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1. What is the CWZ Test? 

The **CWZ Test** is a fire performance evaluation specified under **BS 6387**, used to determine a cable's ability to maintain **circuit integrity** under fire conditions. **CWZ** is the highest classification, requiring cables to pass three separate tests: 

• C (Combustion): Resistance to direct flame exposure.
• W (Water): Resistance to fire combined with water spray.
• Z (Shock): Resistance to fire combined with mechanical impacts.

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2. Breakdown of the CWZ Tests 

1. Resistance to Fire (C): 
- The cable is exposed to a flame at **950°C for three hours**. 
- This tests the cable's ability to maintain electrical continuity during prolonged fire exposure. 

2. Resistance to Fire and Water (W): 
- The cable is exposed to flame at **650°C for 30 minutes**. 
- A water spray is applied for an additional **15 minutes** to simulate firefighting conditions. 
- The cable must remain operational during this test. 

3. Resistance to Fire and Mechanical Shock (Z): 
- The cable is exposed to flame at **950°C for 15 minutes**. 
- During the test, a steel bar strikes the cable every **30 seconds**, simulating falling debris. 
- The cable must maintain circuit integrity under combined fire and impact. 

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3. Applications of CWZ-Tested Cables 

Cables achieving the **CWZ classification** are used in critical systems where maintaining circuit integrity during fire conditions is essential. Typical applications include: 

• Emergency Lighting Systems: Ensures lights remain operational during evacuation.
• Fire Alarm Systems: Prevents system failure during fire incidents.
• Tunnels and Subways: Supports ventilation and communication systems under fire conditions.
• High-Rise Buildings: Protects power supply circuits for firefighting systems.

Example Use Case: 
A CWZ-compliant cable used in a tunnel lighting system ensures that: 
- The lighting system remains functional during fire exposure (C). 
- Water from sprinklers or hoses does not short-circuit the cables (W). 
- Mechanical impacts do not disrupt the circuit (Z). 

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4. CWZ vs. Modern Standards 

While **BS 6387** remains a reference for fire-resistant cables, newer standards like **BS 8519** provide more stringent testing. 

Key Differences: 

• **BS 6387 (CWZ):** Tests are conducted on separate cable samples for each condition (fire, water, and shock).
• **BS 8519:** Tests are conducted on the same cable sample, applying fire, water, and mechanical impacts simultaneously, providing a more realistic assessment of cable performance.

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5. Conclusion 

The **CWZ Test** evaluates cables' fire resistance through separate tests for **combustion (C)**, **fire and water (W)**, and **fire with mechanical shock (Z)**. This classification ensures cables can withstand extreme fire conditions, making them suitable for **critical infrastructure** like hospitals, tunnels, and emergency systems. 

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