How to Measure Earth Loop Resistance Using Ammeter and Voltmeter?

Electrical Technology

How to Measure Earth Loop Impedance Using Ammeter and Voltmeter?

Purpose of measuring earth loop resistance is essentially to determine the resistance of the path through which current flows towards the earth in the event of a short circuit, or to find the resistance of the earth continuity conductor. This is done so that the circuit protective device, such as a circuit breaker or fuse, can act immediately and decisively in the event of a short circuit.

It is crucial to note that as the earth loop impedance decreases, the value of fault current increases, causing the circuit breaker or fuse to act more promptly. If the earth loop resistance is high, the circuit breaker or fuse may not open quickly during a fault due to the lower fault current flow. This could lead to hazardous situations. Therefore, it is essential to measure the earth loop resistance value using this test so that the operation of the circuit breaker or fuse in the event of a fault can be ensured.

This test can be conducted using an earth loop resistance tester, megger, digital ground/earth resistance tester or with the use of a voltmeter and ammeter.

In this tutorial, we will be using an voltmeter and ammeter to measure and calculate the earth loop resistance for specific circuit(s). It is important to ensure that the value of the current flowing through the earth loop is not more than 1.5 times the current rating of the circuit.

Before conducting this test, it is necessary to perform the insulation resistance test between the neutral and earth to ensure that the neutral is properly insulated from the earth.

Measuring Earth Loop Resistance using Ammeter and Voltmeter

To measure the earth loop resistance, one cable of the circuit is used as the return path (utilized as the return path for fault current) and connected to the farthest point of the earth continuity conductor. The procedure for measuring earth loop impedance using an ammeter and voltmeter is demonstrated in the following fig.

The maximum voltage from the transformer secondary windings is 40 volts. The testing leads coming out of the secondary side of the transformer are connected to the termination point of the cable and bonded to the ground rod / earth electrode (as shown in the diagram).

This completes the circuit, and the current flows through the continuity conductor. The value of this current is adjusted to 1.5 times the circuit current rating through a rheostat so that the current does not exceed 25 amperes.

For example, if the circuit current rating is 10 amperes, the transformer supplies 15 amperes to the circuit. Afterward, the voltmeter and ammeter readings are noted, and the total earth loop resistance value is determined using Ohm’s law formula.

= Reading of Voltmeter ÷ Reading of Ammeter

R_L = V ÷ I

Since the resistance of the wiring cable used as the return path is included in the total value of this resistance, the resistance of the wiring cable should be subtracted from the total resistance value to determine the actual resistance value of the earth continuity conductor. The resistance of a specific cable and wire, determined by its length and size, can be found by using the resistance tables for the cable’s conductors.

The value of earth loop resistance should not be more than one ohm (1-Ω) in the case of a copper conductor and not more than half an ohm (0.5Ω) in the case of a steel conduit.