## 1 Introduction  
The operation of power systems relies on communications networks, which themselves rely on the power supply, so that power systems and their communications networks form a mutually dependent cyber-physical system. However, loss of communication can lead to power failures and power failures can lead to loss of communications, with consequential cascading failures [1]. Disasters can cause severe damage to the communications networks and also cause overload situations due to the high demand for communications during emergencies. Hybrid redundant communications systems [2] can fail if the redundant systems both depend on the grid power supply. The aim of this paper is to show how 5G/LTE-based PLMNs can provide reliable communications in a microgrid during blackouts. Existing works on reliability of communications in power grids and microgrids do not address communication systems that continue to work with a widespread loss of mains power but assume that mains power is present [2; 3; 4; 5]. In the context of a microgrid, having reliable communications during a blackout means that the microgrid can continue to manage itself during blackout, i.e., even when the distributed energy resources (DER) in the microgrid are connected to a de-energised grid, the distributed controllers at the DER sites can continue to communicate with each other. It was shown in [6] how an islandedmicrogrid can recover from blackout on its own if its devices can communicate with each other. The approach of this paper is to use LTE/5G-based public land mobile networks (PLMN) to provide reliable communications for the exemplary microgrid blackout recovery use case. Being able to recover from blackout means that the failure modes of the power system and the communications system are decoupled, so that the cyber physical system (CPS) they form together is less vulnerable to catastrophic failure. Our focus on how a particular concrete communications technology can be made reliable in the worst-case fills a gap in the existing work on communications reliability in smart grid applications in that it focuses on how to practically apply a specific technology to provide reliable communications in a specific use case. The theoretical study of the topological conditions between power grid stability and communication failures performed in [7] treats communication failures without considering their cause, developing a communications topology to cope with link failures. A distributed architecture for control of a distribution grid is presented in [8], and the mitigation of failures of communications to devices at different levels of the control hierarchy is considered. However, neither of these works considers the case where there is a global communications failure, such as would occur in a blackout. Communications reliability is treated in an undifferentiated way, i.e., considering the overall communications system reliability without considering reliability under different network operating conditions, but only the overall reliability requirements of different applications [3]. There is, to the authors' knowledge, no literature specifically considering communications reliability during blackout. Equipping smart grid IoT devices with two UMTS Subscriber Identify Module (USIM) cards for different mobile networks will ensure continuity of service in case one network fails [9], but this method would not work if a power outage affected both mobile networks. Literature treating specific communication requirements of smart grid applications, such as latency [4], implicitly assumes that the underlying basic communications system is actually working. This paper addresses the issue of actually ensuring a basic communications service during blackout. Ultra-reliable communications (URC) in 5G wireless systems is discussed by [PERSON] [10] outlines the problem of multiple devices trying to share communication resources but does not propose a solution and does not consider how to provide reliable communications during blackouts.