An 11kV and a 33kV cable termination look similar in photos. Both have stress control tubes, insulation tubes, and rain sheds. But the electrical requirements scale significantly with voltage — and those differences determine the insulation thickness, the number of rain sheds, and the test voltages the termination must withstand.
What Changes Between Voltage Classes
Three parameters scale with voltage:
Insulation thickness. At 11 kV, the termination's insulation tube provides approximately 3.5 mm of recovered wall. At 33 kV, this increases to approximately 5.5 mm. The thicker insulation withstands the higher electric field stress.
Creepage distance. The surface distance along the insulation from the live conductor to the earthed screen must be longer at higher voltage. This is achieved by adding more rain sheds (umbrella skirts) to the termination body. An 11 kV outdoor termination might have 3 sheds; a 33 kV version has 6 or more.
Test voltages. The type test requirements are dramatically different:
| Test | 11 kV Class | 33 kV Class |
|---|---|---|
| Power frequency withstand | 45 kV, 1 min | 45 kV, 1 min |
| Partial discharge | ≤20 pC at 13 kV | ≤20 pC at 26 kV |
| Impulse withstand | 75 kV, ±10 each | 105 kV, ±10 each |
| DC withstand | -52 kV, 15 min | -52 kV, 15 min |
| 4-hour power frequency | 35 kV, 4 h | 35 kV, 4 h |
Note: both classes must pass the same power frequency and DC tests, but the partial discharge test voltage and impulse withstand requirements are higher at 33 kV.
Insulation Distance Comparison
For busbar insulation using our MPG series, the clearance requirements differ dramatically:
| Voltage | Uninsulated Phase-Earth | Uninsulated Phase-Phase | With MPG Insulation |
|---|---|---|---|
| 10 kV | 120 mm | 75 mm | 80 mm / 70 mm |
| 35 kV | 320 mm | 200 mm | 200 mm / 150 mm |
Using MPG busbar tubing at 35 kV reduces the phase-to-earth clearance from 320 mm to 200 mm — a 37% reduction that enables more compact switchgear design.
Partial Discharge: The Critical Metric
Partial discharge (PD) is the leading indicator of long-term reliability in MV cable accessories. IEC 60502-2 requires ≤20 pC for type-tested terminations. Our NRSY-35 and WRSY-35 terminations achieve ≤2 pC — ten times better than the standard requirement.
Why does this matter? PD erodes insulation over time. A termination with 2 pC discharge will last decades. One approaching the 20 pC limit may show insulation erosion within 5-10 years, especially in polluted environments. When comparing suppliers, always ask for the actual PD test value, not just "type tested."
FAQ
Can I use a 33 kV termination on an 11 kV system?
Technically yes — the insulation is more than adequate. But 33 kV terminations are larger, more expensive, and may not physically fit in an 11 kV switchgear cabinet. Use the termination rated for your system voltage.
Why does the partial discharge test voltage change?
The PD test is performed at 1.73 × U₀ (operating phase-to-earth voltage) to simulate the worst-case electrical stress. For 11 kV systems (U₀ = 6.35 kV), the test voltage is approximately 13 kV. For 33 kV systems (U₀ = 19 kV), it rises to approximately 26 kV.
Do I need different rain sheds for indoor vs outdoor?
Yes. Indoor terminations (NRSY) have minimal sheds because they're not exposed to rain or pollution. Outdoor terminations (WRSY) have extended sheds to increase creepage distance and shed water. Using an indoor termination outdoors will fail prematurely in wet or polluted conditions.
Written by Long, Sales Engineer at ZZ Heat Shrink. For 10-35 kV termination specifications, see NRSY/WRSY series. For busbar insulation clearance data, see MPG series. For a quotation, contact our engineers.