## 1 Introduction
In general terms, SSCBs have the following characteristics: fast operation, arc-free, soundless, long operation useful life, and reliability. SSCBs are used for a variety of applications [16]. Different types of silicon semiconductor devices are used for the SSCB gate-turn-off thyristor (GTO), the silicon insulated-gate bipolar transistor (IGBT), integrated gate-commutated thyristor (IGCT), and cathode metal oxide semiconductor controlled thyristor (CS-MCT), each having its advantages and disadvantages. The main characteristic of these switches is their speed, with operating times lower than 100 us; however, SSCBs present high-power losses, and are expensive and also too large for some applications, since they require heat sinks [21]. Another group of SSCBs is devices in which the predominant material is a wide band gap (WBG), such as silicon carbide (SiC) JFETs, SiC metal-oxide-semiconductor field-effect transistors MOSFETs, SiC static induction transistors (SiC) SITs, gallium nitride (GaN) high electron mobility transistors (HEMTs), and gallium nitride (GaN) FETs. WBG semiconductors exhibit superior material properties than those based on silicon, which enables the operation of these power devices at higher-temperature operation, higher blocking voltage capabilities, and higher switching frequencies; however, WBG technology itself is still evolving towards its maturity [22]. HCBs are a combination of the best features of mechanical and solid-state switches in a single device, overcoming their corresponding drawbacks. Indeed, hybrid switches feature small conduction losses, very short operating times, long service life, and high reliability, and do not require special cooling equipment, which shows a new direction in the research and development of switches for engineering applications [16]. However, the switching speed strongly depends on the mechanical parts of the system [23]. After an exhaustive bibliography search, it was found that papers regarding DCMG protection do not focus on the evolution of circuit breakers and its future challenges. In the Ref. [9], an overview on DCMG protections scheme was presented. In the Refs. [5; 21; 24; 25; 26; 27; 28; 29], topics such as topologies, strategies, and protection schemes were covered. In the Refs. [30; 31], protections in DCMGs were described, comparing them with their ACMGs counterparts. Inthe Ref. [32], DC protection schemes were presented, focusing on mathematically derived metrology requirements. In the Ref. [33], benefits and shortfalls of WBG SSCBs and their applications in photovoltaic panel (PV) generators were presented. The authors in the Ref. [34] presented a comprehensive classification of the characteristics of solid-state circuit breakers in which future challenges are addressed; nonetheless, CS-MCT SSCB features are not considered, which is a strong candidate for DCMG protection [35]. The evolution and future challenges of circuit breakers for the protection of DCMGs were the main motivating factors behind this work. We provide a review of the circuit breakers used or potential candidates for DCMG protections that have been recently reported in the specialized literature. The reviewed literature covers low-voltage (<1.5 kV) and medium-voltage (between 1.5 kV and 20 kV) DCMGs [36; 37]. This paper is structured as follows: Section 2 describes the DC Microgrid faults. Section 3 includes the types of DC circuit breakers used in DCMGs, namely, fuses, MCBs, SSCBs, and HCBs. Section 4 describes the evolution of DC CBs. Section 5 presents a summary of future challenges for CBs. Section 6 concludes and highlights the most important aspects of the paper.