Why this distinction exists
In most building fires, smoke inhalation causes more casualties than flame contact. Smoke also blinds escape routes and disables the people trying to use them.
Ordinary PVC insulation contains chlorine. When it burns it releases hydrogen chloride gas, which is toxic to breathe and forms hydrochloric acid on contact with moisture — including the moisture in lungs and eyes, and on electronic equipment well away from the fire itself.
The three grades below represent successive steps in controlling that behaviour.
FR — Flame Retardant
FR compounds are formulated so the cable will not sustain and propagate a flame along its own length. Remove the ignition source and the cable self-extinguishes rather than carrying fire through the building.
FR controls flame spread only. Smoke density and acid gas emission are not restricted, and a burning FR cable will produce dense dark smoke and release HCl.
- Tested for flame propagation to IEC 60332-1 (single vertical wire)
- Bunched cable installations tested to IEC 60332-3
- Suitable for standard residential wiring in open, well-ventilated layouts
FRLS — Flame Retardant Low Smoke
FRLS adds two further controls on top of flame retardance: reduced smoke density and reduced acid gas emission.
Smoke performance is measured as light transmittance through the smoke produced — the higher the percentage, the clearer the escape route remains. Acid gas is measured as HCl released by mass.
- Smoke measured by light transmittance to IEC 61034
- Acid gas emission measured to IEC 60754-1, typically limited to around 20%
- Reduces but does not eliminate halogen content
- Common in commercial buildings, offices and multi-storey towers
LSZH — Low Smoke Zero Halogen
Also sold as HFFR (halogen-free flame retardant), this grade removes halogens from the compound altogether. There is no chlorine present, so there is no hydrogen chloride to release.
Acid gas emission falls to a trace level and light transmittance through the smoke is substantially higher than FRLS. In an enclosed space with a large number of people and a long escape route, that difference is the entire justification for the cost.
- Halogen content essentially eliminated — acid gas typically ≤0.5%
- Highest light transmittance of the three grades under IEC 61034
- Specified for metros, airports, hospitals, tunnels and data centres
- Generally stiffer and higher cost than FR or FRLS
Choosing a grade
The question to ask is not which cable is best, but how difficult it would be to evacuate the space it runs through.
| Grade | Controls | Typical application |
|---|---|---|
| FR | Flame propagation | Standard residential wiring |
| FRLS | Flame, smoke density, reduced acid gas | Commercial and office towers |
| LSZH / HFFR | Flame, smoke, halogen eliminated | Hospitals, metros, airports, data centres |
An open low-rise building with short escape routes is served perfectly well by FR. An underground station is not.
Fire performance claims should be backed by test reports against the relevant IEC standard for the specific compound supplied. Ask for them — the grade name printed on the sheath is not a substitute for test data.