Home  Fuse

Nov 30 2019

Basic Knowledge of Fuse

 The Role of the Fuse

1. Under normal circumstances, the fuse acts as a connection circuit in the circuit.

2. In the case of abnormal (overload), the fuse acts as a safety protection element in the circuit, and safely cuts off and protects the circuit by blowing itself.

figure 1. 

Figure 1.

 Working Principle of the Fuse

When the fuse is energized, the heat converted by the electrical energy causes the temperature of the meltable item to rise. When the normal working current or the allowable overload current passes, the generated heat is radiated to the surrounding environment through the meltable body and the outer casing, and the heat generated by convection, conduction, etc. is gradually balanced with the generated heat. If the generated heat is greater than the amount of heat dissipated, the excess heat gradually accumulates on the meltable item, causing the temperature of it to rise; when the temperature reaches and exceeds the melting point of the meltable item, it will be melted, blown and the current will be cut off and plays a role as a safety protection circuit.

 

 Classification of the Fuse

1. According to the external size, it is divided into: φ2, φ3, φ4, φ5, φ6 and others.

2. According to the blowing characteristics, it is divided into: fast-blown type, medium time-delay blown type, and time-delay type. (it can also be divided into express, strong delay).

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

4. According to safety standards (or areas of use): UL/CSA (North America) specification, IEC (China, Europe, etc.) specification, MIT/KTL (Japan/Korea) specification, etc.

5. Other classifications.

 

 The Terminologies of the Fuse

1. Rated current: The nominal operating current of the fuse tube (the maximum current that the fuse maintains normal operation for a long time under normal conditions).

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

3. Breaking capacity: When a large overload current (such as a strong short circuit) occurs in the circuit, the fuse can safely cut off (break) the maximum current of the circuit. It is the most important safety indicator for fuses. Safe breaking means that something endangers the surrounding elements, components and even personal safety such as splashing, burning, explosion will not happen in the breaking circuit. 

4. Overload capability (load carrying capacity): The fuse can maintain the maximum overload current for working within the specified time. When the current flowing through the fuse exceeds the rated current, the temperature of the meltable item will gradually rise after a period of time and eventually be blown.

The UL standard stipulates that the fuse remains in operation for more than 4 hours, and the maximum unblown current is 110% of the rated current (100% for the miniature fuse tube)

The IEC standard stipulates that the fuse remains in operation for more than one hour, and the maximum unblown current is 150% of the rated current.

5. Fuse characteristics (I-T): The relationship between the load current applied to the fuse and the fuse blowing time.

A. Blowing characteristic curve (I-T curve): A curve formed by the average blowing time coordinate point of the fuse under different load currents in a logarithmic coordinate system in which the load current is the X axis and the blowing time is the Y coordinate. Each type of fuse has a corresponding curve that represents its blowing characteristics, which is a good indication of the fuse's overload performance and it is for reference when selecting the fuse.

B. Blowing characteristics table: A table consisting of several specified representative load current values and corresponding blowing time ranges. All safety standards have clearly stated that this is the most important basis for acceptance of fuses.

For example, fast-blow type such as UL, CSA, MIT/KTLA, is specified as:

In 100% 4 hours(minimum)

In 135% 1 hour(maximum)

In 200% 2 minutes(maximum)

6. Melting heat value (I2t): The nominal energy value that the cut-off current needed to melt and partially carburate the fuse, which is simply the minimum amount of heat required to blow the fuse.

Total I2t=melted I2t+ arcing I2t

The melting I2t (corresponding to the pre-arcing I2t in the IEC standard) refers to the energy required from the melting of the fuse to the moment of arcing; the arcing I2t refers to the energy required for the arcing from the moment it starts to the moment it eventually extinguishes. For low-voltage fuses, the arcing time is very short and often negligible. That is to say, the arcing I2t can be calculated as zero.

Both UL and IEC do not require I2t, but I2t has some help with fuse selection. The I2t measurement of the fuse is calculated as I2t when the fuse's blowing time is less than 10ms (usually 8ms).

7. Voltage drop: The voltage difference across the fuse after thermal equilibrium under rated current conditions.

8. Temperature rise: Under a certain current condition, the difference between the surface temperature of the fuse and the initial temperature of the energization (which can be understood as the ambient temperature) after the heat balance is reached, that is, the temperature rise = the surface temperature of the fuse - the ambient temperature.

 figure 2.

