Fuse I²t and Energy Let-Through: How to Coordinate Fuse Protection with Downstream Equipment

I²t (ampere-squared seconds) is the most important parameter for understanding how a current-limiting fuse protects downstream equipment. Every published fuse datasheet includes pre-arcing I²t, total clearing I²t, and peak let-through current values that define exactly how much thermal and mechanical stress the fuse will pass through to downstream components during a short-circuit event.

This article explains what these values mean, how they relate to each other, and how to coordinate fuse I²t with the withstand ratings of downstream equipment like motor starters, semiconductors, and cables.

What Is I²t?

I²t is the integral of current squared over time during the fuse’s opening operation. It represents the thermal energy delivered to the circuit during a short circuit and has units of ampere-squared seconds (A²s).

For any protective device, the total clearing I²t is the energy that reaches the downstream circuit. For the fuse element itself, the pre-arcing I²t is the energy required to melt the element. The difference (total clearing I²t minus pre-arcing I²t) is the energy dissipated in the arc inside the fuse after the element melts.

Two I²t values are commonly published:

• Pre-arcing I²t (I²t_p): Energy required to begin melting the element. Determined by element mass, material, and geometry.
• Total clearing I²t (I²t_t): Energy delivered until the arc is fully extinguished. Higher than pre-arcing because of arcing time.

Why I²t Matters for Downstream Protection

The thermal energy that a short circuit delivers to a downstream component follows:

“ Energy (J) = I2 × R × t “

Where R is the circuit resistance and t is the fault duration. For a fuse with a given pre-arcing I²t, the downstream component must be able to withstand this energy without damage.

Components with published I²t withstand ratings include:

• Motor contactors and starters (typically 50,000 to 500,000 A²s at various fault durations)
• Semiconductor devices (diodes, thyristors, IGBTs — often 1,000 to 100,000 A²s)
• Cables (limited by adiabatic heating equation I²t = k² × S², where S is conductor cross-section)
• Bus bars and bracing (specified by short-circuit withstand in kA peak or kA²s)

If the fuse’s total clearing I²t exceeds the component’s withstand, the component will be damaged even though the fuse operated correctly.

Peak Let-Through Current (Ip)

In addition to I²t, fuses have a peak let-through current (Ip) — the maximum instantaneous current that passes through the fuse before it opens. For a current-limiting fuse, Ip is much lower than the peak available fault current.

For example, a 200 kA prospective fault current through a 100 A LPJ fuse might have an Ip of only 20,000 A — a 10:1 reduction. This peak let-through is critical for:

• Mechanical stress on bus bars and bracing (proportional to I²)
• Semiconductor peak current ratings
• Cable electromagnetic forces that can damage terminations

Manufacturer datasheets publish let-through curves that show Ip as a function of available fault current at a given fuse rating.

Reading Manufacturer I²t Data

A typical Bussmann LPJ datasheet includes a table like:

Ampere rating	Pre-arcing I2t (A2s)	Total clearing I2t @ 600 V (A2s)	Peak let-through Ip (kA)
30 A	350	1,800	8
60 A	1,400	7,500	14
100 A	4,000	22,000	22
200 A	16,000	90,000	35
400 A	65,000	350,000	55
600 A	145,000	800,000	75

These values are determined by laboratory testing under controlled fault conditions and verified by UL certification.

Coordinating Fuse I²t with Downstream Devices

The standard coordination rule is:

Downstream device I²t withstand ≥ Fuse total clearing I²t at the maximum fault current

If the downstream device has a lower I²t rating than the fuse, you have three options:

1. Use a smaller upstream fuse: A smaller fuse has lower I²t but may nuisance-trip on normal load
2. Use a faster-acting fuse class: Semiconductor fuses (aR) are designed for very low I²t
3. Use a coordination study tool: Bussmann and other manufacturers offer online tools that match fuse I²t to motor starter I²t withstand for known combinations

Selective Coordination Between Fuses

When two fuses are in series (main + feeder), selective coordination ensures that only the downstream fuse operates for faults on its circuit. The upstream fuse must have a higher total clearing I²t than the downstream fuse’s pre-arcing I²t at the fault current.

The rule for coordination:

Upstream fuse pre-arcing I²t ≥ Downstream fuse total clearing I²t × safety margin

A safety margin of 1.5× or 2× is typically applied to account for fuse aging, manufacturing tolerances, and ambient temperature variations.

I²t Considerations for Specific Components

Motor contactors: Most IEC motor contactors have published I²t withstand values (sometimes called “conditional short-circuit current”). Match the upstream fuse’s I²t to the contactor rating using the manufacturer’s coordination tables.

Semiconductor devices (diodes, SCRs, IGBTs): Semiconductor I²t withstand is typically much lower than electromechanical device ratings. Use aR-class semiconductor fuses (Bussmann FWH, FWJ series) specifically designed for low I²t and very fast clearing.

Cables: The IEC 60364 adiabatic equation gives: “ t = (k × S / I)2 “ Where k is the cable material constant (115 for copper with PVC, 143 for XLPE), S is the conductor cross-section in mm², I is the fault current, and t is the maximum fault duration. The fuse must clear before this time.

Practical Example: Coordinating a 200 A LPJ with a 100 A Motor Starter

System: 480 V three-phase, 65 kA available fault current, 200 A LPJ main, 100 A motor starter feeder.

Step 1 — Look up 100 A motor starter I²t withstand: A typical 100 A IEC contactor withstands 50,000 A²s at 65 kA.

Step 2 — Look up 100 A LPJ total clearing I²t at 65 kA: From the curve, total clearing I²t ≈ 22,000 A²s.

Step 3 — Verify: 22,000 A²s < 50,000 A²s ✅ Coordinated.

The starter can safely interrupt the let-through energy from the LPJ fuse. If the calculation had failed (e.g., with a smaller starter), you would either upsize the starter or add a smaller fuse upstream of the starter.

Tools and Software

Bussmann offers the FuseSelector and Coordination tools for free with registration. These tools:

• Calculate available fault current
• Recommend fuse types and ratings
• Generate coordination tables automatically
• Verify I²t matching for common motor starter combinations

For complex systems, third-party tools like ETAP or SKM PowerTools provide full coordination studies with time-current curves, I²t, and Ip verification across the entire electrical system.

Frequently Asked Questions

Does I²t change with system voltage? Yes, slightly. Higher system voltages cause faster arcing extinction, reducing total clearing I²t. Manufacturer datasheets typically publish values at 600 V AC.

Are higher I²t values better? Not necessarily. For coordination purposes, lower I²t is better — it means less stress on downstream equipment. But a fuse with very low I²t may nuisance-trip on inrush.

What is “Ip” vs “I²t”? Ip is the peak instantaneous current (kA). I²t is the energy (A²s). Both matter for different downstream stress types.

Can two fuses with different I²t values coordinate? Yes, if the upstream pre-arcing I²t is larger than the downstream total clearing I²t. Coordination depends on the I²t ratio, not absolute values.

Where to Get I²t Data for Bussmann Fuses

Every FuseStock product page links to the manufacturer’s published datasheet with full I²t, Ip, and time-current curve data. Submit a request for our coordination study support if you need help matching fuse I²t to your specific downstream equipment.

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