In the design of electrical system, designers will often encounter the problem of electrical component selection, such as, fuse selection, relay selection and so on.

Think about a question that many people will ignore: when generally choosing a traditional fuse, the voltage level is required to be higher than the line voltage level, why?

Different voltage levels correspond to different arc extinguishing capacity. The high voltage fuse has strong arc extinguishing capacity, and the low voltage fuse has low arc extinguishing capacity. 

The working state of the fuse is after the melt is fused. If the low voltage level fuse is applied to the high voltage circuit, when the short circuit occurs, the melt breaks off, but the arc cannot be extinguished, causing the circuit to be short circuit for a long time and lose protection.

Another point, about the series of partial pressure. As you all know, the capacitance can raise the voltage level in series, and many people will think that the fuse can raise the voltage level in series, which is wrong.Because, when multiple fuse in series, the resistance value is different, joule heat is different, there will always be a breaker first disconnected, because the single fuse voltage level is not enough, then the disconnected fuse will produce a pull arc, the circuit is in the conduction state, the rest of the fuse will also appear the same situation, the circuit can not be segmented.

According to the idea of series, and in parallel, the two fuses are connected in parallel to increase the current capacity. This is also very difficult to achieve, the two fuses can not guarantee the same resistance, resulting in the internal flow of the current is inconsistent, it is difficult to calculate the current capacity of the two parallel connection. Some manufacturers have made parallel melt integrated fuse, such as Basman, it is also used in reduction.

Of course, it is actually possible to raise the voltage level in series, but to achieve the same action, as follows:

The application of the reference to Eaton's breaker circuit breaker can also reflect this idea. There is one breaker with 4 terminals. Eaton advertised the product as 900V 450A, actually connecting two of the terminals short, and when the two terminals are not in series, it is 450V in series. This is a long time, but Eaton integrates the product.

Another example is that connecting two smart fbreakers in series, connecting their trigger signal lines in parallel by one signal, doubles the voltage resistance.