Circuit breaker time-current characteristic curves are read similar to fuse curves. The horizontal axis represents the current, and the vertical axis represents the time at which the breaker interrupts the circuit. When using molded case circuit breakers of this type, there are four basic curve considerations that must be understood.
- Overload Region
- Instantaneous Region
Illustration: Medium to High-Level Fault Currents–Circuit Breakers
The following curve illustrates a 400A circuit breaker ahead of a 90A breaker. Any fault above 1500A on the load side of the 90A breaker will open both breakers.
The 90A breaker will generally unlatch before the 400A breaker. However, before the 90A breaker can separate its contacts and clear the fault current, the 400A breaker has unlatched and also will open. Assume a 4000A short circuit exists on the load side of the 90A circuit breaker.
The sequence of events would be as follows:
- The 90A breaker will unlatch (Point A) and free the breaker mechanism to start the actual opening process.
- The 400A breaker will unlatch (Point B) and it, too, would begin the opening process. Once a breaker unlatches, it will open. At the unlatching point, the process is irreversible.
- At Point C, the 90A breaker will have completely interrupted the fault current.
- At Point D, the 400A breaker also will have completely opened the circuit. Consequently, this is a non-selective system, causing a complete blackout to the other loads protected by the 400A breaker.