Selection Guide · 11 min read · Updated 2026-10-12
Medium Voltage Cable Construction Explained: Screens, Stranding and Why MV Is Not Big LV
A medium voltage cable is not a low voltage cable with thicker insulation. Above about 3 kV the electric field inside the cable becomes the design problem, and the construction changes to control it: two semi-conductive screens, a much cleaner insulation, and a metallic screen sized for fault current. This article explains what each layer is for, what partial discharge is and why it kills MV cables, and which tests in IEC 60502-2 to ask a supplier to evidence.
The electric field problem that LV does not have
In a cable the electric stress is not uniform across the insulation. It is a radial field, and its maximum value sits right at the surface of the conductor, where the circumference is smallest. The expression is E = U / (r × ln(R/r)), with U the phase-to-earth voltage, r the conductor radius and R the outer radius of the insulation.
Put numbers on it. A 12/20 kV cable with a 240 mm² conductor has a conductor radius of about 5.6 mm and an insulation thickness of 5.5 mm, so R is 11.1 mm and the phase-to-earth voltage is 12 kV. Then E = 12000 / (5.6 × ln(11.1/5.6)) = 12000 / (5.6 × 0.684) = 3.13 kV/mm. A 0.6/1 kV cable with a 2.5 mm² conductor, radius 1.0 mm and 0.7 mm of insulation, works out at 600 / (1.0 × ln(1.7/1.0)) = 600 / 0.531 = 1.13 kV/mm. The MV cable is running at nearly three times the peak stress, continuously, for forty years, and it also has to survive switching surges on top of that.
- Radial stress peaks at the conductor surface, not across the insulation thickness
- MV designs are typically limited to a maximum working stress of a few kV/mm, which is why insulation thickness does not scale linearly with voltage
- Air breaks down at roughly 3 kV/mm at atmospheric pressure, so any void inside an MV cable is a site of discharge
- The same physics is why MV cable needs screens and LV cable does not
Conductor: stranding, compaction and surface finish
MV conductors are almost always Class 2 stranded copper to IEC 60228, compacted before the screens go on. Compacting presses the strands together so that the gaps between them are closed and the outer surface becomes smooth rather than ridged. The effect on the finished cable is a smaller overall diameter for the same cross-section, less compound in the screens and insulation, and a conductor surface that does not imprint its ridges into the layer above it.
It matters so much because the next layer out is in direct contact with the conductor. A ridged conductor presents sharp points, and a sharp point concentrates the electric field exactly where the insulation is thinnest. For the very large conductor sizes used on MV distribution, from about 800 mm² upward, the conductor may be built as a Milliken or segmental construction, in which the conductor is split into insulated segments to reduce the skin effect and make the current distribution across the section more even.
The conductor screen: a layer you cannot see
Between the conductor and the XLPE there is a thin layer of black material that most people never notice, and it is the single most important layer in the cable. It is a semi-conductive compound, a polyethylene loaded with carbon black so that it conducts a little, and it is extruded, not lapped. Its job is to make the inner surface of the insulation electrically smooth.
Without it, the air in the gaps between the conductor strands and the insulation would be stressed to the field calculated above, and at that stress the air would break down. Each breakdown event is a tiny discharge that erodes the insulation a little, and over years the erosion grows into a tree-shaped channel that eventually bridges the insulation. The semi-conductive layer fills those gaps, carries the same potential as the conductor, and removes the voids from the circuit entirely. Because it is in contact with both the conductor and the insulation, it is co-extruded with the insulation and the insulation screen in a single triple extrusion head, then cross-linked together in a continuous vulcanisation line.
Insulation: XLPE, and why cleanliness decides the life of the cable
The insulation is XLPE, cross-linked polyethylene, and in an MV cable the requirement on it is not just electrical strength but freedom from inclusions. A particle of dust, a speck of metal, or a microvoid a few tens of micrometres across is enough to start partial discharge inside the insulation once the cable is energised. Manufacturing therefore controls the compound and the extrusion environment as carefully as it controls the dimensions.
