## 1 Introduction  
A transmission system operator (TSO) is tasked with ensuring the efficient and reliable transmission of electrical energy across a power grid. To achieve this objective, the TSO must implement structured and effective maintenance strategies for equipment located within high-voltage (HV) substations. Proper maintenance is essential not only for preserving the reliability and availability of power system operations but also for extending the service life of substation components. This study focuses specifically on the maintenance of equipment within HV switchyards, which are organized systematically into HV switching bays (SBs). Each SB comprises primary components such as circuit breakers (CBs), disconnectors (Ds), and instrument transformers (ITs) [1]. Surge arresters may also be installed within SBS; however, their maintenance is beyond the scope of this investigation. TSOs typically employ condition-based maintenance (CBM) strategies for HV switching bay equipment. Modern CBM practices rely on condition-monitoring systems to schedule maintenance activities based on the actual status of the devices [2]. Maintenance can be carried out either while the equipment remains in operation (online) or when it is de-energized and disconnected from the power system (offline). All primary devices within an HV SB--CBs, Ds, and ITS--are considered collectively in offline condition monitoring, as this approach requires isolating the entire bay, thereby enabling the simultaneous maintenance of all components. In cases of failure or malfunction, the system unit associated with the affected HV SB is disconnected, and corrective maintenance is carried out without delay. Within a TSO, CBM activities are executed strictly in accordance with the procedures specified in the Maintenance Code [3]. The Maintenance Code [3] establishes the framework for implementing maintenance actions based on manufacturers' recommendations and the TSO's accumulated operational experience. Any deficiencies or internal defects identified during inspections are addressed through corrective interventions in line with this protocol. This study concentrates on the maintenance of circuit breakers, disconnectors, and instrument transformers as the primary devices within HV switching bays. The authors of [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15] explore CBM strategies for circuit breaker maintenance. These authors discuss various partial monitoring tasks for circuit breakers and analyze measures for assessing the condition of these devices. However, none of these studies cover the entire scope of condition-monitoring tasks for circuit breakers comprehensively. The authors of [8] only address failure mode analysis for circuit breakers (CBs) and do not cover condition-monitoring tasks. They conducted a risk assessment based on failure mode analysis with failure probabilities. Moreover, the authors of [9] provide a general overview of asset management, including condition monitoring and health assessment. In the remaining studies, condition or risk assessment analyses for CBs were conducted based on individual maintenance tasks. The authors of [16] addressed the determination of failure rates for disconnectors, while the authors of [17] analyzed measures for assessing the state of transient resistances in HV disconnectors. The authors of [18; 19] analyzed the condition monitoring of the disconnector mechanical operating mechanism. Finally,the authors of [20; 21; 22] examined the condition states of instrument transformers through condition-monitoring activities. The primary objective of this study is to introduce a novel framework for assessing the technical condition of devices in an HV SB, specifically, CBs, Ds, and ITs. The conceptual basis for evaluating the technical condition of 400 kV substation equipment was first presented by the authors of [23], wherein the condition was calculated as a normalized sum of weighted additive criteria.