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.
source:http://electrical-engineering-portal.com
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