A 3.15A slow-blow fuse is an electrical safety device designed to protect electrical circuits and equipment from overcurrent conditions. The "3.15A" indicates its current rating, which is 3.15 amperes. Slow-blow fuses are characterised by their delayed response to overcurrent’s. Unlike fast-blow fuses that react immediately, slow-blow fuses can tolerate short-term current surges while still providing protection against sustained overloads.

These fuses are typically used in applications where precision and time-delayed protection are necessary. When an overcurrent situation occurs, the fuse's delayed response allows it to withstand temporary current spikes that may be common during normal operation. However, if the overcurrent condition becomes sustained and potentially damaging, the fuse will eventually blow to interrupt the circuit, preventing damage to the electrical system and connected devices.