Kyosan Clearup Fuse 25LKA75B

HINODE 25LKA75B | Fast-Acting Semiconductor Protection Fuse — 75A / AC250V / DC350V / 100kA Breaking / 57.5mm / 44mm Centres / Pre-arc I²t 360 A²s

A Fuse for Semiconductors — Why Standard Fuses Don't Qualify

Conventional circuit breakers and slow-blow fuses protect cables and motors. They operate within tens to hundreds of milliseconds after an overload begins. For semiconductor devices — IGBTs, thyristors, MOSFETs, and diodes in inverters, drives, and power supplies — that reaction time is far too slow.

Semiconductor devices can be destroyed by a fault current in microseconds, long before a conventional protective device has begun to open. High-speed semiconductor fuses like 

the 25LKA75B are rated and tested for sub-cycle clearing speed. The fuse interrupts fault current within a portion of the AC half-cycle, before the semiconductor absorbs enough energy to be damaged.

The 25LKA75B is the 75A current rating in the 25LKA/25LKB series, targeting small to medium inverter, UPS, and power supply circuits operating at up to 250VAC or 350VDC.

Key Specifications

I²t — The Critical Number for Semiconductor Coordination

The I²t value is the energy let-through of the fuse during a fault — the integral of current squared over time from fault inception to circuit interruption. This is the figure semiconductor device manufacturers use to specify their component's withstand capability.

For the 25LKA75B:

• Pre-arc I²t = 360 A²s — the energy let-through from fault start to fuse element melting. This is the minimum energy coordination benchmark with the protected semiconductor's peak I²t rating.
• Total I²t = 2,100 A²s — the energy from fault start through arc extinction. This is the total energy the protected circuit must withstand during the full clearing event.

In a properly coordinated design, the semiconductor's rated I²t (from its datasheet) must exceed the fuse's total I²t. The 25LKA75B's 2,100 A²s total I²t defines the maximum semiconductor rating required for coordination at 75A.

Application Scenarios

Variable speed drive (inverter) protection: A 250VAC PWM inverter's rectifier or IGBT module protected by 25LKA75B fuses on the AC input side or DC bus. The fast clearing speed prevents IGBT failure on a short-circuit event; the 360/2,100 A²s I²t values are coordinated against the IGBT's specified I²t withstand.

UPS input protection: A UPS system's semiconductor rectifier bridge protected by 25LKA75B fuses. The fast-acting response protects the rectifier diodes against fault currents during battery charging or input transient events.

Power supply thyristor protection: A controlled rectifier power supply using thyristors (SCRs) on a 250VAC supply. The 25LKA75B provides the fast-clearing protection the thyristors require on fault.

FAQ

Q1: What is the minimum fusing current for the 25LKA75B?

The 25LKA series carries a caution: arc re-ignition may occur if the fusing current is less than 8 times the fuse's rated current. For the 25LKA75B (75A), this means the fuse must only be relied upon for fault currents of at least 600A. Below this level, the arc after fuse element melting may not reliably extinguish. For circuits where fault currents could fall below 8× rated, the fuse should be used in combination with other protective devices.

Q2: What is the difference between the 25LKA75B and 25LKA75SB?

The 25LKA75SB is the version with a fusing indicator — a micro-switch can be mounted on the indicator mechanism. When the fuse opens (blows), the indicator micro-switch changes state, providing an external electrical signal that the control system can detect. The 25LKA75B (this unit) has no indicator. Where fuse-blown status must be signalled back to a monitoring system, the 25LKA75SB is specified; where visual inspection is sufficient, the 25LKA75B is used.

Q3: Can the 25LKA75B be used in DC bus applications above 350VDC?

No. The DC rated voltage is 350VDC (TUV spec) or 400VDC (UL spec, 50kA). For DC bus applications above 400VDC — such as 480/600VAC drives with 650–800VDC DC bus — a higher DC voltage rated fuse from a different series (such as HINODE's 660GHK or 850GHK series) must be specified. Applying a 350/400VDC-rated fuse in a higher DC bus circuit risks failure to interrupt fault current and consequential fire or equipment damage.

Q4: How is the 25LKA75B physically mounted?

The 25LKA75B mounts using bolted connections at two terminal ends separated by a 44mm centre distance. The 57.5mm total length and 44mm bolt centre fit standard semiconductor fuse holder blocks designed for this class of fuse. Confirm the fuse holder's rated current, voltage, and bolt centre dimension matches the 25LKA75B's physical specification before installation.

Q5: What is the relation between HINODE and KYOSAN on these fuses?

HINODE Electric Co., Ltd. (日出電気) and the legacy KYOSAN brand name both refer to products from the same manufacturer. Some distributor listings and older equipment documentation refer to these fuses as KYOSAN 25LKA75B. Both names refer to the same physical fuse product — HINODE is the current company name and brand designation.