Enhancing Interpretability of Data-Driven Fault Detection and Diagnosis Methodology with Maintainability Rules in Smart Building Management

Data-driven fault detection and diagnosis (FDD) methods, referring to the newer generation of artificial intelligence (AI) empowered classification methods, such as data science analysis, big data, Internet of things (IoT), industry 4.0, etc., become increasingly important for facility management in the smart building design and smart city construction. While data-driven FDD methods nowadays outperform the majority of traditional FDD approaches, such as the physically based models and mathematically based models, in terms of both efficiency and accuracy, the interpretability of those methods does not grow significantly. Instead, according to the literature survey, the interpretability of the data-driven FDD methods becomes the main concern and creates barriers for those methods to be adopted in real-world industrial applications. In this study, we reviewed the existing data-driven FDD approaches for building mechanical & electrical engineering (M&E) services faults and discussed the interpretability of the modern data-driven FDD methods. Two data-driven FDD strategies integrating the expert reasoning of the faults were proposed. Lists of expert rules, knowledge of maintainability, international/local standards were concluded for various M&E services, including heating, ventilation air-conditioning (HVAC), plumbing, fire safety, electrical and elevator systems based on surveys of 110 buildings in Singapore. The surveyed results significantly enhance the interpretability of datadriven FDD methods for M&E services, potentially enhance the FDD performance in terms of accuracy and promote the datadriven FDD approaches to real-world facility management practices.


Motivation.
Aligning with the fast development of artificial intelligence (AI) technology, data-driven fault detection and diagnosis (FDD) plays an essential role in modern smart building maintenance and management systems [1]. However, while the data-driven FDD models are often viewed as black-box models, the interpretability of FDD models hinders the methods to be widely applied to real-world applications [2,3]. Expert rules and standards are helpful for data-driven FDD methods to be adapted to real-world scenarios. The expert rules and standards not only increase the interpretability level of the data-driven FDD methods but also improve the FDD performance in terms of diagnosis accuracy rates. According to our literature survey, the rele-vant expert knowledge is considered as a research gap in the field and is highly demanded to detect and diagnose possible faults in building equipment and services [4][5][6]. In this study, we are interested to concretize the expert knowledge, using maintainability rules and standards for FDD, both regionally and globally, of different building mechanical & electrical engineering (M&E) services, including HVAC systems, plumbing & sanitary, fire safety, electrical and elevators & escalators systems, and their critical components. services of building infrastructure and development [7,8]. According to the different approaches replying to different types of evident information, FDD methods are categorized into data-driven FDD, physical model based FDD and mathematical model based FDD methods [9]. Data-driven FDD builds computational models based on historical sensor data, while different types of building faults are recognized as classes [10]. A physical model based FDD understands the whole building system using physics-based models and usually requires a significant amount of prior knowledge for faults identifications [11,12]. A mathematical model based FDD methods also requires prior physics knowledge to define a rule space for an inferencing method searching for the corresponding faults [13,14].
Data-driven fault detection and diagnosis methods represent the next-generation facility management and maintenance techniques adopting modern AI techniques, such as sensor networks [15,16], data analytics [17], big data [18,19], machine learning (ML) [20,21], cybernetic intelligence (CI) [22,23] and Internet of things (IoT) [24,25] and etc. For different building infrastructures, such as heating, ventilation air-conditioning (HVAC), plumbing, fire safety, electrical and elevator systems. In the era of big data, smart building and smart city, data-driven FDD usually serves as one of the most important applications utilizing big data and one of the hottest research topics in the fields of smart city and industry 4.0 [26][27][28].
Compared with traditional physical model based and mathematical model based methods, data-driven FDD methods are usually more efficient, robust and accurate in detecting and diagnosing various building faults, while the machine learning (ML) techniques, such as the neural networks, are constructed for predictive analysis. The ML techniques are generally much more efficient and effective than traditional PM-FDD, MM-FDD and manual classification methods. For example, for HVAC FDD, the existing works showed FDD accuracy rates over 99% for typical chiller faults and 93% for air handling unit (AHU) faults [29][30][31][32]. Traditional approaches, such as the sensitivity test, can only achieve accuracy rates close to 83% for chiller faults and around 80% for AHU faults [33][34][35][36][37]. The 10% to 15% improvement on different FDD approaches saves the energy wasted in buildings significantly, enhances the overall building performance and maintains a sustainable environment for building infrastructure maintenance.
However, the interpretability of the data-driven FDD method has always been the problem for data-driven FDD methods and hinders the data-driven FDD techniques to be widely adopted in real-world applications. While the prediction accuracy and efficiency of the data-driven FDD methods improved significantly in recent years, the internal structures of the AI approaches become more complex, resulting in more challenges for model interpretation [38][39][40][41]. The data-driven FDD models were also tentatively called black-box models in many existing publications [42][43][44], which we believe is not accurate. Many datadriven FDD models are indeed interpretable. For example, Yan et al. [45] presented a decision tree model for FDD of air handling units (AHUs). The decision tree structure is interpretable with if-else rules. However, the if-else rules were not easily recognizable for experts deriving standards for AHU maintenance.
It is evident that the expert knowledge, experience, rules, ISO standards and maintenance guidelines are valuable information and can deeply influence the performance of data-driven FDD methods. Zhao et al. [46] demonstrated that the additional expert knowledge inputs can greatly enhance a Bayesian belief network (BBN) data-driven FDD model's performance by increasing the FDD accuracy for various chiller faults. Li et al. [47] improved [46] by integrated expert knowledge into a diagnostic Bayesian network (DBN) for AHU fault FDD. The reasonings of the AHU FDD were plotted by local casual graphs. The main shortcoming of the works [46,47] is that the expert knowledge inputs were generally generated based on the authors' hypotheses.

