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Abstract: Practice has shown that fuses are simple and effective protection devices for phase-to-phase short circuits, single-phase short circuits, and overloads in low-voltage motors. However, if the type and parameters of the fuse are not selected properly, or if it is not maintained properly, it will also fail to provide the desired protective effect. The following considerations regarding the selection and use of fuse protection for low-voltage motors are presented for reference. Keywords: low voltage, motor, fuse, maintenance. Practice has shown that fuses are simple and effective protection devices against phase-to-phase short circuits, single-phase short circuits, and overloads in low-voltage motors. However, if the type and parameters of the fuse are not selected properly, or if it is not maintained properly, it will also fail to provide the desired protective effect. The following precautions regarding the selection and use of fuse protection for low-voltage motors are provided for reference. 1. Selection issue (1) If the expected short-circuit current is not too high (e.g., less than 4 kA), from an economic perspective, fuses of the RM10, RL6, and RL7 series can be given priority. On the one hand, users can easily disassemble and assemble the melt by themselves. On the other hand, they can serve both as short-circuit protection and as overload protection. (2) If a high short-circuit current is expected, fuses with higher breaking capacity should be selected, such as those in the RT12, RT14, and RT15 series. (3) Fuses for protecting motors generally do not require a high capacity or current-limiting function; rather, it is desired that their melting coefficient be relatively low. Therefore, zinc melts and lead-tin alloy melts are preferable. (4) The RM1, RM2, and RC1 series of fuses have been discontinued. Therefore, ceramic plug-type RC1A series fuses are generally not allowed to be used for motor protection. If it is necessary to use one due to certain constraints, fuses with a rated current of 15A or less can only be used reluctantly as overload protection for motors of 7.5kW or less. 2 Parameter selection (1) The rated voltage of the fuse should match the operating voltage of the motor. The operating voltage of a fuse is related to its fuse tube length and insulation strength. Fuses must not be used in circuits with a voltage higher than their rated voltage, nor should large fusing elements be installed in small fuse holders. (2) The rated current of the fuse should be greater than the maximum operating current that flows through the motor circuit over a long period of time. The current-carrying and contact parts of its shell will not be damaged by the operating current. The rated current of the fuse must not be less than the rated current of the fuse element. (3) The maximum breaking current of the fuse should be greater than the maximum short-circuit current that flows through it. It is used to ensure that the fuse is not damaged when cutting off fault currents. (4) The rated current of the fused component should be selected based on the following three conditions: ① Selection under normal operating conditions: The starting current of the motor can reach (4–8)IeD, with a starting duration of approximately 5–10 seconds. Under these conditions, the fuse should neither age nor blow. The specific characteristics of the fuse should be determined in accordance with the curves provided by the manufacturer; tests show that when the rated current of the fuse is approximately half of the maximum current it can carry, the above requirements are met. The rated current of the fuse can be selected using the following formula: Ie·rj ≥ Iq/K. Here, Iq is the motor starting current, which is generally (4–8)Ie – that is, 4 to 8 times the motor’s rated current. K-proportionality coefficient. It is generally 1.5 to 2.5. For motors that are not started frequently, 2.5 should be used, while for motors that are started frequently, 1.5 should be used. Wound-rotor motors have a low starting current, so the coefficient can be reduced to 1.25. ②It should be selected in such a way that there is a temporal coordination with the control electrical devices. When a fuse is used in conjunction with an electromagnetic contactor, it is necessary to ensure that the fuse first cuts off the short-circuit or overload current, with the contactor then breaking the circuit under no-load conditions. It is known that the contactor’s operating time is 0.04–0.06 seconds. To this end, the fuse’s blowing time is required to be between 0.02 and 0.03 seconds, with a reliability factor of ? Kk = (0.04 – 0.06s / 0.02 – 0.03) = 2. According to fuse blowing tests, when the short-circuit current reaches (20–25)Ie·rj, the fuse’s blowing time meets the requirement of 0.02–0.03 seconds. Therefore, the melt current is selected using the following formula. Ie·rj≥Idmax/20～25; where Idmax is the maximum short-circuit current that can pass through the fuse. ③ To meet the selection requirements between higher and lower level protections: it is necessary to ensure proper selection. The operating time of the lower-level protection must be shorter than that of the upper-level protection, and the operating value of the upper-level protection must be greater than that of the lower-level protection. The melt current should be selected based on the data on its protection characteristic curve and its actual error. If a matching tolerance of 10% is considered for the fuse blowing time, that is, +5% to -5%, then the following condition must be satisfied: t1 ≥ 1.05 + ζ%/0.95 – ζ% × t2. Here, ζ% represents the error in the fuse blowing time; this value can be found in the product manual. If it cannot be determined, 50% is generally assumed as a value. t1 refers to the blowing time of the fuse of the next higher rating, as determined from the characteristic curve corresponding to the fault current value, in seconds. t2 refers to the blowing time of the fuse used for motor protection, which is also determined from the characteristic curve corresponding to the fault current value, in seconds. Under normal circumstances, it is assumed that t1 ≥ 3t2. If it is not possible to find the protection characteristic curve, the value of Ie·rj1 can be determined using the following formula: Ie·rj1 ≥ Kph·Ie·rj2. Here, Ie·rj1 and Ie·rj2 represent the rated currents of the upper-level and lower-level fuses respectively (i.e., those used for motor protection), while Kph is the coordination factor, which is typically taken as 1.8 to 2.5. When a fuse has a filler, a smaller coefficient is used; when there is no filler, a larger coefficient is applied. Generally, for adjacent fuses of the same model and with the same fuse material, the rated current of the upper-level fuse is 2 to 3 grades higher than that of the lower-level fuse. 3 Use and Maintenance (1) Before installation, check whether the rated voltage, rated current, and maximum breaking capacity of the fuse meet the required specifications. ? (2) During installation, it is necessary to ensure good contact between the melt and the contact blade, as well as between the contact blade and the blade holder, in order to prevent incorrect operation due to excessively high melt temperature. At the same time, care must be taken to prevent mechanical damage to the melt. (3) During installation, care should be taken to ensure that the temperature of the medium surrounding the fuse is as consistent as possible with the temperature of the medium surrounding the motor, in order to avoid errors in the protection function. (4) The installation must be reliable to prevent poor contact in one phase, which could result in a situation equivalent to an open circuit in that phase and cause the motor to operate with an incomplete phase connection and thus burn out. (5) If it is found during inspection that the fuse is damaged or blown, it should be replaced, making sure that the specifications of the new fuse installed are identical to those of the one that was removed, in order to ensure reliable operation. (6) Replacing the melt or melt tube must be done while de-energized. (7) The material and cross-sectional area of the fuse’s connection wires, as well as its temperature rise, must all comply with the specified requirements; these should not be changed arbitrarily to avoid accidental operation. (8) Dust accumulated on fuses should be removed promptly. For fuses equipped with an operation indicator, they should also be checked regularly; if it is detected that a fuse has tripped, it should be replaced immediately