case means an active wire breaks loose and touches the metal grid of a tower.

At central offices

When conducting a grounding audit of a central office there are three different measurements required.

Before testing, locate the master ground bar (MGB) within the central office to determine the type of grounding system. The MGB will have ground leads connecting to the multi-grounded neutral (MGN) or incoming service, the ground field, water pipe and structural or building steel (see Fig. 3).

Fig. 3: The layout of a typical central office.

First, perform the stakeless test on all the individual grounds coming from the MGB (see Fig. 4). The purpose is to ensure that all the grounds are connected, especially the MGN. It is important to note that you are not measuring the individual resistance, but the loop resistance of what you are clamped around. Connect the earth ground tester and both the inducing and sensing clamps, which are placed around each connection to measure the loop resistance of the MGN, ground field, water pipe and the building steel. Second, perform the 3-pole fall-of-potential test of the entire ground system, connecting to the MGB (see Fig 5). To get to remote earth, many phone companies use unused cable pairs going out as much as a mile. Record the measurement and repeat this test at least annually.

Fig. 4: Stakeless testing of a central office.

Thirdly, measure the individual resistances of the ground system using the selective test of the earth ground tester (see Fig. 6). Connect the tester. Measure the resistance of the MGN; the value is the resistance of that particular leg of the MGB. Then measure the ground field. This reading is the actual resistance value of the central office ground field.

Fig. 5: Perform the 3-pole fall-of-potential test of the entire ground system.

Now move on to the water pipe and repeat for the resistance of the building steel.You can easily verify the accuracy of these measurements through Ohm’s Law. The resistance of the individual legs, when calculated, should equal the resistance of the entire system given (allow for reasonable error since all ground elements may not be measured).

Fig. 6: Measure the individual resistances of the ground system using the selective test.

These test methods provide the most accurate measure of central offices because it gives you the individual resistances and their actual behaviour in a ground system. Although accurate, the measurements would not show how the system behaves as a network because, in the event of a lightning strike or fault current, everything is connected.