The IEC 61850 series of standards for electrical substations harness the potential of Ethernet based wired networks as the backbone for intra-substation data communication. However, the flexibility of the standard, particularly the IEC 61850-90-5, enables the integration of emerging communication technologies such as wireless 5G cellular network for both intra and inter-substation data communication. In this paper, we evaluate and compare the reliability of a digital substation by analyzing different network architectures used based on wired Ethernet networks and wireless 5G networks. Reliability Block Diagram (RBD) approach have been implemented to conduct the comparison and the availability and Mean Time to Failure (MTTF) have been evaluated. The results show that the wireless 5G networks have a positive impact on the availability of the substation but the MTTF and Mean Time to Repair (MTTR) was found to decrease when compared to the wired Ethernet networks. However, such results indicate that the wireless 5G network is a sensible upgrade for the digital substation when considering the advantages and flexibility offered by the technology.