On the fuse of new energy vehicles

1. Function of fuse

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

(2) Under abnormal (overload) conditions, the fuse, as a safety protection element in the circuit, safely cuts off and protects the circuit through self breaking.

2. Working principle of fuse

When the fuse is energized, the heat converted by electric energy increases the temperature of the melt. When the normal working current or overload current is allowed to pass, the generated heat can be radiated to the surrounding environment through the melt and shell, and the heat dissipated through convection and conduction is gradually balanced with the generated heat.

If the heat generated is greater than the heat emitted, the melt temperature can be increased; When the temperature reaches or exceeds the melt melting point, the melt can melt, fuse and cut off the current, which plays the role of safety protection circuit.

3. Classification of fuses

(1) According to the overall dimensions, it is divided into: φ 2, φ 3, φ 4, φ 5, φ 6, etc.

(2) According to the fusing characteristics, it is divided into: fast fusing and delayed fusing (express, medium and strong delay).

(3) According to the breaking capacity, it can be divided into low breaking, high breaking and enhanced breaking.

(4) According to the safety standard (or use area), it can be divided into UL / CSA (North America), IEC (China, Europe, etc.), MIT / KTL (Japan / Korea), etc.

4. Characteristics and terminology of fuses

(1) Rated current: the working current of the fuse (under normal conditions, the maximum current for the fuse to maintain normal operation for a long time).

(2) Rated voltage: the working voltage of the fuse (the maximum voltage that the fuse can safely withstand when disconnected). When selecting fuses, the rated voltage shall be greater than the input voltage of the protected circuit.

(3) Breaking capacity: when excessive overload current (such as strong short circuit) is connected to the circuit, the fuse can safely break (break) the maximum current of the circuit. This is the most important safety index of fuse. Safety circuit breaker means that there will be no sputtering, combustion, explosion and other phenomena endangering the surrounding components, parts and even personal safety in the circuit breaker.

(4) Overload capacity (carrying capacity): the fuse can maintain the maximum overload current within the specified time.

UL standard stipulates that when the fuse works for more than 4 hours, the maximum non fusing current is 110% of the rated current (100% for micro fuse tube)

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

(5) Fusing characteristic (I-T): the relationship between fuse loading current and fuse fusing time.

Fusing characteristic curve (I-T curve): in the logarithmic coordinate system with load current as X axis and fusing time as y coordinate, it is formed by connecting the average fusing time coordinate points of fuse under different load currents. Each type and specification of fuse has a corresponding curve, which can represent its fusing characteristics. This curve well describes the overload performance of fuse.

Fuse characteristic table: a table composed of several representative load current values and corresponding fusing time range. All safety standards have been clearly defined, which is the most important basis for acceptance of fuses.

Such as UL, CSA, MIT / KTLA and other specifications of fast fusing type, the provisions are as follows:

In100% 4 hours minimum.

In135% 1 hour maximum.

In200% 2 min max.

(6) Melting thermal energy value (i2t): the nominal energy value required to melt the fuse link and partially evaporate the cut-off current, which is simply the minimum thermal energy value required to melt the fuse.

I2t = molten i2t + flashover i2t.

Melting i2t (equivalent to pre arcing i2t in IEC standard) refers to the energy required from melting to arcing; Arcing i2t refers to the energy required from the beginning of arcing to the final extinction of arcing. For low-voltage fuses, the arcing time is very short and is often ignored, that is, the arcing i2t can be calculated as zero.

Neither UL nor IEC requires i2t, but i2t helps to select fuse. The i2t calculation of fuse is the i2t calculation when the fuse fusing time is less than 10ms (usually 8ms).

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

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

5. Safety standards and marking of fuse tubes

(1) UL, CSA standards: safety standards in the United States, Canada and other North American regions; The standard of small current fuse tube is ul248-1 / 14 and csa248-1 / 14.

Safety signs:

UL / cslist (listing mark) is a product safety mark that has passed the test in full accordance with UL / csa248-1 / 14 standard.

UL / csarerecognized (approval mark), some product safety signs are tested according to UL / csa248-1 / 14 standard.

CSA mutually recognized names / safety signs through UL test.

(2) JIS standard: Japanese electrical safety standard. The standard of small current fuse tube is jisc6575.

Safety sign: PSE.

(3) IEC standards: IEC standards, safety standards used in Europe and China. The standard of small current fuse tube is integrated circuit 60127, gb9364 (China).

Safety signs:

CCC China.

Semko, Sweden.

Germany VDE.

UK, BSI.

Italy, IMQ.

6. Factors affecting fuse life and evaluation of fuse life

(1) Factors affecting fuse life:

Operating ambient temperature:

Excessive ambient temperature will damage the life of the fuse. For delay type (slow fusing) fuses, such as tin ball type, when the temperature is about (150 ~ 170 ℃), the temperature at which the melt (wire) of the fast fusing fuse begins to oxidize violently is about (175 ~ 225 ℃).

Therefore, it is recommended that the delay fuse should not work above 150 ℃ for a long time, and the fast fuse should not work above 175 ~ 225 ℃ for a long time.

Pulsating current:

Continuous pulse impact will produce thermal cycle, resulting in fuse diffusion, oxidation, thermal stress and even acceleration. The fuse will gradually age with the increase of pulse energy and times. Generally, it shall be less than 20% to ensure that the fuse can withstand more than 100000 shocks.

other:

Such as the pipe clamp in contact with the fuse, the length and cross-sectional area of the connecting wire, etc. The contact resistance between fuse and pipe clamp is large, which will damage the service life. As specified in UL standard, the contact resistance between fuse and pipe clamp during test is less than 3 ω。

(2) Effects of fuse aging:

After the fuse is aged, it will not produce the current that should be cut off, and the fuse will not fuse. Fuse aging is equivalent to the decrease rather than increase of rated value (current), resulting in the disconnection of the circuit under small overload current or pulse.