## 3 DC Circuit Breakers
### Fuses  
The fuse consists of an element in the form of a metallic conductor with a pair of contacts between them, and a box or cartridge to carry the fuse element. Depending on the voltage level, the cartridge is usually fitted with a device using material such as quartz sand for arc extinction inside [46; 47]. The principle of operation of the fuse is the heating effect of the electric current. If the current passes through a conductor with a certain resistance, the loss due to the resistance of the conductor dissipates in the form of heat. Under normal operating conditions, the heat produced in the fuse element is easily dissipated into the environment due to the current flowing through it. When a fault occurs, such as a short circuit, the current flow through the fusible element exceeds the prescribed limits. This creates excess heat, which melts the fuse and breaks the circuit [48].  
Fuses are usually made of copper or silver and are mainly installed in series with the line. During a fault, the heat from the increased current blows out the fuse, causing the  
Figure 2: Types of faults: (**a**) line–line fault, (**b**) line–ground fault.  
line to open. Fuses are used as the simplest and most economical form of protection in DC systems [21].  
The selection of a fuse for an ACMG requires a response time in the range of 10-100 ms to operate and interrupt the fault; however, the nature of a DCMG requires a maximum operating time of 0.5 ms, which represents a limitation [18; 49]. For the selection of fuses, it must be guaranteed that the time constant of current rise during the fault is lower than a certain limit, since a slow rise in temperature allows the heat-absorbing material to extinguish the arc [19]. Fuses are ideal for applications in low-inductance DC systems, because the time for the fuse to blow out must be minimal [50]. Although a fuse is a very simple and inexpensive form of protection, it has several disadvantages: it must be replaced after operation and does not have the possibility of discriminating between a transient and a permanent fault [50]. Additionally, when a fault occurs in a single line, fuses only isolate the failed pole, leaving the other pole active; despite this, fuses are considered a good option to protect batteries and photovoltaic systems when trip time and cost are considered, as well as the protection of load-feeders working together with mechanical switches and relays [39]. In the Ref. [51], fuses were considered as a viable alternative to mechanical DC breakers; however, fuses installed in DCMGs must be provided an auxiliary device to extinguish the arc produced in the opening of the fuse during a fault. The authors in the Ref. [51] recommended a time constant to faults of >6 ms; however, this could decrease the fuse's ability to interrupt the current and extinguish the arc. In the Ref. [52], a detailed analysis of fuses used in power converters was presented. Fuses were found to be an effective means of protection, although the required amount of capacitance at the output of the voltage-balancing converter can be high, which affects the total cost of the system; therefore, in terms of power converters, the ideal application is to use the fuses as backup protection for the main switch. Fuses are not recommended as backup protection in DCMGs with ring configuration, since in this case, there are bidirectional current flows which require a communication system and the isolation of the cable in case of a fault [52].