Preventive Testing Code for High-Voltage Equipment
High-voltage equipment (10kV) is the core of the hotel's power distribution system, and its insulation condition and mechanical performance are directly related to power supply safety. According to the provisions of Chapters 5 to 8 of DL/T 596 "Preventive Testing Code for Electrical Equipment," the preventive testing of 10kV high-voltage equipment shall cover the following key items and be carried out at the specified intervals. 10kV vacuum circuit breaker testing (interval: once every 3 years) (1) Insulation resistance test: Use a 2,500V megohmmeter to measure the insulation resistance of the circuit breaker. Standard: overall insulation resistance ≥1,000MΩ (at 20°C); insulation resistance between contacts ≥1,500MΩ. Test conditions: the circuit breaker is in the open position; test phase-to-phase, phase-to-ground, and between contacts separately. (2) Circuit resistance test: Use a 100A or 200A DC circuit resistance tester (micro-ohmmeter) to measure the contact resistance of the circuit breaker's main contacts. Standard: for a circuit breaker with a rated current of 1,250A, the circuit resistance ≤100μΩ; for 2,500A, ≤60μΩ. An abnormal increase in circuit resistance (exceeding the standard value by 20% or increasing by more than 50% compared with the previous test) indicates contact wear or poor contact. (3) Mechanical characteristic test: Use a high-voltage switch characteristic tester to measure closing/opening time, three-phase asynchronism, and bounce time. Standard: closing time ≤100ms (asynchronism ≤2ms), opening time ≤60ms (asynchronism ≤2ms), bounce time ≤3ms. 10kV power transformer (dry-type transformer) testing (interval: once every 1-3 years) (1) Insulation resistance test: Use a 2,500V or 5,000V megohmmeter to measure the insulation resistance between high voltage, low voltage, and ground. Absorption ratio (R60s/R15s) ≥1.3 (at 10-40°C). Temperature conversion: insulation resistance decreases as temperature rises; the temperature conversion coefficient shall be implemented in accordance with Appendix E of DL/T 596. (2) DC resistance test: Use a DC resistance tester (10A) to measure the DC resistance of the high-voltage side/low-voltage side windings. Unbalance rate: between lines ≤2%, between phases ≤4% (compared with factory data). (3) Core grounding current test: Use a clamp-on ammeter to measure the core grounding current. Standard: ≤100mA (normal operation); when it exceeds 100mA, a multi-point core grounding fault may exist. 10kV power cable testing (interval: once every 1-3 years) (1) Insulation resistance test: Use a 5,000V megohmmeter to measure the insulation resistance of the cable. Standard: for a 1km-long XLPE cable, insulation resistance ≥1,000MΩ (20°C). (2) DC withstand voltage test (optional): Apply a DC voltage of 4Ue (35kV) for 15min without breakdown. Note that DC withstand voltage testing may carry a risk of damage to XLPE cables; it is recommended to use very low frequency (VLF) withstand voltage testing instead (0.1Hz, 3Ue, 30min). (3) Partial discharge test (recommended): For important power supply circuits (such as transformer incoming cables), partial discharge testing is recommended, with partial discharge level ≤10pC. Test safety requirements: Safety fences and warning signs shall be set up in the high-voltage test area, and unauthorized personnel are strictly prohibited from entering during the test. Before and after the test, the equipment under test shall be fully discharged (grounding discharge time ≥3 times the charging time). Test results shall be compiled into a formal report, including test data, judgment conclusions, and signatures of test personnel.
