Fuse Fundamentals and Selection Guide

Introduction

The function of a fuse is to provide protection of discrete components, or of complete circuits,by reliably melting under current overload conditions. There are various type of fuses and theyare categorized by current, voltage and their timing. Their timing can be divided into very fastacting, fast acting or slow blow type.

Terminology and Functions

CURRENT RATING: The nominal amperage value of the device. It is established by the manufacturer as a value of current which it can carry, based on a controlled set of test conditions.

RERATING: For 25°C ambient temperatures, it is recommended that they be operated at no more than 75% of the nominal current rating established using the controlled test conditions.

ITS CHARACTERISTICS: The characteristic of its design refers to how rapidly it responds to various current overloads. Its characteristics can be classified into threegeneral categories: very fast-acting, fast-acting, or Slo-Blow. The distinguishing feature of Slo-Blow is that these have additional thermal inertia designed to tolerate normal initial or start-up overload pulses.

TIME-CURRENT CURVE: The graphical presentation of the fusing characteristic, time-currentcurves are generally average curves which are presented as a design aid but are not generallyconsidered part of its specification. Time-current curves are extremely useful in definingit, since they with the same current rating can be represented by considerably differenttime-current curves. Its specification typically will include a life requirement at 100% of rating and maximum opening times at overload points (usually 135% and 200% of rating).A time-current curve represents average data for the design; however, there may be some differences in the values for any one given production lot.

VOLTAGE RATING: The voltage rating, as marked,indicates that it can be relied upon to safely interrupt its rated short circuit current in a circuit where the voltage is equal to, or less than, its rated voltage.

SELECTION GUIDE

The steps below are just some basic guidelines in the selection of a fuse for a certain application. It is recommended that tests are done to verify its performance.

Typical Selection Factors

1. Normal operating current

2. Application voltage (AC or DC)

3. Ambient temperature

4. Overload current and length of time in which it must open.

5. Maximum available fault current

6. Pulses, Surge Currents, Inrush Currents, Start-up Currents, andCircuit Transients

7. Physical size limitations, such as length, diameter, or height

8. Agency Approvals required, such as UL, CSA, VDE etc.

9. Considerations: mounting type/form factor, ease of removal, axialleads, visual indication, etc.

10. Fuseholder features.

NORMAL OPERATING CURRENT: The current rating of it istypically derated 25% for operation at 25°C to avoid nuisance blowing.For example, a fuse with a current rating of 10A is not usually recommendedfor operation at more than 7.5A in a 25°C ambient.

VOLTAGE: The voltage rating of it must be equal to, or greater than,the available circuit voltage.

AMBIENT TEMPERATURE: The current carrying capacity tests ofthem are performed at 25°C and will be affected by changes in ambienttemperature. The higher the ambient temperature, the hotter it willoperate, and the shorter its life will be. Conversely, operating at a lowertemperature will prolong its life. It also runs hotter as the normaloperating current approaches or exceeds the rating of the selected device.Practical experience indicates fuses at room temperature should lastindefinitely, if operated at no more than 75% of catalog rating.

Example: Given a normal operating current of 2.25 amperes in anapplication using a Very Fast Acting type at room temperature, then:

Catalog Rating = Normal Operating Current/0.75

OVERLOAD CURRENT CONDITION: The current level for whichprotection is required. Fault conditions may be specified, either in termsof current or, in terms of both current and maximum time the fault canbe tolerated before damage occurs. Time-current curves should beconsulted to try to match the fuse characteristic to the circuit needs,while keeping in mind that the curves are based on average data.

MAXIMUM FAULT CURRENT: The Interrupting Rating of itmust meet or exceed the Maximum Fault Current of the circuit.

PULSES: The general term “pulses” is used in this context to describethe broad category of wave shapes referred to as “surge currents”,“start-up currents”, “inrush currents”, and “transients”. Electrical pulseconditions can vary considerably from one application to another.Different device constructions may not all react the same to a given pulsecondition. Electrical pulses produce thermal cycling and possiblemechanical fatigue that could affect the life of the device. Initial or start-uppulses are normal for some applications and require the characteristic ofa Slo-Blo type. Slo-Blo types incorporate a thermal delay design toenable them to survive normal start-up pulses and still provide protectionagainst prolonged overloads. The start-up pulse should be defined andthen compared to the time-current curve and I2t rating for the device.Application testing is recommended to establish the ability of the devicedesign to withstand the pulse conditions.

FUSE HOLDER RERATING: For 25°C ambient temperatures, it is recommended thatfuseholders be operated at no more than 60% of the nominal currentrating established using the controlled test conditions specified byUnderwriters Laboratories..

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