Short Answer: LS, HF and LTx Are Three Different Properties
LS (Low Smoke) — during combustion the cable emits less smoke. HF (Halogen Free) — the insulation and sheath materials do not contain halogens, and during combustion no corrosion-active halogen-containing gases are emitted. LTx (Low Toxic) — the combustion products have reduced toxicity. These are three different indicators, each confirmed by a separate test.
A cable with the LS index does not necessarily contain no halogens: PVC compound of reduced fire hazard reduces smoke formation, but chlorine remains in its composition. A cable with the HF index does not emit halogen-containing gases, but its smoke formation may not be as low as that of specialised LS versions. LTx does not replace either LS or HF: low toxicity of combustion products is a separate property. The requirements for these indicators are established by GOST 31565-2012, in force in Kazakhstan since 01.01.2014, with an amendment of 29.09.2025.
What the Marking Means
The letter indices in the cable marking are the result of classification according to specific fire hazard indicators.
LS — smoke formation during combustion and smouldering. Test according to GOST IEC 61034-2: the cable burns in a closed chamber, and the reduction in light transmission is measured. For cables with the LS index, the reduction in light transmission must not exceed 50%.
HF — absence of halogens. Two indicators are checked. The first is the content of halogen acid gases calculated as HCl when tested according to GOST IEC 60754-1: not more than 5.0 mg/g. The second is the corrosion activity of combustion products according to GOST IEC 60754-2: conductivity of the aqueous solution not more than 10.0 µS/mm, acidity value (pH) not less than 4.3.
LTx — low toxicity of combustion products. The indicator is the equivalent toxicity indicator of combustion products (PTPM). For LTx its value must be more than 120 g/m³. For LS and HF this indicator must be more than 40 g/m³. The methodology for determining toxicity is according to GOST 12.1.044.
The additional indices ng and FR designate non-propagation of combustion in grouped installation and fire resistance. The difference between them is analysed in a separate material: fire-resistant or non-flame propagating cable. A full table of index combinations — in the article LS, HF, LSLTx, FRLS and FRHF: full table of differences.
Which Fire Properties Are Tested
GOST 31565-2012 establishes five fire hazard indicators. Three of them are related to smoke, halogens and toxicity.
| Designation / property | What it means | What is actually checked |
| PKA (corrosion activity indicator) | Content of halogen acids and corrosion activity of combustion products | Content of halogen acid gases calculated as HCl — not more than 5.0 mg/g according to GOST IEC 60754-1. Conductivity of the aqueous solution — not more than 10.0 µS/mm, pH — not less than 4.3 according to GOST IEC 60754-2. |
| Smoke formation | Reduction in light transmission during combustion and smouldering | For LS — reduction in light transmission not more than 50%. For HF — not more than 40%. Test according to GOST IEC 61034-2. |
| PTPM (equivalent toxicity indicator of combustion products) | Toxicity of combustion products of polymeric materials, taking into account their mass fraction | For LS and HF — more than 40 g/m³. For LTx — more than 120 g/m³. Methodology according to GOST 12.1.044. |
Source: clauses 5.4–5.9 of GOST 31565-2012; test methods — GOST IEC 61034-2, GOST IEC 60754-1, GOST IEC 60754-2, GOST 12.1.044.
Please note: LS and HF are checked by different methods. Smoke formation is measured in a chamber with a light source. Halogen content — by burning a sample and analysing the combustion products for acid content. Toxicity — by a biological method using laboratory animals. Each property is confirmed by a separate test report.
How the Versions Differ
Let us compare LS, HF and LTx by the specific characteristics that affect the choice of cable.
| Characteristic | LS | HF | LTx |
| Smoke formation | Reduction in light transmission not more than 50% | Reduction in light transmission not more than 40% | Not standardised separately. Depends on the base version. |
| Halogen content | Not standardised. PVC compound contains chlorine. | HCl content not more than 5.0 mg/g. Conductivity not more than 10.0 µS/mm, pH not less than 4.3. | Not standardised separately. Depends on the base version. |
| Toxicity of combustion products | PTPM more than 40 g/m³ | PTPM more than 40 g/m³ | PTPM more than 120 g/m³ |
| Main purpose | Reduction of smoke on escape routes | Protection of electronic equipment from corrosion-active gases | Reduction of toxic impact on people during a fire |
Source: clauses 5.4–5.9 of GOST 31565-2012. PTPM — equivalent toxicity indicator of combustion products.
