Selection Guide · 11 min read · Updated 2026-10-12
Common Cable Manufacturing Defects and How to Inspect for Them
The defects that cost money in a cable are the ones you cannot see: a conductor a few per cent under size, insulation that is thin on one side, a void inside the wall, a compound that was never properly cross-linked. All of them are detectable, and each has a specific test that finds it and a specific number that settles the argument. This article lists them in order of how much they cost the buyer, with the arithmetic behind the checks that catch them.
Most defects in a cable cannot be seen
A cable is a concentric assembly of layers, and only the outermost one is visible. Every one of the serious defects hides behind the sheath: the conductor, the thickness of the wall at its thinnest point, the voids inside the insulation, and the chemistry of the compound. A cable can look undamaged on the drum, measure the correct overall diameter, and still be a cable that will fail in service or that does not carry the current its marking claims.
Buyers ask about this for a specific reason, and it is a reasonable one. The commercial pressure in the industry is on the two materials that cost the most, copper and compound, and both can be reduced in ways that pass a visual inspection. The good news is that each of those reductions leaves a measurable trace, and the tests that find them are standardised, cheap and quick. What follows is the list in the order that matters.
- Reduced conductor cross-section, the most serious because it is deliberate and it changes the ampacity
- Insulation thinner than the standard allows, or eccentric, so the thin side is where the failure starts
- Voids and contaminants in the insulation, which start partial discharge at medium voltage
- A compound that was not properly cross-linked, or was substituted with a cheaper grade
- Short length on the drum, which is not a defect in the cable but is a defect in the delivery
Reduced cross-section: the defect with a number on it
This is the one buyers ask about most, and it exists for a straightforward reason: copper is the largest cost in a power cable and a few per cent of it is a few per cent of the selling price. It is done in several ways. The strand count can be reduced while the individual wire diameter is kept. The wires can all be drawn slightly smaller. Or the conductor can be compacted harder than the standard intends, which reduces the diameter and the amount of compound used without changing the marking on the drum.
The consequence is not cosmetic. A 10 % reduction in cross-section is an 11 % increase in resistance, which is 11 % more heat for the same current, and it is a direct reduction in the short-circuit withstand because the adiabatic equation works on the actual area. The cable still looks right, still fits the gland, and still carries a marking saying 4 × 95. There are two ways to catch it, and they are complementary: measure the resistance, and weigh the conductor.
| Nominal cross-section (mm²) | Nominal copper mass (g/m of conductor) | Mass if the area is 5 % short (g/m) | Mass if the area is 10 % short (g/m) |
|---|---|---|---|
| 4 | 35.6 | 33.8 | 32.0 |
| 16 | 142.2 | 135.1 | 128.0 |
| 50 | 444.5 | 422.3 | 400.1 |
| 95 | 844.6 | 802.4 | 760.1 |
| 240 | 2133.6 | 2026.9 | 1920.2 |
The DC resistance check: what it catches and what it misses
Resistance is the reference test, and it is the one in IEC 60228. The measurement has to be made with a four-terminal method, because the resistance of a short length of conductor is in the milliohm range and the resistance of the test leads is not; a two-terminal measurement on 1 m of 240 mm² cable, which is about 75 µΩ, measures the leads and the contact resistance more than it measures the cable. Use a Kelvin connection, a micro-ohmmeter, and a sample long enough for the instrument: 1 m for small sizes, 5 or 10 m for large ones.
Temperature is the part that is most often mishandled, and it is the part that decides whether the result means anything. Copper changes by 0.393 % per kelvin, so a sample measured at 30 °C and not corrected reads 3.9 % high, which is precisely the size of the fraud being looked for. Correct to 20 °C with R₂₀ = Rt / (1 + 0.00393 × (t − 20)), and measure the temperature on the conductor, not in the room. With that done, and with a good instrument, a competent laboratory detects an area reduction of about 2 %; in practical conditions, with the temperature known to a couple of degrees, the reliable threshold is 3 to 5 %.
