Fire-resistant cable sizing - FAQ

According to their behaviour in fire conditions, cables can be classified in three categories

• Standard cables: They do not have any specific performance and when affected by a fire they propagate it easily producing harmful smokes and hot gases all over the building.

• Low Fire-Hazard cables (LFH): They are characterized, at first, by a low smoke emission when burning, thus, without impacting the visibility during the escape and the emission of harmful substances is extensively reduced:

  • A low level of smoke release in case of fire is key to ensure a safe escape of people. When burning LFH cables ensure a visibility that is 5 to 10 times higher than standard designs. The visibility is higher than the 10 m recommended for a safe evacuation.

  • A low emission of harmful substances (smokes, particles) reduces the risk on health at short, mid or long term. 

  • The smoke released is not corrosive making easier the escape of people without impacting the building structures or electrical devices.

In order to reduce the harmful emissions and the smoke propagation and opacity, the materials composing the cables must not contain the following elements:

• Fluorine (FEP, PVDF)
• Chlorine (PVC, CPE)
• Bromine (BFR)
• Iodine

The absence of these elements, called Halogen, improves significantly the cable behaviour in fire.

Low Fire-Hazard cable do not initiate the fire when affected by a low ignition source (spark, lighter...),  here they are claimed being flame retardant. For the highest category, LFH cables are fire retardant. The fire damage is contained on a short distance ensuring a propagation when vertical not higher than a floor height. The heat release is also limited.

• Fire resistant cables: Fire safety systems such as emergency lighting, video surveillance, voice speakers, alarm signals, sprinkler pumps, smoke extraction systems, etc. shall remain operational during evacuation and firefighting. Therefore, their cabling must maintain their functionality, transmitting energy or signals despite exposure to fire at extreme high temperature. Cables shall be resistant to fire. Depending on countries, different designs can be proposed. They are adapted to the local regulations, installation rules, product and test standards.

The various stakeholders shall be made aware of such risks, as was shown in a recent survey organized by Europacable at EU level among the main stakeholders. The objective was to understand how and if people were aware of specific rules to size fire resistant power cables considering fire conditions.

880 answers were received from all the European countries with 80% installing fire safety circuits at least once a year. 

• 50% of the them were considering the sizing of a fire-resistant installation as a standard installation when only 29% are taking into account the theoretical fire temperature. 
• It was also pointed out that only 21% of the “population” was using the cable length installed in the fire zones to adapt the cable designs.

The necessity of informing with trainings, calculation method and software was clearly highlighted in the survey.

The contribution of cables is key to warrant the supply of energy and the transmission of information to security systems. Fire resistant cables ensure their functionality even at extreme temperatures.

Therefore, selecting a cable range that is classified as fire resistant is mandatory when designing fire safety electrical circuits. 

Next, the cable size, its cross section shall be defined. The application calls for a different calculation than the one used for cables operating in normal conditions (70°C or 90°C being used for the maximum conductor temperature). When considering fire resistant cables, the sharp increase of the electrical resistance with temperature shall be considered. This can lead to voltage drops downgrading the performance of the electrical systems.  

Whatever the performance required, it is the responsibility of individual countries to define their own regulation. When dealing with fire safety in buildings, the national regulations defined the classification to be adopted. This classification refers to performances that shall be reached ensuring the correct fire safety level depending on criterion such as building types, conditions of evacuation, people density…

When dealing with resistance to fire for electrical installation and especially cables, the classification expresses a minimum time of integrity. This is the minimum duration a cable will maintain the electrical continuity of circuits in fire conditions. As with other fire safety equipment, fire-resistant cables are classified as having a minimum time of integrity, usually 30, 60, 90 or 120 minutes.

This classification refers also to test methods to be adopted. They make possible assessing the cable according to different scenario. Again, it is the responsibility of individual countries to select the test standard(s) to be implemented. There is not yet a full harmonization even though in some areas we see positive trend (e.g. Europe with the CPR reaction and resistance to fire). Nevertheless, one of the main concept is based on a specific temperature-time curve, ISO 834, that represents a cellulosic fire representative of fire in buildings. After 30, 60, 90 or 120 minutes, the temperatures is respectively of 842°C, 945°C, 1006°C and 1049°C. The other option is to applied a constant attack at a nominal temperature for the full duration requires by the test method.

