How to Check Whether a Circuit is Earthed with a Digital Multimeter
① Distinguish live wire, neutral wire and earth wire: Set the multimeter to the AC voltage range higher than 220V. Insert the red test lead into the voltage jack and leave the black test lead disconnected. Insert the red test lead into each socket hole and observe the readings. The highest reading corresponds to the live wire, the lower reading to the neutral wire, and nearly unchanged reading to the earth wire. If two readings are low and one is high, it means the earth wire is not grounded and connected to the neutral wire. No further measurement in Step 2 is required.
② Measure the earth wire: After identifying live, neutral and earth wires, keep the multimeter in the same AC range above 220V. Insert the red and black test leads into the voltage jack and COM jack respectively. Use the two test leads to measure the voltage between live wire and neutral wire, and the voltage between live wire and earth wire. If these two voltage values are roughly equal, the earth wire is grounded. If the voltage between live wire and earth wire is nearly zero or extremely low, the earth wire is not grounded.
Method 2: ① Set the digital multimeter to the resistance range of 20kΩ or higher, then measure the insulation between phases respectively.
② Test for circuit earthing by measuring the outer casing of electrical equipment. If the circuit is shorted to earth and touches the equipment casing, the resistance value is generally several hundred ohms or even zero.
③ It is difficult to measure the resistance between circuit and earth with a regular multimeter. An insulation resistance tester is recommended for this measurement.
