HowStuffWorks "Basic Circuit Breaker Design"
There are two types of breakers. MOST inexpensive ones use a bi-metallic strip that needs to get hot enough to bend away from the contact to trip the switch to the "tripped" position.
Since the "quickness" of the trip depends on how much heat is being produced, a small overload can take several minutes to heat the strip enough to release the switch. The higher the overload current the quicker it will trip.
Some breakers use an electromagnet to pull the trip lever. Once the overload current reaches a point where the magnet is strong enough, the circuit will open.
Even with the electromagnet style, there are tolerances (say 30 amps +
1 amp). The circuit may open early or late depending on where the actual current flow needed to trip it falls.
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