By EECTECH | 01 July 2020 | 0 Comments
Principles and methods of fuse selection
The fuse is a kind of protection device with simple structure, convenient use and low price. It is widely used in low-voltage power distribution systems and control circuits. It is mainly used as a short-circuit protection component, and is often used as an overload protection component for a single electrical device.
1. General principles for fuse selection
(1) Determine the type of fuse according to the conditions of use.
(2) When selecting the specifications of the fuse, you should first select the specifications of the melt, and then select the specifications of the fuse according to the melt.
(3) The protection characteristics of the fuse should be well matched with the overload characteristics of the protected object.
(4) In the power distribution system, the fuses at all levels should match each other. Generally, the rated current of the upper-level melt is 2 to 3 times larger than the rated current of the next-level melt.
(5) For the fuse to protect the motor, attention should be paid to the influence of the motor starting current. The fuse is generally only used as short-circuit protection of the motor, and the thermal relay should be used for overload protection.
(6) The rated current of the fuse should not be less than the rated current of the melt; the rated breaking capacity should be greater than the maximum short-circuit current that may occur in the circuit.
2. Selection method of general purpose fuses
(1) Selection of fuse type.
The type of fuse is mainly selected according to the load condition and the size of the short circuit current of the circuit. For example, for the protection of lighting circuits or motors with small capacity, RCIA series plug-in fuses or RM10 series unfilled closed tube fuses should be used; for circuits with large short-circuit currents or occasions with flammable gas, they should be used RL series screw fuse with high breaking capacity or RT (including NT) series filled enclosed tube fuse; for protecting silicon rectifier devices and thyristors, fast fuse should be used.
The form of the fuse should also consider the use environment. For example, the common term of tubular fuse is large-scale and large-capacity substation occasions; the common term of plug-in fuse is no vibration occasions; spiral fuse is mostly used for machine tool power distribution ; Electronic equipment generally uses fuse holders.
(2) Selection of melt rated current.
1) For resistive loads such as lighting circuits and electric heating equipment, because the load current is relatively stable, it can be used as overload protection and short-circuit protection, so the rated current Irn of the melt should be equal to or slightly greater than the rated current Ifn of the load, namely:
2) The starting current of the motor is very large, so short circuit protection should only be used for the motor. For the protection of a single motor that works for a long time, considering that the melt cannot be fused when the motor starts, that is
In the formula, when starting with light load or when starting time is short, the coefficient can be close to 1.5; when starting with heavy load, starting time is longer or starting more frequently, the coefficient can be close to 2.5.
3) For fuses that protect multiple motors, considering that the fuse will not be blown when there is a peak current, the rated current of the melt should be equal to or greater than 1.5~2.5 times the rated current of the largest motor, plus the ones used at the same time The sum of the rated current of the remaining motors, namely
In the formula, Irn——the rated current of the motor with the largest capacity among multiple motors, A;
Ifnmax——The sum of the rated current of the remaining motors, A.
It must be noted that due to different motor load conditions, the starting conditions are also different. Therefore, the above coefficients are only used as reference data when determining the rated current of the melt, and the precise data should be determined in practice according to the use conditions.
(3) Selection of the rated voltage of the fuse.
The rated voltage of the fuse should be equal to or greater than the rated voltage of the circuit.
Leave a Reply
Your email address will not be published.Required fields are marked. *