The function of fuse:
Normally, the fuse acts as a connection in the circuit.
In the case of abnormal (overload), the fuse as a safety protection element in the circuit, through its own fusing safety cut off and protect the circuit.
The working principle of fuse:
When the fuse is energized, the heat converted by electric energy causes the temperature of the meltable to rise. When the normal working current or the allowed overload current passes through, the heat generated will pass through the meltable, and the outer shell will radiate to the surrounding environment. The heat emitted and generated will gradually reach balance through convection, conduction and other ways. If the heat generated is greater than the heat emitted, the excess heat will gradually accumulate on the meltable, making the meltable temperature rise. When the temperature reaches and exceeds the melting point of the meltable, it will melt the meltable, fuse and cut off the current, which plays the role of safety protection circuit.
Classification of fuse:
According to the appearance of the size is divided into :φ2,φ3,φ4,φ5,φ6 and other.
According to the characteristics of the fuse is divided into: fast fuse type, medium delay fuse type, delay fuse type.(also can be divided into express, strong delay).
According to the breaking ability can be divided into: low breaking type, high breaking type (can also be divided into enhanced breaking type).
According to the safety standard (or use area), it is divided into :UL/CSA(North America) specifications,IEC(China, Europe, etc.) specifications,MIT/KTL(Japan/Korea) specifications, etc.
Other categories.
Characteristic terms for fuses:
Rated current: nominal working current of the fuse tube (under normal conditions, the maximum current that the fuse can maintain normal operation for a long time).
Rated voltage: nominal operating voltage of the fuse (the maximum voltage that the fuse can safely withstand at the moment of disconnection). When selecting a fuse, the rated voltage of the selected fuse should be greater than the input voltage of the protected circuit.
Breaking capacity: When there is a large overload current in the circuit (such as strong short circuit), the fuse can safely cut off (break) the maximum current of the circuit. It is the most important safety indicator for fuses. Safety breaking refers to the phenomenon that spatter, combustion, explosion and so on do not occur in the breaking circuit to endanger the surrounding elements, components and even personal safety.
Overload capacity (carrying capacity): The maximum overload current that the fuse can maintain for a specified period of time. When the current flowing through the fuse exceeds the rated current, after a period of time the melt temperature will gradually rise and finally be blown.
The UL standard stipulates that the fuse shall be maintained for more than 4 hours and the maximum non-fusing current shall be 110% of the rated current (100% for miniature fuse tubes).
The IEC standard stipulates that the maximum non-fusing current is 150% of the rated current when the fuse is maintained for more than 1 hour
Fuse characteristics (I-T): The relationship between the load current applied to the fuse and the fuse blowing time.
A, fuse characteristic curve (I-T curve): in the logarithmic coordinate system with load current as X axis and fuse time as Y coordinate, the curve is formed by the coordinate points of average fuse fuse time under different load current. Each type of fuse has a corresponding curve representing its fusing characteristics. This curve describes the overload performance of the fuse well. For reference when choosing fuse.
B, circuit breaker characteristics table: A table consisting of several specified representative load current values and corresponding circuit breaker time ranges. Safety standards have been specified, which is the primary basis for the acceptance of fuses.
For example, UL,CSA,MIT/KTLA fast fusing type specifications, as follows:
Minimum In 100% 4 hours
In135% 1 hour Max
In 200% 2 min Max
Fusion heat energy value (I2T): The nominal energy required to melt and partially vaporize the cut-off current of the fuse. Simply put, it is the minimum heat energy required to blow the fuse.
Total amount I2t= melting I2t+ flying arc I2t
Where, melting I2t(equivalent to pre-flying arc I2t in IEC standard) refers to the energy required from melting to the beginning of the flying arc; Arc I2t is the energy required from the beginning of the arc to the end of the arc. For low-voltage fuses, the flight-arc time is very short and often negligible, that is, the flight-arc I2t can be calculated as zero.
Neither UL nor IEC requires I2t, but I2t can help with fuse selection. The I2t of a fuse is calculated when the fuse's blown time is less than 10ms(usually 8 ms). We have the I-T curve of each specification on the sample, and the corresponding specification I2t reference value for reference when choosing fuse.
