Unlike fuses and heat coils, which break the circuit, these series connected self-restoring thermally activated overcurrent protectors (OCPs) automatically reset when the electrical event causing the overcurrent stops, without the need for manual intervention. Self-restoring thermally activated overcurrent protector (OCP) components operate by the increasing in resistance value, which reduces the circuit current when the overcurrent exceeds a given value for a sufficient time. The resistance transition is caused by the component body reaching a critical temperature caused by the i2R heating of the overcurrent flowing through the component. The generic name for components with this type of action is positive temperature coefficient (PTC) thermistors. Being thermally operated, these PTC thermistors generally do not operate for short duration electrical transients, such as coupled lightning currents, but will operate for longer term AC and DC overcurrents. There are two types of material used to make PTC thermistors; ceramic and polymer. Many of the component parameters apply to the both types of material. Some parameters are specific to the material used and these differences are explained. This Recommendation describes PTC thermistor construction, operation, production, ratings, characteristics and gives application examples.