company news about Key parameters of the fuse

Ambient temperature:

Ambient temperature refers to the air temperature around the fuse. This temperature should be distinguished from room temperature. Since the fuse is usually operated under closed conditions (inside the casing) or installed near the heating device (such as resistance, transformer, etc.), the ambient temperature is usually higher than room temperature. Fuses work by heat accumulation and fusing, so they are designed with a temperature derating factor in mind, which is usually shown in diagrams in the specifications.

Breaking capacity

Breaking capacity, also known as short-circuit capacity, refers to the maximum current that the fuse can reliably break at rated voltage. Under fault or short-circuit conditions, fuses may be subjected to transient overload several times or even tens of times more than the rated current. Safe operation requires that fuses remain intact (no explosion or body rupture) and clear faults. The expected fault current of the circuit where the fuse is to be placed must be less than the rated breaking current specified in the standard; otherwise, when the fuse is blown due to the fault, there will be continuous arc, ignition, fuse burning, fuse melting together with the contact, and fuse marking unrecognizable. According to the different design, the cut-off current will vary from 35A to 200kA, the breaking current capacity will decrease with the increase of the working voltage, and vice versa, in view of the specification usually only define a certain or a few voltages of the breaking current, for the actual conditions have special requirements, you can contact the manufacturer to obtain the corresponding data.

Rated current

The current rating indicates the current carrying capacity of the fuse under limited test conditions. Each fuse is marked with a current rating, which can be a number, letter, or color marker. The meaning of each marker can be found through the product data sheet.

Rated voltage

The voltage rating of the fuse must be greater than or equal to the maximum voltage for reliably breaking the short circuit current. Due to the low resistance of the fuse, the voltage drop at both ends of the fuse is small in normal operation, and the voltage rating of the fuse becomes important only when the fuse is attempting to fuse with the generation of an arc. This voltage is mentioned in the breaking capability. After the fuse element has melted, the fuse must be able to break quickly, extinguishing the arc, and preventing the open circuit voltage from re-triggering the arc through the broken melt.

Derating coefficient

For ambient temperatures of 25oC, it is recommended that fuses operate below 75% of their rated current under the limited test conditions described in UL/CSA/ANCE(Mexico) 248-14 "Supplementary Overcurrent Protection -- Fuses" to clearly define the necessary general test criteria. Applicable to the production and manufacturing of sustainable control products to prevent fire and other hazards. Some of the common variables included in these standards are: fully sealed base, high contact impedance, air flow, instantaneous spike, and connection cable variation (diameter and length). Fuses are inherently temperature-sensitive devices, and even small variables under controlled test conditions can greatly affect their expected life at 100% load. Therefore, the wiring engineer should clearly understand that the purpose of controlling the test conditions is to ensure that the fuse manufacturer can produce a consistent product that meets the standard. The 75% derating is necessary to compensate for these variables and to ensure the long life cycle of the line design is fault-free. In addition, IEC fuses do not require derating, and their standards are taken into account when defining the current.

impedance

The impedance of a fuse is usually negligible throughout the circuit. For milliampere fuses, however, the impedance can reach several ohms, and the voltage drop will be noticeable in low voltage lines, which needs to be considered. Most fuses are made of material with a positive temperature coefficient, so you can refer to cold resistance and hot resistance, and the actual working impedance is somewhere in between.

The cold resistance is measured when the fuse applies no more than 10% of the rated current. The hot resistance is calculated from the voltage drop in the steady state as it flows through the rated current. The impedance error of the fuse can be limited to a certain range, which increases the cost.

Time current curve

The time current curve is usually an average value and can be used as a design tool, but is not a required part of the specification. Because fuses of the same current specification can exhibit quite different time-current fusing characteristics, time-current curves are very helpful in defining fuses. The fuse specification will usually define 100% rated current and maximum off time for overload (135% and 200% rated current, depending on the fuse standard). The time current curve represents the mean of the design, but there may be deviations from lot to lot for a given product, so once the fuse has been selected, test samples are needed to verify actual performance.

Fuse integral I2t

The fuse integral, also known as the fuse value I2t, is the energy required to test the fuse element of the fuse. This energy value can be used as a reference for the lifetime algorithm. There are two main ways to calculate it.

8 ms algorithm, pulse current applied to the fuse, measured the time required for the fuse to occur, if not within 8 ms or less time can occur, pulse current value will continue to increase, testing until the fuse occurs within 8 ms. The purpose of this test is to ensure that the accumulated heat is not enough to transfer away from the fuse in a short time, so that all the heat I2t is used for fusing. Once the current and time are determined, the I2t required for melting can be easily calculated.

Another way to calculate I2t is the measured time at 10 times the rated current. The algorithm is the same, and the result is obtained by integration.