Fusing Factor is an important term that is defined for protective devices specially fuses. You might think that the fusing factor is defined only for fuses, but this is not true. It is defined for other protective devices such as Circuit Breaker (MCBs).
It is the ratio of Minimum Current causing operation of the device to the Rated Current (Nominal Current of the device).
The minimum amount of current at which the device is operated. When this current passes through the protective device it interrupts the circuit.
The amount of current which can be passed safely without the unnecessary operation of a protective device. This is the nominal current that passes from the device at a normal condition.
The Rated Current is always less than the Minimum Current causing operation.
Fusing Factor Related to Fuse:
It is the ratio of Fusing Current of fuse to the Current Rating of Fuse element (Fuse Rating).
It is the minimum amount which when passed for a prescribed time (Testing Time), the fuse operates. Due to the flow of this current, the fuse element melts and interrupts the circuit.
Current Rating of Fuse Element:
This is the current which can safely pass through the fuse without operating it. At this current, the fuse element does not melt. The Current Rating of Fuse Element is always less than the Fusing Current.
Typical Values of Fusing Factor:
The value of the fusing factor is always greater than 1. It usually ranges from 1.2 – 2. Its low value is not preferred. If it is 1.2 then it means the Operating current is 20% more than the Rated Current. The operating current is more than rated current, therefore, the conductor is selected to bear the temperature at a current above the rated current.
For overload protection, its typical value is 1.45. The reason to select this is it will not heat up at the rated current and it will not cause deterioration to the insulation of the conductor when passing the current above-rated current and below operating current.