Approach.
In this study, we reviewed the recent publications on data-driven FDD for building mechanical & electrical engineering (M&E) services, including HVAC, plumbing, fire safety, electrical and elevator systems. Different M&E faults were surveyed over 110 buildings in Singapore, including commercial, hotels, industrial, institutional, clinical and residential buildings, for all three stages of infrastructure management life-cycles, in all design, construction and management stages. The expert knowledge of M&E FDD is converted into maintainability rules and international/local standards in Singapore. It is evident that the conveyed maintainability rules greatly enhance the interpretability of the data-driven FDD approach and potentially improve the diagnosis accuracy.
We propose two data-driven FDD methods integrating the maintainability rules for general facility management in buildings, particularly focusing on FDD. The two specific data-driven FDD methods integrating maintainability rules are 1. data-driven expert rules for decision making in smart building facility FDD; and 2. maintainability rules as inputs for data-driven FDD systems. The actual implementations of the two proposed approaches were omitted, while there were existing implementations such as [46,47]. The main aim of this study is to specify the expert knowledge pool of M&E FDD using maintainability rules shown in Section 4. The specification greatly enhances the interpretability of the existing M&E FDD methods.
(ii) Identifying expert rules and standards for various M&E faults in buildings. A total of 110 buildings in Singapore, including commercial, hotels, industrial, institutional, clinical and residential buildings were surveyed, over all three stages of infrastructure management life-cycles, in all design, construction and management stages, collecting necessary FDD information based on experts' knowledge and international/local standards in Singapore. In this way, typical faults for the major M&E equipment are surveyed with detailed experts' rules and standards stated in tables. This main contribution impacts the literature for data-driven FDD approaches targeting building M&E services significantly (iii) Enhancing the interpretability of the existing datadriven FDD methods for building infrastructure faults. The interpretability of the data-driven FDD methods has been a bottleneck problem for a long time. The surveyed expert rules and standards bridge the gap between theoretical FDD strategy and realworld practices. The interpretability enhancement greatly improves the practicality of the data-driven methods in Industry 4.0 [48] and Construction 4.0 [49].
(iv) Potentially improving the diagnosis accuracy of the existing data-driven FDD methods for building infrastructure faults. According to the literature study, such as the works of [46,47], expert knowledge, e.g., the maintainability rules enhance the FDD performance significantly in terms of accuracy. The diagnosis accuracy improvements are justified by various publications [46,47,[50][51][52]].

