FRLS Cable vs LSZH vs FR: What Each Grade Buys You in a Fire

FR stops flame travelling along a cable run but still smokes like PVC when it burns. An FRLS cable cuts smoke and acid gas to tested limits yet stays PVC underneath. LSZH removes halogens entirely and holds acid gas under 0.5 percent. Enclosed, high-occupancy spaces need LSZH. Life-safety circuits need fire survival construction.

Three letters on a cable datasheet decide what the inside of a stairwell looks like four minutes into a fire. Clinical reviews of enclosed-space fires put smoke inhalation behind 60 to 80 percent of sudden deaths at the scene. The flame rarely reaches people. The smoke almost always does.
FR, FRLS and LSZH are the industry’s three answers to that problem, and tenders keep treating them as interchangeable. That is how the wrong grade ends up in metro shafts and hospital risers, and why a share of the FRLS cable sold in India would not pass the tests the letters refer to.
Here is what each grade promises, where each belongs, and how to buy the chemistry instead of the label.

FR vs FRLS Cable vs LSZH

The grades separate on three measured properties: whether flame travels along the run, how much smoke the compound emits, and how much halogen acid it releases. Ordinary PVC fails all three. FR fixes flame travel. The FRLS cable adds tested smoke and acid limits. LSZH nearly removes both, because the chemistry underneath is different

Property	Ordinary PVC	FR	FRLS Cable	LSZH Cable
Resists flame spread	No	Yes	Yes	Yes
Oxygen index (typical)	23 to 25%	29% minimum	29% minimum	29% minimum
Smoke emission	Very high	High	Reduced (tested)	Minimal (tested)
Halogen acid gas	25 to 30%	25 to 30%	Around 20% max	Below 0.5%
Base chemistry	PVC	Modified PVC	Modified PVC	Halogen-free polyolefin
Governing references	IS 694 / IS 7098	Plus IS 10810 flame tests	Plus smoke and acid gas tests	IS 17048, IEC 60754, IEC 61034
Typical use	Being phased out of quality specs	General wiring	High-rises, hospitals	Metros, tunnels, data centres
Relative cost	Base	Small premium	Moderate premium	Higher premium

Figures are typical of a compliant product. Confirm exact values on the manufacturer’s type-test reports.

What Each Grade Promises

Each grade answers a different question about the same fire. Read them that way and most of the selection logic writes itself.

1. FR: Will the cable carry fire down the corridor?

FR is PVC modified with flame-retardant additives so a burning cable does not carry fire along its own run. The numbers behind the claim:

Limiting Oxygen Index of 29 percent minimum, against 23 to 25 for ordinary PVC, so the compound starves and self-extinguishes in normal air
Temperature index of at least 250°C, keeping that behaviour honest at fire temperatures
Verified under IS 10810 (Part 61) and IEC 60332-1 on a vertical sample

The catch is that FR is still PVC. In a large sustained fire it burns, and when it burns it smokes like PVC. FR protects the cable route. It does nothing for the people breathing near it.

Sensible for general wiring, open industrial areas and low-occupancy spaces, and no further. When visibility starts to matter, the FRLS is the step up.

2. FRLS Cable: Will people see the exit?

Burning PVC throws dense black smoke, and in an enclosed corridor visibility drops to near zero within minutes. The exit was never far. People could not find it. An FRLS cable (Flame Retardant Low Smoke) is built against exactly that:

Smoke density held inside IS 10810 (Part 63) and ASTM D2843 limits
Halogen acid capped near 20 percent by weight under IS 10810 (Part 59), against 25 to 30 for ordinary PVC

It is still PVC chemistry, so it reduces the hazards rather than removing them. That makes the FRLS cable a middle rung: right for high-rises, hospitals, hotels, schools, control rooms and panel wiring, and the wrong answer for enclosed, evacuation-critical spaces.

3. LSZH Cable: Will the air stay breathable, and will the electronics survive?

PVC contains chlorine, and burning chlorine becomes hydrogen chloride gas: toxic on its own, hydrochloric acid on contact with the moisture in eyes, throat and lungs. The same acids settle on switchgear and server boards far from the fire zone, where corrosion losses regularly outrun the flame damage.

A zero halogen cable removes the chlorine, fluorine and bromine entirely, so there is nothing to release. The compound is a polyolefin base loaded with mineral fillers such as aluminium trihydroxide, which give off water vapour as they burn, cooling the flame and diluting combustible gases.

The tested numbers:

Acid gas below 0.5 percent under IEC 60754-1, against roughly 20 percent for an FRLS cable
Smoke inside the IEC 61034 3-metre cube limits
Bunched-cable flame performance to IEC 60332-3, the harder test that simulates loaded trays
IS 17048 covers the halogen-free construction in India

Two honest limitations: LSZH compounds are less flexible than PVC in cold weather, and LSZH cables cost more. Both are small against what the material buys. This is the grade metro tenders, airports, underground stations, data centres and hospital specifications keep mandating, with the National Building Code pushing low-smoke requirements for high-occupancy buildings.

Fire Survival: Does the alarm keep ringing?

FR, FRLS and LSZH describe how a cable behaves while it burns. Fire survival (FS) construction keeps the circuit working inside the fire, using mica glass tape over the conductor and testing to IEC 60331 and BS 6387, including variants with water spray and mechanical shock at temperatures up to 950°C.

Alarms, emergency lighting, smoke extraction and fire pumps take FS cable, usually with an LSZH outer sheath. An FR or FRLS cable that merely burns cleanly does not qualify.

