LSZH armoured cable: what the standard actually governs

The point most often missed about LSZH armoured cable: the armour is steel or aluminium wire and does not burn, but an armoured construction needs an inner sheath, so there are two layers of halogen-free compound. At the same conductor size the non-metallic mass is 40–60 % higher than the unarmoured equivalent, and CPR testing counts the whole cable’s non-metallic content. Same compound, same factory, and the armoured version often lands one class lower. That is a structure problem, not a material problem.

This page covers 0.6/1 kV and 1.9/3.3 kV thermosetting-insulated LSZH armoured cables: SWA steel wire, AWA aluminium wire and STA steel tape, and how each relates to CPR class. Mineral-insulated and fire-resistant cables are out of scope, as is medium voltage above 6 kV. Class figures follow BS EN 50575.

How the classes are cut

CPR classes reaction to fire, not fire resistance. BS EN 50575 grades cable from Aca to Fca, and below B2ca three add-on scales apply: s1–s3 for smoke, d0–d2 for flaming droplets, a1–a3 for acidity. The main class comes from EN 50399: peak heat release rate, FIGRA, and total heat release over the first 1200 s. An armoured cable carries more non-metallic mass, so it releases more heat and both figures rise. Switching to an LSZH compound fixes halogen and smoke, not the class itself.

The test method decides whether classes can be compared at all

The documents have separate jobs. BS 6724 covers armoured construction, sheath thickness and mechanical tests; BS 7211 is the unarmoured counterpart; both are thermosetting insulation with an LSZH sheath. BS EN 50575 supplies the CE marking and the class. IEC 60332-3-24 is the bunched-burn method, IEC 61034-2 measures smoke density, and IEC 60754-1 and 60754-2 measure acid gas total and pH with conductivity. A report citing BS 6724 but not EN 50575 has no CPR classification behind it.

The pass criteria can be checked one by one. IEC 61034-2 requires at least 60 % transmittance; IEC 60754-2 requires pH of 4.3 or above and conductivity of 10 µS/mm or below; IEC 60754-1 caps acid gas at 0.5 % by mass. IEC 60332-3 grades by non-metallic volume per metre: 7 L/m for class A, 3.5 for B, 1.5 for C and 0.5 for D. Armoured cables usually sit in the C or D band for installed volume, which is regularly overlooked at selection.

The class on the certificate is not the cable in your hand

The common accident is a certificate that does not match the cable on site. Certification is done against a frozen construction: change the inner sheath thickness, change the compound, even change the armour wire diameter, and heat release moves, so the class has to be retested. The second is quoting an unarmoured report for an armoured product, with 40 % more non-metallic mass. The third is bunched installation, which is harsher than the single-cable test: B2ca on paper, and a full tray still propagates.

Read four fields in a report. First the full cable designation and construction, including armour type and inner sheath thickness. Second the test standard and its version, since EN 50399 revisions change how FIGRA is calculated. Third the non-metallic volume, which decides the bunched rating that was actually demonstrated. Fourth the add-on combination: in B2ca-s1a,d0,a1 the s1a covers smoke, d0 droplets and a1 acidity, and they are not equally strict, so quoting B2ca alone loses half the information.

How to read a test report

• • All three reports present: 60754-1 acid gas, 60754-2 acidity and conductivity, 61034-2 smoke
  • The fire class stated to the letter, with the Cat A/B/C used and the sample volume that goes with it
  • Sheath continuous and short-circuit temperature, listed separately from the insulation rating
  • Minimum installation temperature, since −25 ℃ and −40 ℃ compounds sit at different prices
  • Whether circuit integrity is also required, in which case EN 50575 alone will not answer it

Related: LSZH cable; IEC 60754 cable; .

Frequently asked questions

SWA or AWA?

Start with single-core AC: if it is single-core, it has to be AWA, because steel armour carries eddy currents and hysteresis loss, dropping ampacity a step and heating the armour. Where tensile load matters, risers, direct burial, long spans, take SWA at 1200–1500 MPa against roughly a tenth of that for aluminium. In corrosive ground, aluminium needs a barrier against copper, or galvanic corrosion eats the armour first.

Why does an armoured cable land a class below the unarmoured equivalent?

The inner sheath. At the same conductor size the non-metallic mass is 40–60 % higher, and EN 50399 counts the whole cable, so the same compound releases more heat. There are only three ways up: thinner inner sheath within the standard’s minimum, a higher-grade compound, or a different construction such as AWA to cut mass. If the job needs B2ca and armouring, ask the factory whether that construction has actually been tested.

What separates BS 6724 from BS 7211?

Both are thermosetting insulation with an LSZH sheath; 6724 is the armoured construction and 7211 the unarmoured one. Electrical tests are broadly the same, mechanical tests are not: 6724 adds armour tensile and wrapping tests that 7211 does not run. A 7211 report therefore proves nothing about the mechanical performance of a 6724 cable, and the reverse is equally true.

Can the armour serve as the protective conductor?

Yes, if the equivalent section is checked against Table 54.2 of BS 7671. Steel armour usually works out at a few square millimetres of copper equivalent: fine for a short run, nowhere near enough for fault current over distance, where a separate PE is needed. Bonding both ends also creates circulating current, most visible on inverter-fed circuits, and that has caused failures in pump stations and tunnels.