## 2 Current Status of the Preventive Condition-Based Maintenance of Devices in an HV Switching Bay  
An HV SB device can operate in several states. Most of the time, it operates normally, serving the purpose for which it was installed. It can also be in a non-operational state. According to IEEE 859-2018 [27], the device state is defined as follows: \"The component or unit state is a particular condition or status of a component or a unit, which is important for outage reporting statuses\". The state of a device depends on the state of its components. The operation of the device, including maintenance, according to its state in an HV SB is illustrated in Figure 1. The state of a device, as determined by condition assessment, can be either satisfactory or, at least, deficient. In a satisfactory state, no deficiencies or internal defects are detected. In a deficient state, either deficiencies or internal defects are identified. A deficiency indicates that the device can still operate with full or reduced functionality, albeit with a moderate risk of fault or failure. On the other hand, an internal defect signifies that a part of the device is not in perfect condition. Still, the device remains at least partially capable of performing its intended functions. The risk of fault or failure in this state is significant. The state of a device is influenced by internal (such as aging) and external factors (such as operational conditions). The following sections will discuss the deficiencies and internal defects in a deficient state, where variable DID (deficiency and/or internal defect) will be used in subsequent discussions. If no deficiencies or internal defects are present, the device operates normally (normal operation). Even with detected deficiencies or internal damage, a device may remain in service if its performance remains acceptable. This is referred to as the operating state. During the device's operation, the TSO can conduct planned events to assess the device's condition or implement appropriate (repairing) corrective measures. However, detected or undetected deficiencies or internal defects may result in the failure of the device or its components. As defined by the authors of [28], failure is the loss of the ability to perform as required. Failure is an event that results in a fault, which is the resulting state. A fault occurs when a device transitions from a functional to a non-functional state, potentially due to an internal issue--such as an insulator breakdown resulting in a short circuit. Faults are categorized as technical, operational, and system faults. In the case of a fault, protection mechanisms are triggered, and the primary system unit containing the faulty device is disconnected. In the event of a failure, the system unit becomes unusable and is excluded from the power system, resulting in an unplanned event. In this case, according to the TSO protocol, an intervention follows where the state of the switched-off device is verified as a non-operational state. A device's failure also necessitates maintenance, as the device is not performing its required function as planned. The maintenance procedures in HV SBs include a standardized process for assessing the condition of HV SB devices, as illustrated in the flow chart in Figure 2. Since maintenance activities are performed uniformly across all device types (CBs, Ds, and ITs), the same procedure applies to each of them. The maintenance processes are categorized into substation inspections (SIs) (performed without shutdown); comprehensive inspections, also referred to as global checks (GCs) (performed with shutdown); and measurements (Ms) and tests (Ts). The following is a detailed explanation of the maintenance tasks for monitoring the condition of HV SBs.