The easiest way to measure ground resistance using clamp meter, but be carefull!

Why clamp meter / tester for grounding?

The ground clamp meter / tester is an effective and time-saving tool when used correctly because the user does not have to disconnect the ground system to make a measurement or place probes in the ground.

The method is based on Ohm’s Law, where:

R (resistance) = V (voltage) / I (current)

The clamp includes a transmit coil, which applies the voltage and a receive coil, which measures the current. The instrument applies a known voltage to a complete circuit, measures the resulting current flow and calculates the resistance (see figure 1).

The clamp method requires a complete electrical circuit to measure. The operator has no probes and therefore cannot set up the desired test circuit. The operator must be certain that earth is included in the return loop. The clamp tester measures the complete resistance of the path (loop) that the signal is taking. All elements of the loop are measured in series.

The method assumes that only the resistance of the ground electrode under test contributes significantly. Based on the math behind the method (to be reviewed below), the more returns, the smaller the contribution of extraneous elements to the reading and, therefore, the greater the accuracy.

In addition, it includes the bonding and overall connection resistance. Good grounding must be complemented by “bonding”, having a continuous low-impedance path to ground. Fall of potential measures only the ground electrode, not the bonding (leads must be shifted to make a bonding test).

Because the clamp uses the grounding conductor as part of the return, an “open” or high resistance bond will show up in the reading.

The clamp ground tester also allows the operator to measure the leakage current flowing through the system. If an electrode has to be disconnected, the instrument will show whether current is flowing to indicate whether it is safe to proceed.

Unfortunately, the clamp ground tester is often misused in applications where it will not give an effective reading. The clamp method is effective only in situations where there are multiple grounds in parallel. It cannot be used on isolated grounds as there is no return path.

Unlike with fall of potential testing, there is no way of proofing the result, meaning the results must be taken on “faith.” The clamp ground tester does fill a role as one tool that the technician could have in his “bag”, but not the only tool.

Clamp ground testing theory and methodology

Understanding how and why the clamp method works helps in understanding where it will and will not operate, and how to optimize its use. As mentioned, the clamp test method is based on Ohm’s Law (R = V/I).

The following graphics will show and explain the following:

• Series circuit,
• Parallel circuit,
• Parallel-series circuit and
• The math used to determine the total current and resistance

Series Circuit

In a series circuit (figure 2), total current and total resistance are calculated as follows:

I_t = I₁ = I₂ = I₃
R_t = R₁ + R₂ + R₃

Parallel Circuit

In a parallel circuit (figure 3), total current and total resistance are calculated as follows:

I_t = I₁ + I₂ + I₃
R_t = 1/ (1/R₁ + 1/R₂ + 1/R₃)

Parallel-Series Circuit

In a parallel-series circuit (figure 4), total current and total resistance are calculated as follows:

I_t = I₁ + I₂ = I₃ = I₄ + I₅
R_t = 1/ (1/R₁ + 1/R₂) + 1/ (1/R₃ + 1/R₄)

Clamp Test Methodology

The head of a clamp ground tester includes two cores (see figure 5). One core induces a test current and the other measures how much was induced. The input or primary voltage of the test current inducing core is kept constant, so the current actually induced into the test circuit is directly proportional to the loop resistance.

The important thing to remember with clamp testing is that clamp ground testers effectively make loop resistance measurements. Clamp measurements are loop measurements. For the clamp method to work there must be a series- parallel resistance path (and the lower the better).

The following figure shows a pole ground configuration, one of the most effective applications of the clamp ground tester.

The circuit diagram for this configuration follows (based on a clamp ground tester being clamped around pole 6):

The clamp ground tester is clamped around one of the electrodes and then measures the resistance of the entire loop. The remaining ground electrodes are all in parallel, and, as a group, are in series with the ground electrode being measured. If the clamp tester is clamped around pole #6, the measurement of the resistance of the entire loop would be calculated using the following equation:

R_loop = R₆ + (1/ (1/R₁ + 1/R₂ + 1/R₃ + 1/R₄ + 1/R₅))

For six similar ground electrodes with a resistance of 10 Ω each, the measurement of the total loop resistance would be:

R_loop = 10 + (1/ (1/10 + 1/10 + 1/10 + 1/10 + 1/10))
R_loop = 10 + (1/ (5/10))
R_loop = 10 + 2

R_loop = 12 Ω

The measurement of the loop resistance is relatively close to the resistance of the ground electrode being tested. If there were 60 similar ground electrodes with a resistance of 10 Ω each, the measurement of the total loop resistance would be:

R_loop = 10 Ω + 0.17 Ω = 10.17 Ω

Using the six electrode example, if electrode number 6 had a resistance of 100 Ω and all the other electrodes had resistances of 10 Ω, the measurement of the loop resistance would be:

R_loop = 100 + (1/ (1/10 + 1/10 + 1/10 + 1/10 + 1/10))
R_loop = 100 + (1/ (5/10))
R_loop = 100 + 2

R_loop = 102 Ω

In the following example, the clamp ground tester would indicate the bad ground. If the 100 Ω electrode was one of the electrodes not being measured, the impact on the overall measurement would be minimal:

R_loop = 10 + (1/ (1/10 + 1/100 + 1/10 + 1/10 + 1/10))
R_loop = 10 + (1/ (41/100))
R_loop = 10 + 2.44

R_loop = 12.44 Ω

NOTE // Please note that the measured resistance will always be higher than the actual resistance of the ground electrode being tested. Any error present is on the side of safety, as resistance guidelines are for maximum ground resistance.

This means that if the measured resistance is below target level for the ground electrode, the operator can be assured that actual resistance will also be below the target.

To conclude //

In summary, remember that a clamp ground tester measurement is a measurement of the resistance of the entire loop. There must be a loop resistance to measure. If there isn’t a loop to measure the operator can create one with a temporary jumper lead. The greater the number of parallel paths, the closer the measured value will be to the actual earth resistance of the electrode under test.

Remember that the earth path must be in the circuit to measure ground resistance. This caveat sounds obvious, but if you have metal structures involved there may be a connection through that, rather than the earth mass.

Examples of measuring ground rod resistance using clamp meter

Reference // Guide to Clamp Ground Testing by MEGGER