How to Read a Fuse Time-Current Curve: Melting, Clearing & I²t

A fuse time-current curve (TCC) shows how the operating time changes as overcurrent increases. To read one, find the required current on the logarithmic horizontal axis, move vertically to the curve for the exact fuse rating, then move horizontally to the logarithmic vertical axis to read time. Reverse that path when you know the required time and need to find the corresponding current.

That tracing method is only the first half of the job. Before using the result, identify whether the sheet shows minimum melting (pre-arcing), average melting, or total clearing. A nominal average-melting line is not a guaranteed clearing limit, and a TCC does not replace the fuse’s voltage rating, breaking capacity, I²t, peak let-through, or manufacturer coordination data.

The Six Items to Identify Before Reading the Curve

Do not begin at the axes. Begin with the document title block and legend.

확인	What to record	Stop if
퓨즈 제품군	Manufacturer and exact series or class	The curve belongs to a different series
퓨즈 정격	The specific ampere curve or band	The required rating is absent and interpolation is not authorized
커브 유형	Minimum melting, average melting, or total clearing	The curve type is not stated or cannot be confirmed
전류 기준	Amperes, kiloamperes, or multiples of rated current	The axis units are unclear
Time basis	Seconds, cycles, or another declared unit	You cannot convert the project requirement to the plotted unit
Conditions and notes	AC/DC basis, test voltage, ambient assumptions, preloading, tolerance, frequency, or other manufacturer notes	The intended system falls outside the published conditions

A curve taken from a visually similar fuse is not a substitute. Two fuse-links with the same rated current can have different utilization categories, element designs, time-current behavior, and let-through performance. The VIOX fuse-link terminology guide explains why the replaceable fuse-link and the complete fuse system must also be specified separately.

How to Read Current-to-Time in Five Steps

Use this sequence when the design question is:

At a stated current, approximately how long will this fuse take to melt or clear?

Step 1: Select the exact curve

Find the line or band labeled for the required ampere rating within the correct fuse family. A sheet may show many ratings together. Closely spaced curves are easy to confuse, especially on a logarithmic chart.

예상 결과: one identified curve or band tied to a documented manufacturer part family and ampere rating.

중단 조건: do not estimate a missing curve between adjacent ratings unless the manufacturer explicitly permits that method or supplies the intermediate data.

Step 2: Locate current on the horizontal axis

Read the axis label carefully. The x-axis may show:

actual current in amperes;
prospective current in kiloamperes; or
current as a multiple of rated current, such as 2 × In or 10 × In.

If the axis uses multiples, calculate the multiple before entering the chart:

Current multiple = applied current / fuse rated current

For a 20 A fuse carrying 100 A, the applied current is 5 × In. This arithmetic identifies a point on the axis; it does not predict the operating time without the correct curve.

예상 결과: one current value expressed in the same units as the chart.

중단 조건: do not treat amperes and multiples of In as interchangeable.

Step 3: Move vertically to the curve or band

From the selected current, trace a vertical line upward until it meets the correct fuse curve.

If the graph shows a single average-melting line, the intersection gives a nominal value. If it shows a band or separate minimum-melting and total-clearing boundaries, record both intersections.

예상 결과: one intersection for a nominal line, or two intersections for a time range.

중단 조건: if the vertical line crosses several fuse ratings, confirm the legend again before continuing.

Step 4: Move horizontally to the time axis

From the intersection, trace horizontally to the y-axis. Read the time using the logarithmic divisions.

A point halfway between 1 second and 10 seconds is not 5.5 seconds. On a base-10 logarithmic scale, the geometric midpoint is approximately 3.16 seconds. Use the minor grid divisions supplied on the actual sheet rather than linear visual interpolation.

예상 결과: a nominal time or a bounded time range, with the curve type written beside it.

Step 5: Record the result with its qualifier

A useful engineering note looks like this:

Fuse: [manufacturer, series, rating]
Applied current: [A or multiple of In]
Curve type: [minimum melting / average melting / total clearing]
Read result: [time or range]
Document: [datasheet number and revision]
Conditions/limits: [relevant notes]

Do not record only “the fuse opens in 0.2 s.” Without the curve type and source, another reader cannot tell whether 0.2 s is minimum melting, nominal melting, or maximum total clearing.

How to Read Time-to-Current

Sometimes the design requirement is reversed:

What current is required for this fuse to operate within a target time?

Start at the required time on the vertical axis, move horizontally to the selected curve or band, then move vertically down to the current axis.

