How To Identify Slow Blow Fuse? (Slow vs Fast-Blow)

How To Identify a Fuse on a Circuit Board? (Types & Features) | howtoimprovehome.com

Fuses protect circuits from overcurrent and short circuit situations; the fuse acts by opening the circuit and disconnecting it from the power source. There are two types of fuses a fast blow and a slow blow. So, how to identify a slow-blow fuse or a fast-blow one?

How To Identify Slow Blow Fuse? (Slow vs. Fast-Blow)

  1. Search around the fuse for a large sticker or label; this label will determine whether the fuse is a slow or fast blow; these labels are put there by the fuse’s manufacturer.
  2. When you find the label look for a letter on it; for example, if there is an letter on the label, it’s a fast-blow fuse; meanwhile, if there is S or letter on the label, it’s a slow-blow fuse. You may find other letters on the label; these letters indicate special fuse uses.
  3. If you didn’t find any stickers or labels around the fuse, feel the fuse’s box or body to search for an imprinted letter on it. The letters will have the same meaning as stickers or labels.
  4. The last place to search for letters is the fuse itself; sometimes, the letters are imprinted on the fuse, and the letters have the same meaning as the sticker or labels; the letter refers to a fast-blow fuse; meanwhile, or letter refers to slow-blow fuse. You can use a magnifying glass to see the letters.
  5. Check the wire filament within the glass tube of the fuse; if it’s a thick wire and has a tiny spring on one of its ends, it’s a slow-blow fuse. If it’s a thin wire, it’s a fast-blow fuse.

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What Is the Difference Between Slow and Fast-Blow Fuses?

The main difference between slow and fast-blow fuses is the time they take to act in case of a short circuit or overcurrent. In most cases where electric circuits fail, the reason is short circuits and overcurrents and the hazards that short circuits or overcurrent can do, such as starting a fire.

Overcurrent or short circuit causes a lot of loss to non-recoverable assets, damage to many devices and equipment, and injuries to people near the circuit. The best solution for a short circuit or overcurrent is cutting the circuit’s power during overcurrent or short circuits. Fuses are the components responsible for this role.

The fuse has two types depending on the time it takes to act in case of a short circuit or overcurrent; the two types are fast and slow-blow fuse; each type has its features and is different from the other in some aspects, for example:

Slow-Blow Fuse

Slow-blow fuse is different from a fast-blow one in the time it takes to act during overcurrent and short circuits; the slow-blow fuse is slower. The main use for slow-blow fuses is to delay the tripping to avoid false tripping. During start-up time, many devices or appliances consume a very high current for a short time.

Since this only continues for a very short time, it’s called transient current; for example, the capacitor across SMPS bulk filter input requires a lot of currents when starting up, which is much larger than the normal current it consumes. It’s the same thing when it comes to high-power motors.

The motor requires a huge amount of current to startup, which is considered normal in any circuit and doesn’t count as a fault. Therefore, a slow-blow fuse is used to avoid the fuse tripping when there is a transient current; however, if the circuit faces an overcurrent, the fuse will act and protect it.

Fast-Blow Fuse

This type of fuse is very fast and acts quickly when an overcurrent or a short circuit happens; the fuse acts and disconnects the circuit immediately from the power source. Most household applications and devices use this type of fuse; some certain types of electronic and electric circuits can’t tolerate high currents.

The power should be cut off immediately to avoid damage to the circuits or the devices, so the fast-blow fuse is more recommended for these circuits.

Can You Replace a Slow-Blow Fuse With a Fast-Blow Fuse and Vice Versa?

No, because each fuse has its different uses and features; although both fuses have the same job, which is to protect the circuit against high currents and short circuits by disconnecting the circuit from the power source, they use different ways to serve this purpose.

When the overcurrent stays for a time in the circuit, the slow-blow fuse interferes and disconnects the circuit from the power source, so the slow-blow fuse needs time to act. On the contrary, the fast-blow fuse acts immediately when it senses an overcurrent or a short circuit situation, so it doesn’t need time to act.

Therefore, the fuse type depends on the circuit requirements; if the circuit needs to have a transient current to power up, then the slow-blow fuse is the best choice, as it has a delay in acting; meanwhile, if the circuit can’t tolerate the high currents or short circuit situations and need to be disconnected immediately.

The fast-blow fuse is the recommended option for this circuit to protect the connected devices and the circuit’s components from any damage.

Are Time-Delay Fuse and Slow-Blow Fuse the Same?

Yes, they are the same; both terms can refer to the same fuse type. This type has delayed acting and was designed for a circuit that has a transient current when starting up. The slow-blow fuse can tolerate overcurrents for a short time and several times.

The time delay-fuse or slow-blow fuse differs from the normal fuse in having a spring connected to the connector attached to solder with a low melting point. The fuse is capable of tolerating up to loads 100-200 times its rating, but when the overload continues overloading the circuit for a short time, the fuse acts.

The heat of the fuse increases, and the solder starts to melt down until the spring is disattached, and then the fuse opens the circuit. Therefore, no current will pass through the circuit, and the circuit will be disconnected from the power source.

How Long Does It Take a Fuse To Act?

It takes a very fast-blow fuse about 0.1 seconds when a current higher than its rating pass through to act; meanwhile, it takes a fast-blow fuse about 1 second to act, and in last place, the slow-blow fuse which takes about 10 seconds to act, because it can tolerate high currents for a short time.

Conclusion

To sum up, to identify a slow-blow or a fast-blow fuse, you can follow some easy steps:

  1. Search around the fuse for a large sticker or label; this label will determine whether the fuse is a slow or fast blow; these labels are put there by the fuse’s manufacturer.
  2. When you find the label look for a letter on it; for example, if there is an letter on the label, it’s a fast-blow fuse; meanwhile, if there is S or letter on the label, it’s a slow-blow fuse. You may find other letters on the label; these letters indicate special fuse uses.
  3. If you didn’t find any stickers or labels around the fuse, feel the fuse’s box or body to search for an imprinted letter on it. The letters will have the same meaning as stickers or labels.
  4. The last place to search for letters is the fuse itself; sometimes, the letters are imprinted on the fuse, and the letters have the same meaning as the sticker or labels; the letter refers to a fast-blow fuse; meanwhile, or letter refers to slow-blow fuse. You can use a magnifying glass to see the letters.
  5. Check the wire filament within the glass tube of the fuse; if it’s a thick wire and has a tiny spring on one of its ends, it’s a slow-blow fuse. If it’s a thin wire, it’s a fast-blow fuse.

After identifying which fuse type you have, you can now determine the type of circuit you have; for example, if you have a fast-blow fuse, your circuit can’t tolerate high currents or overloads because it will damage the circuit’s components and the devices connected to it. On the other hand, if you have a slow fuse, you have a different type of circuit. 

Circuits that use slow-blow fuses can tolerate high currents for a short time and can experience them several times. When the overload continues for some time, the solder melts down, and the spring disconnects from the connector, making the circuit open. 

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