What is the acceptable IR value?

       The acceptable insulation resistance (IR) value depends on the type of electrical equipment, its voltage rating, and industry standards. Here are the general guidelines for common electrical devices and systems:
　　1. Low-voltage equipment (≤ 1 kV)
　　For most low-voltage electrical devices (e.g., motors, cables, switchgear), the minimum acceptable IR value is 1 MΩ at room temperature.
　　A common rule of thumb: IR (MΩ) ≥ Rated voltage (kV). For example, a 400V device should have an IR value of at least 0.4 MΩ.
　　New or well-maintained equipment typically has IR values well above 10 MΩ.
　　2. Medium-voltage equipment (1 kV – 36 kV)
　　The minimum acceptable IR value is usually 10 MΩ or higher.
　　For medium-voltage cables and transformers, standards like IEC or IEEE often require IR values > 100 MΩ for reliable operation.
　　3. High-voltage equipment (> 36 kV)
　　High-voltage devices (e.g., power transformers, transmission lines) demand much higher IR values, typically > 1000 MΩ (1 GΩ).
　　The IR value should also remain stable over time; a rapid drop in IR indicates insulation degradation.
　　4. Special considerations
　　Temperature impact: IR decreases significantly with rising temperature. Most standards specify measurements at 20°C – 25°C, or require temperature correction for accurate comparison.
　　Megger test standards: When using a megohmmeter, test voltage is matched to equipment rating (e.g., 500V megger for 400V devices, 2500V megger for 10kV devices).
　　DC systems: For DC circuits (e.g., battery systems), the acceptable IR is generally > 5 MΩ to prevent leakage.
　　Key industry references
　　IEC 60034 (rotating electrical machines)
　　IEEE 43 (insulation resistance testing of rotating machinery)
　　IEC 60502 (power cables)

相见恨晚。