First, the working principle of the current fuse

When a current fuse is energized, the heat converted by electrical energy causes the melt temperature to rise.When the normal working current or allowed overload current passes through, the heat generated will radiate to the surrounding environment through the melt and outer shell, and the heat emitted by convection and conduction will gradually reach the balance with the heat generated.If the heat generated is greater than the heat emitted, the excess heat will gradually accumulate in the melt, causing the melt temperature to rise.When the temperature reaches and exceeds the melting point of the melt, it will make the melt melt, fuse and cut off the current, played the role of safety protection circuit.

Second, the role of the current fuse

1. Under normal circumstances, the current fuse plays the role of connecting the circuit in the circuit.

2. Under abnormal (over load) conditions, the current fuse, as a safety protection element in the circuit, can be safely cut off and protect the circuit by fusing itself.

Iii. Classification of current fuses

1. Designated by size as follows: phi 2, phi 3, phi 4, phi 5, phi 6 and others.

2. According to the fusing characteristics, it can be divided into fast fusing type, medium delay fusing type and delay fusing type.(can also be divided into express, strong delay).

3. According to the breaking ability, it can be divided into: low breaking ability, high breaking ability (also can be divided into enhanced breaking ability).

4. According to safety standards (or regions), it can be divided into UL/CSA(North America) specifications, IEC(China, Europe, etc.) specifications, MIT/KTL(Japan/Korea) specifications, etc.

5. Other categories.

Iv. Parameters of current fuse

1. Rated current: nominal operating current of the current fuse tube (the maximum current at which the current fuse can maintain normal operation for a long time under normal conditions).

2. Rated voltage: nominal working voltage of the current fuse (the maximum voltage that can be safely borne when the current fuse is disconnected).When selecting a current fuse, the rated voltage of the current fuse should be greater than the input voltage of the protected circuit.

3, breaking ability: when there is a large overload current in the circuit (such as a strong short circuit), the current fuse can safely cut (break) the maximum current of the circuit.It is the most important safety index of current fuse.Safety breaking refers to the phenomenon that no splash, combustion, explosion and other hazards to the surrounding elements, components and personal safety occur in the breaking circuit.

4. Overload capacity (carrying capacity) : the maximum overload current that the current fuse can maintain working within the specified time.When the current flowing through the current fuse exceeds the rated current, the melt temperature will rise gradually after a period of time until it is finally fused.

UL standard: the maximum non-fusing current is 110% of the rated current (100% of the miniature current fuse) when the current fuse is maintained for more than 4 hours.

IEC standard stipulates that the maximum non-fusing current is 150% of the rated current when the current fuse is maintained for more than 1 hour

5. Fusing characteristics (i-t) : the relationship between the load current added by the current fuse and the fusing time of the current fuse.

A. Fuse characteristic curve (i-t curve) : in the logarithmic coordinate system with the load current as the X-axis and the fuse time as the y-coordinate, the curve is formed by connecting the average fuse time coordinate points of the current fuse under different load currents.Each type of current fuse has a corresponding curve to represent its breaking characteristics, which well describes the overload performance of current fuse.It can be used as reference for current fuse.

B. fusing characteristics table: a table consisting of several specified representative load current values and corresponding fusing time ranges.All safety standards have been clearly defined, which is the main basis for acceptance of current fuses.

For example, UL, CSA, MIT/KTLA fast fuse type specifications are specified as follows:

Minimum In 100% 4 hours

In135%h Max

Max In 200% 2 minutes

6. Melting heat energy value (I2T) : the nominal energy value needed to melt the fuse body of the current fuse and partially vaporize to cut off the current, which is simply the minimum thermal energy value needed to fuse the current fuse.

Total I2t= melting I2t+ arc I2t

Where, melting I2t(equivalent to IEC standard pre-arc I2t) refers to the energy required from melt melting to the moment of arc starting;Arc I2t refers to the energy required from the moment the arc starts to the moment the arc finally goes out.For the low-voltage current fuse, arc flitting time is very short, often can be ignored, that is, arc flitting I2t can be calculated by zero.

Neither UL nor IEC requires I2t, but I2t helps with fuse.I2t calculation of current fuses is the I2t calculation of current fuses when the breaking time of current fuses is less than 10ms(usually 8 ms).Our samples have i-t curves of various specifications and I2t reference values of corresponding specifications for reference when selecting current fuses.

7. Voltage drop: the voltage difference between the two ends of the current fuse after reaching the thermal balance under the condition of rated current.

8. Temperature rise: under certain current conditions, the difference between the surface temperature of the current fuse and the initial temperature of electrification (which can be understood as the ambient temperature) after reaching the thermal balance, that is, temperature rise = the surface temperature of the current fuse -- the ambient temperature.

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