Fire in a public place can cause significant loss of human life and damage building structures and other properties. Further, in the case of fire, the primary cause of death is the inhalation of toxic gas and the dense smoke that blocks the view, preventing easy escape. So, how important alarm systems are! And you are supposed to use the right cables because the alarm system will stop working if electric cables get damaged. As a result, companies are working and investing in developing fire-resistant cables that can deliver the best performance in case of fire.
Where do you need fire-resistant cables?
How long a cable can work in the case of fire is its resistant-to-fire capability. The higher the resistance to fire capability of a cable, the longer the fire fighting installations can work. Safety and rescue equipment needs a tight power supply; otherwise, the systems will not work in extreme conditions.
In addition, Fire-resistant cables maintain the electrical circuit’s integrity in safety and rescue equipment. In addition to this, they also limit the emission of smoke and gas, protecting human life and equipment. You can use fire-resistive cables in different systems such as:
- Sprinkler systems
- Emergency lighting speakers
- Booster pump systems
- Rescue elevators
- Smoke exhaust systems
- Fire and smoke detector systems
- Alarm horns
Construction of fire-resistant cables
A fire-resistant cable generally has the following cable construction:
- Stranded or flexible copper conductor (annealed)
- Fire-resistant barrier – MICA tape
- Primary insulation materials such as XLPE or EPR, or EVA
- Bedding and cable sheath materials such as Flame retardant Low Smoke Zero Halogen and Poly Vinyl Chloride
Types of fire-resistant cables
Cable hazards from the fire include ignition, flame propagation, and the evolution of smoke and toxic gas. So, fire-resistant cables are supposed to be:
Fire retardant cables:
You can resist fire with these cables. However, if the cable burns, it destroys entirely and has no circuit integrity. Systems connected to these cables go out of work. Further, These cables cannot assure service during the fire, but you can use them to prevent the spread of fire.
These cables maintain the electrical continuity of the circuits in the case of fire. You can use these cables in all types of fire alarm systems. With these cables, it is possible to evacuate buildings, start alarm signals and activate fire extinguishing systems.
In the case of fire, the quality and quantity of gas emitted are also responsible for the loss of people and property. Therefore, companies also need to work to prevent poisonous gas emissions and reduce the smoke density. One of the popular materials that companies use in the insulation of fire-resistant cables is PVC or polyvinyl chloride. PVC has a chlorine compound and thus shows excellent fire-retardant properties.
LSHF Cables | LS0H Cables | LSZH Cables
LSF emanates thick black smoke and fumes because Poly Vinyl Chloride is present in them. So, you need Halogen Free Cables or their variations, LS0H (zero Halogen with low smoke) and LSZH (zero Halogen with low smoke). No PVC is in these cables. Hence no harmful fumes are given off in case of fire. In addition, when burnt, not only do LSHF Cables exude a very small amount of smoke, but also, this smoke has no chemicals in it.
Image: Flammability test cable
Levels of fire-resistant cables
As a matter of fact, you will find that cables with no enhanced fire properties can quickly burn or ignite in the case of fire. On the other extreme, you will see fire survival cables. The choice of cable for a particular application depends on the degree of toleration and the level of performance needed.
Image: electric fire cables from the mains voltage overload
How to test fire-resistant cables
Several tests can evaluate the performance of individual materials and whole cables. Fire test methods for cables are of two types:
- Firstly, those which test the whole cables
- Secondly, those which evaluate individual materials.
Flame Propagation Test:
With this test, you can measure the vertical propagation of flames for a single electrical insulated cable or conductor under fire conditions.
First, suspend a 600mm long cable vertically in a draught-free enclosure for the vertical flame tests. Now, expose its lower end to the gas burner exposed to the 45 degrees horizontal. Apply flame for 1-8 minutes, depending on the cable diameter. The single insulated cable or conductor passes the test when the distance between the onset of charring and the top support’s a lower edge. On the other hand, if charring reaches a point greater than 540mm from the top support’s lower edge.
Flame Spread Test:
This test measures the resistance to the spread of flames in vertically mounted bunched cables and wires under fire circumstances.
In this test, you can simulate a real environment of cable installation. Here, you use bunched cables around 3.5mm long in a vertical arrangement. You can use ribbon-type propane or an air burner as the ignition source, providing a fuel input of 73.7 Mj/hour. First, fasten the cables at the foot of a ladder and arrange the burner horizontally. Now, apply the flame for 20-40 minutes, depending on the test category. The bunched cables and wires can pass the test when the maximum charred portion has not reached more than 2.5m above the burner’s bottom edge.
The categories differ based on test duration, method of sample mounting, and the volume of non-metallic material.
Fire Resistance Test
With this test, you can evaluate wires and cables designed as fire survival cables suitable for emergency lighting, alarm circuits, and other emergency services. This test helps assess whether such cables can maintain the integrity of electrical circuits at temperatures ranging from 650 degrees Celsius to 950 degrees Celsius for up to 3 hours.
While performing the test, first, mount the energized cable in a test chamber and expose it to a flame from a ribbon-type burner at an appropriate temperature. This test has many variations, and depending on cable performance in different categories, you can rate the cable. The different categories include
- Resistance to fire alone
- Resistance to fire with water
- Resistance to fire with mechanical shock
Acid Gas Emission Tests
You can use a tube furnace to generate fire gas. Analyze these gases in various ways. Some subsidiary parts of this method determine the fire gases’ halogen acid content and acidity.
Determination of halogen acid gas content:
This test determines the halogen acid gas except hydrofluoric acid evolved after the combustion of halogenated polymer compounds.
First, heat a burner to 8000 degrees and place 1 gram of the sample inside it. Now, you will see that HCl gas evolves from it and gets absorbed in the water chamber. Now, measure the water with acidity. If the Hydrochloric acid is less than 5mg/g, the cable sample contains LSHF.
Determination of acidity:
The test evaluates the degree of acidity in the gases evolved during cable combustion by measuring their pH and ionic conductivity.
Smoke Emission Test
Importantly, the evaluation of smoke is another critical indicator of cable performance. You can use several methods for this based on optical or gravimetric techniques. You can perform a 3-meter cube test to determine the quantity of electrical cables emitted during a fire.
First, project a light beam from one window to a photocell enclosure attached to another window. Now, connect a recorder to this photocell enclosure and adjust it to register 0% for full obscuration and 100% for luminous transmissions. Then, place a one-meter cable in a 3m3 enclosure and put fire in the container. Finally, record the minimum light transmission. Express the result in the percentage of light transmitted.
If the value exceeds 60%, the sample passes the test.
Limiting Oxygen Index or (LOI)
A limited Oxygen index indicates a material’s flammability. Indeed, it refers to the minimum oxygen percentage in an oxygen/nitrogen mixture needed to support a given material’s combustion at room temperature. You can perform the test with a small vertical test sample over a temperature range of 250C and 1500C under specific test conditions. You can measure the result as an oxygen index at specific temperatures.
To allow burning, flame retardant cables and materials need an oxygen level higher than the atmosphere, i.e., 21%. A flame retardant material having an oxygen index of 26 or greater is considered self-extinguishing. The oxygen index of cables with PVC as outer sheath ranges from 28% to 32%, and for LSHF cables, it is 33% to 45%.
Several safety circuits and other life support circuits must function in the case of fire. Thus, choose the cables for fire exposed electric installations wisely. You can contact Cloom for superior Cable design and reliable fire-resistant cables. We deal in a wide range of cable assemblies and wiring harnesses to suit all applications.