Cable insulation is the non-conductive, extruded or wrapped polymer layer that electrically separates a live conductor from neighbouring conductors and from its surroundings. Four ratings decide which material is suitable: the continuous service temperature, the dielectric strength, the behaviour in a fire and the resistance to oil, chemicals and movement. PVC covers the standard case up to 70 degrees Celsius, cross-linked polyethylene reaches 90 degrees, silicone 180 degrees and PTFE 260 degrees. This article explains the layer structure of a cable, compares the insulating materials by their ratings and shows which flat insulating materials GOBA cuts and stamps from its range of insulation materials for the cable environment.
Core insulation and cable sheath are two different layers
The technical term covers two layers that are often confused in everyday use. The core insulation lies directly on the copper and handles the electrical separation, its wall thickness following the rated voltage. The cable sheath encloses all cores together and works mechanically: it takes abrasion, oil, UV radiation and tensile load. One material can do both jobs, PVC serves as core insulation and as sheath, but the compound differs considerably.
Both layers age faster than any other part of the cable. Copper barely changes over decades, while the polymer around it steadily loses plasticisers and molecular chains to heat, oxygen and UV light. When a cable fails, the insulation is almost always the reason.
Cable structure: five layers from the inside out
A multi-core cable consists, from the inside out, of the conductor, the core insulation, the stranding with filler elements, the shield and the sheath. Not every cable has all five layers, a simple installation cable does without a shield. The order stays the same.
Conductor
The conductor is usually bare or tinned copper, and aluminium in underground cables with large cross-sections. IEC 60228 sorts it into classes: class 1 is a solid single wire, class 2 a stranded conductor for fixed installation, class 5 a flexible and class 6 a highly flexible conductor for moving applications. The finer the strand, the smaller the permissible bending radius, see the limit values in the article on the minimum bending radius.
Core insulation
The core insulation sits directly on the conductor. Its wall thickness follows the voltage class, in a 0.6/1 kV cable it ranges from 0.6 to 2.0 millimetres depending on the cross-section. It also carries the core identification, in low voltage under DIN VDE 0293-308 with green-yellow for the protective conductor and blue for the neutral, and in control engineering often as printed numbers under DIN 47100. In medium and high voltage cables a semi-conductive inner layer sits between conductor and insulation and smooths the field peaks at the rough conductor surface.
Stranding and filler elements
Several cores are stranded together, either in pairs, in layers or as a star quad. The stranding keeps the cable flexible and, in data cables, cancels out the interference between neighbouring pairs. Filler elements made of polymer cords or fleece tapes close the gaps between the cores so that the cable stays round and the sheath rests at an even thickness. Between the stranding and the sheath there is usually a wrapping layer of film or fleece tape that holds the assembly together and separates the sheath from the core bundle when it is stripped.
Shield
The shield keeps electromagnetic interference away from the signal and drains the emissions of the cable itself to earth. Common designs are an aluminium-polymer laminate foil for high frequencies, a copper braid for low frequencies and mechanical robustness, or a combination of both. How much a shield attenuates depends on its coverage and its bonding to earth, the basics are covered in the article on EMC shielding.
Sheath
The outer sheath is the wear layer of the cable. It absorbs abrasion, tension, oil, cleaning agents and sunlight, and therefore determines the service life in the field more than the core insulation does. In a drag chain and on a robot arm polyurethane wins, in the ground polyethylene, in the control cabinet PVC and in a tunnel a halogen-free compound. In underground and medium voltage cables an armour of steel tape or steel wire often sits under the outer sheath to guard against excavation damage.
Cable insulation materials compared
The values below apply to typical commercial grades. The dielectric strength is given as a thickness-related value under IEC 60243, measured on thin samples. It falls as the wall thickness grows, so a cable with twice the insulation thickness does not withstand twice the voltage. The background and the test setup are covered in the article on dielectric strength.
