In order for an overcurrent protective device to operate properly, the overcurrent protective device ratings must be properly selected. These ratings include voltage, ampere and interrupting rating. Of the three of the ratings, perhaps the most important and most often overlooked is the interrupting rating.
If the interrupting rating is not properly selected, a serious hazard for equipment and personnel will exist. Current limiting can be considered as another overcurrent protective device rating, although not all overcurrent protective devices are required to have this characteristic.
National Electrical Code (NEC) requires the ratings found below:
The voltage rating of the overcurrent protective device must be at least equal to or greater than the circuit voltage. The overcurrent protective device rating can be higher than the system voltage but never lower.
For instance, a 600V fuse or circuit breaker can be used in a 208V circuit. One aspect of the voltage rating of an overcurrent protective device is a function of its capability to open a circuit under an overcurrent condition. Specifically, the voltage rating determines the ability of the overcurrent protective device to suppress and extinguish the internal arcing that occurs during the opening of an overcurrent condition.
If an overcurrent protective device is used with a voltage rating lower than the circuit voltage, arc suppression and the ability to extinguish the arc will be impaired and, under some overcurrent conditions, the overcurrent protective device may not clear the overcurrent safely. The voltage rating is required to be marked on all overcurrent protective device labels.
Every overcurrent protective device has a specific ampere rating. In selecting the ampere rating of the overcurrent protective device, consideration must be given to the type of load and code requirements. The ampere rating of a fuse or circuit breaker normally should not exceed the current carrying capacity of the conductors.
For instance, if a conductor is rated to carry 20A, a 20A fuse is the largest that should be used. As a general rule, the ampere rating of a fuse or a circuit breaker is selected at 125% of the continuous load current. Since the conductors are generally selected at 125% of the continuous load current, the ampacity of the conductors is typically not exceeded.
However, there are some specific circumstances in which the ampere rating is permitted to be greater than the current carrying capacity of the conductors. A typical example is the motor circuit; dual-element fuses generally are permitted to be sized up to 175% and an inverse time circuit breaker up to 250% of the motor full-load amperes.
NEC® Article 100 defines interrupting rating as: The highest current at rated voltage that a device is intended to interrupt under standard test conditions. An overcurrent protective device must be able to withstand the destructive energy of short-circuit currents. If a fault current exceeds the interrupting rating of the overcurrent protective device, the device may actually rupture, causing additional damage.
NEC® 110.9, requires equipment intended to interrupt current at fault levels to have an interrupting rating sufficient for the current that must be interrupted. Protective devices such as fuses and circuit breakers are designed to clear fault currents and, therefore, must have short circuit interrupting ratings sufficient for all available fault levels. Equipment such as contactors and switches have interrupting ratings for currents at other than fault levels, such as normal current overloads and locked rotor currents.