Mercury 5 x 20mm T125mA Slow Blow Fuse - 250V

The Mercury T125mA is a 5 x 20mm glass tube slow blow fuse rated at 125mA and 250V, designed to protect sensitive electronics from sustained overcurrent faults while tolerating brief inrush surges during startup. The glass body houses a thin wire element that deliberately delays melting when current exceeds the rated 125mA, allowing short-duration spikes to pass without interruption.

Used in audio equipment, control circuits, power supplies and instrument repair, slow blow fuses prevent nuisance blowing during motor startups, capacitor charging cycles and transformer inrush events where current can briefly spike to several times the steady-state load. The 5 x 20mm size is a standard IEC dimension, fitting fuse holders in most consumer electronics, musical equipment and measurement devices manufactured from the 1980s onwards.

The metal end caps provide electrical contact and mechanical retention inside a spring-clip fuse holder. The transparent glass tube allows visual inspection of the internal wire element — an intact wire indicates a good fuse, while a broken or discoloured element shows the fuse has blown. The 250V rating handles mains-derived circuits in UK and European equipment operating on 230V AC supplies.

What's the difference between slow blow and fast blow?

A slow blow fuse tolerates brief overcurrent surges (typically lasting milliseconds to a few seconds) before melting, making it suitable for circuits with inrush loads like motors or transformers. A fast blow fuse melts almost immediately when current exceeds its rating, providing faster protection but blowing more easily during startup surges.

What equipment uses 5 x 20mm fuses?

This size appears in amplifiers, mixing desks, DJ equipment, guitar pedals, synthesisers, computer power supplies, test equipment and older domestic appliances. Check your equipment's service manual or the markings on the existing fuse holder to confirm size and rating.

Can I replace a T125mA with a higher-rated fuse?

No. Installing a higher-rated fuse (e.g. T250mA or T500mA) removes the protection the original circuit was designed around and risks damage to components or fire in the event of a genuine fault. Always replace with the same rating.

How do I know if the fuse has blown?

Look through the glass tube. An intact fuse shows a continuous thin wire running between the two end caps. A blown fuse shows a broken or vaporised wire, often with blackening or clouding of the glass where the element melted.