BS 6724 covers thermosetting insulated armoured cables with low emission of smoke and corrosive gases when affected by fire. Read that sentence carefully: the standard is about emissions. It is not about whether the cable keeps carrying current through a fire. Those are two separate properties and two separate pieces of paper.
Structurally, this is a BS 5467 cable with the bedding and the outer sheath swapped from PVC to a halogen-free low-smoke compound. That is the whole difference — and it is also most of the price difference, because the compound costs more and processes more slowly.
Emissions limits are set by the IEC test standards listed below, not by a single number in BS 6724. Which ones your project requires determines what evidence you should be asking for.
Two cables, six layers, one substitution. If you are comparing quotations, this is the only column where the difference should appear.
| Layer | BS 5467 | BS 6724 |
|---|---|---|
| Conductor | Class 2 stranded copper | Identical |
| Insulation | XLPE, 90 °C | Identical |
| Filler | Non-hygroscopic compound | Identical |
| Bedding | PVC | Halogen-free compound |
| Armour | Galvanised steel wire (SWA) | Identical |
| Outer sheath | PVC, black | LSZH, low smoke |
| Designation | N2XRY | N2XRH |
| Smoke & acid gas | Not controlled | Controlled to IEC 60754 / 61034 |
| Typical use | Plant rooms, sub-mains, direct burial | Public buildings, tunnels, enclosed escape routes |
"LSZH" on its own is a word. These are the documents that turn it into a specification — and they are exactly what to ask for when a quotation looks too cheap.
| Test | What it measures | Why it matters on site |
|---|---|---|
IEC 60754-1 | Halogen acid gas content | Halogen gases form hydrochloric acid with water — it attacks metal structures, switchgear and electronics in the same room. |
IEC 60754-2 | Acidity and conductivity of evolved gases | The quantitative version. This is the number that proves the compound is genuinely halogen-free rather than "reduced halogen". |
IEC 61034 | Smoke density under the 3 m cube test | Smoke, not flame, is what stops people getting out of a building. Measured as light transmittance. |
IEC 60332-1-2 | Flame propagation, single vertical cable | The baseline. A single cable should not carry flame up itself. |
IEC 60332-3-22 / -23 / -24 | Flame propagation, bunched cables, Cat A / B / C | The one that matters in a real cable tray, where cables are touching in a bundle. Categories differ by volume of combustible material. |
The construction lines are the same as any armoured LV cable. The extra lines are the ones that decide whether the quotation actually answers your specification.
4 C × 240 mm² ← cores and size
XLPE / SWA / LSZH ← insulation / armour / sheath
600/1000 V ← voltage class
to BS 6724 ← the standard, by number
+ IEC 61034, IEC 60332-3-24 ← the clauses your spec cites
01 · THE WORD WITHOUT THE NUMBER
"LSZH" without a test standard is unverifiable at delivery. If the consultant rejects the drum, there is nothing in the purchase order to appeal to.
02 · SMOKE MISTAKEN FOR SURVIVAL
Life-safety circuits specified as "LSZH" alone will not survive long enough to matter. This is the most expensive mistake on this page, and it is discovered during commissioning.
03 · SHEATH IN PLACE OF REPORT
A halogen-free sheath is a material property. A third-party emissions report is evidence. Projects asking for the evidence should raise it at tender, not at delivery.
No — and this is the most common misunderstanding in the whole specification. LSZH describes what the cable emits when it burns: little smoke and no corrosive halogen gas. It says nothing about whether the cable keeps working. That property is circuit integrity, tested under BS 6387 or BS 8491. A cable can be one, the other, or both. Life-safety circuits usually need both.
Conductor, XLPE insulation and steel wire armour are the same. The bedding and the outer sheath change from PVC to a halogen-free low-smoke compound. One substitution, two standards. See BS 5467 →
IEC 60754-1 for halogen acid gas, IEC 60754-2 for acidity and conductivity, IEC 61034 for smoke density, and IEC 60332 for flame propagation — with the 60332-3 series and a category (A, B or C) where cables are bunched. A quotation naming none of these has not yet answered a specification.
The halogen-free compounds are generally stiffer and less tolerant of tight bends than PVC. Confirm the bending radius for your size rather than assuming it carries over from PVC. On long underground pulls this affects drum set-out and how the run is planned — a planning matter, not a reason to avoid the cable.
Yes, but it is then a different standard: the unarmoured halogen-free equivalent is BS 7211. Send the installation route — burial, duct, tray — and the right construction follows from that rather than from preference.
Above 300 mm² it becomes a project question rather than a catalogue one. Send the line with the quantity and we will confirm what is achievable and on what lead time, instead of quoting a size we cannot actually deliver.
The PVC-sheathed version of the same cable. Where mechanical protection matters more than emissions.
BS 6387What to look at when the circuit has to keep working. Not a quality ranking.
IEC 60502-1Where a project cites IEC instead of BS, and what changes on the drawing when it does.
Send the construction, the emission clauses your consultant cited, and the quantity. You will get an answer on feasibility and a quotation you can defend at delivery.