Voltage Definitions U, Uo Um in Power Cables (IEC 60038 – IEC 60071-1 – IEC 6050

Tacettin İKİZ · March 26, 2025, 04:05:18 PM

QuoteVoltage Definitions U, Uo Um in Power Cables (IEC 60038 – IEC 60071-1 – IEC 60502 – IEC 60183)

- U₀: Rated r.m.s. voltage between each conductor and the screen or sheath.  
- U: Rated r.m.s. voltage between any two conductors (nominal system voltage).  
- Um: Maximum r.m.s. system voltage between any two conductors, used for insulation design.

QuoteStandardized Um / U Ratios (Based on IEC 60038 and IEC 60183):

Code Select

| U₀ / U (kV)   | Um (kV) | Um / U |
|--------------|---------|--------|
| 0.6 / 1       | 1.2     | 1.20   |
| 1.8 / 3       | 3.6     | 1.20   |
| 3.6 / 6       | 7.2     | 1.20   |
| 6 / 10        | 12      | 1.20   |
| 8.7 / 15      | 17.5    | 1.17   |
| 12 / 20       | 24      | 1.20   |
| 18 / 30       | 36      | 1.20   |
| 26 / 45       | 52      | 1.16   |
| 36 / 60       | 72.5    | 1.21   |
| 64 / 110      | 123     | 1.12   |
| 76 / 132      | 145     | 1.10   |
| 87 / 150      | 170     | 1.13   |
| 127 / 220     | 245     | 1.11   |
| 220 / 380     | 420     | 1.11   |

QuotePurpose of Um:
- Defines insulation level
- Used for impulse and AC test voltages
- Ensures coordination with possible overvoltages
- Applied in insulation thickness and type selection

QuoteFrom IEC 60071-1 – Voltage Stress Calculations:

Code Select

- Peak power-frequency (opposite polarity) = Um × √2 / √3 ≈ 0.816 × Um  
- Peak lightning impulse stress (opposite polarity) = 0.7 × Um × √2 / √3 ≈ 0.571 × Um

QuoteCable Factory Example:
For a 12/20 (24) kV cable:
- U = 20 kV
- Um = 24 kV
- 1-minute AC test = approx. 48 kV
- Lightning impulse test = approx. 125 kV (per IEC)

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Voltage Definitions U, Uo Um in Power Cables (IEC 60038 – IEC 60071-1 – IEC 6050

Tacettin İKİZ