Cross-linking is done in a continuous vulcanisation line, either catenary, where the cable hangs in a long curve, or vertical, where it travels straight up and down through the heating and cooling zones. Both produce the same cross-linked material; the vertical line supports the heavier cables with large conductors without the risk of the hot conductor deforming under its own weight and drifting off centre. There is one further requirement after cross-linking: the insulation must be checked for cross-linking completeness with the hot set test, and for MV work the insulation must also be free of the by-products of the cross-linking reaction, which is one reason manufacturers hold cable before testing it.
| Rated voltage U0/U (kV) | Typical conductor size in the table | Nominal insulation thickness (mm) |
|---|---|---|
| 3.6/6 | 50 mm² | 2.5 |
| 6/10 | 95 mm² | 3.4 |
| 8.7/15 | 150 mm² | 4.5 |
| 12/20 | 240 mm² | 5.5 |
| 18/30 | 400 mm² | 8.0 |
The insulation screen: strippable versus bonded
Outside the insulation there is a second semi-conductive layer. It is there for two reasons: it confines the electric field to the insulation by providing a smooth, defined equipotential surface at the outer radius, and it provides the surface against which the metallic screen sits. It is co-extruded with the other two layers, so the three leave the extrusion head as one bonded assembly.
The practical question is whether that outer screen can be removed for jointing, and there are two answers. A strippable screen is formulated to peel off the insulation cleanly, which makes terminations and joints quick and is the common choice up to about 18/30 kV. A bonded screen is chemically fused to the insulation and has to be removed by abrasion and heat, which takes longer but leaves a stronger interface with far better partial discharge performance at the cut. Where a joint is badly made, the interface between the insulation and its screen is where it fails, so the choice is usually made on the specification of the jointing system as much as on the cable.
- Strippable: peels off, faster jointing, standard for most MV distribution work
- Bonded: stronger interface, better partial discharge performance, needs abrasion and heat to remove
- The three inner layers are co-extruded and cross-linked together, whatever the outer screen type
- Screen removal is the most common site of an MV joint failure; train the jointers or use a qualified contractor
The metallic screen: copper tape versus copper wires
Outside the semi-conductive insulation screen sits the metallic layer, which has three jobs: it carries the earth fault current, it provides a defined return path for the capacitive charging current, and it contains the electric field inside the cable. It is normally earthed at both ends of the circuit, which keeps the screen at earth potential and ensures that any fault current has a metallic path to operate the protection.
There are two common forms. Copper tape is a thin tape, typically 0.1 mm, applied helically with an overlap; it is economical and adequate for the fault levels of many distribution networks. Copper wires, laid helically with an open counter-wound copper tape to hold them, give a much larger metallic cross-section and are used where the fault level is higher or where the cable has to carry a higher charging current. The short-circuit rating of the screen is quoted by the manufacturer for each construction; a 25 mm² equivalent copper wire screen is typically rated in the region of 2 to 3 kA for 1 s, and the actual figure for the cable being bought should come from its datasheet rather than from a rule of thumb.
| Property | Copper tape screen | Copper wire screen |
|---|---|---|
| Typical construction | 0.1 mm tape, helically applied with overlap | Wires plus an open counter-wound copper tape |
| Fault current capacity | Lower, adequate for many distribution networks | Higher; sizes commonly 16, 25, 35 and 50 mm² equivalent |
| Flexibility | Stiffer | More flexible |
| Use with single-core cables | Both are non-magnetic, so both are usable | Preferred for higher fault levels |
| Cost | Lower | Higher |
| Typical application | Distribution cable at moderate fault level | Primary substation feeders, high fault level networks |
Partial discharge: the failure mechanism MV cables are designed against
Partial discharge is a small electrical breakdown that does not bridge the whole insulation. It happens inside a void, at a contaminant, or at a protrusion, where the local field exceeds what that tiny region can withstand. Each discharge is only a few picocoulombs, but each one erodes a little of the surrounding insulation, and the channels they carve grow over years until the insulation is bridged and the cable fails. It is the dominant ageing mechanism in extruded MV cable, and it is invisible from the outside.