Literature Review for Interpretability Study of the Existing M&E Services FDD Methods
Intelligent facilities management is one of the important topics for smart city design, smart building maintenance system development, Industry 4.0 and Construction 4.0. Techniques based on AI and data-driven approaches attract increasing attention from various perspectives. Besides the effectiveness and robustness of the data-driven approaches for datadriven FDD, the shortcomings and issues, such as the interpretability of the data-driven model and the efficiency for data-driven FDD algorithms, were raised in recent years. Yan et al. [45] introduced a decision tree induction (DTI) based FDD method for detecting and diagnosing AHU faults. The proposed method is data-driven, and interpretable with a post-pruned binary tree structure. The main concern of [45] is that the derived rules do not explicitly map to expert reasoning available in the HVAC system design. Most of the DTI rules were still unreadable from the perspective of HVAC engineers. Mulumba et al. [50] worked on a Kalman filter-based FDD reasonings for AHU faults. The method works for various AHU faults and is also considered a data-driven approach. The shortcoming is again that the Kalman filter rules do not map correspondingly to HVAC experts. Srinivasan et al. [51] showed the importance of explainable AI (XAI) for chiller fault detection systems to gain human trust. Li et al. [52] developed an explainable one-dimensional convolutional neural networks (CNN)based fault diagnosis method for building HVAC systems.
Besides the interpretability study of FDD for HVAC systems, there are existing data-driven FDD approaches proposed for other M&E service systems. Kumar et al. [53] developed a deep learning detecting defects in sewerage systems. The deep learning structure relies on the CNN for object detection in images. The image processing technology using CNN is more interpretable using expert knowledge compared to other ML techniques. Gonzalez-Jimenez et al. [54] surveyed the existing fault diagnostic methods to examine faults for electric drives and revisited the general workflow using ML techniques for electric drive FDD. The main drawback of the data-driven FDD method as concluded in [54] is the lack of interpretability and the lack of explanations for specific phenomena in every particular electric drive. Gavan et al. [55] proposed to integrate expert rules and data-driven FDD methods to develop a positive energy building in France. The project has a nice workflow chart utilizing expert rules for building data analysis and FDD practices. However, it is an ongoing project and the performance of the proposed workflow is yet to be verified.
All the above-surveyed existing works showed that there are already quite many efforts on integrating expert rules and reasonings into the existing data-driven FDD methods to enhance the interpretability of the methods as well as improve the FDD performance on the classification accuracy for building maintenance problems. However, there still exist gaps between expert rules and data-driven methods, such as neural networks. The gap is mainly from the reasoning of AI and the ordinary reasoning of human beings. The most appropriate matching and fitting using the expert rules with the modern data-driven FDD methods remain unknown and desired further explorations, such as the current study. The current study expands the scope of the FDD methods and greatly enhances the applicable area of data-driven FDD methods in building services.

Integrating the Maintainability Rules into Data-Driven FDD for M&E Services
Two types of data-driven FDD strategies are available in general for the concept of next-generation AI-technology integrated smart buildings for facility management and maintenance. The first strategy is named post-caution maintenance. This strategy is widely adopted for modern buildings when expert knowledge of precautions is lacking. Without sufficient rules and guidance in the stages of design and construction, the only option left is monitoring the facilities regularly using physical, mathematical or AI-driven models and detecting potential errors with frequent data analysis. The expert knowledge and rules are added as an additional layer of the ML model for performance enhancement. Existing examples of post-caution maintenance include [46,47].   Fire detection and alarm systems Passenger lifts and service lifts -Guide rails for lift cars and counter-weights -T-type 6 Journal of Sensors   Figure 1: Knowledge-based system for precaution maintenance that constructs the knowledge pool using maintainability rules. 8 Journal of Sensors

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The second type of the data-driven FDD strategy is to involve the expert knowledge in the FDD monitoring of the entire life-cycle of all facilities, or precaution maintenance. With the experts in the fields of, e.g., project management, construction, interior design and quantity survey, relevant regional and global standards, such as SS, BS, ISO, EN, AS and ASTM (Table 1) can be adopted in the precaution of potential risks in the early stage of the FDD. However, there are generally gaps between those standards and the real-world maintenance strategy, i.e., lacking clear guidance of different maintenance rules for different elements of the M&E system. The main contribution of this study is providing systematic and comprehensive maintainability rules for all kinds of M&E elements.
In this section, we demonstrate two data-driven FDD strategies with the maintainability rules for precaution and post-caution maintenance, which apply the maintain-ability rules as the inputs and the knowledge pool, respectively. These two strategies serve as examples of the usage of the maintainability rules listed in the Tables in Section 4.

