A slow blow fuse is different from a fast acting fuse in its capability to withstand transient pulse currents, i.e., it can withstand the surge current upon power-on/off, thus ensuring the equipment works normally. Therefore, slow blow fuses are often called time-delay fuses. Technically, a slow blow fuse features a higher I2t value, and it requires more energy to blow, so it is more capable of withstanding pulses compared with a fast acting fuse of same rated current.
When an overcurrent occurs in a circuit, the breaking time of a slow blow fuse takes longer than that of a fast acting fuse because of the larger I2t. Is it less protected this way as some people are worried? The answer is no. Once the circuit fails, the overcurrent will last and corresponding energy released will go beyond the I2t of the fuse until it blows out. The timing difference of slow blowing and fast acting is not significant to their protection. Slow blowing will affect the protection performance only when sensitive components existing in the protected circuit need to be protected.
Due to the previous difference, slow blow and fast acting fuses are applied to different circuits. Fast acting fuses must be used in purely resistive circuits (no or fewer surges) or the circuits where IC and other sensitive components need to be protected, while slow blow fuses are preferably used in capacitive or sensitive circuits where surges occur upon power-on/off and power input/output. Apart from circuits for IC protection, most applications with fast acting fuses can be replaced with slow blow ones to enhance anti-surge capability. Contrarily, replacement of applications with slow blow fuses to fast acting ones may cause the fuse to break as soon as the equipment is switched on and fails to work.
Furthermore, economic consideration is also an indirect factor for selection because a slow blow fuse is much expensive than a fast acting one.
A restorable fuse is essentially a PTC thermistor. It is totally different from a fuse because of its operating principle: it protects a circuit as the resistance of the PTC material is positively correlated to temperature and jump at Curie point.
For the similarities: they both can be used for circuits for overcurrent protection and applied to a number of similar fields and circumstances. Both can be used or replaced in some circumstances. For example, both products are outstanding for battery protection applications where overcurrent protection requirement is not so strict. For protection of certain ICs and important components or at power input/output ends, only the disposable fuse may be competent. The impedance requirement is also higher for these parts. However, the PTC thermistor will be the best choice for the circumstances when products should avoid being damaged from overheat or overcurrent protection at interfaces where hot-swapping is frequent. Yet in most circumstances, the two products are greatly different or cannot replace one another. The following table shows the primary differences between the two.
Owing to the similarities and differences, the two products are available in the market for different purposes.
One Time fuse
Resettable PTC fuse
The internal resistance of a fuse is an index reflecting its consumed power in the circuit. The higher the resistance, the more power consumed. For fuses, two parameters can be used to check this index: voltage drop or cold resistance.
Voltage drop: the voltage drop reading after the fusing element reaches thermal balance under rated current. To measure it, apply rated current to the fuse and wait until the reading is stable. It may take a few minutes. It is a little inconvenient and time-consuming.
Cold resistance: the reading of cold resistance basically under no load condition, easy and fast.
Therefore, the cold resistance method is always preferred for fuse check. The load of less than or equal to 10% means basically no load or heating condition, so there will be no thermal balance process, while the ambient temperature itself has a certain effect on fuse performance. All performance indexes of a fuse are parameters in ordinary atmospheric condition, i.e. 25°C. The cold resistance of a fuse, if not measured under the above-mentioned conditions, may not be accurate.
The interrupting capacity of a fuse is a safety index. It is rarely related to the electrical performance. Thus it is often neglected by the manufacturer or user. Some people are even unaware of this technical index.
As a safety precaution, a fuse needs to guarantee the security of a circuit and other parts at any time, so it has to be ensured safe in any event, especially during the fuse operation process, when no hazard should occur, including persistent arcing, times of conduction, burning, splashing, and even explosion. Interrupting capacity is an index reflecting guaranteed safety.
The unit of measurement is the Ampere, indicating the maximum current that can be safely interrupted by a fuse. As long as the circuit overcurrent does not exceed the interrupting capacity, the fuse can safely interrupt the current without producing any hazard. On the other hand, although the fuse could break circuit current, absolute safety can not be ensured. Therefore, interrupting capacity is not a dispensable index.
In order to guarantee fuse safety, we also need to consider safety indexes like interrupting capacity, rated voltage and security certification, except for essential performance parameters including current capacity, blowing characteristic, pulse withstand capability and ageing rate, impedance and power consumption, and temperature derating.
For European standard miniature tube fuses, fuses of the same profile and dimension can be divided into low and high interrupting rating fuses. The interrupting capacity of the former is specified as 10 times of rated current or 35A, whichever is larger, while the interrupting capacity of the latter is as high as 1500A. North American tube fuses have no such division. The specified interrupting capacity index is much higher, 10,000A for 125V, 35~1500A for 250V, which depends on the rated current of the fuse. The interrupting capacity of SMT chip fuses is always around 50A.
A resistor is available for a certain load. If the current flowing through the resistor goes beyond the limit, it will be burned and the circuit will be open. At this point, a resistor is similar to a fuse, i.e., the current can be interrupted upon overcurrent. This seems like a fuse function. In this way, some manufacturers change the fuse in its original design to cheap fuse resistors to keep cost down, presenting customers with superficial security.
However, is a fuse resistor able to substitute a fuse to have the same function? The answer is obviously no.
As we know, a good fuse must possess three essential functions: protection, load bearing and safety, while a fuse resistor may not be able to work well in all three aspects.
The protection function covers overload protection and short circuit protection. Specifically, a fuse must operate within the specified time when an unexpected overcurrent, even an extremely short overcurrent, occurs in the circuit, so as to protect the circuit and parts. A fuse resistor may not be as accurate as the fuse in terms of technical parameters like starting time and operation duration, and thus cannot ensure overcurrent protection. It only works to some extent for short current.
The load bearing function of a fuse is ensured by the I2t value, which enables a fuse to withstand pulse shocks of a certain energy from switching operations. We should calculate and evaluate this value before fuse selection. Nevertheless, a fuse resistor has no similar technical indexes, so it may be blown by a pulse if the value is small or fail to protect the circuit if it is large.
As for safety, a fuse can be assured through rated voltage, interrupting capacity and other indexes, especially safety compliance certification by an authorized third party. A fuse resistor is not a safety component, so it does not need certification. Its safety is therefore questionable. Furthermore, it has insufficient protection, and it’s hard to ensure no hazards occur during the breaking process.
In conclusion, a fuse resistor may blow upon overcurrent, but it cannot work the same as a fuse. In most applications, it will not be appropriate to replace a fuse with a fuse resistor to cut cost.