How Do You Choose Between FR, FRLS and LSZH?

Three questions settle it, taken in order: how hard is evacuation, what does the smoke damage besides people, and does anything need to keep working during the fire.

1. Evacuation first

Ground-floor shop with two exits: FR is defensible. Fifteenth floor, single stairwell, two hundred occupants: an FRLS cable at minimum. Underground platform with one escalator: LSZH, no debate. The slower and more crowded the escape, the harder the case for a low smoke cable, and in enclosed spaces, for zero halogen.

2. Then the equipment

If the space holds servers, switchgear, PLCs or medical electronics, halogen acid will corrode hardware far outside the fire zone. For data centres and control rooms, LSZH protects the balance sheet as much as the occupants.

3. Then the circuits that must not stop

Fire alarms, emergency lighting, smoke extraction, fire pumps: fire survival construction, full stop.

And the cost objection: It is real and it is small. LSZH cost more than FRLS, which costs more than FR, and cabling is a small slice of a project’s electrical budget. The upgrade is a fraction of that slice, buried in walls and shafts for 25 years where it cannot be inspected and is expensive to replace.

Nobody standing in a smoke-filled stairwell has ever wished the project had saved two percent on cable.

Check the project documents before applying any of this. Metro, airport, refinery, and institutional specifications often mandate the grade outright. The framework is for projects where the specifier still has discretion.

Where the Specification Goes Wrong

The misprinted label. Because the FRLS cable commands a premium, FRLS is the most freely misprinted label in the Indian cable market. Plenty of cable sold as FRLS is ordinary PVC with different lettering on the sheath. The properties live in the compound chemistry, and chemistry is invisible on inspection.

The LSF look-alike. A second trap wears a similar badge: the low smoke low halogen cable, sold in places as LSF. It is modified PVC. Treat it as FRLS-class performance regardless of the sheath print. Only a certified halogen-free compound earns LSZH-class treatment.

The single-grade tray. The grade gets written for the power cable runs and nothing else, but everything shares the route:

Instrumentation cables and signalling pair cables in metro shafts and risers
Shielded twisted pair cable serving communication and control systems
The twisted pair cable network running through data centre ceiling voids alongside power

All of it burns in the same fire. One ordinary PVC run bundled with LSZH cables is a fuel line through an otherwise clean installation, so the mandate should cover instrumentation cables and comms runs, including shielded twisted pair, with the same rigour as the power cables themselves. And the performance has to run through the whole construction: a low smoke cable sheath over ordinary insulation behaves like ordinary cable once the sheath is breached.

The paperwork defence. It is short. Ask for type-test reports against the specific clauses:

IS 10810 Parts 61, 63 and 59 for an FRLS claim
IEC 60754-1 and IEC 61034 for LSZH
The BIS licence number where the standard applies

Buy FRLS cables and halogen-free cables from manufacturers whose compounding and testing you have reason to trust. The letters cost nothing to print.

The Standards, In One Place

IS 694 and IS 7098: base product standards for PVC and XLPE insulated cables
IS 10810 (Part 61): flame propagation test behind the FR claim
IS 10810 (Part 63) and ASTM D2843: smoke density tests behind the FRLS cable label
IS 10810 (Part 59): halogen acid gas measurement for PVC-based grades
IEC 60332-1 and IEC 60332-3: single-cable and bunched-cable flame tests
IEC 60754-1: halogen acid quantification for LSZH, below 0.5 percent
IEC 61034: 3-metre cube smoke density test
IS 17048: Indian standard for halogen-free flame retardant (HFFR) cables
IEC 60331 and BS 6387: circuit integrity tests for fire survival cables

Frequently Asked Questions

1. What is the full form of FR, FRLS and LSZH?

FR is Flame Retardant. FRLS is Flame Retardant Low Smoke, meaning an FRLS cable pairs FR behaviour with tested smoke and acid gas limits. LSZH is Low Smoke Zero Halogen, also written LS0H or HFFR (Halogen-Free Flame Retardant).

2. What does low smoke zero halogen LSZH actually mean?

The compound contains no chlorine, fluorine or bromine, so it cannot release halogen acid gas when it burns, and it is formulated to emit minimal smoke. Acid gas stays below 0.5 percent under IEC 60754-1, and smoke is tested under the IEC 61034 3-metre cube method.

3. Is an FRLS cable the same as an LSZH cable?

No. An FRLS cable is modified PVC that reduces smoke and acid gas to tested limits. A low smoke zero halogen cable is a halogen-free compound that nearly eliminates both. LSZH is the strictly higher grade. In an enclosed space, the difference is measured in survivability.

4. Is LSZH mandatory in India?

There is no single blanket mandate. LSZH cables are specified in metro rail projects, airports and many institutional and high-rise tenders, and National Building Code provisions push low-smoke requirements for high-occupancy buildings. Always check the project specification and local fire norms, since many tenders now specify LSZH over the FRLS cable for enclosed spaces.

5. What is the difference between LSZH and fire survival cable?

LSZH defines how cleanly a cable burns. Fire survival defines whether the circuit keeps operating during a fire. Life-safety circuits such as fire alarms, emergency lighting and smoke extraction need fire survival cables, usually with an LSZH sheath, tested to IEC 60331 or BS 6387.

6. Which grade should I use for house wiring?

For ordinary homes, FR is the common baseline and an FRLS cable is a sensible upgrade, particularly for flats in high-rise buildings where escape routes are shared. Whatever the grade, the wiring should carry BIS certification to IS 694.