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Interpretation of the method for judging short circuit and grounding
If you want to check for short circuits in a line, the first step is to turn off the power supply. Then, disconnect the load switches and use a multimeter to measure the resistance between the two wires. Under normal conditions, a higher resistance value is better. To check for grounding, use the ohm setting on the multimeter to measure the resistance between each wire and the ground. Again, the higher the resistance, the better.
It should be noted that using a multimeter to detect short circuits or grounding is not always accurate. In cases where the resistance is very low, a multimeter can detect it. However, if the resistance is slightly higher, the multimeter may not provide reliable results. For low-voltage circuits (such as 380V), it's recommended to use a 500V megohmmeter instead. The resistance between lines or between a line and ground should be above 0.38 megohms; otherwise, the circuit is consideredä¸åˆæ ¼ (unqualified).
First, separate the live wire from the neutral wire. For the ground wire, set the multimeter to AC voltage mode, and choose a range higher than 220V. Insert the red test lead into the voltage socket, while the black test lead remains disconnected. Then, insert the red lead into one of the sockets and observe the reading. The highest reading corresponds to the live wire, the lower reading is the neutral wire, and the ground wire should show little to no voltage. If two readings are low and one is high, it could indicate that the ground wire is not properly grounded or is connected to the neutral line, and further testing may not be necessary.
Next, switch the multimeter to the "short circuit" test function (if available). Connect the red and black test leads to the circuit’s ground and the main power ground, respectively. If the test shows a short circuit or a very low resistance, the line is grounded. Otherwise, it is not.
To check for leakage and grounding, set the multimeter to 200MΩ. Connect one end of the test lead to the device casing or ground wire, and the other end to the line being tested. Make sure your hands do not touch the test leads during measurement to avoid errors.
Set the multimeter to the resistance mode (20K or 200KΩ). Turn off the main and load power supplies. Connect one test lead to the live wire and the other to the ground wire, then repeat the process with the neutral wire and ground wire. If the resistance is 7.3 MΩ or higher, or 14 MΩ or more, 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 range (or diode mode) to see if it's close to 0 ohms (or shows 0 on the diode display). If it is, this suggests a short circuit. Similarly, measuring the resistance between the wire and ground can confirm if it is grounded. While this method works, it may not always be practical, especially in situations where turning off the power is inconvenient. Is it feasible to perform these measurements continuously?
The fundamental concept of voltage is the potential difference. As long as the voltage between two lines is zero, you can use the resistance setting to check for shorts. For example, if you want to determine whether there is a short between Line A and Line B, there might be a voltage (e.g., 220 volts) between them. The potentials on the lines are Potential A and Potential B. Most people would think that using the resistance setting directly requires disconnecting the power supply on both lines first. While this approach is not wrong, it can be overly cautious.
Set the multimeter to the AC voltage setting, select the highest range (e.g., AC 1000 volts), and measure the voltage between Line A and Line B. If there is a significant voltage (e.g., 200 volts), it indicates that the potentials are not equal, meaning the two lines are not at the same potential and are not shorted together.
If the AC voltage measurement shows no voltage between the two lines, for safety, switch to the DC voltage setting (e.g., 1000 volts) and measure again to confirm there is no DC voltage. This confirms that the potentials are equal. However, even if they are equal, it doesn’t necessarily mean there is no voltage relative to the neutral line. For instance, if both lines are 220 volts relative to the neutral, the voltage between them could still be zero. At this point, you can use the lowest resistance setting to measure the resistance between the two wires. If it’s close to 0 ohms, the lines are shorted.
To check for grounding, apply the same logic by treating the ground wire like any other line. However, when checking for grounding, it’s common to use a megohmmeter to measure insulation resistance (typically 5 MΩ). This type of measurement requires the power to be turned off.