Knowledge-Based Rule System Integrated Data-Driven FDD for M&E Faults.
A knowledge-based system is a fundamental AI system that makes decisions purely based on rules. A traditional knowledge-based system comprises a large set of if-else rules that builds a decision tree and processes FDD queries efficiently. A semantic of a typical knowledge-based FDD system is shown in Figure 1. The collected sensor data is evaluated by maintainability rules. The evaluation results lead to the various maintenance decisions following a tree-alike structure.
For example, following the escalator maintainability rules stated in Table 2: 'The landing area of escalators and The annual inspection and testing should be performed by an independent authorised examiner (AE). Bar access to the escalator or passenger conveyor with suitable devices and notices/ signage displaying "no access/no entry" during maintenance, repair works, or inspections, in accordance with BS EN 115-1, SS 626 or equivalent. Adopt the inspection criteria for safety of escalators in accordance with JIS A 4302. Refer to procedure for ride quality measurements of escalators and moving walks in accordance with BS ISO 18738-2 or equivalent. 9 Journal of Sensors passenger conveyors should have a surface that provides a secure foothold for a minimum distance of 0.85 m (measured from the root of the comb teeth)', the measurement data collected from the sensor can be easily evaluated as 'satisfactory' or 'unsatisfactory'. Different evaluation results will arrive to different decisions for automated maintenance.
A knowledge-based FDD system integrating traditional data-driven FDD framework treats the knowledge pool ( Figure 1) as an expert system [9,56]., where if-else rules are derived from maintainability rules. Following the existing rule-based system structures proposed in the related fields, such as [45,57,58], decisions for labeling various faults can be reached. The accuracy and performance of such FDD systems depend on the precision and reasonings of the rules. Compared with the existing rule-based systems, the maintainability rules proposed in this study are more precise Historical data (faulty) Future sensor data Training Training Multi-class ML model Figure 2: A typical FDD framework uses machine learning (ML) models to classify sensor data.    Journal of Sensors Comply with air conditioning plant optimisation strategies (e.g. high-efficiency chillers, aggressive condenser water reset, medium temperature chilled water loop, chilled water VFD pumping, etc.) in accordance with SS564-1 or equivalent.
Use refrigerants with zero ozone depleting potential (ODP) or global warming potential (GWP) of less than 100 [7]. Recommend installation of leak detection system in critical areas of plant rooms. Cary out proper commissioning of chiller plant and set reasonable points. Perform postinstallation monitoring of the installed instrument's performance through the BMS (building automation system) or EMS (energy measurement system).
Document as-built drawings (including concealed services) for building user phase. Prepare and handover maintenance checklists for service and repair of each instrument during commissioning. Refer to measures for recommended monitoring procedures for chiller efficiency in accordance with SS 591 or equivalent. Inspect temperature controllers and thermostatic control valve for any malfunctions and then reset.
Perform routine inspection of condenser pipes. Carry out cleaning and servicing (de-scaling) if/ when necessary.
Routinely inspect insulation for any damaged/ worn-out layers. Daily logging of chiller system to ensure system operates at optimum conditions. Any deviation from the intended chiller operation or alarms need to be attended promptly.
Perform annual shutdown or overhauling in accordance with manufacturer's guidelines (ANSI/ASHRAE/ACCA standard 180).  Provide adequate reach to all parts of the cooling tower for cleaning so as to prevent bacterial growth.

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Specify locally green certified cooling tower water treatment system [8], but not limited to automated non-dispersant, non-chemical treatment etc. as it will minimise the downstream maintenance issues. The automated system with the capability of checking of the conductivity and other recommended parameters can be also linked to the building management system (BMS).

