Question 37 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is A. **Why Option A is Correct:** Circuit breakers are protective devices designed to interrupt fault currents and protect electrical systems and equipment from damage. Even when initially selected and installed correctly (correctly rated and properly installed), various factors over time can degrade their performance. These factors include: * **Environmental Stress:** Dust, humidity, temperature extremes, and corrosive atmospheres can affect internal components, linkages, and contacts. * **Mechanical Wear:** Repeated operation (though infrequent for primary protection devices) can cause wear on moving parts. * **Electrical Stress:** Previous fault interruptions, even within the breaker's rating, can cause contact erosion and degradation of insulation. * **Vibration and Loosening:** Connections and calibration settings can loosen over time due to vibration. Periodic testing (including visual inspection, mechanical operation checks, and electrical tests like contact resistance and trip-time testing) is essential to identify these degradations and confirm that the breaker still operates within its specified parameters. The primary goal of ensuring the breaker operates correctly is to guarantee that it will reliably provide the **original degree of protection** it was intended to deliver when a fault occurs. **Why Other Options are Incorrect:** **B) to insure they can trip faster as they increase in age:** This is incorrect. Circuit breakers are designed to trip within a specific, controlled time range (based on their time-current characteristics). Age and degradation typically lead to **slower** tripping times or complete failure to trip, not faster operation. Even if the breaker could trip faster, tripping faster than its designed curve could cause nuisance tripping or improper coordination. **C) to insure they do not exceed their interrupting capacity:** This is incorrect. The interrupting capacity (IC) is a fixed maximum fault current the breaker is designed to safely clear. Testing does not change the installed IC of the breaker; rather, testing ensures that the breaker's internal mechanisms and contacts are still capable of safely clearing a fault current up to its rated IC. Exceeding the interrupting capacity is a function of the electrical system's available fault current, not the breaker's internal condition. **D) to insure they will be able to withstand at least 125% of applied voltage:** This is incorrect. Circuit breakers are rated for specific nominal system voltages (e.g., 480V, 13.8 kV). They are typically required to withstand higher instantaneous voltages (like transients and switching surges, which are handled by their basic insulation level, or BIL), but the 125% factor usually relates to continuous **current** capacity in conductors and overcurrent devices (NEC Article 210.19/215.2), not continuous voltage tolerance for the protective device itself. Testing focuses on current handling and mechanical reliability, not demonstrating a 25% continuous over-voltage tolerance.