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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 should be turned off first, and then the load switches should be disconnected. Using a multimeter, measure the resistance between the two wires. Under normal conditions, a higher resistance is better. For grounding checks, use the ohm function of the multimeter to measure the resistance between each wire and the ground. Again, the higher the resistance, the better.
It's important to note that while a multimeter can detect very low resistance (indicating a short or ground), it may not be accurate for slightly higher resistance values. In low-voltage circuits (such as 380V), it's recommended to use a 500V megohmmeter instead. The insulation resistance between lines or between lines and ground should be above 0.38 megohms; otherwise, the circuit is considered faulty.
To begin, separate the live wire from the neutral wire. For the ground wire, set the multimeter to AC voltage mode with a range higher than 220V. Insert the red probe into the voltage socket and leave the black probe unconnected. Then, insert the red probe into one of the sockets on the outlet and observe the reading. The highest reading corresponds to the live wire, the lower reading is the neutral, and a near-zero reading indicates no ground connection.
If you get two small readings, it suggests that the ground wire is not properly grounded or is connected to the neutral line. In such cases, further measurements may not be necessary.
Next, switch the multimeter to the "short circuit" test mode (if available). Connect the red and black probes to the circuit’s ground and the mains’ ground, respectively. If the reading shows a short circuit or very low resistance, the line is grounded. Otherwise, it is not.
To check for leakage or grounding, 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. Make sure not to touch the test leads during measurement to avoid errors.
Set the multimeter to the resistance mode at 20K or 200KΩ. Turn off both the main and load power supplies. Connect one test lead to the live wire and the other to the ground wire, noting the resistance value. Then connect one test lead to the neutral wire and the other to the ground wire, observing the resistance again. If the resistance is 7.3 or higher, or 14 or higher, it could indicate a resistance issue or a zero-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. A near-zero reading suggests a short. Similarly, checking the resistance between the wire and the ground can confirm grounding. While this method works, it may not always be practical, especially when power cannot be turned off.
The key idea behind voltage is the potential difference. If the voltage between two wires is zero, you can use the resistance mode to check. For example, if you want to test for a short between line A and line B, there might be a voltage (like 220 volts) between them. The first thought is to turn off the power before measuring, but this approach is overly cautious.
Alternatively, set the multimeter to AC voltage mode at the highest range (e.g., 1000V) and measure the voltage between line A and line B. If a high voltage is detected, it means the potentials are different, so the lines are not shorted. If no voltage is present, switch to DC voltage mode (e.g., 1000V) to double-check. If there is no DC voltage, the potentials are equal, but they could still be at 220V relative to the neutral line.
In that case, use the lowest resistance setting to measure between the two wires. If the resistance is close to 0 ohms, the wires are shorted. As for grounding, treat the ground wire like any other line. However, to check for proper grounding, it's best to use a megohmmeter to measure insulation resistance (typically around 5 megohms), which requires the circuit to be powered off.