XLPE vs LSZH Cable: Insulation vs Sheath (BS 6724)

01The five-second misunderstanding that costs projects

Enquiries still arrive asking for a quotation for "XLPE cable" and another for "LSZH cable", as though the two were rival product families to be weighed against each other. They are not. XLPE describes the material wrapped around the conductor. LSZH describes the behaviour of the outer sheath when it burns. A single cable routinely has both, and the standards that matter in the UK write them into the same construction.

Treating them as alternatives produces two symmetrical errors, and both are expensive:

• Over-specifying. Specifying an LSZH construction for an outdoor industrial route where no fire strategy calls for it, and paying a premium for a property nobody needs.
• Under-specifying. Specifying "XLPE cable" on a tunnel or high-rise route, receiving PVC-sheathed cable that meets the letter of the request, and discovering at hand-over that the sheath is not what the fire strategy required.

The confusion is not carelessness. The two acronyms sit side by side in datasheets and in cable designations, and both are used as shorthand for "the modern one". Separating what each term classifies is the whole of the problem.

02XLPE and LSZH are not the same kind of thing

A cable is a stack of layers, each doing a different job. Reading the two acronyms against that stack is what resolves the apparent conflict.

From the conductor outwards, a typical armoured LV cable runs: conductor, insulation, bedding or inner covering, armour, outer sheath. XLPE belongs at the insulation position. LSZH belongs at the outer sheath — and, in a properly constructed halogen-free cable, at the bedding as well, because a halogen-free over-sheath wrapped around a PVC bedding is a compromise the standard does not ask for.

The temperature and halogen rows are the two that generate the most misplaced confidence. A 90 °C rating says nothing about fire behaviour, and a halogen-free insulation says nothing about smoke density.

Why the cross-linking matters

Polyethylene is a thermoplastic: heat it and it softens. Cross-linking converts it into a thermoset network that retains its mechanical integrity at temperatures well above the softening point of the base polymer, which is what allows a 90 °C continuous conductor rating against the 70 °C of a standard PVC-insulated cable. The practical consequence in procurement is ampacity: for the same conductor size, a 90 °C insulation carries more current than a 70 °C one, which is why XLPE displaced PVC in power distribution long before fire performance entered the conversation.

Why "zero halogen" is a test result, not a material name

LSZH compounds are typically polyolefin-based resins loaded with inorganic flame-retardant fillers such as aluminium trihydroxide or magnesium hydroxide, which release water vapour when heated and dilute the combustion gases. But the chemistry is not the definition. A cable qualifies as halogen-free, low-smoke, low-corrosivity because it has been tested to the relevant methods and met the limits the applicable cable standard sets. A supplier who cannot produce those results has an assertion, not a classification.

03What BS 6724 and BS 8573 actually specify

The clearest way to see the relationship is through the two British Standards that combine the layers. One covers the armoured construction, the other the unarmoured one, and the difference in scope matters more than most buyers realise.

BS 8573 scope from the published standard record: non-armoured, thermosetting-insulated cables of rated voltage 600/1 000 V for fixed installations, producing lower levels of smoke and corrosive products than cables to BS 7889, with a maximum sustained conductor temperature of 90 °C and a short-circuit limit of 250 °C for 5 s. Its exclusion of ground burial is explicit in the scope.

The first two rows of that table share conductor, XLPE insulation and steel wire armour, and differ in the bedding and the over-sheath. That single change is what moves the cable from BS 5467 to BS 6724. It is also why the same cross-section carries the same current in both: the sheath choice is a fire-performance decision, not an electrical one. On our own range, a 4 × 70 mm² copper XLPE steel-wire-armoured cable is published at 246 A in air at 30 °C and 188 A direct buried at 20 °C in the PVC-sheathed version and in the halogen-free version alike.

04The five separate tests behind an LSZH claim

This is the section that decides whether a "low smoke zero halogen" line on a datasheet means anything. LSZH is not one property measured one way. It is a bundle of independent results, produced by independent test methods, and a cable can pass some and fail others.

Five results, five methods. IEC 60754-1 and IEC 60754-2 are test methods for halogen acid gas content and corrosivity; IEC 61034-2 is the smoke density method; the IEC 60332 series covers flame propagation. Whether a cable qualifies as low smoke zero halogen depends on the limits set by the applicable cable standard or project specification, not on any one of these results alone.

The specific error worth naming

Halogen content gets quoted as if it were a smoke rating. It is not. A compound can be entirely halogen-free and still produce dense smoke, because smoke density is a separate measurement with a separate method — IEC 61034-2, run in a chamber, with a light-transmittance result. A datasheet that shows only a halogen-free test report and then summarises the cable as "low smoke" has told you one of the two things it appears to be claiming.

The same trap sits in the flame tests. A single-cable test to IEC 60332-1-2 and a bunched test to IEC 60332-3 are not strengths of the same property. A real cable route is a bunched condition, which is why a specification that names a bunched category is asking a question that a single-cable test cannot answer. Our armoured halogen-free construction is classified to IEC 60332-3 Cat. C; a Cat. A or Cat. B requirement is a different construction and is quoted separately rather than assumed.

05How to read the construction code on the drum

The construction code printed along the cable is the shortest honest summary of what you have bought, and it separates the two terms for you if you read it layer by layer.

• CU/XLPE/SWA/PVC — copper conductor, XLPE insulation, steel wire armour, PVC sheath. The UK armoured standard for this construction is BS 5467. The sheath is not halogen-free.
• CU/XLPE/SWA/LSZH — the same build with a halogen-free low-smoke bedding and over-sheath. The UK standard is BS 6724.
• CU/XLPE/LSZH, unarmoured — no armour. The UK standard is BS 8573, and the scope excludes burial.

What a field test cannot tell you

A flame applied to the end of a cable proves nothing about its classification. It is not one of the five methods, it produces no quantified result, and it cannot demonstrate compliance with any of them. Additives change burning behaviour in ways a field observation cannot rank; the test may also damage the product and release fumes in the process. Verification runs through the marking, the product standard, the test report and the manufacturer's traceability — not through a lighter.

Where the cable marking and the datasheet agree on the construction code and the standard number, and the test reports cover the five properties above, the claim is supported. Where any of those four is missing, the honest position is that the claim is unverified until the documentation is produced.

06When LSZH is required, and when it is not

The sheath decision follows the fire strategy of the installation, not the preference of the buyer. Where people have to evacuate through the space the cable occupies, or where sensitive equipment shares the space, the smoke and acid gas the cable itself can produce become design inputs. Where neither applies, the halogen-free premium buys nothing.

Routes where the sheath is a design input

• Road and rail tunnels. Enclosed, high-occupancy and evacuation-critical, where smoke visibility governs the design.
• Metro and railway stations. Station power and auxiliary supplies in enclosed public spaces, where the armour requirement and the fire requirement arrive together — which is the case BS 6724 exists for.
• Basements, plant rooms and below-ground switchrooms. The failure modes being designed out are smoke logging and corrosive gas damage to adjacent equipment.
• Data centres and control rooms. Acid gas from a cable fire is a direct threat to the electronics in the next room, not only to people.
• Hospitals and other buildings with staged evacuation. Where the cable route passes through areas of refuge.
• High-density tray routes in enclosed buildings. Where the bunched condition is the real condition, which is why the bunched flame classification matters more here than the single-cable one.

Routes where it usually is not

Outdoor and industrial routes with no occupancy exposure, no enclosed accumulation and no fire strategy naming the cable do not need a halogen-free sheath. An outdoor tray run at a plant, a buried feeder between buildings, or a general industrial distribution route is normally well served by the PVC-sheathed XLPE construction — and there the UV and abrasion performance of the sheath, rather than its combustion products, is the property to check.

07The cost question: what the LSZH premium actually buys

A halogen-free construction typically costs more than an otherwise comparable PVC-sheathed cable, and the size of the premium varies with the construction, the cross-section, the certification route and the market at the time. Those are the variables that move it, and anyone quoting a single percentage for the difference is averaging across too many cables to be useful.

What can be compared without a price list is what the premium is buying and what the alternative costs when the specification changes later:

Current ratings are the published manufacturer figures for the two constructions at 4 × 70 mm², at the stated reference conditions. This article deliberately quotes no price or price percentage — those depend on copper, construction and market at the time of enquiry, and a stale number is worse than none.

The commercial argument is not that halogen-free cable is cheaper over twenty-five years. It is that the comparison is asymmetric. The premium is a known cost at tender. The consequence of under-specifying is a re-cable of a route that by then runs through a finished building, with the access, reinstatement and business interruption that implies. Where a fire strategy calls for the property, the retrofit is not optional — so the decision to pay for it has already been made, and the only question is when.

Where the strategy does not call for it, the premium is avoidable, and paying it is a decision to spend money on a property the installation does not use. Both directions matter: the correct specification is the one that matches the fire strategy, not the more expensive of the two options.

08What we publish, and what we will not pretend to publish

The scope of this article is the two terms and the standards that combine them. Three limits are worth stating plainly.

It does not quote compound grades as our product specification. Datasheets in this market commonly name the insulation grade and the sheath grade of a halogen-free construction, and a grade is a more precise statement than "LSZH" — but our published product data specifies the construction and the standard, not a trade name, and we are not going to present a grade as our specification in order to sound more specific.

It does not give a price premium. Halogen-free constructions usually cost more than comparable PVC ones and the difference moves with the market; a percentage quoted here would be wrong within a quarter, and there is no version of the number that helps a buyer make the decision.

And it does not treat a fire test as something you can run on site. The five results in Table 3 come from laboratories and standardised apparatus. A field trial with a flame does not reproduce any of them.