Preventive Testing Code for Low-Voltage Power Distribution Systems
Preventive testing of low-voltage power distribution systems is a routine maintenance measure to ensure the daily power supply safety of five-star hotels. Although its voltage level is lower (380/220V), because of its wide coverage and numerous user devices, its failure rate is far higher than that of high-voltage systems. According to the provisions of Chapter 12 of DL/T 596 "Preventive Testing Code for Electrical Equipment" and Chapter 8 of GB 50150 "Standard for Handover Test of Electrical Equipment in Electrical Installation Engineering," the preventive testing of low-voltage power distribution systems shall include the following contents. Low-voltage distribution cabinet/distribution box testing (interval: once every 1-2 years): (1) Insulation resistance test: Use a 500V or 1,000V megohmmeter to measure the insulation resistance of busbars, switches, and lines. Standard: insulation resistance per kV of working voltage shall not be less than 1MΩ; for a 380V system, insulation resistance ≥0.5MΩ. The insulation resistance of distribution cabinets shall be ≥5MΩ. During testing, disconnect the fuses of all outgoing circuits or open the circuit breakers. In humid areas (the plum rain season in the south), the insulation resistance value may decrease somewhat, but shall not be lower than 0.5MΩ; otherwise, moisture removal treatment shall be carried out. (2) Switch/circuit breaker operation test: The operating mechanism shall operate flexibly and reliably, and the closing/opening indications shall be correct. For frame-type circuit breakers (ACB) and molded-case circuit breakers (MCCB), conduct 3 manual and 3 electric closing/opening tests each, with no jamming and no abnormal noise. Trip unit setting verification: the settings of the long-time delay trip (L), short-time delay trip (S), and instantaneous trip (I) shall be consistent with the design. (3) Circuit resistance test: Use a 200A DC micro-ohmmeter to measure the contact resistance of the main circuit (busbar connection points, circuit breaker contacts). Standard: contact resistance at busbar lap joints ≤10μΩ (when the copper bar contact area is ≥100×10mm). (4) Leakage protection test: Test all circuits equipped with leakage protection (RCBO/RCD/ELCB). Use a leakage protection tester (accuracy ±5%) to measure the operating current and operating time. Standard: for an RCBO with an operating current of 30mA, the actual operating current shall be between 18-30mA (not greater than 30mA), and the operating time ≤100ms (required by Article 5.5.2 of GB 50054). The test button of the residual current device (RCD) shall be manually pressed at least once a month. Low-voltage cable line testing (interval: once every 2-3 years): (1) Insulation resistance test: Use a 500V or 1,000V megohmmeter to measure line-to-line and line-to-ground insulation. Standard: ≥1MΩ (for flame-retardant cables, this may be relaxed to ≥0.5MΩ). (2) DC withstand voltage test: For cables of important circuits (fire power supply, emergency lighting, elevator power supply, etc.), apply a DC voltage of 1,000V (for cables with rated voltage 0.6/1kV) for 5min without breakdown. Capacitor cabinet testing (interval: once every year): (1) Insulation resistance test: Use a 2,500V megohmmeter (after short-circuiting the two capacitor poles, measure to ground). (2) Capacitance measurement: Use a capacitance meter to measure the capacitance of each phase; the deviation shall be within -5%~+10% of the rated value. (3) Discharge resistor check: discharge resistance value (usually 1MΩ), discharge time ≤3min (from √2Ue down to below 50V). The Marriott MIDS standard requires hotels to conduct a comprehensive preventive test of the low-voltage power distribution system at least once a year, and test records shall be kept for no less than 3 years.
Relay Protection Device Calibration and Setting
Relay protection devices are the last line of defense for the power distribution system. Their correct setting and reliable calibration are key technical measures to ensure selective fault removal and prevent accident expansion. According to the provisions of Chapter 20 of DL/T 596 "Preventive Testing Code for Electrical Equipment" and DL/T 587 "Code for Operation and Management of Microcomputer-Based Relay Protection Devices," the calibration interval for relay protection devices is usually once every 1-3 years (for microcomputer-based protection devices, it may be extended to 3 years). The relay protection configuration and calibration key points of the 10kV power distribution system of a five-star hotel are as follows: Types of relay protection configuration: (1) Overcurrent protection (50/51): used to detect phase-to-phase short-circuit faults. The setting current value is set at 1.2-1.5 times the rated current of the protected equipment (overcurrent stage I high setting) and 1.05 times (overcurrent stage II low setting). Time grading coordination: the time difference between upper and lower protection levels is 0.3-0.5s (0.3s for microcomputer-based protection, 0.5s for electromagnetic protection). (2) Zero-sequence overcurrent protection (50N/51N): used to detect single-phase grounding faults. The setting current value is usually 10%-20% of the rated current. Time setting: coordinated with outgoing line protection, 0.3-0.5s delay. (3) Undervoltage protection (27): used for undervoltage protection. The voltage setting value is usually set at 40%-70% of the rated voltage. (4) Overvoltage protection (59): the voltage setting value is usually set at 110%-120% of the rated voltage. (5) Transformer protection: including instantaneous protection (internal transformer fault, no delay), overcurrent protection (backup protection, delay 0.3-0.5s), gas protection (Buchholz relay, for oil-immersed transformers), and temperature protection (dry-type transformers, alarm temperature 135°C, trip temperature 150°C). Protection device calibration contents: (1) Appearance inspection: the protection device appearance is intact, the LCD display is normal, and the indicator lights are normal. (2) Power-on inspection: after power-on, the protection device self-check passes with no error messages. Clock calibration (error with standard time ≤±1s). (3) Setting verification: Use a relay protection tester (single-phase or three-phase, accuracy 0.1%) to verify the setting of each protection function of the protection device. When 0.9 times the setting value is applied, the protection shall not operate; when 1.1 times the setting value is applied, it shall operate reliably. The operating time measurement error shall be ≤±2% or ±30ms (whichever is greater). (4) Trip circuit inspection: inspect the trip output circuit of the protection device; the circuit breaker shall operate normally and the signal feedback shall be correct. Each protection operation shall have a corresponding central signal alarm. (5) Interlocking logic test: simulate various fault conditions (high-resistance grounding, low-resistance grounding, three-phase short circuit, two-phase short circuit, etc.) to verify the correctness of the protection device's logic judgment and trip output. Setting value management: (1) Protection setting values shall be issued by the power supply company or a qualified design institute in the form of a setting calculation sheet. (2) An approval system must be established for modification of setting values. (3) After each calibration, a setting value freeze sheet shall be kept (including setting values, calibration date, and calibration personnel signature). The Marriott MIDS standard requires that the relay protection devices of the hotel's 10kV power distribution system be calibrated at least once a year (microcomputer-based protection may be extended to 2 years), and calibration reports shall be archived for reference.
