Mercury 5 x 20mm F4A Quick Blow Fuses - 4A 250V (Pack of 5)

Mercury 5 x 20mm F4A quick blow fuses provide fast-acting overcurrent protection at 4A and 250V. The glass tube body sits between two nickel-plated brass end caps, with a thin wire element inside that interrupts current flow within milliseconds of a fault condition. Quick blow (or fast-acting) fuses respond faster than slow-blow or time-delay types, making them suited to circuits where the load has minimal inrush current and where immediate protection is needed.

These fuses fit standard 5 x 20mm fuse holders in audio amplifiers, DJ mixers, guitar amps, PA controllers, test equipment and control circuits. The 4A rating handles loads up to approximately 960W at 240V mains, though lower-voltage DC circuits use the same fuse body for much smaller wattages. The glass tube allows visual inspection — a broken element appears as a gap or dark deposit on the inner wall.

Each fuse is IEC certified and carries the F4A designation, which specifies both the 4A current rating and the fast-acting blow characteristic. The pack contains five fuses, making it practical to keep spares on hand for workshop or installation work. The nickel-plated brass end caps resist corrosion and provide consistent contact in the fuse holder.

What's the difference between quick blow and slow blow fuses?

Quick blow fuses interrupt current within milliseconds of exceeding the rated amperage, while slow blow (time-delay) fuses allow brief overcurrent surges before blowing. Quick blow types suit loads with no inrush current, such as low-power electronics. Slow blow types suit motors, transformers and filament bulbs that draw high current at switch-on.

Can these be used in 12V DC circuits?

Yes. The 250V rating is the maximum voltage the fuse can safely interrupt. It can be used in lower-voltage AC or DC circuits at the same 4A current rating. In a 12V DC circuit, 4A equates to approximately 48W.

How do I know if the fuse has blown?

The glass tube allows visual inspection. A blown fuse shows a broken wire element inside the tube, often with a dark deposit on the inner wall where the element vaporised. An intact fuse shows a continuous wire running between the two end caps.

Are these compatible with standard panel-mount fuse holders?

Yes. The 5 x 20mm dimension is a standard format across most panel-mount fuse holders, inline fuse holders and PCB-mount clips designed for glass tube fuses.