Definition
An electrical fuse designed to tolerate brief current surges above its rated value without opening, but to melt and break the circuit if the overcurrent persists. It protects equipment that draws a large momentary inrush current at startup — such as motors, solenoids, or capacitive loads — while still cutting the circuit during a sustained fault.
Plain English
A fuse that does not blow the instant current spikes briefly. It waits a short moment to see if the surge settles down. If the high current keeps flowing, it melts and stops the circuit.
Context Anchor
Seen in aircraft electrical-system maintenance, especially when checking or replacing circuit protection for equipment that may draw extra current at startup.
Derivation
The name describes the behavior: the fuse is deliberately slow to blow. 'Blow' is the long-standing electrical term for a fuse melting and opening the circuit. The 'slow' part distinguishes it from a standard or 'fast-blow' fuse that opens almost immediately.
Why Pilots Care
Maintains reliable power to essential systems by avoiding nuisance trips from normal surges while still protecting against sustained faults.
Intuition Check
Do not read “slow-blow” as a fuse that is old, weak, or slow to fail. It means the fuse is designed to tolerate a brief normal surge, then open if the overload continues.
Example Sentence 1
The landing gear motor circuit uses a slow-blow fuse so the brief startup surge from the motor does not pop the fuse on every gear cycle.
Example Sentence 2
When checking the electrical panel, the mechanic confirmed the slow-blow fuse was the correct type for the avionics bus.