Prepare and submit an operation and maintenance (O&M) manual
For the chemical dosing system after the successful commissioning of the system. Perform monthly checks for legionnaire, scaling and corrosion in/of the condenser system. Disinfect and manually de-sludge cooling towers if required.
Monitor water usage in cooling towers and review the efficiency.
Ensure optimum scheduling of temporary shutdown and standby of cooling towers.
Discharge used water from cooling owners into a sewerage system.
Water efficiency Consider collection of condensates for re-use.
Take into consideration the cycles of concentration of the cooling owner used in order to reduce blow-down frequencies.
Avoid uncontrolled water losses due to drift, splash, overflow etc.
Ensure proper installation of float operated inlet valve to reduce frequencies of overflow.
Link cooling towers in order to maintain water balance.
Consider using solenoid operated inlet valves for cooling towers.
Legionella outbreak Outbreak on air-water surfaces of cooling tower Avoid redundant pipework, bends, and loops for cooling system design. Allow easy access to all parts of the system for maintenance [9]. Minimise drift by enclosing cooling tower pond. Take the wind direction/distribution and the surrounding environment into account when designing the cooling system.
Use non-corrosive, chemical-resistant, nonporous, smooth, opaque (to block sunlight) material to inhibit growth and proliferation of microorganisms.
Design of cooling towers must be capable for preventing dirt accumulation and water stagnation. Perform proper commissioning to ensure safe operation of cooling towers. Develop cooling tower maintenance manual including cleaning/ water treatment/decontamination procedures and handover during commissioning.
Ensure system is clear of dirt/debris/organic matter and clean before operation.
It is presupposed that the control of legionella bacteria in cooling towers is in compliance with applicable statutory and regulatory requirements. Cooling tower's water temperature should be monitored since elevated temperatures and moisture at air-water surfaces provide ideal conditions that may serve as a nutrient source for legionella growth.
The chemical concentration limits of cooling tower effluents should be checked and in compliance with the relevant local standard.
If cooling tower is not in use, it should be kept drained and dry. If not in use for more than 5 days, it should be drained, cleaned and disinfected before operating. Regular testing for legionella bacteria should be carried out and get water samples from the cooling tower pond in 13 Journal of Sensors 14 Journal of Sensors    Maintain proper water pressure in piping system to avoid bursting from over pressurisation.
Carry out thorough cleaning and disinfection of service pipes on a monthly basis, and clean the main pipes semi-annually. If required, removal of blockage with manual cleaning method (e.g., plunger, drain rod, spring auger) should be performed.
Carry out chemical de-scaling quarterly (care should be taken so that it does not harm the pipes or jointing by giving consideration to chemical type or contact time). Perform maintenance schedule to check for clogged outlets in accordance with BS 8558, SS 636 [11].
Isolate all leaks and faulty water pipes and fittings first to prevent water wastage before they are being repaired by a licensed plumber. All repairs should be done as soon as possible. 18 Journal of Sensors Semi-annual pressure test should be carried out for sewage pumps.
Additional bracing or anchor block support at bends and branches should be provided in order to withstand the hydraulic thrust. Specify variable speed drives (VSD) that are able to handle both maximum and variable demand. Ensure that the pump is mounted on an isolation bed (e.g., 150 mm insulated padding) and that no site adjustment in height or position should be done. The installation of pumps should be adopted in accordance with manufacturer's instructions.

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Ensure proper construction of isolation beds and installation/mounting of equipment.
Special attention should be given for tightness of joints, and alignment of bearings and pipes. Testing and commissioning of the auto and manual interchange of duty and standby pumps should be carried out.
Lubricate pump parts with oil or grease in accordance with manufacturer's instructions.
Carry out routine checks, clean motor starter and all heavy current contacts, and replace worn parts.
Visually check for any damage or missing parts (screw, nuts, strainer, etc.).
Ensure that after installation, pump is tested with power on mode for any unusual vibration, noise, leakage or burnt smell.
Sensor self-closing delayed-action basin taps To specify the display contact numbers of maintenance staff so that users can report any faulty, leakages or damaged taps to them for immediate rectification. To ensure the display contact numbers of maintenance staff so that users can report any faulty, leakages or damaged taps to them for immediate rectification. Carry out routine checks to ensure that sensor selfclosing delayed-action tap can function properly and when the tap is activated, the water is automatically cut-off after a pre-set period of time.
The tap should automatically stop functioning during power failure in accordance with SS 448.
Self-closing delayed-action basin taps To specify the display contact numbers of maintenance staff so that users can report any faulty, leakages or damaged taps to them for immediate rectification. To ensure the display contact numbers of maintenance staff so that users can report any faulty, leakages or damaged taps to them for immediate rectification. Carry out routine checks to ensure that self-closing delayed-action tap can function properly and that the water is automatically cut-off after a pre-set time in accordance with SS448.
Dual-flush low-capacity flushing cisterns To specify the display contact numbers of maintenance staff so that users can report any faulty, leakages or damaged taps to them for immediate rectification. To ensure the display contact numbers of maintenance staff so that users can report any faulty, leakages or damaged taps to them for immediate rectification.

