Slow Blow VS Fast Blow Fuse – Choosing the Best for Power Circuit Overcurrent Protection


Fuse devices play important roles in circuits during current surges. They interrupt any inrush current, therefore preventing further overflowing current from destroying other circuit components. While there are many types of fuses, today, we will only discuss the slow blow VS fast blow fuse. Besides their definition, you’ll also learn the several criteria to consider before choosing or knowing a specific fuse. 

Slow Blow VS Fast Blow Fuse: Definition

A fast blow fuse is a fuse that instantly bursts/disconnects when you pass high power voltage through it. It is a common type in our day-to-day electrical equipment.

fast blow fuse

(fast blow fuse)

Conversely, a slow-blow fuse is a fuse that withstands high voltage levels for shorter periods without short-circuiting itself. In other words, if the voltage changes suddenly or a short circuit occurs, the fuse will not blow immediately. Instead, it’ll tolerate the high currents, enabling you to switch off the power source or finish a task. 

Slow Blow VS Fast Blow Fuse: The Uses

Generally, a fast fuse is ideal for home appliances because of the appliances’ high sensitivity to electricity flow. Often, the fast-blow fuse allows the home appliances to last by immediately shorting itself rather than short-circuiting the electronic circuit.

home appliances

(home appliances)

Contrarily, a slow blow fuse is recommendable in the motor industry. When you start a motor engine, a high electrical current momentarily passes through its electric circuit. Under normal circumstances, the fuse blows. But, using a slow blow fuse enables the circuit to bypass and withstand the high current flow. Thus, the motor starts with ease. 

Slow Blow VS Fast Blow Fuse: How to select

Undeniably, fast-acting and slow fuses perform the same function but differ based on build, structure, and duration of action. Luckily, we have outlined a stepwise means of choosing a suitable fuse for your electrical device. 

Step One

The first identification process depends on the voltage type you aim at protecting. Usually, manufacturers rate fuses by either an AC or DC voltage. Also, some fuses can have a dual rate which means they’ll work for AC and DC voltages. But, the ratings of fuses will differ depending on the overall power use. 

AC and DC voltage symbols

(AC and DC voltage symbols)

Therefore, select fuses based on a specific power range and voltage of circuits to prevent catastrophic outcomes. 

Step Two

Next, check on the amperage of the circuit (what the fuse protects in all applications like fluorescent lighting and motors). 

A fuse specific to a particular circuit permits the fuses to protect and shield devices from an overload or inrushing current. 

Step Three

Thirdly, select a fuse based on its size. For this criterion, use a fuse holder specific to your type of fuse to fit in your fuse device.

Slow Blow VS Fast Blow Fuse –5-amp fuse holder

(5-amp fuse holder)

Step Four

Nowadays, some fuses give you the flexibility of replacing burnt-out parts. So, instead of swapping the whole fuse, see if you can give back the burnt element. 

As we all know, manufacturers make fuse elements using replaceable conductive metals. Just ensure you don’t damage the cartridge during the changing process. 

Slow Blow VS Fast Blow Fuse –cartridge glass fuse

(cartridge glass fuse)

Step Five

Finally, as you buy your fuse, ensure you get one with an inspection window. The observation/inspection window helps you know if your circuit has some issues, such as if you have a burnt fuse or not. 

How to Tell if a Fuse Is a Fast- or Slow-Blow?

A fuse often sacrifices itself by suffering the damages affected by an increase in an electric current flowing through a circuitry. Thus, it is logical to want to replace it instead of buying the whole device. 

The guideline below will help you distinguish between a slow and fast blow fuse.

Step One

First, check for stickers or large labels on the fuse. The manufacturers’ labels often indicate the speed of fuse blow.

Step Two

Secondly, read to see if the manufacturer’s label fuse type is slow or fast. If the writing is S or T, it indicates slow-blow, but if it is F, it refers to a fast-blow fuse. 

Slow Blow VS Fast Blow Fuse –labeling on fuses

(labeling on fuses)

There may be other letters, but they are unique fuses in particular devices. 

Step Three

Then, if you cannot find a label or sticker, feel for any raised letters on the glass tube case. The imprinted letters often house the fuse and show its type. 

Step Four 

Next, use a magnifying glass to identify the fuse. Here, you’ll be seeing if its details are an inaccessible location or small in size. 

Step Five

Lastly, check the wire filament in the tube of the fused glass by looking through it. If you see a thick wire with a tiny spring at one end, it’s a slow-acting fuse. However, if there’s a thin fuse wire, then it’s a fast-blow fuse.

Slow Blow VS Fast Blow Fuse –slow blow and fast blow fuses

(slow blow and fast blow fuses)


To summarize, fuses help in overcurrent situations or when there are fault currents. Additionally, the different fuse ratings suit different applications. That’s why we have shown you how to distinguish between a slow-blow and a fast-blow fuse for various uses.

If you still find it hard to differentiate between the two, do not hesitate to contact us for professional help.

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