Practice has proved that fuses are simple and effective protection devices for phase-to-phase short circuit, single-phase short-circuit fault and overload of low-voltage motors. However, if the type and parameters of the fuse are not properly selected or the maintenance is unfavorable, the expected protection effect will not be achieved. The following precautions are presented for the selection and use of fuse protection for low voltage motors for reference.
1 selection problem
(1) If the short-circuit current is not expected to be too large (for example, less than 4kA), the RM10, RL6, and RL7 series fuses are preferred from the economical point of view. On the one hand, the user can easily disassemble the melt by himself. On the other hand, they can be used for both short-circuit protection and overload protection.
(2) If the short-circuit current is expected to be large, fuses with higher breaking capacity, such as RT12, RT14, RT15 series fuses, should be used.
(3) The fuse for protecting the motor generally does not require a large capacity and a current limiting effect, but it is desirable that the melting coefficient is appropriately small. Therefore, zinc melt and lead-tin alloy melt should be used.
(4) The RM1, RM2 and RC1 series fuses have been eliminated. Therefore, porcelain plug-in RC1A series fuses are generally not allowed for motor protection. If it is not restricted by conditions, it can only be used with a fuse with a rated current of 15A or less, as an overload protection for motors below 7î€5kW.
2 parameter selection
(1) The rated voltage of the fuse should be in accordance with the operating voltage of the motor. The operating voltage of the fuse is related to the length of the fuse and the strength of the insulation. It is not possible to use the fuse in a circuit above its rated voltage, nor to install a large fuse into a small dissolved tube.
(2) The rated current of the fuse should be greater than the maximum operating current that the motor circuit has passed for a long time. The current carrying portion and the contact portion of the housing are not damaged by the operating current. The rated current of the fuse shall not be less than the rated current of the fuse.
(3) The ultimate breaking current of the fuse should be greater than the maximum short-circuit current flowing. It is used to ensure that the fuse is not burned when the fault current is cut off.
(4) The rated current of the fuse should be selected according to the following three conditions:
1Select according to normal working conditions:
The motor starting current can reach (4~8) IeD, and the starting duration is about 5~10s. Under these conditions, the fuse should neither age nor melt.
The specific fuse characteristics should be based on the curve supplied by the manufacturer. It is known from the test that the rated current of the fuse is about half of the maximum passing current, which can meet the above requirements.
The rated current of the fuse can be selected as follows
Ie·rj≥Iq/K
In the formula Iq-motor starting current, generally (4 ~ 8) IeD is 4 to 8 times the rated current of the motor.
K-scale factor. Generally it is 1.5 to 2.5. For motors that do not start frequently, take 2.5, and for motors that start frequently, take 1.5. Winding
The motor starting current is small and the factor can be reduced to 1.25.
2 should be selected in accordance with the control of the electrical equipment in time:
When the fuse is used with the electromagnetic contactor, the fuse should be cut off for short circuit or overload current, and the contactor will be disconnected after no load. The contactor action time is known to be 0.04 to 0.06 s. For this reason, the fuse has a fuse time of 0.02 to 0.03 s, and its reliability coefficient is up to 
Kk=(0.04-0.06s/0.02-0.03)=2
According to the fuse blowing test, when the short-circuit current reaches (20 to 25) Ie·rj, the fuse time can satisfy 0.02 to 0.03 s. Therefore, the melt current is selected as follows.
Ie·rj≥Idmax/20～25
Idmax--the maximum short-circuit current through the fuse
3 According to the selectivity requirements between the protection of the upper and lower levels:
To ensure selectivity. The action time of the lower level protection must be less than the upper level protection, and the action value of the upper level protection is greater than the lower level protection. The melt current should be selected based on the data on its protection characteristic curve and its actual error. If the matching margin of the fusing time is considered to be 10%, that is, +5% to -5%, the following conditions must be met.
T1≥1.05+.ζ%/0.95-ζ%×t2
In the formula ζ%--fuse fuse time error, found by the product specification, if not found, generally considered 50%
T1--corresponding to the fault current value, the melting time of the upper melt found from the characteristic curve, s
T2--corresponding to the fault current value, the next stage, that is, the blown time of the motor-protected melt, which is found from the characteristic curve, s
In general, consider t1 ≥ 3t2.
If the protection characteristic curve cannot be found, it can also be selected as follows.
Ie·rj1≥KphIe·rj2
In the formula, IIe·rj1, Ie·rj2-- are the rated currents of the upper and lower (ie motor protection) melts, respectively.
Kph--coordination coefficient, generally takes 1.8 to 2.5. When the fuse has filler, take a smaller coefficient. When there is no filler, take a larger coefficient. Under normal circumstances, the adjacent fuses of the same type and the same melt material, the rated current of the upper fuse is 2~3 larger than the lower one. grade.
3 use maintenance
(1) Before installation, check whether the rated voltage, rated current and ultimate breaking capacity of the fuse are consistent with the requirements. 
(2) When installing, ensure that the melt and the contact knife and the contact knife and the tool holder are in good contact, so as to prevent the melt temperature from being too high and malfunction. At the same time, care must be taken not to subject the melt to mechanical damage.
(3) When installing, it should be noted that the temperature of the medium around the fuse is as consistent as possible with the temperature of the medium around the motor to avoid errors in the protection characteristics.
(4) The installation must be reliable, so as to avoid a bad contact between the two phases, and a situation equivalent to one-phase open circuit occurs, causing the motor to run out of phase and burn.
(5) If the melt is found to be damaged or melted during the inspection, the melt should be replaced, and the specification of the new melt to be replaced should be the same as that of the replaced one to ensure the reliability of the operation.
(6) The replacement of the melt or the melt pipe must be carried out without electricity.
(7) The material and cross-sectional area of ​​the fuse connection line and its temperature rise should be in accordance with the regulations, and should not be changed arbitrarily to avoid malfunction. 
(8) Dust accumulated on the fuse should be removed in time. For fuses with action indicators, check frequently and find that the fuse has been activated and should be replaced in time.
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