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Do Power Module Fuses Choose Slow-break Or Fast-break?

Pulished on Nov. 10, 2022

A fast-acting fuse will blow as long as the current exceeds its rated value, and can only be used for short-circuit protection, while the slow-blow fuse will remain intact when a non-fault pulse current occurs in the circuit, protecting the long-term current overload, and has short-circuit and overload protection functions.

The main difference between a fast-blow fuse and a slow-blow fuse is its ability to withstand instantaneous pulse current. Technically, slow-blow has a large melting thermal energy value, and the energy required for a outdoor fused cutouts to blow is relatively large. For fuses with the same rated current, In terms of slow blow, the ability to withstand pulses is much stronger than fast blow. Because slow-blow fuses are larger than fast-blow fuses of the same specification, when an overcurrent occurs in the circuit, the fuse time will be slower than the fast-blow fuse.

Cut-out Fuse

Cut-out Fuse

Slow Blow Fuse

Slow blow fuse requires much higher time than the Fast Acting one. But why does one require a delay in a fuse blowing up? This delay is to decrease the chances of false tripping. During startup, many circuits or components require very large current for a very short period. This time is very less. It is called a transient current. For example, high power SMPS uses a bulk filter capacitor across input that requires huge startup current for a short period, much larger than the average running current. The same thing happens for high power motor circuits because to start a motor, a huge startup current is required. The truth is that high startup current is not considered as a circuit-level fault, rather it is a normal circuit operation. But if the fuse has a very low I2t, the fuse will trip on the transient current and disconnect the circuit. This is the situation where the fuse requires a time delay. This is why a time delay fuse or slow blow fuse is used.

But this doesn't mean that the fuse will not protect the circuit during overcurrent or in short circuit situations. When a circuit gets shorted or the overcurrent situation happens due to some kind of circuit-level fault, it remains the same even after the I2t of the fuse exceeds, the fuse then disconnects the circuit from the source.

Fast Acting Fuses

As discussed before, this type of fuse acts very fast and if overcurrent occurred, it will instantly disconnect the circuit from the source. This type of fuse has very low I2t compared with slow blow fuse.

The majority of the application is in consumer or household equipment. Generally, different kinds of electrical or electronic circuits don't tolerate sudden current changes. In this situation, fast Acting is required to protect the source circuit from the load. Fast Acting fuse makes this thing happen instantly.

Slow Blow Vs Fast Blow Fuse

Is it possible to replace a faulty slow blow fuse with a Fast Acting fuse or vice versa? The answer is NO. Because two fuses have different properties and different applications.

These two fuses serve the same purpose that is disconnecting the circuit during overcurrent or short circuit situations but these two fuses serve this same purpose using different methods. Slow Blow Fuse disconnects the circuit only when the overcurrent situation continues to stay in the supply line. Therefore, it is not an instantaneous job. But the major difference is a Fast Acting fuse will immediately disconnect the circuit as soon as the overcurrent is detected. For the Fast Acting fuse, it is an instant job.

Thus, whenever a circuit is using a slow blow fuse, that means, the circuit may have the inrush or transient startup current which needs to be considered by the slow blow fuse. If the fuse is changed with a Fast Acting fuse, the fuse will blow during the startup of the circuit. The same thing is true for the Fast Acting fuse. If a circuit has a fast Acting fuse, it means the circuit does not tolerate any high transient current changes and needs to be immediately disconnected. Thus, changing a fast Acting fuse with a slow blow fuse will be vulnerable, as the circuit will not become immune to the high transient changes and could lead to a total failure in those situations.

Because of their differences, fast-blow and slow-blow fuses are often used in different circuits, such as: purely resistive circuits (no or few surges) or circuits that need to protect sensitive components such as ICs. Fast-blow fuses must be used, and Capacitive or inductive circuits (surges when switching on and off), power input and output parts are best to use slow blow fuses. In addition to protecting the IC's circuit, in most cases where fast-blow fuses are used, slow-blow fuses can be used instead to improve their anti-interference ability. However, if you use a slow-blow in a place with a slow-blow, it will cause the fuse to break when you turn it on.

Fuse Cutout

Fuse Cutout

In practical applications, power modules often have inrush current or inrush current, that is, the instantaneous current of some circuits will be several times higher than usual at the time of switching. Generally, the current peak is high and the appearance time is short. The normal ordinary fuse cutout switch cannot bear this kind of current. If it is used, it will cause the circuit to fail to start normally. Replacement with a large-size current fuse will cause the circuit to fail to provide protection when overload current occurs. Therefore, choosing a slow-blow fuse can solve this problem and avoid the module's instantaneous current generated at the instant of startup exceeding its limit value. The module does not blow at the instant of startup, but continuously blows beyond the limit value.

It should be noted that if the fuse is too small, it will easily cause mis-fuse. If it is too large, it will not be protected, and it will easily cause a circuit input short circuit and cause power supply interruption.

How Fuses Distinguish between Fast Break And Slow Break?

Some people may pay more attention to the fast and slow disconnection of fuses, so how to distinguish between fast and slow disconnection of fuses has clearly become the focus of everyone's attention. The fuse cutout supplier lead us to analyze how to distinguish fast interrupts from slow disconnections and explain the knowledge of certain fuses in the circuit.

Fuse generally has its special characteristics, such as different letters according to the fuse speed, slow fuse (T), middle fuse (M), fast fuse (F); according to international standards are divided into VDE (European regulations), UL (American Standard), PSE (Japanese Standard), etc.