Cable is the application where getting the colour wrong stops being a cosmetic problem.

An insulation compound has one job above all others – to not conduct. A sheathing compound has a different job, to survive sunlight, abrasion and water for decades. The masterbatch you put in each has to respect that difference, and the most common mistake in the industry crosses exactly that line.

The rule that matters most: carbon black is conductive

Carbon black is not simply a black pigment. It is a conductive carbon, and it is the same material used deliberately to make semi-conductive screening layers in medium and high voltage cable.

Which means:

Carbon black belongs in cable sheathing. It must never be used to colour insulation.

At sufficient loading, carbon black forms a percolating conductive network through the polymer. In a sheath that is irrelevant – a sheath is mechanical and environmental protection, not electrical. In insulation it directly attacks volume resistivity, the property the insulation exists to provide.

This matters because it is easy to do by accident. Black is the cheapest and most available colour, general-purpose black masterbatch is everywhere, and a shop running both insulation and sheathing on the same site can very easily let the wrong drum reach the wrong line.

Black insulation, where it is genuinely required, uses a non-conductive black pigment system rather than a standard carbon black concentrate. If a supplier quotes you “black masterbatch” for an insulation grade without asking which it is for, ask them the question they should have asked you.

What else insulation grades demand

Beyond conductivity, insulation colour has requirements that general-purpose colour does not.

Very low ionic content. Chloride and sulphate ions, and residual salts from pigment manufacture, migrate under an electrical field and in the presence of moisture. In wet conditions this promotes water treeing, the slow branching degradation that ends the life of buried XLPE cable. A pigment that is fine in a bucket is not necessarily fine in a cable.

Copper deactivation. Polyethylene in contact with copper degrades faster than polyethylene on its own – copper ions catalyse oxidation. Cable-grade compounds carry a metal deactivator for exactly this reason. A colour masterbatch must not dilute or interfere with that package, and any pigment carrying trace copper is entirely the wrong thing to put next to a copper conductor.

Heat ageing. Cable is qualified by long-term ageing – typically ovens at elevated temperature for days or weeks, with retained tensile and elongation measured afterwards. Some organic pigments shift shade badly over that ageing, and some carriers oxidise. A colour that passes on day one and fails after ageing has failed.

No heavy metals. Lead chromate yellows and cadmium reds are excellent, cheap, heat-stable pigments and they are the reason “cheap yellow” still exists. They are also restricted under RoHS and equivalent regimes, and a cable that must be RoHS-compliant cannot carry them. Ask for a RoHS declaration in writing.

Colour identification is a standards question, not a preference

Core colour identification in Indian cable is governed by standards – IS 694 for PVC insulated cables up to 1100 V, IS 1554 for PVC insulated armoured cables, IS 7098 for XLPE – and those standards specify which colours identify which cores, including the reserved earth colour.

Two practical consequences:

Colour is a functional specification with a tolerance. “Red” is not a shade you choose from a card; it is a colour that must be identifiable, consistent between drums, and stable over the cable’s life. Ask your supplier for a ΔE tolerance and hold them to it.

The reserved earth colour must not appear anywhere else. Green and green-yellow are reserved for protective earth. Using an adjacent green on a non-earth core is a safety issue, not a styling one.

Confirm the current amendment of the relevant standard before you finalise a shade set – these standards are periodically revised and the older colour conventions still circulate.

PVC, XLPE and LSZH each need different colour

The three main cable polymer systems behave differently enough that the same masterbatch cannot serve all three.

PVC. Highly filled and highly plasticised, processed at temperatures where the polymer is genuinely near its own decomposition. Colour for PVC has to survive that, and it must not interfere with the heat stabiliser package – the calcium-zinc or lead stabiliser system that keeps PVC from degrading. A pigment that reacts with the stabiliser causes discolouration and, at worst, accelerates degradation. PVC colour is usually supplied on a PVC-compatible carrier rather than a polyolefin one.

XLPE. Cross-linked polyethylene. The colour system must not interfere with the cross-linking chemistry – with peroxide-cured XLPE, anything that scavenges radicals competes directly with the reaction that forms the network. Nor may it introduce moisture, which matters for silane-cured systems. This is a real formulation constraint, not a theoretical one.

LSZH – low smoke zero halogen. These compounds are extremely heavily loaded with mineral flame retardant, commonly aluminium or magnesium hydroxide, at loadings that dominate the formulation. There is very little polymer left to carry pigment, colour development is poor, and the mineral filler is itself white or off-white, which fights every shade you try to achieve. LSZH colour needs to be formulated for LSZH. A standard polyolefin colour concentrate will disappoint.

Sheathing: where UV comes in

Outdoor and buried sheathing needs UV protection, and this is where carbon black is not just permitted but preferred – a well-dispersed fine-particle carbon black at a suitable loading is the most cost-effective UV screen available.

The requirements mirror those for pipe: fine grade for genuine UV screening rather than mere blackness, and proper dispersion, since agglomerates protect nothing and act as stress concentrators in a sheath that will be pulled through ducts.

Coloured sheathing – used for circuit identification and for utility colour conventions – needs a full HALS package instead, because coloured pigments provide little UV screening. Expect a coloured outdoor sheath to cost more to protect than a black one.

What to send your masterbatch supplier

  • Whether the compound is insulation or sheathing – state it explicitly
  • Base polymer: PVC, PE, XLPE (peroxide or silane cured), LSZH, TPE
  • Voltage grade and the standard you are qualifying to
  • Heat ageing conditions and retained-property requirements
  • RoHS or other restricted-substance requirement
  • Volume resistivity requirement, if insulation
  • Colour references with a ΔE tolerance
  • Outdoor exposure and required life, if sheathing
  • Flame retardant type and loading, for LSZH

FAQs

Can I use black masterbatch in cable insulation? Not a standard carbon black concentrate. Carbon black is electrically conductive and reduces volume resistivity – the property insulation exists to provide. Carbon black belongs in sheathing. Black insulation, where required, uses a non-conductive black pigment system.

Why does copper affect polyethylene cable insulation? Copper ions catalyse the oxidation of polyethylene, so PE in contact with a copper conductor degrades faster than PE alone. Cable compounds include a metal deactivator, and the colour system must not interfere with it or introduce trace copper of its own.

What is water treeing and can pigment cause it? Water treeing is slow branching degradation in polyethylene insulation under electrical stress in wet conditions. Ionic contamination accelerates it, so cable-grade pigments must have very low residual chloride, sulphate and salt content.

Which standards govern cable core colours in India? IS 694 for PVC insulated cables up to 1100 V, IS 1554 for armoured PVC cables and IS 7098 for XLPE, among others. Confirm the current amendment, and note that green and green-yellow are reserved for protective earth.

Why is colouring LSZH compound so difficult? LSZH compounds carry very high loadings of mineral flame retardant, usually aluminium or magnesium hydroxide. There is little polymer left to develop colour and the filler is itself pale, so shades come out weak and dirty unless the masterbatch is formulated specifically for LSZH.

Do coloured cable sheaths need more UV protection than black ones? Yes. Carbon black is an excellent UV screen in its own right, so a black sheath gets substantial protection from the pigment. Coloured pigments screen very little UV, so a coloured outdoor sheath needs a full HALS package to reach the same life.