Figure 2.

 Safety Standards and Signs for Fuse Tubes

1. UL, CSA standards: North America regional safety standards such as the United States, Canada; small current fuse tube standards are UL248-1/14, CSA248-1/14.

Safety sign:

--- UL/CSA LIST (Listing Sign), the product safety sign passed the test in accordance with UL/CSA248-1/14.

--- UL/CSA RECOGNIZED (Approved Sign), the product safety mark passed the test partly in accordance with UL/CSA248-1/14.

2. JIS Standard: Japanese Electrical Safety Standard. The standard for small current fuse tubes is JIS C6575.

Safety sign:

--- T

--- PSE

Both signs were valid before the end of 2006, after which only the “PSE” mark was valid.

3. KTL Standard: Korean Electrical Safety Standard.

Safety sign:

--- K

4. IEC standards: International Electrotechnical Commission standards and safety standards used in Europe and China. The standard for small current fuse tubes is IEC60 127, GB 9364 (China).

Safety sign:

CCC --- China

SEMKO --- Sweden

VDE --- Germany

BSI --- UK

IMQ --- Italy

 

Ⅵ Factors Affecting Fuse Life and Evaluation of Fuse Life

1. Factors Affecting the Life of the Fuse

a. working environment temperature:

Excessive ambient temperature is detrimental to the life of the fuse. Time-delay (slow-blow) fuses, such as tin ball type, begin to spread to the wire when the temperature is approximately 160℃ (150-170℃); the meltable item (wire) of the fast-blow fuse begins to violently oxidize at a temperature approximately equal to 200℃ (175 to 225℃). As the fuse is oxidized from the outside to the inside, multiple times of diffusion, thermal stress fatigue, etc., the life of the fuse will be gradually shortened. Therefore, it is recommended that the time-delay fuse should not work above 150℃ for a long time, and the fast-blow fuse should not work above 175~225℃ for a long time.

b. pulse current:

Constant pulse shock will cause thermal cycling, which will cause the diffusion, oxidation, thermal stress, etc. of the fuse to be generated and even accelerated. The fuse will age as the pulse energy and frequency increase. The impact resistance life of the fuse depends on the I2t of the pulse as a percentage of the fuse's own I2t; normally, it should be less than 20%, so that the fuse can withstand more than 100,000 times of impact.

c. other:

Such as the tube clamp in contact with the fuse, and the length and cross-sectional area of the connecting wire. The contact resistance between the fuse and the pipe clamp is large, which is detrimental to the service life. The UL standard specifies that the contact resistance between the fuse to the tube clamp is less than 3mΩ during the test. When the contact resistance is large, the tube clamp does not dissipate heat but generates heat and transmits it to the fuse.

2. Effect of the Use of the Fuse After Aging

After the fuse has aged, the situation that the current should be cut off and the fuse is not blown will not happen. When the fuse ages, it is equivalent to a drop in the rated value (current) rather than a rise, so there is no safety problem in the circuit, but the circuit is cut off under a small overload current or pulse.

3. Test Evaluation of Fuse Life

The "endurance test method" is specified in the IEC standard, and there is no similar regulation in the UL standard.

The durability test in the IEC standard is the life test by using the DC power supply test at normal temperature:

a. The voltage drop is measured until the temperature is stable under the rated current;

b. 1.2 times of rated current for 1h, cut off current for 15min and circulate for 100 times;

c. Power on 1.5In for 1h and measure voltage drop;

d. Measure the voltage drop with method a.

Requirements: The voltage drop change before and after the test should not exceed 10%, and the sign is still clear and identifiable, and the end cap solder joint does not show any deterioration.

 figure 3.

Figure 3.

 Fuse Suitable Circuit

1. Very fast and fast-blow type fuse tubes: Suitable for circuits with relatively constant current, or circuits with low inrush current, and there are shock-resistant fragile components in the circuit.

2. Medium time-delay and time-delay blown fuse tubes: Suitable for circuits with normal inrush current, and there are no shock-resistant fragile components in the circuit. Lightning-resistant fuse tube for special circuits that need to withstand lightning strikes, such as telephones.

3. Breaking current fuse tube: Suitable for circuits where large short-circuit current may occur.

4. Oxygen resin package and plastic case type fuse tube: suitable for installation of dense components or circuits where contact short circuits may occur.

