The qualification criteria for fire resistance rating testing of oil-immersed transformers are based on NB/T 11307.2-2024, combined with standards such as GB 2536, GB/T 2408, GB/T 5169, and GB/T 16172. The criteria are comprehensively assessed from four dimensions: insulating oil, solid materials, overall combustion, and fire hazard level. The specific qualification standards that can be directly used in reports/tender documents are as follows.
I. Insulating Oil Safety Indicators (Core Requirements)
1. Closed-Cup Flash Point (GB/T 261, GB 2536)
New Mineral Insulating Oil: ≥135℃
Operating Oil: Temperature drop ≤5℃ compared to new oil, but not lower than 130℃
High Flash Point Oil (Ester/Silicone Oil): ≥250℃ (according to product standards)
2. Arc Test in Oil (NB/T 11307.2)
Apply corresponding arc energy according to voltage level (66kV/2000kJ～750kV/27000kJ)
Pass/Fail Criteria: No continuous combustion, no oil spraying, no explosion, no abnormal sudden increase in tank pressure
3. Basic Oil Quality Indicators (Fire Protection Related Items)
Breakdown Voltage: 15kV The following voltage levels are acceptable: ≥30kV; 15～35kV ≥35kV; 66～220kV ≥40kV; 330kV ≥50kV; 500kV ≥60kV
Acid value: ≤0.1mgKOH/g (new oil); ≤0.3mgKOH/g (operating oil)
Moisture content: ≤30mg/L (new oil); ≤50mg/L (operating oil)
II. Flame retardant indicators for solid insulation materials (GB/T 2408, GB/T 5169)
1. Vertical burning (UL 94, GB/T 2408)
Core components (insulating cylinder, lead wire insulation, support components): V-0 level
Single afterflame ≤10s, total afterflame ≤50s, no dripping ignition of degreased cotton
Non-critical components: V-1 level (single afterflame ≤30s, total afterflame ≤250s)
2. Horizontal burning (GB/T 2408)
All solid materials: HB grade, burning rate ≤75mm/min
3. Glow wire test (GB/T 5169.10)
Key components: 960℃/30s, no ignition / self-extinguishing within 30s after ignition, no ignition of surrounding materials
4. Needle flame test (GB/T 5169.5)
Apply needle flame for 10s, after removal, residual flame ≤30s, no dripping material igniting the underlying absorbent cotton
III. Overall combustion performance indicators (GB/T 16172, ISO 5660-1)
1. Heat release rate (cone calorimetry)
Peak heat release rate (PHRR): ≤200kW/m²
Total heat release (THR): ≤10MJ/m²
2. Smoke density and toxicity
Smoke density rating (SDR): ≤75
Smoke toxicity: Reaches ZA2 grade (low toxicity), no release of highly toxic gases
3. Combustion drippings: No molten drippings ignited the combustibles below during the entire test.
IV. Overall Fire Hazard Assessment (NB/T 11307.2-2024, Core Judgment)
The standard classifies the fire hazard of oil-immersed transformers into levels I to V (level I is the lowest, level V is the highest):
Minimum acceptable level: ≤ Level III (generally applicable)
High safety requirements (inside buildings/densely populated areas): ≤ Level II
Level V: Deemed unacceptable, requires rectification and retesting
Comprehensive Judgment Dimensions (Weighted Scoring)
Arc Test: Combustion duration, flame height, oil spray/explosion risk (weight 40%)
Material flame retardancy: UL 94 rating, glow wire/needle flame results (weight 30%)
Heat release: PHRR, THR, smoke density (weight 20%)
Structural protection: Tank strength, explosion-proof measures, oil containment/collection design (weight) 10%)
V. On-site Fire Safety Compliance (Not a laboratory test, but a necessary condition for qualification)
No oil seepage or leakage, oil level normal
Fire separation distance meets GB 50016 and GB 50229 requirements
Equipped with oil containment pools/collection pits, fire sand, and fire extinguishers (2000kVA and above with water spray)
No signs of overheating or arcing, connection point temperature/temperature rise compliant
VI. One-sentence qualification criteria
For oil-immersed transformers to pass the fire safety test, the following must be met: closed-cup flash point of insulating oil ≥135℃, no continuous burning/oil spraying from electric arc in oil, solid materials reaching V-0/HB grade, overall heat release ≤200kW/m², fire hazard level ≤Ⅲ, and on-site fire prevention facilities compliant.
