Top 3 influential factors when choosing fire-resistant cables

When choosing fire-resistant cables, it is crucial to consider several factors that can influence the severity of a fire or explosion incident, such as toxic smoke, smoke density, and the speed of fire spread. Ensuring that fire-resistant cables meet the requirements of each specific application, installation method, and comply with standards and regulations is essential.

1.1 Fire spread speed

Fire typically starts at a single point. The characteristics of the cables and the operating environmental conditions directly affect the speed at which the fire spreads to other areas, potentially increasing the severity of the fire.

1.2 Amount of smoke produced

The density of smoke produced depends on the chemical composition of the cable material when burned. Such smoke can be hazardous to health when inhaled, impairing visibility and hindering first aid efforts. Therefore, fire-resistant cables are often made of halogen-free materials, which do not produce toxic fumes when burned.

>>See more: Why should halogen-free cables be used?

2.1 Conductor material and core structure of fire-resistant cables

Aluminum has a melting point of 685℃, while fire-resistant cables must withstand temperatures up to 750℃. Thus, aluminum conductors are unsuitable for fire-resistant cables. Copper, with a melting point above 1000℃, is suitable for fireproof cables.

Copper wires with a cross-section of 1.0mm² or less do not melt at high temperatures but are prone to breakage due to local overheating and slight external influences. Therefore, they are unsuitable for fireproof cables.

An uneven conductor surface can damage the mica tape and compromise the cable's performance. Cables with a large cross-section are difficult to make fire-resistant. Round core conductors are preferred for fire-resistant cables because their smoother surface exerts a more uniform force on the mica tape, reducing damage and cost compared to fan-shaped structures.

>>See more: A glance at conductors' cross-section and their applications

2.2 Mica tape selection

Mica is the material mainly used for fire-resistant cables. Compared to standard cables, mica is able to withstand higher temperatures, has high stability, ensuring the safety of the electrical system.

2.3 Insulation material of fire-resistant cables

Commonly used materials include PVC, PE, and XLPE. PE and XLPE are more effective than PVC at high temperatures. When burned, they produce fewer solid residues, which minimizes damage to the mica tape.

The main components of PVC are PVC resin and CaCO₃ that adhere to the outside of the mica tape according to its combustion mechanism. Its residual combustion solid mixture Cao-CaCl₂ is in a relatively stable state and adheres to the outside of the mica tape during combustion. It will create internal stress, compress the mica tape, damage the mica tape and affect its fire resistance.

PE and XLPE, being pure hydrocarbons, primarily produce CO₂ and water vapor upon combustion, with minimal solid residues that do not damage the mica tape. Therefore, while PVC, PE, and XLPE can all be used as insulation materials, PE and XLPE are preferable for fire-resistant cables.

Fire-resistant cables need to strictly comply with national, regional, and international standards and regulations to ensure the safety of property and human lives. There are many different standards, depending on the specific country and region:

3.1 IEC standards

IEC (International Electrotechnical Commission) classifies the fire performance of electric cables into many different classes. The IEC Standard for Flame Retardancy evaluates the flame propagation of single wires (IEC 60332-1, IEC 60332-2, IEC 60332-3) and bundled cables (IEC 60332-3).

The IEC 60332-1-2 flammability test requires the fire-resistant cables to be fixed vertically and exposed to a Bunsen burner at a 45° angle from 60 minutes (≤25 mm) to 120 minutes (25<D≤50mm). Damage to fire-resistant cables must stop at least 50mm below the fixing clamp and the fire-resistant cables must be self-extinguishing.

The IEC 60332-3 flammability test requires the cables to be placed permanently on a ladder, either close or far depending on the type of fire. The temperature is determined by the specific volumes of propane and air. The duration can range up to 20 minutes, depending on the cable type. Fire damage is visible up to 2.5m from the bottom of the burner to the top.