| Cross-section (mm²) | Maximum R at 20 °C (Ω/km) | R for a nominal cross-section (Ω/km) | Area reduction the tolerance permits |
|---|---|---|---|
| 1.5 | 12.1 | 11.5 | about 5 % |
| 4 | 4.61 | 4.31 | about 6.5 % |
| 16 | 1.15 | 1.08 | about 6 % |
| 50 | 0.387 | 0.345 | about 11 % |
| 95 | 0.193 | 0.182 | about 6 % |
| 240 | 0.0754 | 0.0718 | about 5 % |
| 400 | 0.0470 | 0.0431 | about 8 % |
- The resistance check catches every way of reducing the conductor area, including copper-clad aluminium and copper-clad steel
- It does not catch short length on the drum: resistance per kilometre is normal on a short drum
- It does not catch a strand count below the standard's minimum if the total area is still correct
- It does not catch compound substitution, thin or eccentric insulation, or a fake sheath material
- A result sitting exactly on the standard's maximum is a warning, because a well-made conductor measures several per cent below it
Eccentricity and minimum insulation thickness
Insulation is extruded over a moving conductor, and if the conductor is not held exactly central in the die the wall comes out thicker on one side and thinner on the other. The overall diameter is unaffected, which is why an eccentric cable passes every check that measures the outside. Two numbers describe it: the minimum thickness, and the eccentricity itself, which is the difference between the thickest and thinnest points as a proportion of the thickest.
The electrical consequence is concentrated at the thin side. The electric stress is highest at the conductor surface, and where the wall is thin the stress per millimetre is higher still; that is where the insulation ages fastest and, at medium voltage, where partial discharge begins. It is also where a spark test finds the cable, and where a joint made with a mechanical tool cuts through. IEC 60502-1 sets the minimum thickness at 0.9 times the nominal minus 0.1 mm, and an eccentricity limit of about 15 % is commonly applied alongside it.
| Measurement | Cable A | Cable B |
|---|---|---|
| Maximum thickness measured (mm) | 0.78 | 0.85 |
| Minimum thickness measured (mm) | 0.70 | 0.65 |
| Minimum allowed, 0.9 × 0.7 − 0.1 (mm) | 0.53 | 0.53 |
| Eccentricity, (max − min) / max | 10 % | 24 % |
| Result against a 15 % eccentricity limit | Pass | Fail, although the minimum thickness passed |
Voids and contaminants in the insulation
A void is a small gap inside the insulation or at its interface with a screen, and a contaminant is a particle that should not be there. Both matter for the same reason: at the electric stresses present in medium voltage cable, the gas inside a void breaks down long before the insulation around it does, and each breakdown erodes a little more. This is the partial discharge mechanism, and it is why IEC 60502-2 sets the acceptance level at no more than 10 pC at 1.73 U0.
It is worth being precise about where this matters. In a 0.6/1 kV cable the working stress is around 1.1 kV/mm, and a small void is usually harmless for the whole service life. In a 12/20 kV cable the stress is around 3.1 kV/mm and the same void is the start of the failure. So voids are inspected for in two different places: on the production line by spark testing, per IEC 62230, which catches pinholes that reach the surface during extrusion and is applied to every metre; and in the laboratory by partial discharge testing, which finds the voids that are buried and is applied to medium voltage cable.
Strand count, compaction and a diameter that looks right
A conductor is made of wires, and the standard specifies both the resistance and the minimum number of them. The two are independent requirements, and a cable can satisfy one while failing the other. A conductor with fewer, thicker wires can meet the resistance limit and still be non-compliant, and it will also be stiffer and less flexible than the cable the designer specified. The check is quick: count the wires, measure one of them, and compute the area as the number of wires × πd²/4.
Compaction is the related issue. A compacted conductor has its wires pressed together so the section is closer to a solid circle, which gives a smaller overall diameter for the same cross-section and uses less of everything extruded over it. That is legitimate and it is done deliberately. What is not legitimate is using the smaller diameter of a compacted conductor as cover for a conductor that is genuinely undersized, and the two are separated by the resistance and mass checks rather than by looking at it.
| Cross-section (mm²) | Minimum number of wires | Typical actual construction |
|---|---|---|
| 16 | 7 | 7 wires |
| 25 | 7 | 7 wires |
| 35 | 7 | 7 to 19 wires |
| 50 | 19 | 19 wires, often compacted |
| 95 | 19 | 19 wires, compacted |
| 120 | 37 | 37 wires, compacted |
| 240 | 61 | 61 wires, compacted |
| 400 | 61 | 61 wires, compacted |
Ageing and cross-linking: what the compound tests reveal
A compound can be substituted or under-processed in ways that no dimensional check finds. The two tests that matter are the ageing test and, for XLPE, the hot set test, and they answer two different questions. The hot set test asks whether the insulation was properly cross-linked: a sample is held at 200 °C under a load of 20 N/cm² for 15 minutes, and it must not stretch more than 175 % and must not retain more than 15 % permanent set afterwards. A cable that has not been through the cross-linking line for long enough will fail this, and it will then deform under load in service.
