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Use a multimeter to check line short circuit and ground
Universal meters are commonly used to check for short circuits in home wiring or to identify which wires are live. To test for a short circuit, the power supply should be turned off first. Then, all load switches should be disconnected, and the resistance between the two wires should be measured using the multimeter's ohm setting. Normally, a higher resistance value is better. When checking for grounding, use the ohm range of the multimeter to measure the resistance between each wire and the ground. Again, a higher reading is preferable. It’s important to note that while a multimeter can detect very low resistance values (indicating a short or ground fault), it may not be accurate for small resistances. In low-voltage systems (up to 380V), it's recommended to use a 500V megohmmeter for more accurate insulation testing between lines and to ground. The resistance should be above 0.38 megohms; otherwise, it’s consideredä¸åˆæ ¼.
To begin, separate the live wire from the neutral wire. For the ground wire, set the multimeter to AC voltage mode, selecting a range higher than 220V. Insert the red probe into the voltage socket, leave the black probe unconnected, and insert the red probe into one of the socket’s slots. Observe the reading. The highest reading corresponds to the live wire, the lower reading is the neutral, and no reading indicates no ground connection. If both readings are low, it suggests the ground wire is not properly grounded or is connected to the neutral line, and further testing may not be necessary.
Switch the multimeter to the "short circuit" test function (if available). Connect the red and black probes to the circuit's ground and the main power ground. If the result shows a short circuit or very low resistance, the line is grounded. Otherwise, it is not. For leakage and grounding checks, set the multimeter to 200MΩ. Connect one end of the test lead to the device's casing or ground wire, and the other end to the line being tested. Avoid touching the test leads during measurement to prevent errors.
Set the multimeter to the resistance setting (20K or 200KΩ), turn off the main and load power supplies, then connect one test lead to the live wire and the other to the ground wire. Check the resistance value, then repeat the process with the neutral wire and ground wire. If the resistance reads 7.3 or higher, or 14 or higher, it may indicate a resistance issue or a neutral line leakage.
Many electricians recommend disconnecting all power sources and measuring the resistance between two wires using the lowest resistance setting (or diode mode) to see if it's close to 0 ohms (or shows 0 on the diode display). If so, the wires are likely shorted. Similarly, checking the wire against the ground can confirm grounding. This method works well, but it may not always be practical to shut off the power. Is continuous measurement feasible? It depends on the situation.
Voltage essentially represents a potential difference. If the voltage between two wires is zero, you can use the resistance setting to check for a short. Suppose you want to check for a short between Line A and Line B. There might be a voltage (e.g., 220V) between them. People often think you need to disconnect the power first before measuring. While this approach is correct, it can be overly cautious.
Directly set the multimeter to AC voltage mode, select the highest range (e.g., 1000V), and measure the voltage between Line A and Line B. If there is a significant voltage (e.g., 200V), it means the potentials are different, so the lines are not at the same potential and are not shorted. If there is no voltage, switch to DC voltage mode (e.g., 1000V) to double-check. Even if there is no DC voltage, the lines could still be at the same potential, such as both being 220V relative to the neutral. In that case, use the lowest resistance setting to measure between the two wires. If the resistance is close to 0 ohms, they are shorted.
To check for grounding, treat the ground wire like any other wire. However, for a proper ground test, it's best to use a megohmmeter to measure the insulation resistance (typically around 5 megohms). This requires the system to be powered off for accurate results.