Voltage drop: The voltage difference between the two ends of the fuse after reaching thermal equilibrium at rated current.
Temperature rise: Under certain current conditions, the difference between the surface temperature of the fuse after reaching thermal balance and the initial temperature of power-on (can be understood as the ambient temperature), that is, the temperature rise = the surface temperature of the fuse -- the ambient temperature.
Safety standards and marks for fuse tubes:
UL,CSA standards: safety standards of the United States, Canada and other North American regions; Small current fuse tube standard is UL248-1/14,CSA248-1/14.
Safety mark:
-- -- UL/CSA LIST(Label) Indicates the product safety label that has passed the UL/CSA248-1/14 test.
-- -- UL/CSA RECOGNIZED(Recognized marks), which are partially certified according to UL/CSA248-1/14 product safety marks.
--- UL test passed,CSA mutually recognized list name/approved safety mark, equal to
JIS Standard: Japanese Electrical safety standard. Small current fuse tubes are JIS C6575 standard.
Safety mark:
--- T
--- PSE
Both marks are valid until the end of 2006, after which only the "PSE" mark is valid.
KTL standard: Korean electrical safety standard.
Safety mark:
--- K
IEC standards: International Electrotechnical Commission standards, safety standards used in Europe and China. Small current fuse tube standard is IEC60 127,GB 9364(China).
Safety mark:
CCC -- China
SEMKO - Sweden
VDE -- Germany
BSI -- UK
IMQ -- Italy
Vi. Factors affecting Fuse life and evaluation Fuse life:
Factors affecting fuse life:
Working environment temperature:
High ambient temperature damages the life of the fuse. Delay type (slow fusing type) fuse such as tin ball type, the temperature is equal to about 160℃(150~170℃) tin begins to spread to the wire; The temperature at which the melt (wire) of a fast fusing fuse begins to oxidize violently is about 200℃(175~225℃). With the oxidation of fuse from outside to inside, multiple diffusion, thermal stress fatigue and so on, the life of fuse will be gradually shortened.
Therefore, it is suggested that the delay fuse fuse should not work for a long time above 150℃, and the fast fuse fuse should not work for a long time above 175~225℃.
B. Pulse current:
Continuous impulse impact, will produce thermal cycle, resulting in fuse diffusion, oxidation, thermal stress, and even accelerated. The fuse will gradually age as the energy and number of pulses increase. The impact life of the fuse depends on the percentage of I2t of the pulse in I2t of the fuse itself; Usually, it should be less than 20%, so that the fuse can withstand more than 100,000 shocks.
c, other:
For example, the pipe clamp in contact with the fuse, and the length of the connecting wire, cross-sectional area, etc. The contact resistance between the fuse and the pipe clip is large, which damages the service life. According to the UL standard, the contact resistance between the fuse and the pipe clip during the test is less than 3mΩ. When the contact resistance is high, instead of dissipating heat, the clamp generates heat and transmits it to the fuse.
The influence of aging fuse on use:
After the fuse is aged, there is no danger that the current should be cut off and the fuse will not blow. When the fuse is aged, it is equivalent to a decrease in rating (current) rather than an increase, so there is no safety problem in the circuit, but it will be cut off at a small overload current or pulse.
Test and evaluation of fuse life
The IEC standard provides for a "durability test method ", while the UL standard does not.
The durability test in IEC standards is called the life test. Its method is to test at normal temperature using DC power supply:
a, frontal current until the temperature is stable voltage drop;
b,1.2 times rated current 1h cut-off current 15min. Cycle 100 times;
c, power 1.5In 1h to measure voltage drop;
d, measure voltage drop with method a.
Requirements: The voltage drop change before and after the test should not exceed 10%, and the label is still clear and identifiable, and the end cap solder spot does not appear any deterioration.
Seven, fuse applicable circuit:
1, super fast and fast fusing type fuse tube: suitable for more constant current circuit, or small surge current circuit, and there are weak shock resistance components or components in the circuit.