This is why IEC 60502-2 includes a partial discharge test on the complete cable, and why the acceptance limit is set so low. The test is applied at 1.73 U0 and the discharge magnitude must not exceed 10 pC. A cable that is well made with clean, void-free, triple-extruded layers measures close to the noise floor of the test set; a cable with a void or a contaminant shows a clear signal at a level that no amount of extra insulation thickness will cure. The test is the single most informative item on an MV test report, and it is the one to ask for by name.
- Partial discharge erodes insulation from the inside and eventually bridges it
- IEC 60502-2 requires the discharge magnitude at 1.73 U0 to be no more than 10 pC
- Extra insulation thickness does not fix a partial discharge problem; only manufacturing quality does
- Water treeing is a related mechanism in the presence of moisture; modern tree-retardant XLPE and water-blocking constructions address it
What IEC 60502-2 requires, and what to ask for
IEC 60502-2 covers power cables with extruded insulation from 3.6/6 kV up to 18/30 kV; above that range the standards are IEC 60840 and then IEC 62067. It specifies the construction, the dimensions, and the type, sample and routine tests, and a cable that conforms to it will carry the voltage designation, the construction, the standard number and the manufacturer on its sheath.
When you evaluate an offer, the documents matter as much as the cable. Ask for the partial discharge test record with the measured values, the routine AC voltage test record, the measured minimum and average insulation thickness, the hot set result for the insulation, and the short-circuit rating of the metallic screen. A supplier who has the line and the laboratory will send these as a matter of course; one who has to chase them will take a long time to answer.
| Test | What it demonstrates | What to look for on the report |
|---|---|---|
| Partial discharge, IEC 60502-2 | Freedom from voids and contaminants in the insulation | Measured magnitude at 1.73 U0, not exceeding 10 pC |
| Routine AC voltage test | The insulation withstands the rated voltage with margin | Applied at 3.5 U0 for 5 minutes, with the result recorded |
| Conductor resistance, IEC 60228 | The conductor cross-section is what it claims to be | Measured value at 20 °C against the maximum for Class 2 |
| Insulation thickness measurement | The thin spot meets the standard, and the wall is concentric | Minimum and average thickness, and the eccentricity |
| Hot set test on the insulation | The XLPE is properly cross-linked | Elongation under load and permanent set within the standard limits |
| Screen short-circuit rating | The screen survives the network fault level | The kA rating for 1 s for the exact construction offered |
Frequently asked questions
Why does MV cable have semi-conductive screens and LV cable not?
Because the electric stress at the conductor surface is far higher. A 12/20 kV cable runs at about 3.1 kV/mm against 1.1 kV/mm for a 0.6/1 kV cable, and air breaks down at roughly 3 kV/mm. Without screens, the voids between the conductor strands would break down and erode the insulation. The screens fill those voids and make the inner and outer surfaces smooth.
What is a partial discharge test and what is the limit?
It measures the small discharges inside voids or at contaminants in the insulation, which erode the cable from the inside over years. IEC 60502-2 requires the magnitude not to exceed 10 pC at a test voltage of 1.73 U0. Ask for the measured value rather than a pass statement, because a well made cable reads close to the noise floor of the test set.
Copper tape or copper wire screen for MV cable?
Copper tape, typically 0.1 mm and applied with an overlap, is economical and suits moderate fault levels. Copper wires with a counter-wound tape give a larger metallic cross-section and are used where the network fault level is higher. Match the screen to the fault level and the required short-circuit rating from the manufacturer's table, in kA for 1 s.
What voltage range does IEC 60502-2 cover?
It covers extruded insulated power cables from 3.6/6 kV up to 18/30 kV, which is the usual medium voltage distribution range. Higher voltages move to IEC 60840 for 30 kV to 150 kV and IEC 62067 above that. The standard sets the construction, the dimensions and the type, sample and routine tests.
Can MV cable be installed in the same way as LV cable?
The pulling and handling rules are similar, but the jointing and terminating are not. The semi-conductive screen has to be removed to a defined length, the cut has to be clean and smooth, and the stress control at the termination has to match the cable. MV terminations are a specialist task, and a badly made one is the most common cause of an early MV cable failure.
Need this worked out for a specific installation?
Send load current, route length, installation method, ambient temperature and the applicable standard — we will return a sized construction with the volt-drop and derating arithmetic shown.