Fault Description and Initial Information Collection
At 14:32 on July 15, 2023, Elevator No. 1 (Otis Gen2, 10 floors/10 stops, rated speed 1.75 m/s, rated load 1,600 kg) at a five-star hotel in Shenzhen (hereinafter referred to as Hotel S) experienced an entrapment fault during operation. The elevator was traveling down from the 7th floor to the 4th floor when it suddenly made an emergency stop between the 5th and 4th floors; there were 5 guests in the car (including 1 foreign national and 1 child). The total entrapment duration was 38 minutes, from fault occurrence to the safe rescue of all occupants. Initial fault information collected included: the elevator control cabinet recorded fault codes 'DBSS Door Open Zone Fault' and 'DZ Sensor Lost'; elevator surveillance video showed smooth operation before the fault with no obvious swaying or abnormal noise; the 5 passengers reported hearing a slight 'click' before the fault, followed by an emergency stop and severe swaying, with lighting uninterrupted; machine room temperature and humidity records showed a rainstorm in Shenzhen that day, with machine room temperature 28.5°C and relative humidity 82% (exceeding the standard required range of 40%–70%); the last maintenance record was July 11, 2023 (4 days before the fault), covering routine inspection, guide rail lubrication, and safety device function testing, with records showing everything normal. In accordance with Clause 14.2.1.1 of GB 7588 'Safety Rules for the Construction and Installation of Electric Lifts,' an elevator entrapment fault constitutes a serious accident hazard and requires immediate technical investigation. In accordance with Clause 7.4 of TSG T7001 'Regulation for Lift Supervisory Inspection and Periodical Inspection,' the elevator user unit shall report to the special equipment safety supervision and administration department within 24 hours after the fault occurs.
RCA Analysis Process and Fault Mechanism Study
A systematic RCA (Root Cause Analysis) method was adopted, using the 5-Why analysis combined with the fishbone diagram tool to conduct layer-by-layer in-depth analysis across five dimensions: Man, Machine, Material, Method, and Environment. First layer (direct cause): The door zone (DZ) sensor signal loss recorded by the elevator control cabinet caused the control system to be unable to confirm whether the car was in the door zone, triggering safety circuit disconnection and an emergency stop. Second layer (physical cause): The DZ sensor itself functioned normally, but the contacts of the door lock relay (KAD-1 type) in its signal transmission path had poor contact, causing signal interruption. Disassembly inspection found a black oxide layer and slight arc burn marks on the relay contact surfaces, with contact resistance rising from the standard value of ≤50 mΩ to approximately 380 mΩ. Third layer (system cause): Further inspection found that the KAD-1 relay was installed at the bottom of the control cabinet, adjacent to the machine room floor drain. The machine room air-conditioning system operated under high load during the rainstorm, and there was slight leakage at the condensate pipe joint, with dripping water falling directly onto the area above the relay. Clause 13.6 of GB 7588 stipulates that the elevator machine room should be dry, ventilated, and free of water leakage hazards. Fourth layer (management cause): During the routine maintenance on July 11, the maintenance unit did not check the relay contact condition as required by Clause 5.2.3 of GB/T 31821 'Specification for Elevator Maintenance.' In the maintenance work order, the 'electrical component inspection' item only checked 'visual inspection' without contact resistance measurement. Fifth layer (root cause): (1) Improper relay selection—the KAD-1 relay is a general-purpose type (protection rating IP40), unsuitable for high-humidity environments, while the machine room is adjacent to the coastline (about 2 km from Shenzhen Bay) with year-round high humidity; (2) Lack of maintenance quality control—the maintenance system did not include electrical contact resistance measurement as a mandatory inspection item; (3) Machine room environment management deficiency—the air-conditioning condensate discharge pipe was not cleaned periodically, and the drain outlet had no drip prevention measures. Fault mechanism reconstruction: Under high-humidity conditions, a water vapor film adsorbed onto the relay contact surfaces → long-term frequent operation (average about 200 times/day) intensified contact arcing → formation of oxide layers and corrosion pits → increased contact resistance → contact voltage drop exceeded the allowable value (allowable voltage drop in a DC 24V system ≤3V) → signal transmission interruption → control system determined door zone sensor fault → safety circuit cut off → emergency stop and entrapment.
