Mercury 5 x 20mm F6.3A Quick Blow Fuses - 250V

The Mercury F6.3A is a 5 x 20mm quick blow cartridge fuse with a 250V rating and a 6.3A breaking threshold. The glass body with nickel-plated brass end caps houses a fine-gauge wire that melts within milliseconds when current exceeds the rated 6.3A, disconnecting the circuit before downstream components overheat or fail.

Designed for equipment protection in AV amplifiers, powered speakers, DJ mixers, lighting controllers, and other mains-powered audio and electrical gear where rapid fault isolation is critical, the quick blow construction prioritises speed over inrush tolerance. This makes the fuse suitable for resistive loads and circuits where startup current spikes do not exceed the 6.3A threshold. For circuits with higher inrush (motors, transformers, capacitive PSUs), a time-delay or slow-blow fuse may be more appropriate.

The 5 x 20mm dimension is an international standard IEC format, making these fuses compatible with the majority of UK and European equipment using cartridge-style fuse holders. The glass body allows visual inspection of the internal wire, so a blown fuse can be identified without a continuity meter. Each fuse weighs approximately 1g and the nickel-plated brass caps provide corrosion resistance and reliable contact within spring-loaded holders.

What does 'quick blow' mean?

Quick blow (also called fast-acting or F-type) means the fuse responds to overcurrent within milliseconds. When current exceeds 6.3A, the internal wire melts almost instantly, cutting power to protect downstream components. This differs from slow-blow or time-delay fuses, which tolerate brief current spikes without breaking.

What equipment uses 5 x 20mm fuses?

The 5 x 20mm format is standard in audio amplifiers, DJ equipment, powered speakers, lighting controllers, bench power supplies, and many mains-powered electrical devices. Check your equipment's fuse holder or manual to confirm the required size and rating before replacing.

Can I use this to replace a 5A or 10A fuse?

No. Fuse ratings must match the equipment specification. Using a 6.3A fuse in a circuit designed for 5A risks under-protection; using it in place of a 10A fuse means the fuse will blow prematurely under normal load. Always replace with the exact amperage rating specified by the manufacturer.

How do I know if the fuse has blown?

The glass body allows visual inspection. A blown fuse shows a broken or discoloured internal wire, and the glass may be blackened by the arc. If the wire appears intact and continuous, test with a multimeter in continuity mode to confirm.