Low Voltage Load Switches and Fuses

1. Classification of low-voltage switch names.
The standard of low-voltage and high-voltage load switch is GB14048.3 (IEC60947-3) power main switch, isolator, isolating switch and fuse constitute electrical products. Among them, the normative definition of the name of the low-voltage related power main switch. In the beginning, the main power switch is a main power switch electrical equipment that can connect and overcurrent all normal load currents, and can connect to short-circuit fault currents. An isolator is an electrical switch that achieves a safety protection effect, only disconnecting and closing the full current. The isolation switch is the functional composition of the main power switch and the isolator to achieve safety protection functions, and can connect and overcurrent all normal load currents.
The fuse constitutes the electrical product refers to the fuse group and the fuse disconnector. These two electrical products are important low-voltage switch products of Siemens, Schneider and other manufacturers. The difference between these two types of products is shown in the figure below. One is in series with the fuse, and the other is the fuse on the moving disconnector.
Mains main switch/isolator/isolating switch definition isolating switch fuse and fuse isolating switch distinguish isolating switch.
Isolation switch installation requirements:
Taking Siemens load isolation switch product line as an example, the next three sub-product lines are divided into.
(1) The load isolating switch (manipulation and emergency stop switch) is a standard isolating switch, which has safety protection function, can connect and overcurrent all normal load currents, and has a certain professional ability of magnetic blowing. For example, its 3KA load disconnect switch can cut 8 times the rated current.
(2) The isolation switch fuse group is a standard isolation switch fuse group, which has the functions of overvoltage protection and overcurrent maintenance after being formed with the fuse. According to the principle of its internal structure circuit, there are two safety protection mouths, which are beneficial to remove the fuse.
(3) The fuse isolating switch is the same as the standard name, and is suitable for the start, stop and maintenance of the cable coaxial line and the motor circuit. The main power switch itself only has overcurrent and connection rated current, and the fuse has a high professional capability of overcurrent and short circuit fault current. The difference with the isolation switch fuse group is that this machine is used in coaxial circuit, the maximum rated current is 630A, and the isolation switch fuse group has a maximum rated current of 3150A, suitable for three-phase five-wire, similar to ACB and MCCB .
2. Low voltage fuse.
(1) Definition
Pre-arc time: Also known as pre-arc time, it refers to the time interval from the current value to the start of the fusing of the sufficient fusing electronic devices to the time interval caused by the electric isolation. In the whole process of fusing, the pre-arcing time accounts for most of the fusing time.
The electric arc ignition time refers to the interval from the moment when the electric arc is created to the moment when the electric arc is finally eliminated. Generally speaking, the electric isolation time only accounts for a small part of the fusing time, and can even be ignored.
Therefore, according to the requirements of GB13539.1, when the pre-arc time exceeds 0.1s, the pre-arc time current trend graph is used for verification. If the pre-arc time is less than 0.1s, the I2t characteristic is used for verification. The 0.1s here is close to the overcurrent time. Considering that the time phase is very short and the heat supply is less, the I2t characteristic is used.
g fuse link: overcurrent maintenance fuse link, which can melt the fuse link to all currents in the middle of the rated overcurrent current.
A fuse link: Overcurrent maintenance fuse link, which can separate at least the current on the fuse current time curve from all currents in the middle of the rated overcurrent current. It must be noted that A fuse link is only used for overcurrent maintenance, and a suitable high-voltage switchgear should be selected for common problem current less than at least fuse current.
Types of fast fuses for semiconductor devices are shown below:
(2) The current impulse of the fuse.
For the mutual cooperation of the upper and lower fuses, if the lower fuse is broken due to the short-circuit fault current, the upper fuse also bears the same short-circuit fault current, will it directly affect its application period? The following is the description.
For the inrush current, first calculate the integral of the inrush current I and T (denoted as A), and then compare the pre-arc I2T of the fuse (the instructions for use are generally preI2T, denoted as B). If A=50%B, fully consider about 100 times of impact, if A=1/3B fully consider about 1000 times, if A=0.25B fully consider countless times. In the same way, if the temperature resistance change value is less than 10% of the actual impact service life, the number of shocks at that time meets the service life requirements.
(3) Select the fuse.
In addition to the protection of the secondary circuit, fewer fuses are used in low-voltage circuits. However, in some places where high breaking capacity is required, such as the high-voltage side of the transformer, the low-voltage capacitor cabinet, the diesel generator side, especially the low-voltage capacitor, the inrush current can reach 100 times the rated current.
Motor circuit: The cooperative relationship between the motor circuit fuse, AC contactor, and thermal relay (motor protection equipment) can be seen from the following. Initially, the current corresponding to the intersection of the thermal relay and the fuse should be less than the overcurrent current of the AC contactor, that is, beyond this current, the AC contactor cannot overcurrent, but the fuse. In addition, the welding current of the AC contactor exceeds the overcurrent current. At this time, the maximum current that can be used by the fuse cannot exceed the welding current of the AC contactor, otherwise it will lead to the welding of the AC contactor. The I2t of the fuse cannot exceed the short-term current I2t of the AC contactor.

Post time: Apr-13-2022