A capacitor is not a fuse. Unlike a fuse, a capacitor's failure behavior is undocumented. When a fuse fails, it breaks the circuit and the circuit is safe. When a capacitor fails, it may either break the circuit or short it. To prevent a capacitor from failing, you shouldn't exceed its rating.
Most capacitor fuses have a maximum power frequency fault current that they can interrupt. These currents may be different for inductive and capacitively limited faults. For ungrounded or multi-series group banks, the faults are capacitive limited.
The fuse for an individual unit in a capacitor bank must withstand the energy contributed to the failed unit by other capacitors in the same phase group. Short circuit (interrupting) – Must be greater than the short-circuit current that will ow when the capacitor unit is shorted.
The capacitor must be able to absorb this energy with a low probability of case rupture. Fuses are usually applied with some continuous current margin. The margin is typically in the range of 1.3 to 1.65 per unit. This margin is called the fusing factor.
Internally fused units consist of elements that are each protected by a series connected fuse inside the capacitor enclosure. As an element fails, the internal fuse protecting that element clears. After the fuse clears, the voltage on elements in parallel with the failed element rises and the voltage on the capacitor unit rises.
Capacitor current-limiting fuses can be designed to operate in two different ways. The COL fuse uses ribbons with a non-uniform cross section. This configuration allows the fuse to be used to interrupt inductively limited faults. The pressure is generated by the arc contained in the sealed housing.