FR, FRLS or LSZH cable: what is the difference?

In most building fires, the flame is not what kills people. Smoke is. Ordinary PVC insulation, when it burns, releases dense black smoke and hydrogen chloride gas. The smoke removes visibility in an escape route within seconds; the gas attacks the lungs and, afterwards, corrodes every piece of surviving electronic equipment in the building.

Flame retardant grades address the wrong half of the problem. They slow the spread of fire along the cable, which is genuinely useful - but they are still PVC, and they still produce the smoke. Halogen-free cable addresses the smoke, which is why it is specified in buildings where evacuation is slow or crowded.

Separately from the halogen question, insulation can be cross-linked by passing the finished cable through an electron beam. This links the polymer chains together and changes the material's behaviour under heat: it stops softening and flowing, so it resists melting under short circuit and tolerates a higher continuous temperature - typically 105 °C against 70 °C for standard PVC.

In practice that means a cross-linked cable can carry more current in the same size, lasts longer in a hot conduit, and does not deform where it passes over a sharp edge. Both APAR Anushakti and RR Kabel Firex use it.

Halogen-free cable costs more per metre, and on a bill of quantities it is compared against FR PVC on exactly that basis. The comparison is misleading, because the two products are not doing the same job - one manages flame spread and the other manages survivability during evacuation.

If a project has a genuine budget constraint, the sensible compromise is not to downgrade everything uniformly. It is to specify LSZH on the circuits that run through escape routes, shafts, basements and enclosed corridors, and accept FRLS elsewhere. That targets the spend at the places where smoke actually traps people.