7.6Designing with fuses#
Fuses are also used as electrical protection devices. They have a central element or elements that will melt due to the heat generated by current flowing through the element. The time / current characteristic of the fuse indicates how long the fuse will carry specific currents before rupturing and opening the circuit. There are no clearly defined thermal and magnetic components as for circuit breakers.
The silver element type technology of HRC fuses provides a consistent definition of rupture points for both 400 ms and five-second disconnect times. Consequently, both times are appropriate for use with fuses.
The conventional fusing current is the value of the current which causes the operation of the fuse-link within the conventional time.
| Rated current I (A) N | Conventional time (hr) | Conventional current (A) | |
|---|---|---|---|
| Non fusing | Fusing | ||
| 1≤I ≤63 N | 1 | 1.25 I N | 1.6 I N |
| 63≤I ≤160 N | 2 | 1.25 I N | 1.6 I N |
| 160≤I ≤400 N | 3 | 1.25 I N | 1.6 I N |
| 400≤I N | 4 | 1.25 I N | 1.6 I N |
Table in words
- For a rated current I N of 1≤I ≤63 N, the conventional time is 1 hr, the non fusing conventional current is 1.25 I N, and the fusing conventional current is 1.6 I N.
- For a rated current I N of 63≤I ≤160 N, the conventional time is 2 hr, the non fusing conventional current is 1.25 I N, and the fusing conventional current is 1.6 I N.
- For a rated current I N of 160≤I ≤400 N, the conventional time is 3 hr, the non fusing conventional current is 1.25 I N, and the fusing conventional current is 1.6 I N.
- For a rated current I N of 400≤I N, the conventional time is 4 hr, the non fusing conventional current is 1.25 I N, and the fusing conventional current is 1.6 I N.
For fuselinks up to 63 A, the fuse will rupture within one hour when passing a current of 1.6 x its rated current (IN). This is called the conventional fusing current (I₂) and is the current ensuring effective operation of the device; hence:
