BIL (Basic Impulse Level)
The ability of transformer insulation to withstand high-voltage surges from lightning or switching, measured in kV peak.
BIL defines the transformer insulation system's ability to withstand high-voltage impulse surges from lightning or switching, expressed in kV peak using a 1.2/50 microsecond waveform tested during lightning impulse testing.
BIL determines insulation coordination and must align with surge arresters. Ratings: 30 kV (low voltage) to 1,500+ kV (EHV transformers). Factors include system voltage, altitude (see altitude derating), lightning exposure.
QDTB designs insulation systems with appropriate BIL margins, with enhanced options for severe environments. Our international standards comparison guide covers BIL requirements across IEC and IEEE.
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Related Terms
Dielectric Strength
The maximum electric field an insulating material can withstand before breakdown, measured in kV/mm.
Lightning Impulse Test
A high-voltage test simulating lightning surges to verify transformer insulation withstands transient overvoltages.
Insulation Class (A/E/B/F/H)
Standardized temperature ratings for transformer insulation materials defining maximum operating temperatures.
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A metallic strip conducting electricity within switchgear, transformers, and distribution equipment.
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A factory-assembled compact substation integrating transformer, HV/LV switchgear, and protection in a weatherproof enclosure.
More in “Insulation & Testing”
Dielectric Strength
The maximum electric field an insulating material can withstand before breakdown, measured in kV/mm.
Insulation Class (A/E/B/F/H)
Standardized temperature ratings for transformer insulation materials defining maximum operating temperatures.
Lightning Impulse Test
A high-voltage test simulating lightning surges to verify transformer insulation withstands transient overvoltages.
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