This direction is useful when checking:

whether a known fault current reaches a required operating-time region;
whether a short inrush pulse enters the fuse’s melting region;
which published rating within one fuse family is closest to a defined time-current requirement.

The answer remains tied to the published curve type. A current read from an average-melting line does not prove maximum clearing within that time.

Worked Example: Reading a Published Mersen AJT30 Curve

Mersen’s one-page 시간-전류 곡선을 읽는 방법 uses the AJT time-delay Class J family. The sheet is labeled melting time-current data, and Mersen describes its continuous curves as nominal average-melting time.

For the published AJT30 curve:

Enter the x-axis at 160 A.
Move vertically to the 30 A curve.
Move horizontally to the time axis.
The published reading is 10초.

Reading in the opposite direction:

Enter the y-axis at 0.1 second.
Move horizontally to the 30 A curve.
Move down to the current axis.
The published reading is 320 A.

Question	Published curve path	Result on Mersen’s example
How long at 160 A?	160 A → AJT30 curve → time axis	10 s
What current for 0.1 s?	0.1 s → AJT30 curve → current axis	320 A

These values illustrate how to read that named manufacturer curve. They are not universal values for every 30 A fuse, and they represent the published average-melting characteristic—not a blanket guarantee that every AJT30 will completely interrupt the circuit at exactly those times. Voltage rating, interrupting rating, application, and the rest of the manufacturer’s data still govern the final choice.

Minimum Melting, Average Melting, and Total Clearing Are Different

The labels around the curve determine what the number means.

Curve or term	무엇을 나타내는가	Appropriate use	증명하지 못하는 것
Minimum melting / minimum pre-arcing	Earliest time at which the fuse element may melt at a stated current	Ride-through and coordination boundary checks	Final interruption time
Average melting	Nominal or typical element-melting behavior	General comparison and approximate behavior	Guaranteed minimum or maximum
아크 시간	Time from element melting until the current is interrupted	Understanding the gap between melting and clearing	Total response from fault inception
Total clearing	용단 시간과 아크 시간의 합	Maximum clearing boundary when published as such	Equipment protection without checking let-through and withstand data
Tolerance band	Range bounded by relevant minimum and maximum curves	Conservative time-range interpretation	A single exact operating time

Eaton defines clearing time as the total interval from the beginning of the overcurrent until final circuit interruption, consisting of melting time plus arcing time. Mersen’s selective-coordination guidance likewise distinguishes minimum-melting and total-clearing curves and uses those boundaries—not an average line—to evaluate separation between devices.

Terminology can vary by region and manufacturer. “Pre-arcing” is commonly used for the period before the element melts; some literature uses “melting time.” Read the definitions printed in the exact data source rather than assuming every catalog uses identical labels.

Why Both Axes Are Logarithmic

Fuse behavior spans a wide range of current and time. A single chart may need to display long-duration overload behavior and sub-cycle high-current behavior. Logarithmic axes compress those decades into a readable graph.

On a base-10 axis:

Major interval	Typical minor values
1 to 10	2, 3, 4, 5, 6, 7, 8, 9
10 to 100	20, 30, 40, 50, 60, 70, 80, 90
0.01 to 0.1	0.02, 0.03, 0.04, 0.05, and so on

The visual spacing between 10 and 20 is not the same as between 20 and 30. When the chart resolution is insufficient for a decision, obtain the manufacturer’s digital data, coordination software, or a clearer curve instead of guessing from a small image.

Turning a Curve Reading into an Engineering Check

A time-current reading becomes useful only when compared with the circuit requirement. Three profiles normally matter.

1. Normal load and inrush profile

Plot or describe the magnitude and duration of expected starting, magnetizing, charging, or other transient current. Confirm that the profile stays outside the manufacturer’s melting boundary with the required design margin and operating conditions.

Do not solve nuisance operation simply by increasing the ampere rating. That change can weaken conductor or equipment protection. Check the fuse family, time-delay behavior, ambient conditions, holder thermal limits, and the actual inrush waveform.

2. Protected conductor or equipment limit

Compare the fuse’s clearing boundary with the applicable conductor or equipment withstand information. The fuse should interrupt the damaging overcurrent before the protected item’s permitted thermal or damage limit is exceeded.

A generic TCC cannot establish this on its own. The required limit must come from the applicable design method, equipment data, or authority for the target market.

3. Upstream and downstream protection

For two fuses in series, a conventional curve check uses the downstream device’s total-clearing boundary 그리고 upstream device’s minimum-melting boundary. Where the downstream total-clearing curve remains separated from the upstream minimum-melting curve over the evaluated range, the downstream fuse is expected to operate first within that plotted range.

This is a screening rule, not a universal coordination certificate. Mersen notes that conventional TCC analysis may not accurately represent selectivity when current-limiting devices and instantaneous breaker operation are involved. Manufacturer selectivity tables, tested combinations, peak-current data, I²t data, or a formal protection study may be required.

For the separate technology comparison, use VIOX’s fuse vs MCB response-time guide. For breaker B/C/D and electronic trip curves, use the circuit-breaker trip-curve guide.

Where I²t Fits—and Where the TCC Stops

A TCC relates current magnitude to time. I²t describes the integral of current squared over time:

I²t = ∫ i²(t) dt

Because fault current changes during interruption, the integral is more accurate than multiplying one assumed constant current by one time value.

Fuse data may distinguish:

melting or pre-arcing I²t: energy integral up to element melting;
arcing I²t: energy integral during the arc;
total clearing I²t: the combined integral through final interruption.

Use the exact manufacturer definitions and test conditions. Eaton’s fuse-technology guidance emphasizes that pulse I²t and fuse melting I²t comparisons must account for the application waveform and product guidance. Littelfuse also separates time-current curves from peak let-through curves: the latter address the actual peak current passed during current-limiting interruption.

A TCC alone is not enough when the decision depends on:

semiconductor or power-electronic withstand;
high prospective short-circuit current in the current-limiting region;
peak electromechanical stress;
cascading, backup protection, or a tested selective combination;
a mixed fuse-and-breaker system entering the breaker’s instantaneous region;
compliance with a project-specific selectivity requirement.

In those cases, obtain the manufacturer’s I²t tables, peak let-through charts, selectivity tables, exact device curves, and any required study data.

Common Reading Errors and the Corrective Action

오류	실패 원인	Correct action
Reading the nearest fuse rating instead of the exact one	Adjacent ratings can have materially different curves	Obtain the exact rating curve
Treating an average line as a guaranteed limit	Average melting excludes tolerance and may exclude arcing time	Use minimum-melting and total-clearing boundaries
Reading a log axis as linear	The estimated point can be far from the published value	Follow labeled logarithmic subdivisions
Ignoring amperes versus multiples of In	The x-axis point becomes wrong before the curve is read	Convert units explicitly
Assuming “opens” always means total clearing	Manufacturer terminology and curve labels differ	Record the document’s exact term
Comparing curves from unrelated test bases	Apparent separation may not represent the same conditions	Use compatible manufacturer/project data
Using TCC alone in the current-limiting region	Peak current and energy may govern the result	Check I²t, peak let-through, and selectivity data
Choosing only by ampere rating	Voltage, breaking capacity, class, holder, and application remain unresolved	Complete the full fuse specification

Fuse TCC Reading Worksheet

Copy this table into the design record or procurement review.

분야	프로젝트 항목
제조업체
Fuse family / series
Catalog number
정격 전류
Rated voltage and AC/DC basis
Breaking / interrupting rating
Curve document and revision
커브 유형
Horizontal-axis unit
Vertical-axis unit
Applied current or required time
Read melting-time value or range
Read total-clearing value or range
Load/inrush profile checked	Yes / No
Protected-equipment or conductor limit checked	Yes / No
Upstream/downstream coordination checked	Yes / No
I²t data required and checked	Yes / No / Not applicable
Peak let-through data required and checked	Yes / No / Not applicable
Manufacturer selectivity table or study required	Yes / No / Not applicable
Assumptions and unresolved items

A complete fuse specification also requires the correct utilization category or fuse class, physical format, holder compatibility, and application conditions. Start with the VIOX electrical fuse guide for the broader device and selection context; use the IEC 60269 fuse selection guide for the IEC class and system boundary; then use the VIOX 퓨즈 제품 페이지 as the commercial route for product-family evaluation.

Final Verification Before You Approve the Fuse

A defensible TCC result should answer all five questions:

Is this the exact manufacturer family and ampere rating?
Does the result say minimum melting, average melting, or total clearing?
Was the logarithmic scale read in the correct units?
Does the fuse ride through the intended load profile and clear before the applicable damage boundary?
Have I²t, peak let-through, selectivity tables, and other ratings been checked wherever TCC alone is insufficient?

If any answer is uncertain, keep the selection provisional. For a checkable product inquiry, send the system voltage, AC/DC duty, normal current, inrush profile, prospective fault current, fuse class, required operating time, holder format, and coordination requirement to [email protected].

How to Read a Fuse Time-Current Curve: Melting Time, Clearing Time, I²t, and Coordination