Understanding Cable Fire Ratings: FT4, FT6, and LSZH

The Reality of Cable Tray Fires

In an industrial environment, cable trays aren’t just pathways for power and control, they are potential highways for fire. A localized arc flash or equipment fire can quickly travel across a facility if the cables themselves act as fuel. This is why understanding fire ratings like FT4, FT6, and LSZH is critical during installation and retrofits.

FT4 vs. FT6: The North American Standard

In Canada and the US (under CSA and UL), cables are subjected to vertical flame tests:

• FT4 (Vertical Tray Flame Test): This is the baseline standard for cables in trays in most industrial environments. An FT4 rated cable will self-extinguish and limit the spread of fire along the tray. If a cable lacks an FT4 rating, it acts as a wick, spreading the fire rapidly.
• FT6 (Plenum Rated): FT6 cables are subjected to an even more rigorous test that not only limits flame spread but severely restricts smoke generation. They are mandated when cables run through environmental air handling spaces (plenums).

The Hidden Danger: Toxic Smoke and Equipment Loss

While FT4 and FT6 address flame spread and smoke volume, they don’t solve toxicity. Many standard cable jackets are made of PVC (Polyvinyl Chloride). When PVC burns, it releases hydrogen chloride gas.

If this gas mixes with moisture in the lungs, it forms hydrochloric acid, which is lethal to inhale. But the push for LSZH in mining hasn’t just been for life safety. When hydrogen chloride mixes with ambient moisture, it becomes incredibly corrosive. This acid can completely destroy the sensitive electronics inside a switchroom or MCC long after the actual fire is put out.

Low Smoke Zero Halogen (LSZH) cables are engineered to solve this. When they burn, they emit very little smoke and, crucially, zero halogens (no acid gas).

Actionable Takeaways

• Never substitute unrated cable into an industrial tray. Pulling standard house wire or non-FT4 control cable into a tray instantly compromises the fire integrity of that entire run.
• Weigh LSZH against Teck 90 in high-risk areas. While Teck 90 XLPE remains the undisputed workhorse in mining and heavy industry, its standard PVC jacket produces toxic smoke when burned. You generally see LSZH being actively specified for high-risk, confined areas where smoke extraction is difficult, such as:
  • Refuge stations
  • Dead-end electrical cutouts and underground substations
  • Shaft communications and emergency lighting systems
• Check the jacket stamp. The rating (FT4, FT6, LSZH) must be visibly printed directly on the cable jacket. If you can’t verify it, don’t install it in a rated tray.