The table shows that LS and HF solve different tasks. LS combats smoke: less smoke — better visibility during evacuation. HF combats halogen-containing gases: less acid — less corrosion of electronics. LTx combats toxicity: higher PTPM — less toxic impact on people. A cable can combine these properties: ng(A)-LSLTx simultaneously reduces smoke formation and toxicity. But replacing one property with another is not allowed.
Where They Are Applied
The choice of version is determined by the conditions of the facility and the requirements for protecting people and equipment.
Industrial facilities. For cable structures, trestles, galleries, tunnels, versions with non-propagation of combustion are used: ng(A), ng(B). If electronic equipment is present in the room, HF is added. For lines that must operate during a fire, FR is added.
Public buildings. For buildings with mass occupancy, multi-functional complexes, high-rise buildings, ng(A)-HF is used. The absence of halogens reduces the corrosion activity of combustion products. For lines that must maintain operability during a fire, FR is added.
Escape routes. For lighting of escape routes and warning systems, cables with reduced smoke emission are used: ng(A)-LS. Smoke on escape routes is more dangerous than the flame itself, therefore LS is critical here. If these lines must operate during a fire, FR is added.
Fire protection systems. For fire alarms, emergency lighting, smoke extraction, fire-resistant cables are used: ng(A)-FRLS or ng(A)-FRHF. In control and monitoring circuits of building fire protection systems, cables of the FRHF version are used, since the absence of halogens is important for the reliability of electronics.
Facilities with a large number of people and limited mobility. For hospitals, preschool institutions, nursing homes, schools, ng(A)-LSLTx is used. People in these institutions cannot quickly leave the building, therefore the requirements for the toxicity of combustion products are increased. An analysis of this version with areas of application — in a separate material: LSLTx Cable: GOST, decoding, characteristics, application. The differences between VVGng and VVGng-LS are analysed in the article VVGng or VVGng-LS: what is the difference and when LS is needed.
How to Choose the Cable Version
The selection algorithm is built from the conditions of the facility to the specific version.
- Determine the installation method. Single or grouped. For grouped, the ng index with a category is required.
- Determine whether the line must operate during a fire. If yes — FR is required.
- Determine the requirements for smoke emission. If the cable is laid on escape routes or in a room with people — LS is required.
- Determine the requirements for halogens. If there is electronic equipment, server rooms, control rooms in the laying zone — HF is required.
- Determine the requirements for toxicity. For facilities with limited mobility of people — LSLTx.
- Assemble the version. The combination of indices gives a specific type: ng(A)-FRLS, ng(A)-FRHF, ng(A)-LSLTx.
- Verify with regulatory requirements. GOST 31565-2012 establishes the classification, but specific requirements for the facility may be clarified in the design documentation and building norms. If the regulatory situation for the facility is ambiguous, the decision is made by the designer.
Typical Mistakes
“LS and HF are the same thing.” LS reduces smoke formation but does not eliminate halogens. An ng(A)-LS cable contains PVC compound with chlorine. HF is the absence of halogens in the insulation and sheath materials. An ng(A)-HF cable does not emit halogen-containing gases, but its smoke formation may not be as low as that of specialised LS versions.
“LS means low toxicity.” LS and LTx are different indicators. LS is checked for smoke formation (reduction in light transmission). LTx is checked for the equivalent toxicity indicator of combustion products. A cable with the LS index may have a PTPM in the range from 40 to 120 g/m³. Low toxicity requires the LTx index with a PTPM of more than 120 g/m³.
“HF is a fire-resistant cable.” HF designates the absence of halogens, not fire resistance. An ng(A)-HF cable contains no halogens, but when exposed to flame it may char and stop working. Fire resistance is designated by the FR index and confirmed by a separate test for maintaining operability in a flame.
“The more indices, the better.” Each index reflects a specific property and increases the cost of the cable. If the facility does not require low toxicity, overpaying for LSLTx gives no practical benefit. The version is chosen according to the conditions of the facility, not on the principle of “maximum configuration”.
