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How Are Fire Resistance Ratings Classified for Industrial Cable Assem...

Industrial cable assemblies are critical components in countless facilities, powering essential machinery and control systems. Their performance during a fire is paramount, not only for minimizing fire spread but crucially for maintaining critical circuit integrity (CI), ensuring crucial safety systems like fire alarms, emergency lighting, ventilation, and exit signs remain operational long enough for safe evacuation and firefighting efforts. Fire resistance ratings provide a standardized measure of this vital performance.

Unlike flammability ratings (which measure how easily a cable ignites or propagates flame), fire resistance ratings specifically assess a cable assembly’s ability to continue functioning while directly exposed to fire and high temperatures for a defined period. These ratings are defined through rigorous, standardized tests simulating severe fire conditions. The primary classifications for industrial cable assemblies are designated as Class A, Class B, and Class C, typically measured in minutes or hours (e.g., 30 minutes, 60 minutes, 90 minutes, 120 minutes, or 180 minutes). This rating indicates the minimum duration the cable assembly is designed to maintain circuit integrity under the specified test fire conditions.

The classification system (A, B, C) defines the intensity and conditions of the fire test itself:

Class A Fire Resistance:
- Test Fire Severity: Most severe test conditions.
- Typical Temperature: ~950-1000°C flame temperature.
- Applicable Standard Test: Primarily defined within IEC/BS EN 60331-11/-21/-23/-25 (formerly BS 6387 Categories CWZ).
- Rating Example: A cable rated Class A 120 maintains circuit integrity for at least 120 minutes under the intense Class A furnace test conditions.
Class B Fire Resistance:
- Test Fire Severity: Intermediate severity test conditions.
- Typical Temperature: ~750-800°C flame temperature.
- Applicable Standard Test: Primarily defined within IEC/BS EN 60331-11/-21/-23/-25.
- Rating Example: A cable rated Class B 90 maintains circuit integrity for at least 90 minutes under the Class B furnace test.
Class C Fire Resistance:
- Test Fire Severity: Less severe test conditions.
- Typical Temperature: ~650°C flame temperature.
- Applicable Standard Test: Often associated with tests like IEC/BS EN 60331-11/-21/-23/-25 and also UL 2196 “Fire Tests for Circuit Integrity (CI) Cables” in North America.
- Rating Example: A cable rated Class C 60 maintains circuit integrity for at least 60 minutes under the Class C furnace test.

Core Fire Resistance Test Standards:

IEC/BS EN 60331 Series: This is the predominant international (IEC) and European (EN) standard for fire resistance testing of cables designed to maintain circuit integrity. Specific parts of the standard define the test methods for different cable types and installation scenarios (e.g., exposed, buried, in ducts):
- BS EN 60331-11 / IEC 60331-11: Tests for electric cables under fire conditions – Circuit integrity – Part 11: Apparatus – Fire alone at a minimum temperature of 750 °C (Standard severity applicable for Class B and C core definitions; higher temp needed for Class A).
- BS EN 60331-21 / IEC 60331-21: Tests for electric cables under fire conditions – Circuit integrity – Part 21: Procedures and requirements – Cable of rated voltage up to and including 0.6/1.0 kV.
- BS EN 60331-23 / IEC 60331-23: Tests for electric cables under fire conditions – Circuit integrity – Part 23: Procedures and requirements – Electric data cables.
- BS EN 60331-25 / IEC 60331-25: Tests for electric cables under fire conditions – Circuit integrity – Part 25: Procedures and requirements – Optical fibre cables.
UL 2196: This is the primary North American standard for testing the fire resistance of electrical cable assemblies. While the core principle (maintaining circuit integrity under fire) aligns with IEC standards, the test furnace profile and specific procedures differ. UL 2196 ratings are typically given as 1 hour, 2 hour, or 3 hour.

Comparison of IEC/BS EN and UL Fire Resistance Classifications

Feature	IEC/BS EN 60331 Fire Resistance	UL 2196 Fire Resistance (North America)
Classification	Classes A, B, C (Defined by Test Fire Severity)	Time Ratings (1 Hour, 2 Hour, 3 Hour)
Rating Basis	Duration cable maintains CI under specific fire class test	Duration cable maintains CI under UL 2196 furnace test
Fire Severity	A > B > C	UL furnace provides a severe hydrocarbon-like fire curve
Key Standards	IEC/BS EN 60331-11, -21, -23, -25	UL 2196
Common Durations	30min, 60min, 90min, 120min, 180min (combined with Class)	1 Hour, 2 Hour, 3 Hour

What “Circuit Integrity” Means in Testing:

During these fire tests, the cable assembly is installed according to the manufacturer’s instructions within a furnace simulating severe fire conditions appropriate for the classification (A, B, C for IEC/EN or the UL profile). While exposed to high heat and flames, the cable must simultaneously withstand rated voltage without experiencing a short circuit or open circuit failure for the entire duration of the rating. This often involves monitoring conductor temperature rise (ΔT maximum limits) or verifying continued current flow without breakdown.

Key Markings:

Manufacturers clearly mark fire-resistant cable assemblies to identify their specific fire resistance classification and duration. Look for markings like:

Class A 120 (Meets IEC/BS EN 60331 Class A for 120 minutes)
Class B 90 (Meets IEC/BS EN 60331 Class B for 90 minutes)
Class C 60 (Meets IEC/BS EN 60331 Class C for 60 minutes or UL 2196 for 1 hour)
CI 120 / Circuit Integrity 120 (General indication of 120-minute CI performance)
UL 2196 2H (Meets UL 2196 requirements for 2 hours)