5. 350V, 300V fuse tube: suitable for electronic rectifiers and other products.

 

 Precautions for Using the Fuse Tube

1. The rated voltage of the selected fuse should be greater than the input voltage of the protected circuit.

2. The rated current of the UL specification fuse is determined under laboratory conditions and should be used less than 75% of the nominal value in actual use. For example, the circuit operating current is 0.75A, we can select the fuse tube with a minimum rated current of 1A.

3. The rated current of the IEC specification fuse tube can be used at 90% or 100% of the nominal value in actual use. For example, the circuit operating current is 0.9A, and the fuse tube with a minimum rated current of 0.9A or 1A can be selected.

4. Under different operating environment temperatures, the working life of the fuse is different. The higher the temperature, the shorter the working life of the fuse. In actual selection, the rated current of the fuse should be increased according to the coefficient.

5. The breaking capacity of the fuse tube is proportional to its volume and inversely proportional to the rated voltage, that is, the larger the volume or the smaller the rated voltage, the larger the breaking capacity of the fuse tube; the smaller the volume or the larger the rated voltage, the smaller the breaking capacity of the fuse tube. Therefore, if a small-size fuse tube is used, it is necessary to determine that the short-circuit current that may occur in the protected circuit is not too large; if a large short-circuit current may occur in the protected circuit, a larger-size fuse tube with a larger breaking current must be selected.

6. The surge I2t of the protection circuit should be less than 20% of the rated I2t of the fuse tube. The fuse tube can withstand more than 100,000 surges in the protected circuit.

 

 Selection of Fuse Tube

a) Determine the safety sign: According to the market requirements for the product to be sold, select the safety certification sign and safety standard (UL standard or IEC standard fuse tube) of the fuse tube.

b) Determine the dimensions of the fuse tube: Select the dimensions of the fuse tube according to the installation space and the defined safety certification sign and safety standards.

c) Determine the model number: Select the type of fuse tube based on the current characteristics of the circuit being protected. For example, if the current characteristic of the protected circuit is a constant current, the fast-blow type should be selected.

d) Determine the rated voltage: Determine the rated voltage of the fuse tube according to the input voltage of the protected circuit and the requirements for use. For example, if the input voltage of the protected circuit is 220V, the fuse tube with rated voltage above 220V should be selected, 250V, 300V, 350V, etc. can also be selected; but considering the cost factor, it is not necessary to use the rated voltage which is too high.

e) Determine the minimum rated current: According to the stable operating current of the protected circuit and the relevant use loss factor, the rated current of the fuse tube is initially determined. For example, the stabilized working current of the protected circuit is 1A, the UL standard time-delay fuse tube should be selected, and the working environment temperature is about 80℃. The minimum rated current of the fuse tube is selected as: 1A × 1.25 ÷ 0.5 = 2.5A.

f) Determine the minimum I2t of the fuse tube: Determine the I2t of the fuse tube based on the surge I2t of the protected circuit. For example, the surge I2t of the protected circuit is 1 (A2S). To ensure that the fuse tube can withstand more than 100,000 times of impact, the I2t of the fuse tube should be greater than: 1÷0.2=5 (A2S).

g) Determine the rated current of the fuse tube: According to the minimum rated current and the minimum I2t value, check the corresponding model specifications, and take the primary rated current specification that is greater than the minimum rated current value and whose I2t value is also greater than the minimum I2t value as the rated current of the selected fuse tube. For example, based on the above minimum,

(1) If the I2t of the rated current of 2.5A is 4.3A2S and the I2t of 3A is 5.4A2S, take 3A as the rated current of the selected fuse tube;

(2) If the I2t of rated current of 2A is 5.3A2S and the I2t of 2.5A is 7.6A2S, take 2.5A as the rated current of the selected fuse tube.

Related Articles

  • The Selection Techniques of Fuse Resisto...
    Ⅰ Fuse ResistorResistors and fuses are similar in material and construction, and fuse-type resistors have both functions. They can be used as resistors. Once the current is abnormal, they act as ...
  • Basic Knowledge of Fuse
    Ⅰ The Role of the Fuse1. Under normal circumstances, the fuse acts as a connection circuit in the circuit.2. In the case of abnormal (overload), the fuse acts as a safety protection element in th...

0 comment

Leave a Reply

Your email address will not be published.

 
 
   
Rating: