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.
| Semak | What to record | Stop if |
|---|---|---|
| Keluarga fius | Manufacturer and exact series or class | The curve belongs to a different series |
| Kadar fius | The specific ampere curve or band | The required rating is absent and interpolation is not authorized |
| Jenis lengkung | Minimum melting, average melting, or total clearing | The curve type is not stated or cannot be confirmed |
| Asas arus | 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.
Hasil yang dijangkakan: one identified curve or band tied to a documented manufacturer part family and ampere rating.
Keadaan berhenti: 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 currentFor 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.
Hasil yang dijangkakan: one current value expressed in the same units as the chart.
Keadaan berhenti: 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.
Hasil yang dijangkakan: one intersection for a nominal line, or two intersections for a time range.
Keadaan berhenti: 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.
Hasil yang dijangkakan: 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 Cara Membaca Keluk Masa-Arus 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 10 saat.
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.
| Soalan | 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 | Apa yang diwakilinya | Appropriate use | Apa yang tidak dibuktikan olehnya |
|---|---|---|---|
| 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 |
| Masa arka | Time from element melting until the current is interrupted | Understanding the gap between melting and clearing | Total response from fault inception |
| Total clearing | Masa lebur ditambah masa arka | 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 dan 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) dtBecause 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
| ralat | Mengapa ia gagal | 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.
| Bidang | Entri projek |
|---|---|
| Pengeluar | |
| Fuse family / series | |
| Catalog number | |
| Nilai semasa | |
| Rated voltage and AC/DC basis | |
| Breaking / interrupting rating | |
| Curve document and revision | |
| Jenis lengkung | |
| 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 halaman produk fius VIOX 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].
Sumber
- IEC 60269-1:2024 — Low-voltage fuses, Part 1: General requirements
- Mersen — How to Read a Time-Current Curve
- Mersen — Evaluating Selective Coordination Between Current-Limiting Fuses and Non-Current-Limiting Circuit Breakers
- Eaton — Fuse Technology: Terminology, Specifications and Device Selection
- Littelfuse — Fuseology Selection Guide
- Littelfuse — Fuse Fundamentals