The ageing test asks what the material will be like after it has been hot for a long time. Samples are held in an air oven, commonly 168 hours at 135 °C for XLPE and 168 hours at 80 °C for PVC, and the tensile strength and elongation at break are measured before and after. The results have to stay within the variation the standard permits, which is typically ±25 % of the unaged values. A compound with too much filler, or the wrong grade, or one that has been reprocessed, shows up here even though every dimension was correct.
| Test | Conditions | Criterion | What a failure means |
|---|---|---|---|
| Hot set, IEC 60811-507 | 200 °C, 15 minutes, 20 N/cm² | Elongation under load 175 % or less, permanent set 15 % or less | The XLPE was not properly cross-linked |
| Ageing in an air oven, IEC 60811-401 | 168 h at 135 °C for XLPE, 168 h at 80 °C for PVC | Tensile strength and elongation within typically ±25 % of the unaged values | The compound is the wrong grade or has degraded |
| Tensile and elongation before ageing | Room temperature | Minimum values per the cable standard | A weak or overfilled compound |
| Insulation resistance | At the maximum operating temperature | Insulation resistance constant of at least 3.67 MΩ·km for XLPE at 90 °C | A contaminated or damp compound |
| Water absorption | Gravimetric, per the standard | Within the stated mass increase | A compound that will not survive wet locations |
The inspection ladder, and what to demand from a supplier
No single test covers everything, so the practical approach is a short list of cheap checks applied to every delivery, plus the deeper tests applied when there is a reason. The cheapest two are resistance and mass, and between them they settle the question that matters most. Reading the metre mark on the sheath settles the length. Everything else is a laboratory test, and the time to ask for it is before the order is placed, not after the container has landed.
What to demand is simple and it costs the supplier nothing if the cable is right: a retained sealed sample from the actual production batch, the mill certificate tied to the drum numbers, the right to have the testing witnessed, and a stated length tolerance in the contract. A supplier who produces to a standard will have these already. The documents themselves, the inspection arrangements and the commercial terms around them are set out in full in the import guide in this series.
| Defect | Test that catches it | Where | Cost and time |
|---|---|---|---|
| Reduced cross-section | DC resistance at 20 °C, four-terminal | Any electrical laboratory | Low, minutes |
| Reduced cross-section, independent check | Mass of the stripped conductor | Any scales | Low, minutes |
| Short length on the drum | Read the sequential metre mark on the sheath | On receipt | Free |
| Thin or eccentric insulation | Thickness measured around the section | Factory or laboratory | Low |
| Pinholes in the insulation | Spark test during extrusion, IEC 62230 | In-line, every metre | Included in production |
| Voids and contaminants | Partial discharge, 10 pC at 1.73 U0, IEC 60502-2 | Factory laboratory, MV cable | Medium |
| Under-cross-linked XLPE | Hot set test | Third-party laboratory | Low, but days |
| Cheap or substituted compound | Tensile and elongation before and after ageing | Third-party laboratory | Medium, several days |
| Wrong strand count | Count the wires and measure one | Anywhere | Low, minutes |
Frequently asked questions
How can I tell if cable cross-section has been reduced?
Two independent checks. Measure the DC resistance with a four-terminal method, correct it to 20 °C, and compare with the IEC 60228 maximum, or strip a measured length and weigh the copper: the area in mm² is the mass in grams divided by 8.89 times the length in metres. Resistance catches every way of reducing the area; weighing confirms it without a temperature correction.
What percentage of cross-section reduction can a DC resistance test detect?
With a four-terminal measurement, a known conductor temperature and a 5 or 10 m sample, a good laboratory detects about 2 %. In practical conditions, where the temperature is known only to a degree or two, the reliable threshold is 3 to 5 % of area, because copper changes 0.393 % per kelvin. Correct the result to 20 °C before drawing any conclusion.
Can a cable pass the resistance standard and still be undersized?
Yes, by several per cent. The IEC 60228 maximum resistance for a size sits a few per cent above the value for a nominal-area conductor, so a cable that just meets the limit can be a few per cent short of nominal. Treat a measured result sitting exactly on the maximum as a warning sign, and ask for the measured values rather than a pass statement.
What is insulation eccentricity and why does it matter?
It is the difference between the thickest and thinnest points of the insulation wall, expressed as a proportion of the thickest, commonly limited to 15 %. It matters because the electric stress is highest at the conductor surface, so the thin side of an eccentric cable carries the highest stress per millimetre, ages fastest and is where partial discharge starts. The overall diameter looks correct because the thick side hides it.
How do I check that a drum contains the length I paid for?
Read the sequential metre mark printed on the sheath at the end of the drum, which is the quickest method and needs no unwinding. Check the drum tag against the packing list at the same time. For a drum without metre marking, measurement means counting the drum revolutions against the drum circumference, or weighing the contents, both of which are approximate.
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.