Fault Tree Analysis and Key Evidence Chain
Construct a Fault Tree Analysis (FTA) to perform quantitative diagnosis of passenger entrapment faults. Top event: passenger entrapment during elevator operation (probability P=1). Intermediate events include: A - safety circuit disconnection; B - control system logic confusion; C - mechanical jamming; D - electrical component failure. A total of 23 minimal cut sets were identified for bottom events. Through quantitative fault tree analysis, the occurrence probabilities of each bottom event were ranked by importance: electrical component contact failure (importance 0.68) > inadequate maintenance inspection (importance 0.52) > excessive ambient humidity (importance 0.47) > improper relay selection (importance 0.41) > air-conditioning condensate leakage (importance 0.35). Regarding evidence chain construction, the following key evidence was collected: (1) Physical failed relay — SEM (Scanning Electron Microscope) analysis of the contact surface revealed obvious arc erosion pits and copper oxide particles; EDS (Energy-Dispersive Spectroscopy) confirmed the contact material as AgCdO (silver cadmium oxide), exhibiting sulfide corrosion characteristics; (2) Machine room environmental data — temperature and humidity recorder data exported for the 7 days before the fault showed that during the rainstorm period (July 14–15), the machine room relative humidity continuously fluctuated between 75%–88%, far exceeding the 40%–70% range specified in GB 7588; (3) Life testing of relays of the same model — accelerated aging testing of the same batch of KAD-1 relays under 85%RH and 30°C (operating frequency 200 times/day) showed that contact resistance began to rise significantly after the 58,000th operation (design life 1 million operations), and exceeded the failure threshold (300mΩ) after the 82,000nd operation, far below the design life; (4) Maintenance record review — among the 14 maintenance records from January to July 2023, the electrical contact inspection item was only filled in as 'visual inspection normal,' with no actual measurement performed even once. Article 2.2.3 of Annex A of TSG T5002 'Elevator Maintenance Rules' explicitly requires that 'contactors and relay contacts should be inspected, and cleaned or replaced as necessary.' The evidence chain ultimately formed a complete closed loop of causality: excessive ambient humidity (cause) → accelerated deterioration of relay contacts (fault evolution) → lack of maintenance testing (management failure) → contact failure leading to signal loss (direct fault) → safety circuit disconnection (fault mechanism) → elevator passenger entrapment (final consequence).
Establishment of rectification measures and preventive mechanisms
Based on the RCA results, a trinity of rectification measures was formulated: 'equipment modification + management upgrade + environmental optimization.' At the equipment level: (1) Replace all ordinary relays in elevator control cabinets with sealed relays (protection rating IP67), changed to the Omron G7J-2A2B series, with contact material changed to AgSnO₂ (silver tin oxide), which has better corrosion resistance and arc erosion resistance than AgCdO; (2) Add dehumidification modules inside the control cabinets (using Peltier semiconductor cooling dehumidification technology, dehumidification capacity 300ml/day), configured with humidity sensor linkage control to automatically start dehumidification when cabinet humidity >60%RH; (3) Add DZ sensor signal redundancy — install an additional set of backup door zone sensors on the car top and in the hoistway, automatically switching to the backup sensor when the main sensor signal is abnormal. At the management level: (1) Modify the maintenance work order template to add a mandatory inspection item for 'electrical contact resistance measurement' (standard requirement ≤50mΩ), using a digital micro-ohmmeter to measure quarterly and record in files; (2) Establish an Elevator Health Index (EHI) evaluation system integrating four dimensions: contact resistance, vibration, temperature, and humidity, with warning thresholds formulated according to TSG T7001 inspection standards; (3) Improve the quality clauses of maintenance contracts, clarifying the liability for breach of contract by maintenance units failing to perform contact testing as required, with 10% of monthly maintenance fees deducted for each breach. At the environmental level: (1) Improve the air-conditioning condensate drainage system in the machine room, installing stainless steel drip trays and drainage pipes to eliminate leakage hazards; (2) Add dehumidifiers in the machine room (dehumidification capacity 50L/day), configured with constant-humidity controllers set to a target humidity of 50%RH; (3) Add a waterproof layer to the machine room floor, convert drainage ditches to concealed ditches, and reduce moisture evaporation sources. After rectification, data was tracked for 12 months from September 2023 to August 2024: the elevator failure rate dropped from 0.38 times/unit per month before rectification to 0.06 times/unit per month (a decrease of 84%); contact resistance measurements stabilized within the range of 15–35mΩ; machine room humidity was controlled between 45%–58%RH. The RCA analysis case of the elevator passenger entrapment fault at S Hotel was listed by the Special Equipment Safety Supervision Division of the Shenzhen Municipal Market Supervision Administration as an excellent case of elevator safety analysis for 2023 and was shared at the citywide elevator maintenance unit work conference.