Circuit protection selection: If current in a given circuit must not exceed 24 A, which fuse rating is the most appropriate to protect the circuit reliably?

Introduction / Context:
Selecting the correct fuse rating is critical for safety and reliability. A fuse should protect conductors and loads by opening the circuit before damaging currents persist. The rating must coordinate with the maximum desired operating current and allowable transient conditions. Here, the requirement is that current must not exceed 24 A.

Given Data / Assumptions:

• Maximum permissible current in normal operation is strictly below 24 A.
• We want the fuse to open the circuit before currents reach or exceed 24 A for sustained periods.
• No special inrush or time-delay considerations are provided; use a conservative general-purpose selection.

Concept / Approach:

A protective device rating is typically chosen at or slightly below the maximum continuous current that should be allowed. Using a fuse equal to the prohibited limit (24 A) may allow operation at the very threshold; choosing a lower rating (e.g., 20 A) provides a margin that ensures the circuit opens before exceeding 24 A under sustained conditions. Time-current curves complicate real designs, but absent special data, a 20 A fuse is the safer pick.

Step-by-Step Solution:

Verification / Alternative check:

In practice, you would consult device time-current characteristics and load inrush data. For typical steady loads without high inrush, a 20 A fuse ensures compliance with the 'not to exceed 24 A' constraint.

Why Other Options Are Wrong:

24 A risks operating at the limit. 10 A is unnecessarily restrictive and may nuisance-blow. 'A fuse is not necessary' conflicts with the protection requirement. 30 A would permit currents above the stated maximum.

Common Pitfalls:

Ignoring inrush; confusing breaker ratings with fuse fast/slow characteristics; selecting a rating above the maximum allowable current.

Final Answer:

20 A