Mersen A4J80 | 80A Class J Fast-Acting Fuse — Amp-Trap® Expert Pick

Mersen A4J80 — 80 Amp Class J Fast-Acting Fuse | Amp-Trap® A4J Series

When your application demands maximum current-limiting performance in a compact, rejection-type form factor, the Mersen A4J80 is the right choice. This 80-amp, 600V Class J fast-acting fuse is part of Mersen's proven Amp-Trap® A4J series — the industry benchmark for fast-acting, high interrupt current-limiting protection.

The A4J80 delivers a 200kA AC / 100kA DC interrupting rating and is classified as a rejection-type fuse, meaning its blade terminal geometry physically prohibits substitution with inferior, non-current-limiting alternatives. If you're specifying for switchboards, feeder circuits, or panelboard protection — this fuse was built for exactly that.

Full Specifications

Where Pros Use the A4J80

• Feeder circuit protection — fast-acting response limits let-through energy on high-fault systems
• Panelboard and switchboard applications — Class J's small footprint fits standard rejection-type holders
• Capacitor bank protection — fast-acting characteristic responds quickly to capacitive inrush and fault events
• Bus duct systems — high interrupt rating handles available fault currents in large commercial facilities
• Lighting and HVAC branch circuits — reliable backup protection for sensitive downstream equipment

Frequently Asked Questions

What makes Class J fuses different from Class RK1 or RK5?

Class J fuses are physically shorter and thicker than Class R fuses, with a rejection-type blade terminal that prevents cross-installation with lower-rated fuse classes. They also offer a higher interrupting rating — 200kA versus 200kA for RK1 and 100kA for RK5 — along with a smaller physical footprint suitable for modern, compact fuseholder designs.

Is the Mersen A4J80 fast-acting or time-delay?

The A4J80 is fast-acting. It is designed to clear fault currents quickly and does not incorporate intentional time delay. For motor load applications requiring time-delay characteristics, consider the Mersen AJT80 instead.

What fuseholder does the A4J80 require?

The A4J80 requires a Class J rejection-type fuseholder sized for 31–100A fuses. It uses a blade (knife-blade) terminal and clip-mount installation. Mersen and other manufacturers offer compatible Class J holders in both single and multi-pole configurations.

Can the A4J80 be used on DC circuits?

Yes. The A4J80 is rated for 300VDC with a 100kA DC interrupting rating, making it suitable for DC bus protection in certain renewable energy and industrial DC applications.

What replaced Ferraz Shawmut A4J80?

Nothing replaced it — Ferraz Shawmut rebranded to Mersen around 2010. The A4J80 continues under the Mersen brand with the same part number, specifications, and manufacturing standards. It is a direct, identical product.