Flame-Retardant vs Fire-Resistant vs LSZH Cable

What does each term actually describe?

The three terms answer three different questions, so they cannot be ranked against each other. Use them as shorthand only, then replace them in the specification with the test that applies:

• Flame-retardant usually means the cable limits how far flame travels along a bundle of cables. The evidence is a flame-spread test.
• Fire-resistant usually means a circuit is expected to stay energized for a stated time during a fire. The evidence is a circuit-integrity test.
• LSZH (low smoke, zero halogen) usually means low smoke density and low halogen acid gas when the material burns. The evidence is smoke-density and gas-acidity testing.

A cable can meet one of these requirements and fail another, which is why a single label cannot settle a bid comparison. For background on the LSZH material itself, see what LSZH cable is.

How is flame spread tested, and what does a pass prove?

IEC 60332-3-24 evaluates vertical flame spread on vertically mounted bunched cables under defined Category C conditions. The IEC description treats the cable loading, ladder arrangement and flame application as controlled parts of the method. Category C is defined by a test loading of 1.5 litres of non-metallic material per metre of tested cable and a 20 minute flame application. The IEC 60332-3 series uses several categories that differ in the allowed volume of non-metallic material, so the category must be stated on the report.

A pass demonstrates performance under that test arrangement only. It does not demonstrate that the cable will continue transmitting power or signals during a fire. For procurement, record the following:

• the standard number, part and edition year printed on the report;
• the category, such as Category C;
• the tested cable construction and size range;
• the damage criterion reported by the laboratory;
• the laboratory identity, test date and complete result pages.

Do not replace these fields with the word "flame-retardant".

What does a circuit-integrity rating actually prove?

IEC 60331-2 addresses circuit integrity under fire with mechanical shock, for cables within its stated voltage and diameter scope. The standard also directs larger-diameter cables to a different part. A duration printed on a datasheet is therefore incomplete unless the test method, exposure conditions, specimen configuration and cable scope are identified.

The flame used in these tests has also changed over time. The legacy parts 21, 23 and 25 used a flame of at least 750 °C. The current parts 60331-1 and 60331-2 use at least 830 °C with mechanical shock. The duration is set by the product standard or the project; a 90 minute duration is commonly cited for the legacy test, but it should not be assumed for a current report.

Some specifications for large power cables use a different standard. BS 8491, for example, covers large power cables above 20 mm diameter at 600/1000 V, combining flame, mechanical impact and a water jet, with survival classes of 30, 60 and 120 minutes.

Confirm the following before accepting a circuit-integrity claim:

• which cores or optical elements were monitored during the test;
• the applied voltage, where relevant;
• the failure criterion;
• whether mechanical shock or another project-required exposure was included.

"Fire-resistant" is not evidence until these conditions can be traced to a report.

Do low-smoke and LSZH results prove circuit integrity?

No. IEC 61034-1 defines the apparatus for measuring smoke density from cables burning under defined conditions, using light transmittance as the comparison measure. IEC 61034-2 is commonly used to classify low smoke with a minimum light transmittance of 60 %, and some sectors, such as rolling stock, require higher values. IEC 60754-1 addresses halogen acid gas content from cable materials, and a limit of 0.5 % (5 mg/g) is a common LSZH acceptance value. IEC 60754-2 addresses the acidity and conductivity of gases captured in solution; pH 4.3 or higher and conductivity of 10 µS/mm or lower appear as recommended values in its Annex A.

These methods answer different questions. A low-smoke result does not prove low halogen acid gas content, and neither result establishes circuit integrity. The two gas methods may test individual cable components or weighted material combinations, so confirm how the reported specimen represents the complete offered cable. The acceptance requirement must come from the applicable cable specification or project requirement, not from a generic "zero halogen" label.

Values are commonly cited figures from the listed standards. Confirm the edition and the acceptance requirement in the project specification before purchase.

How should a report be matched to the offered cable?

Compare each report with the quotation and datasheet before accepting a supplier declaration. The following identifiers should align:

• manufacturer and production location stated by the report;
• cable designation, rated voltage, conductor count and size;
• insulation, bedding, armour, screen and outer-sheath materials;
• overall diameter or tested size range, where the method depends on it;
• standard number, edition, part, category and exposure duration;
• sample identification, test date, laboratory and complete result pages.

A change in material, sheath or dimensions can change fire behaviour. If the offered construction differs, ask for a documented technical basis showing why the evidence still applies, or for new test evidence. A cable report covers the tested specimen and method only. It does not verify the complete installed circuit, so supports, joints, terminations, glands, penetrations, routing and installation workmanship remain separate interfaces. Record who approves each one and which site inspection or system test closes it. For wider guidance on reviewing test documents, see cable standards and test reports.

This article explains how to structure and review evidence. It does not select a fire-performance level for a particular building, jurisdiction or safety circuit; project designer and authority requirements determine which combination is necessary.

Frequently asked questions