Measurement of Smoke Density of Cables Burning Under Defined Conditions

Introduction

Building, transportation, and industrial cables can have adverse effects in increasing dangerous smoke levels in case of fire incidents. The determination of the amount of smoke density when the material is combusted to produce smoke is important in evaluating the performance of a fire of such materials. This information is used to classify fire hazards and comply with safety laws, enabling manufacturers to produce cables with reduced smoke emissions, thereby increasing occupant protection and aiding evacuation.

Instrumentation

Smoke density measurement is typically carried out using a smoke density chamber (e.g., NBS smoke chamber), a source of radiant heat (normally an infrared or electric furnace), optical sensors (most typically photodiodes or photocells), and a data acquisition system. The chamber is developed in such a way as to make a sealed, controlled combustion environment. The optical system maintains a constant check on the amount of transmitted light across the smoke and shows a measurement of the density of the smoke on a quantitative basis, in real-time.

Principle and Methodology

The given test is usually applied in cases with the application of a radiant heat source that heats the horizontal specimen of cable in a closed chamber. The heat causes pyrolysis and combustion, whereby the smoke that develops is enclosed and quantified. A beam of light is made to move across the chamber, and the extent to which smoke has affected the intensity of the beam is measured as time goes by. Since light transmittance decreases, light transmittance is used to calculate the density of smoke, which is often referenced as specific optical density (Ds). It standardizes the time of the test, the positioning of a sample, and radiant heat flux so that any samples of the different types of cables could be comparable and replicated.

Strengths 

The capacity to simulate realistic conditions of fire and to offer quantitative values of smoke generation is one of the main strengths of this test. It is standard, repeatable, and sensitive to low levels of smoke and, as such, allows comparison of formulations and measurement of regulatory compliance.

Limitations

The method can, however, take all characters, like the flame spread or the evolution of toxic gases, into consideration. It is also the case that results can be geometry-specific as well as depend on installation cabling layouts that may restrict field applicability.

Importance

The process of smoke density testing plays a crucial role in the improvement of fire safety in space, wherein the cabinets are highly frequented with cables and must be evacuated within a critical time frame. The use of low-smoke cables is desirable in public buildings, especially mass transit and data centers, where visibility during the occurrence of fire is necessary during emergency response and for the safety of the occupants.