“Toxicity is tested in the same way as smoke formation.” These are different methods. Smoke formation is measured by an optical method in a chamber. Toxicity is determined by a biological method according to GOST 12.1.044 using laboratory animals. The results do not correlate directly: a cable with low smoke formation may have high toxicity of combustion products.
FAQ
What is the difference between LS and HF?
LS reduces smoke formation: the reduction in light transmission when tested according to GOST IEC 61034-2 is not more than 50%. HF contains no halogens: HCl content not more than 5.0 mg/g, conductivity of the aqueous solution not more than 10.0 µS/mm, pH not less than 4.3 according to GOST IEC 60754-1 and 60754-2. A cable with the LS index does not necessarily contain no halogens. A cable with the HF index does not necessarily have low smoke formation.
What PTPM value is considered low-toxicity?
For cables with the LTx index, the equivalent toxicity indicator of combustion products (PTPM) must be more than 120 g/m³. For cables with the LS and HF indices, this indicator must be more than 40 g/m³. The determination methodology is according to GOST 12.1.044. The higher the PTPM, the lower the toxic impact of combustion products.
Is LTx mandatory for hospitals and schools?
GOST 31565-2012 refers LSLTx to the predominant area of application for preschool and educational institutions, specialised nursing homes for the elderly and disabled, hospitals, and dormitory buildings of boarding-type educational institutions. Specific requirements for the facility are established by the design documentation and building norms. If the regulatory situation is ambiguous, the decision is made by the designer.
How is the halogen content in a cable tested?
According to GOST IEC 60754-1, the amount of halogen acid gases, except hydrofluoric acid, released during combustion of compounds is determined. According to GOST IEC 60754-2, the degree of acidity of the released gases is determined by measuring pH and specific conductivity. Both methods are applied to individual elements of the cable construction: insulation, sheath, filling.
What does a fire hazard class mean, for example P1a.7.2.2.2?
The fire hazard class is a sequence of indicators: flame propagation limit for grouped installation (P1a, P1b, P2, P3, P4), fire resistance limit (1–8), corrosion activity indicator (1 or 2), toxicity indicator (1 or 2). For example, P1a.7.2.2.2 is decoded as P1a, fire resistance limit 7, corrosion activity category 2, toxicity category 2.
Normative Sources Used
- GOST 31565-2012 “Cable products. Fire safety requirements” — in force, introduced 01.01.2014, amendment put into effect on 29.09.2025 — official text on meganorm.ru — confirms the requirements for smoke formation (clauses 5.4, 5.5), halogen content (clause 5.7), toxicity of combustion products (clauses 5.6, 5.9) and their confirmation methods.
- GOST IEC 61034-2-2024 “Measurement of smoke density of cables burning under defined conditions. Part 2. Test procedure and requirements” — in force, introduced 01.07.2025 — official portal of Rosstandart, protect.gost.ru — confirms the smoke formation test method.
- GOST IEC 60754-1-2026 “Test on gases evolved during combustion of materials from cables. Part 1. Determination of the amount of halogen acid gas” — in force with the right of early application, introduced 01.01.2028 — official portal of Rosstandart, protect.gost.ru — confirms the method for determining the content of halogen acids.
- GOST IEC 60754-2-2026 “Test on gases evolved during combustion of materials from cables. Part 2. Determination of acidity (by pH measurement) and conductivity” — in force with the right of early application, introduced 01.01.2028 — official portal of Rosstandart, protect.gost.ru — confirms the method for determining the corrosion activity of combustion products.
- GOST 12.1.044-2018 “Occupational safety standards system. Fire and explosion hazard of substances and materials. Nomenclature of indicators and methods for their determination” — in force, introduced 05.10.2018, with amendments of 13.08.2024 — official text on meganorm.ru — confirms the methodology for determining the toxicity of combustion products.
- TR CU 004/2011 “On the safety of low-voltage equipment” — in force — official portal of the EAEU — confirms the mandatory certification of cables for voltages from 50 to 1000 V.
- Electrical Installation Code of the Republic of Kazakhstan 2015, as amended by the order of the Minister of Energy of the Republic of Kazakhstan dated 31.10.2022 No. 340 — in force — IPS “Adilet” — confirms the requirements for cable installation.
