Manually resettable supplementary circuit protectors are often used within appliances or other electrical equipment. While circuit breakers provide branch protection for overcurrent conditions resulting from short-circuit and circuit overload situations, supplementary protectors provide over- or under-voltage protection, as well as additional overcurrent protection and are used where branch circuit overcurrent protection is not required or is already provided.
Curve ratings determine the level of sensitivity of a protector, with A-curve being the most sensitive and D-curve being the least sensitive.
A-curve supplementary protectors have a magnetic trip point at 2 to 3 times the rated current for protection of sensitive equipment.
Protect computers, semiconductors, and other sensitive equipment that have resistive or slightly inductive loads using these B-curve supplementary protectors, which have a magnetic trip point at 3 to 5 times the rated current.
For inductive loads, including transformers and pilot devices, C-curve protectors have a magnetic trip point at 5 to 10 times the rated current.
When you have highly inductive loads, use D-curve supplementary protectors, which have a magnetic trip point at 10 to 20 times the rated current.