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Journal of Sensors   Inspect condition of overflow warning alarm for the water tank. Inspect the condition of warning alarm which indicates when water levels fall below 50 mm from the invert level of the pipes.

Potable water tanks
Cleaning and disinfection of potable water tanks should be adopted in accordance with SS 636 or equivalent.  Ensure that egress is unobstructed in case of emergency evacuation in accordance with evacuation plan and NFPA 101 or equivalent.

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Journal of Sensors   The horizontal illuminance on the Centre line of any exit cannot be less than 0.5 lux. A fuel supply should be readily available to ensure that emergency lighting operates continuously for the rated period following the failure of normal power supply in accordance with SS 563-2 or equivalent. Ensure the maintenance of emergency power supply systems in buildings. Conduct a monthly manual test of emergency lights and replace batteries or lamps as soon as a fault is detected in accordance with SS 563-2, AS/NZS 2293.2/ AMDT 3 or equivalent.

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Conduct monthly fire simulation test. Simulate failure of main power supply and test the efficiency of the standby battery.
Ensure visual and audible fault signals are activated once the battery is disconnected. 28 Journal of Sensors   [14].

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Adopt the positioning of breeching inlets as close as possible to rising main/hydrant in accordance with BS 9990, SS 575, NFPA 14 or equivalent.
Ensure that the locations are accessible, with no obstructions from parking, loading bays, landscaping, building elements, etc.
Provide protection to hydrants from mechanical damage.
Specify easily visible/identifiable signage and colour in accordance with SS 508-3, ISO 3864-1 or equivalent.
Installation should be secure and safe, with special consideration given for potential sources of damage.
Connections and position of valves should be guided by specifications. Risers must be securely anchored before any pressure or flow test is performed.
Hydrants should be made operable immediately after completion and should be tested to protect the construction site. Conduct monthly checks for any leakage, blockage or corrosion, and for workable line pressure.
Perform thorough inspection of booster pump and associated systems semi-annually. Ensure that a thorough inspection of the hydrants is annually performed by a competent professional.

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Journal of Sensors  Mounting should be carried out in accordance with manufacturer's instructions. To adopt general guidelines in accordance with NFPA 13 or equivalent.
Sprinkler system should be carefully installed to maintain correct orientation without hindrance by supports.
Spare sprinklers and sprinkler spanner should be maintained after installation for future needs. Conduct quarterly visual inspection of all sprinklers for any leakage, damages or grease/dirt in spray nozzle and replace as necessary.
Conduct annual inspection of pipes and hangers for corrosion and mechanical damage (clean, paint or replace as necessary).
Clean quarterly and remove any obstruction affecting efficient discharge from sprinklers. Check for any sign of corrosion or deposit of dirt, paint or foreign material in accordance with NFPA 13 or equivalent.
Practice good housekeeping to avoid stacking of material leading to obstruction of sprinklers. Check for insulation damages (e.g., cracks, blisters, warping) caused by overheating, physical impact or by spillage of cleaning chemicals. Check for potential short circuits or ground faults. Ensure that switchboards are not exposed to direct sunlight or alternative heat sources conduct annual shutdown to eradicate hot spots along the distribution network as witnessed by the owner and certified by a licensed electrical worker (LEW). Provide necessary warning notices/labels at switchboards (e.g., shock hazard warnings).            44 Journal of Sensors and reasonable, consequently providing better results in terms of diagnostic accuracy.

