## 2 Current Status of the Preventive Condition-Based Maintenance of Devices in an HV Switching Bay
Determining a device's condition requires the performance of tasks for assessing the condition of devices in an HV switching bay, as illustrated in the flow chart in Figure 2. Since the maintenance processes are carried out uniformly for all devices in the HV switching bay, the procedure shown in the diagram applies to all devices (circuit breakers, disconnectors, and instrument transformers). The maintenance processes are divided into inspections (without shutdown), comprehensive inspections (with shutdown), and measurements and tests. The devices in the substation and all HV SBs are inspected when inspecting the substation without shutting down the SB. The tasks during the inspection involve primarily determining the condition of the device or its components visually. These tasks include assessing the cleanliness of the components (mainly insulators); checking for mechanical deformation; identifying defects; and examining the condition of connections, screws, and connector joints. The specific tasks may vary between different devices. If no deficiencies or visible defects are detected, the current condition of the devices is considered satisfactory. When a comprehensive inspection (global check) involving the shutdown of the switching bay is planned according to the Maintenance Code [3] (Internal Maintenance Act), all  
Figure 1: Illustration of device states in an HV switching bay. scheduled tasks are carried out to determine the condition of the devices in the HV switching bay. These tasks include inspections and measurements/tests per the maintenance guidelines. The set tasks for a global check involve primarily assessing the device's or its components' condition when it is switched off. These tasks focus mainly on the condition of the terminals, HV connections, screws, connector joints, and supporting structures and the adequacy of the insulators and flanges. For CBs, the condition of the drive mechanism components, the operation of the heaters and ventilation within the drive box, and the SF6 gas pressure are also inspected. In addition to the general tasks, an inspection of the drive mechanism components, contact blades, and jaws is conducted for Ds. The operation of the heaters in the drive box is also checked, similarly to CBs. In addition to general tasks such as sealing, the condition of the primary and secondary connections and the position of the membrane are inspected for ITs. A device's current condition is deemed satisfactory if no deficiencies are found during these inspections. An overhaul and careful, comprehensive inspection are performed for both CDs and Ds. Regular diagnostics must be performed to monitor and determine the condition of the devices. This process is based on the magnitude of the typical parameters or the trend in the changes in the parameters of individual HV devices. These parameters are captured through measurements, which are usually performed in the switch-off state, i.e., during a global check. For devices in the HV SB, an extensive set of measurements is conducted: transient resistances of HV current paths and connection terminals (Ds); inherent times and simultaneity of contact switching (CBs); grounding resistances; electrical properties of the insulation system; and the electrical, chemical, and physical properties of the transformer oil. All measurements are performed according to the specified tasks outlined in the maintenance guidelines. A special measurement type, not attributed to any specific device in the HV SB, is the diagnostic measurement with a thermographic camera for thermal  
Figure 2: Procedure for determining the status of devices in an HV switching bay for a TSO. imaging. If the measurements indicate that the parameters are within the predefined tolerance limits, the device's current condition is considered satisfactory. In addition to measurements, tests are an essential component of diagnostics in determining the condition of the devices.