How to use a clamp ammeter to identify grounding fault points in low-voltage power grids
In low-voltage power grids, it is difficult to identify the specific grounding fault point when a grounding fault occurs in the distribution circuit or electrical equipment. The method of measuring leakage current using a clamp ammeter to determine grounding faults does not require wiring or power outage, and is easy to operate, allowing for quick and accurate identification of the fault point.
The circuit for detecting the grounding fault point in the low-voltage power grid using a clamp ammeter is shown in the following figure.
In the low-voltage distribution network, if a ground fault occurs, the power supply can be tested in phases on the power main switch QS in the distribution room (temporary removal of other two-phase fuses can be used), and then a clamp type ammeter (preferably a clamp type ammeter with a milliampere rating, or a few more turns of the tested wire on the clamp iron core) can be used to connect the neutral grounding wire of the distribution transformer (as shown in the PA1 clamp type ammeter in the figure), Measure the leakage current of each phase circuit separately. The phase circuit with a high leakage current can be classified as a circuit with a ground fault point.
After determining the phase circuit with a ground fault, first detect the main line of the circuit. Check if there are any overhead wires that are broken or connected to wires, telephone lines, broadcasting lines, etc., or if the faulty phase circuit directly collides with the neutral line, resulting in a single-phase grounding short circuit fault. This type of grounding fault point is clearly easy to find. If the fault is not on the main line, a clamp type ammeter can be used to check the branch lines of each phase (as shown in the PA2 clamp type ammeter in the figure). When a branch line has current and the other branch circuits have no current, it can be determined that the branch circuit with current has a ground leakage fault.
Afterwards, the branch line is tested using the "bisection method", which involves finding a convenient testing point from about half of the circuit and using a clamp shaped ammeter to clamp the phase wire. If there is current at the testing point, it indicates that the fault point is on the load side of the testing point; If no current is measured or the current is very weak at this test point, it indicates that the grounding point is on the power side of the test point. After determining which section of the circuit the fault point is in, follow the above method to detect and judge from the middle of this section of the circuit, gradually reducing the scope of ground fault occurrence. When the boundary point between the presence and absence of current is found, this is the ground fault point to be searched for.
