MV cables XRUHAKXS and NA2XS(F)2Y — how they are built and how to read them

Why single-core cables dominate MV networks

For decades, medium-voltage cable lines used three-core cables with paper-oil insulation and later PVC insulation. Today, new 15 and 20 kV lines are almost always designed with three single-core cables insulated with cross-linked polyethylene (XLPE), laid side by side or bundled in a trefoil formation.

There are several reasons for this:

• Easier transport and installation. A single-core cable is lighter and more flexible than a three-core cable of the same cross-section, and longer lengths fit on a drum. Fewer connections mean fewer joints along the route.
• Simpler accessory installation. Joints and terminations for single-core XLPE cables are available in heat-shrink and cold-shrink versions and are installed in a repeatable way.
• Good XLPE insulation properties. Cross-linked polyethylene tolerates high conductor operating temperatures, has low dielectric losses and needs no oil impregnation.
• Flexibility in a fault. Damage to one cable means repairing one phase, not an entire multi-core cable.

In Poland you will find two “families” of designations for these cables: Polish ones such as XRUHAKXS and German (VDE-style) ones such as NA2XS(F)2Y. Structurally the cables are very similar; they differ mainly in how they are named and in manufacturer-specific details.

Construction of a single-core MV cable, layer by layer

Whatever the designation, an XLPE-insulated MV cable has a similar layer structure. From the centre outwards:

1. Conductor — usually stranded, compacted aluminium. In longitudinally watertight versions the gaps between the strands may also be filled with a water-blocking material.
2. Conductor screen — a thin semiconducting layer that smooths the electric field at the conductor–insulation interface.
3. XLPE insulation — the main insulating layer. Its thickness depends on the cable's rated voltage.
4. Insulation screen — a second semiconducting layer, usually easy to strip (“peelable”), which makes installing accessories easier.
5. Water-blocking layer — tapes that swell on contact with water. If the sheath is damaged, the tape swells and limits the spread of moisture along the cable.
6. Metallic screen — copper wires, often with a counter-helical copper tape. It carries capacitive and fault currents and provides touch safety.
7. Outer sheath — polyethylene in both cable types discussed here, resistant to moisture and mechanical damage when laid in the ground.

Every one of these layers matters when joints and terminations are installed: the jointer has to expose them in turn over precise lengths while keeping everything clean and geometrically correct. The quality of that work largely determines how reliable the line will be in service.

How to read the XRUHAKXS designation

The Polish cable designation system uses letters describing successive elements of the construction. XRUHAKXS can be read as follows:

Part of designation	Meaning
X	cross-linked polyethylene (XLPE) insulation
RU	codes relating to longitudinal water sealing of the construction — the exact extent (for example conductor and screen area) is defined by the manufacturer in the data sheet
H	screened cable (insulation screen and metallic screen)
A	aluminium conductor
K	power cable
XS	polyethylene outer sheath

The full designation also gives the number of cores, cross-sections and rated voltage, e.g. XRUHAKXS 1×150/25 mm² 12/20 kV. Because individual manufacturers may interpret the letters and build the cable slightly differently, always check the data sheet of the specific cable — especially when selecting accessories.

How to read the NA2XS(F)2Y designation

NA2XS(F)2Y comes from the German VDE system, which is widely used across Europe. It reads as follows:

Part of designation	Meaning
N	cable made to a standard (standard-type cable)
A	aluminium conductor (no letter means copper)
2X	cross-linked polyethylene (XLPE) insulation
S	copper screen
(F)	longitudinally watertight screen area (swelling tapes)
2Y	polyethylene (PE) sheath

Related designations follow the same logic: NA2XS2Y (no longitudinal water sealing), NA2XSY (PVC sheath) and N2XS(F)2Y (copper conductor). Once you know the system, it is easy to tell these variants apart.

Conductor and screen sizes, and voltage ratings

The notation 1×150/25 mm² means a single-core cable with a 150 mm² main conductor and a 25 mm² copper screen. The conductor size determines the line's current-carrying capacity and voltage drop, while the screen size determines what fault current the screen can safely carry for a short time.

Distribution MV networks most commonly use aluminium conductors from 70 to 240 mm², occasionally larger. Screens are typically 16, 25, 35 or 50 mm². Sizing is the designer's job, taking into account load, installation conditions, the operator's fault-current requirements and room for future network development.