| Material | Continuous service temperature | Dielectric strength | Characteristic | Typical use |
|---|---|---|---|---|
| PVC | -15 to 70 °C, special grades 105 °C | 20 to 40 kV/mm | low cost, flame retardant through chlorine, releases hydrogen chloride and dense smoke in a fire | installation cables, control cables, household appliances |
| PE | -40 to 70 °C | 25 to 40 kV/mm | permittivity of 2.3 and very low moisture uptake, burns readily without additives | coaxial and data cables, underground cables, telecommunications |
| XLPE | -40 to 90 °C, 250 °C under short circuit | 30 to 50 kV/mm | chemically cross-linked, stays dimensionally stable above the melting point | power cables from low to high voltage, photovoltaics |
| PUR | -40 to 90 °C | 20 to 35 kV/mm | highest abrasion resistance in the group, resistant to oil and microbes, almost always used as a sheath | drag chains, robot cables, construction and agricultural machinery |
| Silicone | -60 to 180 °C, 250 °C briefly | 20 to 25 kV/mm | stays soft in the cold, burns to an insulating silicon dioxide ash, sensitive to abrasion | motor connections, furnace and plant engineering, circuit integrity cables |
| PTFE | -190 to 260 °C | 40 to 80 kV/mm | chemically almost inert, paste extruded or tape wrapped and sintered | measurement and sensor cables, aerospace, chemical plants |
| FEP | -100 to 200 °C | 40 to 80 kV/mm | melt extrudable and therefore thinner walled than PTFE, optically clear | thin-wall high temperature cores, medical technology, semiconductor production |
| EPR / EPDM | -40 to 90 °C, special grades 150 °C | 20 to 30 kV/mm | rubber-elastic and ozone resistant, withstands 250 °C under short circuit | rubber-sheathed cables, medium voltage, mining, welding cables |
| Halogen-free compound (HFFR) | -30 to 90 °C | 20 to 30 kV/mm | mineral filled with aluminium hydroxide, therefore harder and more notch sensitive | public buildings, tunnels, rail vehicles, shipbuilding |
One point is regularly stated wrongly online: fluoropolymers are not halogen-free. Halogen-free means, under IEC 60754 and EN 50267, that a material releases no corrosive hydrogen halide gases in a fire. PTFE, FEP and PFA contain fluorine and release hydrogen fluoride under fire. Thermally and chemically they are first class, but they do not count as halogen-free in the sense of the cable standards. Polyethylene, polypropylene, EPR and the filled polyolefin compounds are halogen-free, PVC and the fluoropolymers are not. How a material is classified in a fire is described in the article on UL 94, and the European Construction Products Regulation additionally sorts cables into the euroclasses Aca to Fca under EN 50575.
Thermal classes, test voltage and insulation resistance
IEC 60085 sorts the permissible continuous temperature of an insulating material into thermal classes: class A at 105 degrees Celsius, E at 120, B at 130, F at 155, H at 180, N at 200 and R at 220 degrees Celsius. In cable engineering these classes appear less often than in motor design, where the conductor temperature is used directly, 70 degrees for PVC and 90 degrees for XLPE. The underlying logic is the same and is set out in the article on thermal classes of insulation. Anyone who permanently exceeds the limit temperature shortens the life of the insulation sharply, as a rule of thumb it halves for every 10 kelvin of excess.
Beside the continuous temperature, the short-circuit temperature counts. PVC withstands 160 degrees Celsius for a maximum of five seconds in a fault, XLPE and EPR take 250 degrees. This difference decides the permissible short-circuit current of a cross-section and is easily overlooked when two cables are compared. How quickly the heat leaves the conductor at all depends on the thermal conductivity of the insulation, which for all the polymers named lies between 0.15 and 0.4 watts per metre and kelvin, roughly a thousand times lower than copper.
The insulation is tested in two ways. In the factory, IEC 60502-1 requires a routine alternating voltage test over five minutes for cables from 1 to 30 kV, set at 3.5 kV for the 0.6/1 kV voltage class. In the finished installation, the insulation resistance is measured with direct voltage instead. DIN VDE 0100-600 states clear limits: at least 0.5 megohms at 250 volts test voltage for SELV and PELV circuits, at least 1.0 megohms at 500 volts for circuits up to 500 volts rated voltage and at least 1.0 megohms at 1,000 volts above that. Readings that fall over the years reveal moisture in the cable or aged insulation long before a breakdown occurs. At dirty terminals and cable ends a second ageing path is added, the surface leakage current across the surface.
Which material to choose? Four questions in this order
The harshest load at the installation point decides the material choice. We work through it in this order, because each question narrows the selection more than the next one.
- Temperature first. The highest continuous temperature at the installation point rules materials out hard. Above 70 degrees standard PVC is gone, above 90 degrees XLPE and PUR follow, above 180 degrees only PTFE and FEP remain. For anything above 155 degrees it is worth looking at high-temperature insulation.