Test Interval Management and Test Record Archiving
The interval management of preventive testing and the standardized archiving of test records are management guarantees for ensuring the continuous and effective operation of the testing system. According to the requirements of Chapter 4 of DL/T 596 "Preventive Testing Code for Electrical Equipment" and Chapter 22 of JGJ 16 "Code for Electrical Design of Civil Buildings," an annual test plan shall be formulated for preventive testing of electrical equipment, and test intervals shall be reasonably arranged according to the importance of the equipment, operating years, and environmental conditions. Classified management of test intervals: (1) Class A tests (mandatory annually): low-voltage distribution cabinet insulation resistance test, leakage protection test, grounding resistance test (confirmation of ≤1Ω), emergency power supply (EPS/UPS) function test, and surge arrester leakage current test. Class A tests shall be completed before the thunderstorm season (April-May) to ensure summer power supply safety. (2) Class B tests (once every 1-3 years): high-voltage circuit breaker insulation and circuit resistance test, dry-type transformer insulation and DC resistance test, power cable insulation test, relay protection device calibration, and high-voltage cabinet mechanical characteristic test. Class B tests may be scheduled together with Class A tests to reduce the number of power outages. (3) Class C tests (once every 3-5 years): transformer winding deformation test, cable partial discharge test, high-voltage cabinet temperature rise test, and power quality analysis (harmonics, three-phase unbalance, power factor). Class C tests may be carried out selectively according to equipment operating conditions. Principles for preparing the test plan: (1) Coordinate with the hotel's off-season: five-star hotels usually have an off-season from November to March, and this period may be used to schedule test items requiring power outages. High-voltage equipment testing is recommended to be arranged after the Spring Festival or in September-October. (2) Combine testing with maintenance: preventive testing may be carried out simultaneously with equipment maintenance (such as cleaning distribution cabinets before insulation testing and replacing aging components and lamps), reducing the number of power outages. (3) Prepare emergency plans: during testing, emergency power supply plans shall be prepared (such as activating backup circuits and temporary generator power supply) to ensure that the basic operation of the hotel is not affected. Test record archiving requirements: after each test is completed, a formal test report shall be generated within 5 working days. The report contents shall include: test date, name and number of the equipment under test, test environmental conditions (temperature, humidity), name and number of test instruments (including calibration validity period), test data (including original data sheets), comparative analysis with previous test data, judgment conclusion (pass/fail/warning), handling recommendations (treatment measures and time limits for nonconforming items), and signatures of test personnel and reviewers. Test file management: (1) Establish an equipment test ledger (an Excel or CMMS ledger may be established according to substation equipment numbers). (2) Electronically scan and archive test reports (PDF format, naming rule: equipment number_test date_test item). (3) Retention period of historical test reports: high-voltage equipment ≥5 years, low-voltage equipment ≥3 years (according to Article 22.4.2 of JGJ 16). (4) Establish trend curves for test data of key equipment (transformers, circuit breakers, protection devices) (such as year-by-year insulation resistance variation curves) to assist in judging equipment deterioration trends. The Marriott MIDS standard requires that the hotel's annual preventive testing plan be submitted to the hotel engineering director for approval, and the test completion rate shall serve as an annual KPI assessment indicator for the engineering department (recommended ≥95%).