A slow blow is a type of electrical fuse designed to tolerate brief surges of current without blowing, only opening the circuit when an overload persists for a sustained period. Also called a time-delay fuse, it’s the standard protection device in appliances like air conditioners, dryers, and anything with a motor that draws a large burst of power at startup. If you’ve ever replaced a fuse in a household appliance or a circuit board and noticed the letter “T” printed on it, that was a slow-blow fuse.
How a Slow-Blow Fuse Works
The key difference between a slow-blow fuse and a regular (fast-acting) fuse is what’s inside. A fast-acting fuse has a thin, straight wire element that melts almost instantly when too much current flows through it. A slow-blow fuse uses a coiled element, sometimes combined with a small mass of heat-absorbing material. This coiled construction means the element heats up more gradually. A brief spike in current won’t generate enough sustained heat to melt the coil, but a prolonged overload will.
That design is intentional. Many devices draw far more current in the first fraction of a second after you turn them on than they do during normal operation. This phenomenon, called inrush current, is especially dramatic in motors and transformers. When a transformer first powers up, the initial current surge can be 10 to 15 times larger than its normal operating current, and it can last for several seconds in large units. Toroidal transformers (common in audio equipment and power supplies) can spike to 60 times their running current. Electric motors behave similarly: until the motor spins up to speed and generates its own resistance to the electrical supply, it essentially draws the same current as if the motor were completely stalled.
A fast-acting fuse would blow every time you turned on a device like this. A slow-blow fuse rides out that startup surge and only opens if the high current continues, which signals a real problem like a short circuit.
Where Slow-Blow Fuses Are Used
You’ll find slow-blow fuses protecting circuits that experience predictable startup surges. The most common applications include:
- Air conditioners and refrigerators: Their compressor motors draw heavy current at startup.
- Clothes dryers: The motor that tumbles the drum creates a similar surge.
- Power supplies and amplifiers: Transformers inside these devices produce large inrush currents when first energized.
- Industrial motor controllers: Any equipment driving large motors needs protection that won’t trip on every start cycle.
Fast-acting fuses, by contrast, are used to protect sensitive electronics like circuit boards, microprocessors, and semiconductor components that can be damaged by even a brief overcurrent. These circuits don’t produce startup surges, and any unexpected spike in current is a genuine fault that needs to be interrupted immediately.
How to Identify a Slow-Blow Fuse
Fuses are marked with standardized letters that indicate their speed rating. The international system, based on IEC standards, uses these codes:
- FF: Very fast acting
- F: Fast acting
- M: Medium acting
- T: Slow acting (time-delay)
- TT: Very slow acting
The “T” comes from the German word “Trage,” meaning slow. If you pull a fuse from an appliance and see a “T” printed on the body along with the current rating (like “T 3.15A”), that’s a slow-blow fuse rated for 3.15 amps. You can also often identify them visually: if you look through a glass fuse body, a coiled or spring-like element usually means slow-blow, while a single straight wire means fast-acting.
Industrial fuses use a two-character IEC code that describes both the breaking range and the application. For general-purpose protection in power distribution, you’ll commonly see markings like “gG” or “gL.” Semiconductor protection fuses carry codes like “aR” or “gR,” indicating they’re designed for very fast response to protect components like diodes and thyristors in motor drives and power converters.
Why Using the Wrong Fuse Type Is Dangerous
Swapping a slow-blow fuse into a circuit that calls for a fast-acting one creates a real safety hazard. If a fault occurs in a circuit with sensitive electronics, the slow-blow fuse will sit there absorbing the overcurrent for precious extra time before it opens. That delay can be long enough to overheat components, damage circuit boards, or in worst cases, start a fire. The whole point of a fast-acting fuse in those circuits is to cut power before delicate parts are destroyed.
Going the other direction is less dangerous but still problematic. Putting a fast-acting fuse where a slow-blow is required means the fuse will blow repeatedly during normal startup surges. You’ll find yourself constantly replacing fuses, and the appliance will seem to malfunction even though nothing is actually wrong with it. Some people respond by installing a higher-amperage fast-acting fuse to stop the nuisance blowing, which defeats the overcurrent protection entirely and creates its own fire risk.
Always replace a fuse with the same type and rating specified by the manufacturer. The markings on the old fuse (or in the device’s manual) tell you exactly what you need.
Slow-Blow Fuse Standards
Time-delay fuses sold in the United States are tested and certified under UL Standard 248-14, which covers supplemental fuses for low-voltage applications. This standard defines how long a fuse must withstand specific levels of overcurrent before opening, and how quickly it must open under short-circuit conditions. Fuses carrying the UL mark have been independently verified to meet these requirements. Internationally, the IEC 60269 standard provides unified electrical characteristics so that fuses from different manufacturers and countries are interchangeable when they share the same dimensional and performance class.
When shopping for replacement fuses, look for a UL or IEC marking on the fuse body or packaging. These certifications confirm the fuse will perform within the time-current boundaries the standard requires, not just at the moment of purchase but across the temperature range and conditions it’s likely to encounter in real use.