The function of a circuit protective device is to protect a circuit from damage. Primarily, the devices act to avoid damage to insulation. The two most common methods of protecting low voltage electrical circuits are by means of either circuit breakers or fuses. Each of these types of devices has unique characteristics and properties. Molded case circuit breakers (MCCBs) are the most widely used type of circuit breaker for applications under 1000 V. Thermal-magnetic circuit breakers are a very common type of MCCB, perhaps the most common. Fuses are available with a variety of time-current characteristics. The two most common types are the quick-acting and the time delay. The time delay fuses are particularly suited to motor circuits. There are several commonly recognized methodologies for determining a fault current. Some of the more common procedures are the ohmic method, the point-to-point method, and the per-unit method.