Posted by Peter Kennedy on 1/9/2014 to Circuit Protection
The interrupt rating of a fuse or circuit breaker is the maximum short circuit current that an overcurrent protection device can safely interrupt under test conditions. The interrupt capacity is the highest current the device can safely interrupt at the rated voltage.
In extreme cases a fuse carrying an excessively high current can vaporize and the metal can be deposited on the base of the fuse block in a way that permits it to continue to carry current. Similarly with circuit breakers, if the level of fault current goes above a certain level the circuit breaker can become inoperable while still conducting current. Fuses and circuit breakers all have an interrupt rating as a measure of their ability to cope with these circumstances. For any given device the interrupt capacity may be different for AC and DC current and for different voltages.
For boats, the ABYC standards specify the interrupt capacity requirements.
For DC Systems of 12 or 24 volts the primary circuit protection (Main fuse or breaker next to the battery) is required to have an Interrupt capacity of 3000 Amps for battery banks from 650 CCA (cold cranking amps) up to 1100 CCA capacity and 5000 Amps interrupt capacity for larger battery banks. Secondary circuit protection is permitted to have a lesser interrupt capacity on the assumption that the primary circuit protection will blow before the secondary one is permitted to carry such a large fault current, the allowable range is from 750-3000 amps interrupt capacity depending on battery size.
For AC systems of 120 Volts the requirement is 3000 Amps interrupt capacity for both main and secondary circuit protection. For 240 volt systems the requirement is 5000 Amps interrupt capacity throughout.
If you would like to read up on this in more detail I can recommend this article by Cooper Industries and the detailed ABYC standards are summarized in Table IV of ABYC standard E11
This is an interactive blog, feel free to add your questions or comments.