## 5 Future Challenges  
The bibliographical review shows that CBs in DCMGs are still a technology under evolution and study. Each CB has its own advantages, but also has its drawbacks; the weak points or limitations of CBs are being addressed by researchers around the world to better meet the requirements for their application in DCMGs. At the present time, until the technology is consolidated, it is not possible to know which of the CB will predominate over the others. Their strengths and weaknesses are manifested in different aspects; therefore, it is not possible to define a definite winner in a strict technical sense. Nonetheless, to facilitate the decision-making of engineers and designers of DCMGs, Table 3 summarizes the main advantages and disadvantages of the CBs considered in this paper. It can be seen, for example, that despite the fact that fuses are not expensive, they are unable to distinguish between a transient and a permanent fault; regarding operation speed, MCBs are not as fast as SSCBs; nonetheless, they feature low cost and low power losses. On the other hand, HCBs present low power losses, but are expensive when compared with MCBs or fuses. In particular, the fourth column of Table 3 provides a comparison in terms of operation speed. It can be seen that in general terms, new technologies of CBs have a tendency to be faster, since speed of operation is a fundamental aspect of modern DCMGs. A more detailed account of advantages, disadvantages, and future challenges of each CB approached in this paper is carried out in the following paragraphs which expand and complement the information presented in Table 3.  
Fuses are the simplest and most economical protection device for DCMGs, which can be installed in systems that have low inductance. For DC systems that have high inductance, there is a risk of the production of a permanent arc in the terminals of the fuse after its operation. Fuses are commonly used in low-cost DCMGs for protection against fire or explosion; this breaker does not fully protect the elements of the MG, requiring other complementary protection devices to satisfy the DCMG requirements. Fuses are commonly used to protect battery banks; however, as mentioned, this breaker does not fully protect the battery; basically, in this particular case, it is used to prevent fires or explosions when the battery terminals are short-circuited [21; 50]. The most important challenge for this circuit breaker is the reduction of response time under fault to values below 0.5 ms, and an alternative for reaching this challenge consists of investigating heat-absorbing materials whose properties allow the arc to be extinguished with fast current rises [18; 19; 49]. If response times in fuses are satisfactorily reduced, they would be used as  
Figure 10: Timeline evolution of DC CBs after the definition of DCMGs. back-up protection devices in DCMGs. Another important aspect is the development of fuses that do not have to be replaced after the fault occurs; however, the development of this capacity may increase the price of the fuse, which is currently its main advantage. MCBs is the most extended protection for ACMGs, and this is the reason why it might become one of the most promissory candidates for DCMGs. In general terms, MCBs that are used in ACMGs could be used directly in DCMGs; however, the arc produced in DC circuits is more difficult to extinguish than the arc in AC circuits. Thus, in order to preserve the useful life of the protection, the protection must be derated. In consequence, one of the challenges for the use of MCBs in DCMGs is the improvement of the active current-switching circuits, which permits mitigating the impact of the arc and preserves the useful life of the device. Another important aspect is that MCBs are very slow; in consequence, it is still necessary to achieve better operating times, less than 0.5 ms [31]. If better times are reached, MCBs could be implemented in a coordination scheme that adequately protects the system.