Which protective device is primarily used to prevent overcurrent in electrical systems?

The device primarily used to prevent overcurrent in electrical systems is the circuit breaker. Circuit breakers are designed to interrupt the flow of electrical current when it exceeds a predetermined level, thereby protecting the system from damage due to excessive current. They automatically switch off in response to an overcurrent condition, preventing overheating, potential fires, and damage to electrical equipment.

While fuses also provide overcurrent protection by melting and breaking the circuit when the current exceeds safe levels, circuit breakers offer the advantage of being reset or re-engaged after tripping, allowing for continuous use without replacement. This makes circuit breakers a more versatile and practical choice for modern electrical systems compared to fuses.

Transformers are used to change the voltage level in electrical systems but do not offer overcurrent protection. Isolation switches serve to disconnect parts of the electrical system for maintenance or safety purposes but do not actively protect against overcurrent situations. Therefore, the circuit breaker is the most appropriate answer for protecting against overcurrent conditions in electrical systems.