- Oil and chemicals next. Oil attacks PVC and softens it, PUR and EPR hold up, PTFE resists practically everything. In hydraulic environments and in machining with cutting fluid, this is the second hard filter.
- Movement and bending cycles third. In a drag chain what counts is the abrasion resistance of the sheath and the recovery of the core insulation. A PUR sheath over class 6 highly flexible conductors reaches several million bending cycles, a PVC sheath breaks considerably earlier.
- Fire protection last, but not negotiable. Wherever people escape or equipment has to keep running in a fire, halogen-free materials and proof under EN 50575 are mandatory. Silicone is the only material in the table that keeps an insulating ash structure after burning, which is what makes circuit integrity cables possible.
Our recommendation for the standard case in the control cabinet and in the machine: XLPE as core insulation instead of PVC. The surcharge is in the low single-digit percent range, while the margin from 70 to 90 degrees continuous temperature and from 160 to 250 degrees under short circuit is almost always worth that price. PVC remains sensible where it stays cool, where nobody moves the cable and where the price per metre outranks everything else. For polyethylene in its cross-linked and uncross-linked forms one rule holds: it is the most versatile cable material of all, with a clear weakness in abrasion and in UV light without a carbon black additive.
Cable insulation in practice
In an electric vehicle, orange marked high-voltage cables carry up to 800 volts DC between the battery, the power electronics and the motor. Silicone or cross-linked polyolefin insulation works there under engine bay temperatures and vibration, flanked by flat insulating parts at connectors, bushings and battery modules, as described in the article on high-voltage insulation. Inside the battery module, films and cut parts take over the rest, see battery insulation.
At the motor connection, the cable insulation meets the insulation of the winding. The cable ends in the terminal box, and from there slot insulation, phase separation and layer insulation made of Nomex, polyester film or laminates take over. In major household appliances the 3 mm rule of IEC 60335-1 applies at this point as well, as set out in the article on white goods. In the control cabinet the energy continues over busbars, whose insulation follows different rules than a round cable, as set out under busbar insulation. In photovoltaic plants and wind farms, cables lie outdoors for more than twenty years, and there UV and ozone resistance under EN 50618 decides the service life rather than the dielectric strength. Where a cable has to be protected or separated after the fact, an insulation sleeve supplements the existing insulation.
What GOBA supplies around the cable, and what it does not
GOBA does not extrude cable insulation. We make neither core insulation nor cable sheaths and we do not sell cable by the metre. This boundary is stated early on purpose, because it saves both sides time.
What we do supply are the flat insulating materials that work in the same cable environment. From our range of insulation materials in classes A to C we cut and stamp wrapping and separating layers, insulating films for bushings and terminal compartments, cut parts for wiring harnesses and connector areas, and carrier films for cable wrapping tapes. The materials we process include polyester film, Nomex, Kapton, PTFE film and laminates such as NMN and DMD.
In contract slitting we produce slit rolls from 5 to 1,600 millimetres wide with a width tolerance of plus minus 0.1 millimetres, wound onto cores with an inner diameter of 25, 55, 76 or 152 millimetres. For shielding we slit copper and aluminium foils from 6 to 200 micrometres thick, including laminated PET-aluminium composite films of the kind used as a foil shield under the cable sheath. The details are on the page about slitting EMC shielding film. For drives and battery systems we run the material programme for electromobility and automotive.
The most common objection at this point: if you do not make the cable, why talk to you at all. Because the flat parts around the cable look small on the bill of materials and become expensive when they fail. A separating layer of the wrong size in the terminal compartment, a wrapping layer from the wrong thermal class, a film cut with a frayed edge at a bushing: nobody likes ordering those in batches of 200, and those are exactly what we have been making since 1959. Anyone buying cable by the kilometre belongs with a cable manufacturer. Anyone who needs cut parts, wrapping layers and shielding foils in reproducible quality belongs with us.
GOBA Takeaway
Cable insulation stands or falls on four numbers: continuous temperature, short-circuit temperature, dielectric strength and fire behaviour. Anyone who knows those four for their installation point has made the material choice in ten minutes, usually in favour of XLPE for the standard case, PUR in movement, silicone in the heat and a halogen-free compound in a public building. We supply the flat insulating materials around the cable, cut and stamped to your drawing. Send us the material, the dimensions and the tolerance through custom products and you will usually have an answer the same day.
