Mersen Fuses: The Complete Guide to Industrial Overcurrent Protection
Why Mersen?
Mersen (formerly Ferraz Shawmut) is the world's largest independent manufacturer of industrial fuses. Their products protect equipment from overcurrent and short circuit in industrial plants, commercial buildings, data centres, and renewable energy systems worldwide. If you're specifying overcurrent protection for any application beyond standard residential circuit breakers, Mersen likely has the right fuse for the job.
Mersen Fuse Product Lines
A70QS — Semiconductor Fuses
Ultra-fast fuses designed to protect silicon-based components (SCRs, IGBTs, diodes) in drives, UPS systems, and power electronics. The A70QS series operates in less than 1 millisecond, far faster than any circuit breaker can respond. Critical for protecting variable frequency drives and soft starters.
AJT — Time-Delay Fuses
600V dual-element time-delay fuses. The AJT series handles motor inrush current (up to 5x FLA for 10 seconds) without nuisance opening, while still providing fast short-circuit protection. The standard choice for motor branch circuit protection in Canada under CEC Table 28A.
A6D, A2D — Non-Time-Delay Fuses
Fast-acting fuses for lighting, heating, and transformer protection where no inrush tolerance is needed. A6D (600V) and A2D (250V) are commonly used in lighting panels and transformer primaries.
AJT60 Series — Class J, 600V
Class J fuses are rejection-type (they cannot be installed in non-Class J fuse blocks), which prevents upsizing. They offer 200kA interrupting rating, making them suitable for high-fault-current service entrances and large distribution panels.
EFC — Solar / PV Fuses
DC-rated fuses specifically designed for photovoltaic string protection. The Mersen EFC series is rated 1000VDC or 1500VDC and meets UL 2579 for solar applications.
Fuse Selection Guide
Choosing the right fuse requires matching: voltage rating (must meet or exceed circuit voltage), ampere rating (sized per CEC), interrupt rating (must exceed available fault current), and fuse class (determines physical form and rejection).
| Class | Max Voltage | Interrupt Rating | Typical Use |
|---|---|---|---|
| Class K5 | 600V AC | 200kA | General distribution |
| Class J | 600V AC | 200kA | Motor control, distribution |
| Class L | 600V AC | 200kA | 600A–6000A service feeds |
| Class CC | 600V AC | 200kA | Control circuits, small motors |
Fuse Blocks and Holders
Mersen fuse blocks (SC series, UL98B) and disconnect switches (DMPD series) are matched components that maintain the fuse's interrupt rating and safety certifications. Always use matched blocks and fuses — mismatched combinations can reduce interrupt ratings.
Mersen Fuses at Maple Electric Supply
We stock Mersen AJT, A6D, A2D, A70QS, and Class L fuses, plus Mersen fuse blocks and disconnects. Available same-day in Toronto or next-day across Ontario. Browse our fuses collection or contact our industrial counter for project-specific sizing help.