33 kV Cables

33 kV is one of the voltage ratings listed in IEC 60038 (IEC Standard Voltages) and is included in several British Standards such as BS 6622 (Electric cables – Armoured with thermosetting insulation for rated voltages from 3.8/6.6 kV to 19/33 kV – Requirements and test methods.), BS 7835 (Electric cables – Armoured cables with thermosetting insulation for rated voltages from 3.8/6.6 kV to 19/33 kV having low emission of smoke and corrosive gases when affected by fire – Requirements and test methods)as well as Part 4.10 of BS 7870 (Specification for distribution cables with extruded insulation of rated voltages of 11 kV to 33 kV – single-core 11 kV to 33 kV cables).

Voltage ratings covered by BS 6622 and BS 7835 include:

• 3.8/6.6 (7.2) kV
• 6.35/11 (12) kV
• 8.7/15 (17.5) kV
• 12.7/22 (24) kV
• 19/33 (36) kV

Voltage ratings covered by BS 7870-4.10 include:

• 6.35/11 (12) kV
• 19/33 (36) kV

This range also aligns with those in the Specifications issued by most DNO’s.

The adoption of these standards ensures the safety, performance, and compatibility of electrical equipment and systems in the UK.This voltage level is typically used in large medium voltage power Distribution networks and Industrial installations, over transmission distances up to 30 km.

A 33 kV (kilovolt) voltage rating is applied to a cable designed to operate in a medium-voltage electrical system where the nominal voltage is 33,000 volts.

Although many parts of Europe use IEC standards such as IEC 60502-2, parts of the old British Empire as well as British Standards still use this voltage rating. Historically the specifications issued by the UK DNO’s (Distribution Network Operators) also use this voltage rating in preference to the IEC rating.

When selecting or installing a 33 kV cable, it is essential to adhere to the relevant BS cable standards to ensure the cable’s safety and performance. These standards cover aspects such as cable construction, insulation and sheathing materials, conductor size, and give guidance on installation practices designed to prevent electrical failures and ensure the reliability of the whole of the electrical distribution system. The UK DNO’s issue guidance on the current ratings to be applied to their cables. Always consult the specific standards and local regulations applicable in your region or industry to ensure compliance and safety.

Tratos can supply a range of standard 33 kV cables, as well as bespoke deigns with:

Conductors

• Solid or stranded Aluminium conductors up to 1000 mm2.
• Stranded Copper conductors up to 1000 mm2.

Copper is by far the most widely used material for conductors in electric cables because its high electrical conductivity makes it a good conductor of electricity, only Silver has better conductivity. It is relatively soft and ductile, so it is easy to draw to size, and so it is an ideal material with which to make 33 kV cables.

Aluminium conductors offer a cost-effective alternative without compromising on performance.

Tratos Class 2 conductors are supplied water blocked.

Insulations systems

BS 6622 and BS 7835:

• Extruded semiconducting conductor screens.
• EPR or XLPE insulation
• Bonded or strippable extruded semiconducting insulation screen in combination with a metal layer of aluminium or copper wires, copper tapes or aluminium laminate tapes.

BS 7870-4.10:

• Extruded semiconducting conductor screens.
• EPR or XLPE insulation
• Bonded or strippable extruded semiconducting insulation screen in combination with a metal layer of copper wires.

Both EPR and XLPE are cross-linked materials, having good mechanical properties coupled with excellent dielectric properties. Because of their elastomeric nature they withstand high temperatures without melting or excessively deforming. 

Because of their stable dielectric properties over a wide temperature range, EPR and XLPE are usually the insulation of choice for medium voltage and high voltage cables, although is some applications such as subsea EPR is preferred by some customers. Both materials are included in British Standards as well as Specifications issued by the DNO’s.

The semiconducting compounds used by Tratos are fully compatible with the EPR and XLPE insulation.

Metallic screen

When applied to each individual core the metallic screens ensure a uniform symmetrical stress within the insulation, they can also help electromagnetic interference.

The maximum D.C. resistance of the copper wire screen shall not exceed,

As the manufacturer Tratos can manufacture 33 kV cables with different screen resistance to meet the requirements of the Standard, Installation or Local Regulations.

Assembly and finish

Single core BS 6622 and BS 7835 Unarmoured

• Metallic screened core as above
• Extruded outer sheath in PVC, Polyethylene or Low smoke, zero halogen emission.
• Radial and longitudinal Water blocking is available.

Single core BS 7835-4.10

• Metallic screened core as above
• Extruded outer sheath of MDPE (Type A) or Low smoke, zero halogen emission (Type B).
• Radial and longitudinal Water blocked is available.

These single core unarmoured 33 kV cables are both light and pliable to ease installation in applications where mechanical protection is not required.

Single core BS 6622 and BS 7835 Armoured

• Metallic screened core as above
• Extruded or taped inner covering.
• Aluminium metal wire armour.
• Extruded outer sheath in PVC or MDPE (BS 6622 or Low smoke, zero halogen (BS 7835)
• Radial and longitudinal Water blocking is available.

