BS 7835 XLPE / LSZH Medium Voltage 19/33 (36)kV Cable

What is the BS 7835 XLPE / LSZH Medium Voltage Cable?

At its core, the BS 7835 XLPE / LSZH cable is engineered for medium voltage applications, rated at 19/33 (36)kV. This means it's suited for power distribution where the phase-to-earth voltage (Uo) is 19kV, phase-to-phase (U) is 33kV, and the maximum voltage (Um) hits 36kV. XLPE refers to cross-linked polyethylene, a thermoset material that enhances the cable's thermal and mechanical properties compared to traditional thermoplastics like PVC. LSZH, on the other hand, ensures low smoke emission and zero halogen gases during combustion, reducing toxicity and visibility issues in fires.

Why does this matter in South Africa? Our country faces unique challenges: from the fire-prone tunnels in Gauteng's mines to the underground networks in Durban's humid climate. Traditional cables might release harmful fumes, endangering lives and equipment. The BS 7835 standard, aligned with IEC/EN 60502-2 for power cables, emphasises flame retardancy and low smoke, making this cable a smart choice for installations where public safety is key. It's not just about transmitting power; it's about doing so responsibly in a nation pushing for sustainable development.

Construction: Building Blocks of Reliability

The cable's build is a masterclass in engineering. At the centre: stranded copper conductors for flexibility and conductivity. A semi-conductive XLPE screen smooths electric fields, preventing stress points.

Insulation is XLPE type GP8 per BS 7655, cross-linked for superior heat resistance. Another semi-conductive XLPE screen follows, then a metallic copper tape screen (individual or overall) for fault current handling.

Fillers use PET fibres for shape retention, with binding tape as a separator. Bedding is LSZH for low smoke, and armour varies: aluminium wire (AWA) for single-core to counter induced currents, steel wire (SWA) for multi-core mechanical protection.

The outer sheath is LSZH, ensuring safety in fires. This construction makes the cable suitable for underground burial, ducting, or even in air – versatile for South Africa's diverse terrains.

Electrical Characteristics: Performance Under Pressure

Current carrying capacity is impressive. For a single-core 70mm² in ground (trefoil formation), it's 270A; in air, up to 370A flat. Larger sizes like 1000mm² handle 860A in ground and 1900A in air.

De-rating factors account for real-world conditions. At 35°C air temp, derate by 0.92; for ground thermal resistivity of 1.5 km/W, 0.92. Depth of laying at 1.5m requires 0.94 derating. These ensure safe operation, preventing overheating in buried installations common in rural KwaZulu-Natal or urban Pretoria.

In South Africa, where soil types vary from sandy Karoo to clay-rich Free State, these factors help engineers optimise designs, reducing failures and extending lifespan

Usage Scenarios: 

This cable excels in power networks, underground, and ducted setups where fire risks loom. Think hospitals in Bloemfontein or shopping centres in Port Elizabeth – places where smoke could trap people.

In mining, LSZH minimises fumes in tunnels, aligning with safety regs. For renewables, it's ideal for connecting wind farms in the Eastern Cape or solar plants in the Northern Cape to the grid, handling medium voltage drops efficiently.

Urban upgrades benefit too: replacing old PILC cables in Eskom's networks with XLPE/LSZH reduces maintenance and boosts reliability amid load shedding.

The BS 7835 XLPE / LSZH 19/33 (36)kV cable isn't just wiring; it's a lifeline for South Africa's power woes. From specs ensuring durability to case studies proving its mettle in upgrades and renewables, it offers insightful solutions.