Fuse Selection Guide

Selection Factors:
1. Normal Operating Current: Fuses are typically derated by 25% at 25°C to prevent nuisance blowing. For example, a 10A fuse should not operate above 7.5A at 25°C.
2. Application Voltage: The fuse's voltage rating must be equal to or greater than the circuit voltage.
3. Ambient Temperature: Affects the current-carrying capacity of fuses. Higher temperatures reduce fuse life, while lower temperatures extend it.
4. Overload Current and Duration: The fuse must open within a specified time under overload conditions.
5. Maximum Available Fault Current: The fuse must handle the maximum fault current.
6. Pulses and Transients: Consideration of surge, inrush, and transient currents is necessary.
7. Physical Size Limitations: Constraints such as length, diameter, or height.
8. Agency Approvals: Compliance with standards like UL, CSA, VDE, etc.
9. Mounting and Form Factor: Includes ease of removal, axial leads, and visual indication.
10. Fuseholder Features: Options like clips, mounting blocks, and R.F.I. shielding.

Voltage and Temperature Considerations:
- The voltage rating must meet or exceed the circuit voltage.
- Current-carrying capacity tests are conducted at 25°C. Higher ambient temperatures reduce fuse life, while lower temperatures extend it.
- Fuses should operate at no more than 75% of their catalog rating at room temperature for indefinite life.

Effect of Ambient Temperature:
- A chart illustrates how different fuse types (Thin-Film, Fast-Acting, Resettable PTCs) are affected by ambient temperature changes.
- Example: For a 2.25A operating current using a Very Fast Acting fuse at room temperature, a 3A fuse is recommended.

Melting I2t and Clearing I2t:
- The melting I2t is the energy required to melt the fuse element, expressed in Ampere Squared Seconds (A2 Sec.).
- Laboratory tests determine the nominal melting I2t by applying a pulse of current and measuring the time to melt.
- Clearing I2t includes both melting and arcing phases, with nominal values provided for the melting phase.