Halogen-free cables (LSZH) — when and why (IEC 60754)

The cable fire-classification guide covers the Euroclasses (Aca through Fca) from the CPR for the fire behaviour of cables. A separate but related characteristic is whether a cable sheath is halogen-free — often referred to as LSZH (Low Smoke Zero Halogen).

What halogen-free means

Standard PVC cable sheaths contain chlorine (a halogen). When burned, this releases hydrochloric acid gas (HCl): toxic, corrosive to equipment, and together with smoke it forms a dense, hard-to-penetrate fog that hampers evacuation. A halogen-free sheath lacks these halogen compounds.

IEC 60754-2 sets the quantitative requirements: the combustion product of the material must not drop below a pH of 4.3, and the conductivity value of the combustion product must not exceed the limit of 10 µS/mm. Both values are indirect measures of the amount of acidic, corrosive combustion gases produced.

Smoke density — a separate criterion

Besides being halogen-free, the amount of smoke released is a separate criterion. IEC 61034-2 defines the measurement of smoke gas density for burning cables: the test result must show a light attenuation of at most 40% to be classified as "low smoke". LSZH therefore combines two requirements: halogen-free (IEC 60754-2) and low-smoke (IEC 61034-2).

Note: LSZH is not a formal Euroclass under the CPR — a cable can be halogen-free and still have a lower Euroclass (or vice versa). For the legally required Euroclass of a specific application, the cable fire-classification guide remains authoritative; LSZH is an additional characteristic, often contractually or situationally required.

Practical relevance

In a commercial building with many people present, or a space with valuable electronic equipment (server room, control cabinet with control systems for a horticultural business), halogen-free cabling is relevant for two reasons: less toxic smoke during an emergency (evacuation safety) and less corrosion damage to equipment after a cable fire elsewhere in the same air circuit.

Common mistakes

1. Confusing LSZH with a higher Euroclass — being halogen-free says nothing about the flame-spread rate or heat release of the cable itself; that is regulated separately via the Euroclass.
2. Using halogen-free cable without checking the smoke-density requirement (IEC 61034-2) — a cable can be halogen-free yet still produce a lot of smoke if the low-smoke requirement has not been separately verified.
3. Not distinguishing between sheath material and insulation material — both layers of a cable can independently be halogen-free or not; the complete cable construction is decisive.

Related

• Cable fire classification (CPR)
• Fire-rated cable penetrations