These single core armoured 33 kV cables are an ideal choice for installations where mechanical protection is required.

Three core BS 6622 and BS 7835 Individual core screens Unarmoured

• Metallic screened core as above
• 3 cores laid up with non-hygroscopic fillers.
• Extruded outer sheath in PVC or MDPE (BS 6622) or Low smoke, zero halogen emission (BS 7835).
• Radial and longitudinal Water blocking is available.

Three core BS 7870-4.10

• Three metallic screened and sheathed cables laid up in triplex formation

These three core unarmoured 33 kV cables are both light and pliable to ease installation in applications where mechanical protection is not required.

Three core BS 6622 and BS 7835 unscreened core, Collective screen in the form of an Armour

• Unscreened core as above
• 3 cores laid up with non-hygroscopic fillers.
• Lapped inner covering. (an extruded inner covering can be supplied upon request)
• Galvanised wire collective screen applied as an armour.
• Extruded outer sheath in PVC or MDPE (BS 6622) or Low smoke, zero halogen emission (BS 7835).
• Radial and longitudinal Water blocking is available.

The use of a collective metallic screen rather than an individual metallic screen around each core is preferred by some customers for 11 kV cables as it simplifies terminations.

Three core BS 6622 and BS 7835 Individual core screens Armoured.

• Metallic screened core as above
• 3 cores laid up with non-hygroscopic fillers.
• Extruded inner sheath.
• Galvanised steel wire collective screen applied as an armour.
• Extruded outer sheath in PVC or MDPE (BS 6622) or Low smoke, zero halogen emission (BS 7835).
• Radial and longitudinal Water blocking is available.

These 33 kV cables are popular for installations where maximum mechanical and electrical protection is required.

Sheaths

PVC sheaths should require no introduction, they have been used in the manufacture of electric cables since WW2.

PVC sheaths have good mechanical and ageing properties, with excellent resistance to ozone and weather, and specific resistance to UV can be achieved by including carbon black in the compound.

Cables sheathed with PVC have good resistance to water but for prolonged or permanent immersion, an alternative sheathing material such as polyethylene is required. 

PVC has good resistance to chemicals but can be affected by solvents and some oils which tend to leach out the plasticiser and can cause the PVC to crack. Specific formulations can offer improved performance if required.

PVC compounds are classed as flame retardant, being self-extinguishing when the source of the flame is removed.

MDPE sheaths are popular as sheathings or coverings for both Energy and Telecommunication cables which are to be buried or drawn into ducts during installation. With the move away from lead sheaths Polyethylene is becoming the preferred sheathing material for installations where moisture or water are present continually, or for long periods of time. MDPE coverings also provide corrosion protection in installations where this is a problem. However, when used as a sheath or covering which will be exposed to UV, polyethylene sheaths must be compounded with carbon black or, if coloured, UV stabilisers.

Where increased resistance to radial water penetration is required, Tratos are able to supply cables with a “Glover Barrier” design, a thick polyethylene coated aluminium foil laminate bonded to an MDPE inner or outer sheath.

Low smoke, zero halogen sheaths are now the preferred choice for 33 kV cables which need improved performance in the event of a fire. This has seen requirements for low smoke emission, low corrosivity when burnt, flame retardance and reduced propagation become clearly defined and increasingly specified in many areas of cable installation. This has removed halogenated compounds from many markets to be replaced by LSZH – low smoke, zero halogen – types. 

Depending upon the construction and compliance with local regulations 33 kV cables can be installed in air, laid directly in ground, in cable ducts, indoors or outdoors.

TECHNICAL CHARACTERISTICS

• Voltage Rating Uo/U (Um) 19/33 (36) kV
• Test Voltage  4Uo (BS 7870-4.10) 
• Test Voltage 76 kV (BS 6622 and BS 7835) 
• Temperature Rating
• Fixed: -20°C to +90°C
• Short Circuit Temperature +250°C
• Minimum Bending Radius (BS 7879-4.1) 20 x overall diameter
• Minimum Bending Radius (BS 6622 and BS 7835) 
• 15 x overall diameter single core
• 12 x overall diameter three core

For an in-depth technical specification, please consult our detailed PDF specification sheets.

Tratos manufactures a comprehensive range of DNO-approved 33 kV cables designed to meet the stringent demands of UK distribution network operators, including UKPN, SSE, Western Power Distribution (WPD), Northern Power Grid (NPG), Scottish Power Energy Networks (SPEN), and Electricity Northwest (ENW). These cables are in strict compliance with the construction standards and specifications stipulated by each individual DNO. Our portfolio encompasses specialised cables, including the 33 kV Triplex formation cable, tailored specifically for use by DNOs.

Whether you require technical guidance or assistance in selecting the ideal 33 kV cable for your application, our experts are at your service. As an Independent Distribution Network Operator (iDNO), power grid installer, or contractor, you can rely on our DNO MV specialists for invaluable insights and recommendations. Please do not hesitate to reach out to us for further information, and we will be delighted to support you in meeting your 33kV cable and installation requirements.