Maintainability Rules as Inputs for Data-Driven FDD
Systems. Data-driven FDD applied machine learning techniques to sensor data and performs automated classification with a pre-defined training process on the collected data. A typical data-driven FDD process is shown in Figure 2, where historical data containing both normal operational and faulty conditional data is received by the machine learning (ML) models. Two particular ML models are trained. The binary ML model handles the fault detection for facilities management, which classifies the future sensor data into normal or faulty classes. The multi-class ML model handles the fault diagnosis part, which classifies the faulty sensor data into different types of faults. Traditional FDD methods, as shown in Figure 2, assume completely no background knowledge of the maintainability of the facilities. The maintainability rules that we proposed in this study provide a great opportunity to improve the existing FDD approaches. The simplest way of extending the current FDD framework with the maintainability rules is to treat them as inputs for the ML models. We formalize the proposed extension of the existing FDD framework in Figure 3.
In Figure 3, the traditional FDD framework has been improved by adding maintainability rules as inputs for both training and testing phases. Since ML models, in general, do not require background knowledge for classifications, the maintainability rules are served as additional inputs for both training and testing of the ML models. The maintainability rules have the potentials of enhancing the interpretation capability of the ML models as well as the prediction performance.
A concrete example of the proposed framework shown in Figure 3 is the three-layer Bayesian Belief Network (BBN) adapting the maintainability rules as an additional layer for FDD. The three-layer BBN is a three-layer neural network, calculating the probabilities of label assignment based on evidence and conditional probability. The details of the BBN construction can be found in [46,47]. The internal structure of the BBN is illustrated in Figure 4, where expert knowledge is interpreted using maintainability rules as introduced in the Introduction Section (Section 1) and Section 4. For prediction probabilities calculated by neural networks, the maintainability rules provide evidence that influence the probability calculation. Therefore, the FDD accuracy will be improved significantly.

Maintainability Rules Study for Facility Management in M&E Services
In this section, we summarize the maintainability rules following the expert knowledge of typical components in M&E services, namely, HVAC system, plumbing and sanitary system, fire safety, electrical system and elevator & escalator system collected through survey and interview results over 110 buildings in Singapore, including commercial, hotels, industrial, institutional, clinical and residential build-ings. The maintainability rules summarize the preventive checklist based on expert knowledge as well as standards regionally or globally in all design, construction and operational stages of buildings. The maintainability rules are useful serving as the knowledge pool for the post-caution FDD approach or as the additional maintainability layer for a precaution FDD approach, as explained in Section 3. The maintainability rules for the chiller plant, the cooling tower, the air handling unit (AHU) and the air distribution, terminal system of the HVAC system are summarized in Tables 3-6, respectively. The maintainability guidance for general pumping issues, the water supply system and the water tank of the plumbing and sanitary system ae summarized in Tables 7-9, respectively. The maintainability issues for the fire detection, the fire hydrant system, the sprinkler system and the fire extinguishers of the fire safety (Table 10) are listed in Tables 11-13, respectively. The maintainability rules for the switchgear, the standby generator, the artificial lighting, the lightning protection system (LPS) and earthing are summarized in Tables 14-17, respectively. The general rules for the elevators and escalators, common faults for the elevators and escalators, the elevator safety, energy efficiency for the elevators and escalators and the maintenance for escalators, in general, are summarized in Tables 18-21, respectively.
The details of the regional (Singapore-based) and global standards, such as SS, BS, ISO, EN, AS and ASTM, are listed in Table 1.

Conclusions, Limitation & Future Works
Maintainability rules for M&E systems based on the survey and interview results of 110 buildings including commercial, hotels, industrial, institutional, healthcare and residential buildings are summarized. The maintainability rules are useful to be integrated into the existing data-driven FDD approaches for 1) an extension of the existing FDD algorithm to all M&E facilities in buildings, 2) enhancing the interpretability of the existing AI models and 3) improving the performances of the AI models. In Section 3, we demonstrate two data-driven FDD strategies integrating the maintainability rules, including 1) data-driven expert rules for decision making in smart building facility FDD; and 2) maintainability rules as inputs for data-driven FDD systems.
Based on the literature study, the surveyed maintainability rules will greatly enhance the interpretability of the existing data-driven FDD methods for M&E services and consequently promote the FDD methods to other building facilities and to other industrial areas, such as the Industry 4.0 evolution solutions. Furthermore, existing works show that the expert knowledge potentially improves the datadriven FDD results by adding the rules to the machine learning models, such as the decision trees.
The limitation of this study includes not showing the actual implementation of the maintainability rules integrated FDD framework, which we believe is a repetitive work to the existing publications. The main contribution of this study is first, to further extend the existing studies and concretize the maintainability rules that are used in existing