A cable's rated voltage is written as U₀/U, where U₀ is the voltage between conductor and screen (earth) and U is the phase-to-phase voltage. The most common levels are:

Cable rated voltage U0/U	Typical use
6/10 kV	6 and 10 kV networks, often in industrial plants
8.7/15 kV	15 kV networks
12/20 kV	20 kV networks, and 15 kV networks where the operator plans a future move to 20 kV
18/30 kV	30 kV networks, including the internal networks of some wind farms

Which rated voltage to use in a given network follows from the operator's requirements and the design — in practice many DSOs standardise on 12/20 kV cables even for lines operating at 15 kV.

XRUHAKXS or NA2XS(F)2Y — the practical differences

From a contractor's point of view the two cable types are very similar: aluminium conductor, XLPE insulation, water-blocked copper screen and polyethylene sheath. The differences lie mainly in:

• the designation system and documentation — a VDE-designated cable is often easier to compare with offers from foreign manufacturers;
• construction details — for example the extent of longitudinal sealing (whether it covers the conductor too), the screen design or the sheath thickness used by a particular manufacturer;
• the requirements of a specific operator — DSOs' technical standards often list approved cable and accessory types, and the designer selects the cable accordingly.

For an investor, what matters more than the name is that the cable, accessories (joints and terminations) and installation method form a consistent system that matches the design. A joint or termination must suit the specific construction, conductor size, insulation diameter and screen type.

Laying MV cables — what we pay attention to

The quality of a cable line depends not just on the cable but above all on how it is laid. On site, the key points are:

• Bending radius — must not be smaller than the manufacturer's minimum (for MV cables usually a dozen or more times the cable's outer diameter). Bending too tightly can damage the insulation or screen.
• Pulling tension — when a cable is pulled by winch, the force has to be controlled so as not to exceed the permissible values for the conductor. This is done with hydraulic pullers with tension control — in our fleet, a Tesmec ARS405.
• Installation temperature — the manufacturer specifies the minimum temperature at which the cable may be uncoiled and laid. Below it, the sheath and insulation lose flexibility.
• Bedding, backfill and marking — the cable is laid on a layer of sand or suitable soil, covered, and a warning tape is placed above it. Laying depth is set by the design in line with the operator's requirements.
• Protective ducts — under roads, in drilled crossings and where the route crosses other utilities, the cable runs in ducts. The duct diameter must allow the cable to be pulled in freely.
• Sealed ends — cable ends awaiting joints or terminations are protected with end caps to keep moisture out.

Once laid and fitted with accessories, the line goes through acceptance testing, including insulation tests and sheath voltage tests — more on this in From design to acceptance.

Cable accessories: joints and terminations

The cable itself is only half the line. The other half is the accessories that connect cable lengths and terminate them at equipment. MV lines with single-core XLPE cables mainly use:

• Straight joints — connect two lengths of the same cable type, for example where one drum length ends. On a three-phase line of single-core cables, a set of three joints is installed at each location.
• Transition joints — connect cables of different construction, for example new XLPE cable to older paper-insulated cable, which is common when rebuilding existing networks.
• Indoor terminations — cable ends in switchgear inside buildings and substations.
• Outdoor terminations — cable ends on a pole where the cable line changes to overhead line, usually installed with surge arresters.
• Separable connectors — screened plugs pushed onto the bushings of compact switchgear, for example in ZKSN junction boxes or packaged substations.

Accessories are usually heat-shrink (components shrunk with a torch) or cold-shrink (components held expanded on a spiral core that is removed once in position). Every kit comes with manufacturer's instructions specifying stripping lengths for each layer, how to clean the insulation and how to connect the screen. Accuracy and cleanliness decide whether a joint will run trouble-free for many years — which is why accessory installation is entrusted to experienced jointers.

XRUHAKXS and NA2XS(F)2Y on our projects

We have laid both cable types on contracts confirmed by our clients' reference letters:

Project	Cable	Length
Toruń — undergrounding of the 15 kV line GPZ Toruń Podgórz – Włocławek 3	3×XRUHAKXS 1×150 mm²	5,725 m (route length)
Konin — RS Maliniec 15 kV switching station	3×XRUHAKXS 1×240/50 mm² (6 sections) and 3×XRUHAKXS 1×240/25 mm²	2,890 m in total and 2,520 m
MV/LV network rebuild in 8 villages	3×NA2XS(F)2Y 1×150/25 mm²	5,606 m (route length)

In Toruń and Konin, besides laying the cables, we installed MV cable joints and terminations — connector, indoor and outdoor — and carried out directional drilling under obstacles. On the network rebuild around Międzychód, the NA2XS(F)2Y cable ran partly in protective ducts installed by directional drilling and pipe jacking.

If you are preparing a project with an MV cable line, see how we deliver MV and LV cable lines, or look up terms in our glossary.