2, medium delay and delay blown fuse tube: suitable for the presence of normal surge current circuit, and the circuit does not exist shock vulnerable components or components. Lightning-resistant fuse tube, suitable for special circuits that need to withstand instant lightning strikes, such as telephone sets.
Breaking current fuse tube: Suitable for large short circuit current may occur.
Oxyresin-encased and plastic-clad fuse tubes: suitable for use in circuits where dense components are installed or where contact short circuits may occur.
350V,300V fuse tube: Suitable for electronic rectifier and other products.
Some considerations in the use of fuse tubes:
The rated voltage of the selected fuse shall be greater than the input voltage of the protected circuit.
The rated current of the UL fuse is determined under laboratory conditions and should be less than 75% of the nominal value in actual use. For example, the circuit operating current is 0.75A, the minimum use of rated current 1A fuse tube.
The rated current of the fuse tube of IEC specification can be used at 90% or 100% of the nominal value in practice. For example, if the circuit operates at A current of 0.9A, a fuse tube with a current rating of 0.9A or 1A can be used as a minimum.
The working life of fuse is different under different ambient temperature. The higher the temperature is, the shorter the working life of fuse is. In actual selection, the rated current of the fuse should be increased according to the coefficient. Our product catalog has indicated the temperature influence curve, for the selection of fuse tube reference.
The breaking capacity of a fuse tube is proportional to its volume and inversely proportional to its rated voltage. That is, the larger the volume or the smaller the rated voltage, the greater the breaking capacity of the fuse tube; The smaller the volume or the higher the rated voltage, the smaller the breaking capacity of the fuse tube. Therefore, such as the selection of small size fuse tube, need to determine the protection circuit may appear short circuit current is not too large; If the protected circuit may have a large short circuit current, a larger fuse tube with a larger breaking current should be used. The breaking current of each type and specification is marked in the product catalog for reference when choosing fuse tubes.
The surge I2T of the protection loop should be less than 20% of the rated I2T of the fuse tube, and the fuse tube can withstand more than 100,000 surge shocks in the protected loop.
Selection of fuse tube:
Determine the safety mark: Select the safety certification mark and safety standard for fuse tubes (UL standard or IEC standard for fuse tubes) based on the market requirements for which the product will be sold.
Determine the shape size: Select the shape size of the fuse tube according to the installation space and the determined safety certification mark and safety standard.
Determine the model: According to the current characteristics of the protected loop, select the model of the fuse tube. For example, if the current characteristic of the protected circuit is constant current, the fast fusing type is selected.
Determine the rated voltage: Determine the rated voltage of the fuse tube according to the input voltage and service requirements of the protected circuit. For example, the input voltage of the protected circuit is 220V, then the fuse tube with rated voltage of 220V or above must be selected, optional 250V,300V,350V, etc.; But considering the cost factor, it is not necessary to choose too high rated voltage.
Determine minimum current rating: Preliminarily determine the rated current of the fuse tube based on the stable operating current of the protected loop and the associated service loss factor. For example, the stable working current of the protected loop is 1A, the UL standard delay fuse tube is selected, and the working temperature is about 80℃. Then the minimum rated current of the fuse tube is 1A×1.25÷0.5=2.5A.
Determine the minimum I2T of the fuse tube: Determine the I2T of the fuse tube based on the surge I2T of the protected loop. For example, the surge I2T of the protected loop is 1(A2S). To ensure that the fuse tube can withstand more than 100,000 shocks, the fuse tube I2T should be greater than :1÷0.2=5(A2S).
Determine the rated current of the fuse tube: according to the minimum rated current and minimum I2T value, check the corresponding model specifications in the product catalog, and choose the primary rated current specification that is greater than the minimum rated current value and the I2T value is greater than the minimum I2T value as the rated current of the fuse tube. For example, according to the above minimum values,(1) if I2T of rated current 2.5A is 4.3A2S, and I2T of 3A is 5.4A2S, then 3A is the rated current of selected fuse tube; (2) If I2T of rated current 2A is 5.3A2S and I2T of 2.5A is 7.6A2S, then 2.5A is the rated current of selected fuse tube.