Question 5 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is C **Why Option C is Correct:** Using a multimeter set to the ohmmeter function (measuring resistance) requires that the component being tested is completely isolated from any external power source or parallel components. 1. **De-energization:** An ohmmeter uses a small internal battery to pass a known current through the component and then calculates the resistance based on the resulting voltage drop (Ohm's Law). If the circuit is energized, the external voltage will interfere with the meter's internal measurement circuit, leading to an incorrect reading and potentially damaging the meter. Therefore, the circuit must be de-energized. 2. **Isolation:** If the resistor remains connected within the circuit, other components (such as parallel resistors, capacitors, or coils) will provide alternative paths for the ohmmeter's internal current. This parallel path resistance will combine with the target resistor's value, resulting in a reading that represents the total circuit resistance rather than the true value of the single resistor being tested. To get an accurate reading of the specific resistor, at least one lead must be disconnected (isolated) from the rest of the circuit. **Why the Other Options are Incorrect:** **A) The meter case must be grounded prior to attaching the leads.** Incorrect. While grounding is critical for high-voltage measurements or ensuring personnel safety in some environments, it is not a prerequisite for obtaining a valid resistance reading on a passive component like a resistor using a standard handheld multimeter (ohmmeter). The resistance measurement relies on the potential difference across the component itself. **B) The meter leads must not be twisted so as to cancel out the individual magnetic fields.** Incorrect. Twisting leads (or avoiding twisting them) is a precaution often taken in high-frequency measurements or when measuring very small inductances to minimize electromagnetic interference (EMI) or coupling. For standard resistance measurements of a resistor in a central control console, this concern is irrelevant, as the magnetic fields generated by the small DC current from the ohmmeter's internal battery are negligible and do not affect the resistance reading. **D) The correct polarity must be observed because reverse bias will damage the component.** Incorrect. Resistors are passive, non-polarized components. Unlike diodes, transistors, or electrolytic capacitors, they function the same regardless of the direction of current flow. Observing polarity (forward or reverse bias) is unnecessary when testing a resistor, and applying reverse polarity will not damage it. (Note: The meter itself might slightly prefer one polarity for certain high-precision analog measurements, but the component is unaffected.)