Fire compartmentalization or compartmentation is a vital safety strategy in building to limit the spread of fire and smoke. The concept is based on dividing the building into fire-resistant areas able to contain a fire for a certain period of time defined in the regulation. Each compartment is reinforced by using fire-resistant materials, by installing measures such as fire doors, fire barriers…
The main objective of compartmentation is to contain a fire within a cell without affecting the others. This allows more time for occupants to safely evacuate a building and for fire services to extinguish the blazes. Whilst safety is essential, another common objective of fire compartmentation is to prevent a fire from reaching parts of a building that are of particular value or contain hazardous materials.

Key elements of fire compartmentation include:

• Fire resistant barriers: fire-rated walls, floors, and doors

• Fire stops: seal gaps and penetrations in fire-resistant barriers so that fire and smoke cannot pass through hidden routes, such as ducts, pipes, cable trays

• Smoke  control: limitation of smoke spread

• Fire doors and dampers

In case of fire, considering the temperature time curve ISO 834, The electrical resistance of the copper conductor increases:

• x 4.71 after 30 min (842°C)

• x 5.22 after 60 min (945°C)

• x 5.52 after 90 min (1005°C)

This impacts the performance of the electrical line on the portion that is affected. Finally, the standard way for selecting the cable design, its cross section, is not anymore valid to ensure the efficiency and reliability of the system as a voltage drop is induced. 

This parameter should be considered. A tolerance is usually given in the equipment specification or we refer to a voltage drop not higher than 10%. This information makes possible the correct selection of the cable cross section to be installed.

The Wiedemann-Franz law states that the ratio of the thermal conductivity to the electrical conductivity is the same for all metals at a given temperature. Wiedemann-Franz law is the law which relates the thermal conductivity (κ) and the electrical conductivity (σ) of a material which consists of somewhat freely moving electrons in it.

• Thermal Conductivity (κ): It is the degree (measure) of capacity of a material to conduct heat.

• Electrical Conductivity (σ): It is the degree (measure) of capacity of a material to conduct electricity.

In metals, when temperature increases, the velocity of free electrons increases and that leads to an increase in heat transfer and it also increases the collisions between the lattice ions and free electrons. This results in the drop in electrical conductivity. The law defines the ratio of the electronic role of the thermal conductivity of a material to the electrical conductivity of a material (metal) is directly relative to the temperature.
The Wiedemann-Franz method is therefore based on the laws of physics. This method also takes compartments into account.

In normal electrical applications, up to temperatures around 200 °C, the copper conductor’s electrical resistance varies linearly with temperature. When higher, representing fire conditions, the relation between temperature and electrical resistance becomes non-linear. Gustav Heinrich Wiedemann and Rudolph Franz have proposed a model, the Wiedemann-Franz law, to predict the electrical resistance related to temperature evolution. This can be applied to understand the possible effect of fire on the performance of cables.

It is the responsibility of individual countries to define their own rules. Then, the current installations are meant to comply with existing regulations. 

When dealing with fire safety in buildings, the national regulations defined the classification to be adopted. This classification refers to performances that shall be reached ensuring the correct fire safety level depending on criterion such as building types, conditions of evacuation, people density…

Until such requirement is clearly stated and enforced, the current installations shall follow the regulations in force. However, looking at current trends and the risks entailed, Nexans, as a leading cable manufacturer and fire safety expert, strongly recommends to follow such method and adapt the corresponding voltage drop calculation for fire resistant cables taking into account fire conditions, compartmentalization and equipment specifications.

A risk analysis of the different situations should be envisioned considering:

• Fire resistant cable classification in the country to determine the maximum temperature seen in case of fire
• The building drawings to determine: 
  • the maximum length possibly affected by the fire
  • the total length of the line from the voltage supply up the final equipment
• Voltage drop allowed to ensure a correct efficiency of the fire safety services.

Based on this, the optimized cross section should be determined and compared to the real installation. The potential efficiency loss should be determined and depending on the criticity, specific measures should be considered.