Guidelines for Selection of Fuses
1. Fast acting or Time Delay?
When an equipment or a circuit (especially Power Supply circuit) is switched on, there may be “in-rush” current for a short duration depending upon the type of load. This “in-rush” current versus time should be measured and matched with the time-current characteristics of fuses to select Fast Acting or Time Delay types. In general, the thumb rule given below can be applied :
Fast Acting fuses: These Fuses are used for protection of circuits where little or no current surges are encountered or where high over current or high short-circuit current must
be interrupted quickly.
Time Delay fuses: These fuses can withstand heavy “in-rush” currents. They are used in circuits having high capacitive transients and circuits with large starting currents such as motor and lamp circuits.
2. Nominal Current ( In)
The nominal current rating of a fuse can be decided based on
- Peak “In-rush” current,
- Duration of “In-rush” current and
- “Steady-state” current in the circuit.
It is recommended that the “Steady state” current through the fuse should not exceed 80% of current rating for Fast Acting fuses and 95% for Time Delay fuses to prevent false operation.
Chart 1: Time Current Characteristics
3. Fusing Characteristics
The fuse time current characteristic should be compatible with the time-current characteristic of the load and the time current characteristic of the circuit components to be protected.
- Select a Fast Acting fuse for resistive loads or other loads where no transients or surges are encountered. Where only protection against short circuit hazard is required, for maximum economy, aFastActing fuse rather than a Time Delay fuse can be used. Select the highest amperage rating possible to prevent normal switching surges, transient spikes, etc., from causing premature fuse failure.
- Select a Time Delay fuse where protection against a sustained overload current greater than 50% of normal load is required and high “In-rush” or starting loads are present, as in-capacitive or motor circuits.
Test selected fuse in the intended circuit under all normal circuit conditions that may include transient, “In-rush “, or any other non-steady-state currents.
4. Voltage Rating
For general circuit protection, the voltage rating on the fuse should be equal to, or greater than, the circuit voltage. A fuse may operate at any voltage less than its rated voltage without affecting its fusing characteristics. At very low voltages, however, the fuse’s natural resistance should be taken into consideration.
5. Breaking Capacity
The maximum short-circuit current which can occur under fault conditions should not exceed the rated breaking capacity of the fuse. Fuses with ceramic body have higher breaking capacity than fuse with glass body.
6. General Recommendations
Fuse holders which have contact resistance less than 10 mOhms between fuse holder termination and fuse end cap should be used to preserve the time current characteristics of fuses. Allow for environmental influence on the fuse-link. The higher the ambient temperature, the hotter the fuse will operate and the shorter its life. Chart 2 given below indicates the ambient temperature influence on current carying capacity. In general, Curve ‘A’ applies to Time Delay and Curve ‘B’ to Fast Acting types.
Chart 2: Temperature Shift of Rated Current
