Question 7 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is B **Explanation for Option B (Thermal overload relay):** When a three-phase induction motor experiences an open in one phase (a condition known as single-phasing), the motor will attempt to continue running on the remaining two phases. This causes the current flowing through the windings in the remaining phases to increase significantly, often exceeding the full-load current rating. This excessive current leads to overheating of the motor windings. A **thermal overload relay** is specifically designed to monitor the current draw of the motor. It contains heating elements (or electronic equivalents) that mirror the thermal characteristics of the motor windings. When the sustained current exceeds the set threshold (as happens during single-phasing), the thermal elements heat up and trip the protective contacts, opening the motor circuit and removing power before the sustained overheating can cause permanent damage to the insulation and windings. **Explanation for Incorrect Options:** * **A) Magnetic blowout coil:** Magnetic blowout coils are typically used within circuit breakers or contactors to assist in extinguishing the arc that forms when contacts open, especially under high current conditions. They do not monitor the motor's operating current or temperature and therefore cannot detect or prevent damage from single-phasing. * **C) Overspeed trip:** An overspeed trip (governor) is designed to protect motors or generators against excessive rotational speed. Single-phasing in an induction motor generally causes it to slow down or stall, not overspeed. Thus, this device is irrelevant for protecting against an open phase condition. * **D) Three-pole safety switch:** A three-pole safety switch is a manual disconnecting device (or disconnect switch) used to isolate the motor circuit for maintenance or safety. While it can interrupt power, it is not an automatic protective device designed to sense and trip based on electrical faults like single-phasing or overcurrent.