Methods for Detecting Short‑Circuits in Circuits with a Multimeter
From the perspective of electrical equipment maintenance, short‑circuit faults have the following characteristics: ① The resistance (or impedance) at the short‑circuit point (between the two short‑circuited ends) is zero or close to zero. ② Short‑circuit faults are highly destructive. Once a short‑circuit occurs, direct power‑on inspection is generally prohibited, which is different from open‑circuit faults.
After a short‑circuit fault occurs, protective components of the circuit (such as fuses and circuit breakers) will operate. Since a protective component may control an area composed of multiple loops, when locating electrical short‑circuit faults, it is necessary to first identify the faulty loop within the fault area, and then find the exact short‑circuit point inside the faulty loop.
Methods for Locating Short‑Circuited Loops
A multimeter is commonly adopted for this purpose. The multimeter method measures the resistance of the short‑circuited loop with the multimeter ohm range after cutting off circuit power.
Take Figure 7 as an example. Assume that the fuse element of fuse FU has blown, indicating that a short‑circuit fault has occurred in the area protected by this fuse. This fault area includes three loops No. 1‑3 and the main trunk circuit. With the power supply disconnected, restore the fuse element of FU. Set the multimeter to the "R×10 Ω" ohm range (do not use high‑multiplier ohm ranges to avoid reading errors caused by human‑body resistance and other factors), and connect the multimeter across terminals L and N. Turn off switches S1, S2 and S3 to disconnect each individual loop. If the multimeter reads zero resistance, the short‑circuit fault exists on the main trunk circuit, as shown in Figure 7(a).
If the multimeter shows "∞" or a large resistance value, the short‑circuit fault lies in one of loops 1‑3. Close switches S1, S2 and S3 one by one. If the multimeter displays a definite resistance value when S1 and S2 are closed, and shows zero resistance when S3 is closed, the fault point is located in Loop 3.
