Question 5

The Correct Answer is A. ### Why Option A is Correct **Option A: if possible, open the delta-connections to avoid shunting the phase being tested** In a three-phase delta ($\Delta$) connection, the three phase windings are connected end-to-end, forming a closed loop. When using an ohmmeter to check the resistance of a specific phase winding (Winding 1, for example), the ohmmeter is connected across the terminals of that winding. However, because the circuit is a closed loop, the current from the ohmmeter splits and flows not only through Winding 1 but also through the series combination of Winding 2 and Winding 3. If one phase winding (Winding 1) has an open circuit (infinite resistance), the ohmmeter measurement taken across its terminals will not read infinity. Instead, the ohmmeter will measure the resistance of the path composed of Winding 2 and Winding 3 in series. This shunting effect makes it impossible to definitively isolate and test a single winding for an open circuit if the delta connection remains closed. Therefore, to accurately test the resistance and confirm an open circuit in a specific phase, the delta connection **must be opened** (broken at one or more points) so that each winding can be measured independently without the shunting path provided by the other two windings. ### Why Other Options Are Incorrect **B) measure the voltage across the open connections while testing** An ohmmeter is a device used to measure resistance (impedance at DC). While voltage measurements are critical for energized circuits, an open circuit test using an ohmmeter is performed on a de-energized system. Measuring voltage across the winding terminals during an ohmmeter test (which injects a small DC current) is irrelevant and does not help diagnose the open circuit in the specific winding. **C) test the windings as parallel groups to avoid short circuiting** Delta windings are already connected in a closed loop, which is a form of series/parallel combination relative to a single winding test. Testing them as parallel groups is usually done for resistance balancing in Y-connections, but it doesn't solve the fundamental problem of the delta connection: the shunt path. Furthermore, the goal is to find an **open** circuit, not to avoid a **short** circuit (which is a different fault type). **D) test each phase with all connections intact** This is precisely what must be avoided. As explained in the reasoning for Option A, testing the windings with all connections intact makes it impossible to isolate an individual phase. If one phase is open, the ohmmeter will read the resistance of the two healthy phases in series, masking the fault. Accurate open-circuit testing requires breaking the closed loop.