Electrical wiring is part of fire-protection systems; therefore, this must work reliably for a given period when under fire conditions. Defective or faulty cables are one of the leading causes of fire. Fires often initiate in unsafe and non-standard conditions, so we must understand the cables’ properties.
This article discusses how fire-resistant cables function and perform, including how the reliability and robustness of cables, or lack of these, impact safety.
During the initial stages of a fire, the fundamental first response concept is to prevent or reduce as much as possible the early fire development and propagation, fire extinguishment and to provide for occupants to evacuate under safe conditions. Fire-rated cables are suitable for use under fire conditions where high performance, reliability and protection of life and equipment are required:
- Cables installed as part of fire-protection systems are essential to improving building safety and, most importantly, for mitigating the loss of life.
- Certified fire cables are utilised in almost every residential, commercial building, factory, mall, educational or hospital facility to protect and save life and property.
The importance of fire testing of cables
Many fires occur from an electrical source of ignition. The fire performance of cables is a vital element in guaranteeing the safety and evacuation of people in case of fire.
We all understand that zero risk does not exist; however, risk can be mitigated and minimised. For this reason, fire performance tests are a prescriptive requirement per the rules and regulations of most countries for the construction sector globally.
Electrical cables’ role from a fire-protection aspect includes functionality and materiality expectations:
ν as part of fire protection and life safety systems aiding notification, egress and emergency response/rescue functions
ν from a materiality perspective, their role in a (cause) ignition source and behaviour under fire can contribute to the propagation of fire and compartment fire development – contributing to more significant losses.
During a fire situation, exit signs must be readily recognisable and easy to locate, so visibility must be high enough to support evacuating occupants finding their way out. And life safety and special systems devices (smoke management, exhaust fans, fire/flame, smoke detectors, etc.) must continue to function perfectly for a given time, needing to be connected continuously and uninterrupted to the power network long enough to support evacuation and initial emergency response operations.
In standard conditions, fire-resistant cables offer different performance levels; these cables or systems can provide electrical continuity for 30, 60, 90 or even 120 minutes.
Cables achieving the test requirements successfully are given a nomenclature of PH and survival time values, shown in minutes, for example ‘PH120’, for marking. This categorisation must be easily identifiable on the labels applied to the products to be released into the market.
Types of fire testing
There are various fire-resistant test protocols to assess cable reliability, as fire is of great concern. ESL is fully equipped to carry out the critical fire behaviour tests defined in the following international standards:
Fire Resistance Tests (Circuit Integrity Test)
- BS EN 50200 – Test for resistance to fire of unprotected small cables for use in emergency circuits
- BS 8434-2 – Test for unprotected small cables for use in emergency circuits – BSEN 50200 with a 930°C flame and with water spray.
- IEC 60331 – Tests for electric cables under fire conditions – Circuit integrity
Flame Retardant Tests (Flame Propagation test)
- EN 60332-1-2 – Flame propagation of Single cables/Wires using 1kW Premixed burner
- EN 60332-2-2 – Flame propagation of Single cables/Wires using Diffusion flame.
Circuit Integrity Tests according to:
BS EN 50200
This standard specifies the test method for cables designed to have intrinsic resistance to fire and intended for use as emergency circuits for alarm, lighting and communication purposes.
A single cable is mounted on a particular fibre-glass wall with minimum bending radius and heated with a propane burner at min. 830°C (Nominal Temperature of 842°C). During the test, a rated voltage is applied to the cable and a mechanical shock is applied to the wall every 5 minutes, where the cable is attacked with a force of 25kg.
Annexe E: Test with Water Spray Protocol
The requirement of a water spray when assessing fire-resistance tests may be recommended by codes of practice or specific product standards.
In cases where water spray is required, the water spray should be started 15 minutes after the start of the EN 50200 test while flame at 830+40°C and shock are still being applied. Water application, 15 minutes; it should continue until the end of the test. The test, in which 15 minutes of water spray is applied to the specimen, takes 30 minutes in total. It is to ensure circuit integrity is provided that the voltage is maintained, i.e. no fuse blows or circuit breaker cut.
Enhanced fire-resistance test according to BS 8434-2, water spray will be applied for 60 min. followed by the 60 min. fire with mechanical shock, at 930+40°C temperature. The duration of this test is 120 minutes.
PH120 cables are designed for installations requiring enhanced circuit integrity for up to 120 minutes, including complex or high-rise buildings and public areas requiring a more extended evacuation period.
The criterion of fire-resistance classification is that there may be no cable conductor break or short circuit and no fuse blows in the cable system.
An International standard for Fire Performance tests is IEC 60331, which consists of the following parts under the general title: Tests for electric cables under fire conditions – Circuit integrity:
ESL has invested in state-of-art technology, a fully automated system that monitors continuity and short circuits, which is checked by built-in electronics. This test is conducted on a particular fire test rig, and the cable is energised up to 1,000V.
The cable’s electrical continuity is indicated by a series of light bulbs connected through fuses. A failure will be shown by the fuse blowing and the light failing.
Area of application:
As discussed earlier, and per the UAE Fire and Life Safety Code, fire-resistant cables are essential in ensuring that the emergency and building critical systems are supported, providing vital time for the safe passage of occupants out of the building.
Applications, where a fire-resistant cable would be specified include emergency lighting, sprinkler systems, extraction systems, smoke dampers and shutters, emergency generators, pressurisation fans, and emergency voice and fire alarms, all of which can prove critical during a fire.
Applications for fire-resistant cable are effectively endless.
- Areas where people will remain in occupation for a short time, e.g. schools, shopping malls, mass transit systems like metro stations etc.
- Services where circuit integrity is essential under fire conditions, e.g. Special equipment in a hospital
- Necessary safety circuits, e.g. fire detection, fire alarm, voice alarm etc.
- Power supply to equipment used in firefighting, e.g. sprinkler pumps
- In large buildings, fire strategy involves occupants’ evacuation in a phased manner.
Measurement of flame propagation
Flame-retardant cables shall meet the requirements of IEC 60332 Part 1. (For European use, EN 60332 is identical to the IEC standard). These tests define the cable performance under fire conditions.
In a fire condition, cables can act as a conduit combustion fuel network to propagate the flame along their span to distances to unaffected areas from the fire area.
The key definitions of flame-retardant cables are:
- Cables which don’t spread fire
- Cables which are self-extinguishing
The tests are carried out on a single length of cable supported vertically in a draught-free enclosure with a burner of 1 kW flame at an angle of 45° to the horizontal applied to the lower end of the cable. After a specified time, the heat source is removed, and the cable should not continue to burn after a stated length of time. The extent of charring at the top of the cable is also defined.
What are the points to consider in fire-resistant cable selection?
To safeguard the fire-resistant cable criteria based on the classification report, consultants, designers, specifiers and contractors should ensure that the detailed product is certified by an internationally accredited certification body when selecting the suitable cable.
In the UAE, according to the ‘Regulation on the protection of buildings from fire’, the minimum working time for fire-rated cables during a fire is 120 minutes. However, there is no information about the fire-resistance test standard by which the performance will be determined. Here again, the consultants, designers, specifiers and contractors must be competent in selecting the fire-resistance test standard applicable to the cable’s end-use application and stipulated regulated classification for the identified risk areas or zones of the building/structure.
Thus, when considering the use of cables and fire-rated cables, which is increasing daily in our buildings and living spaces, materiality is of the utmost importance, especially considering the need for fire performance and life and property safety. Using correct and reliable cables in the right area will minimise fire risks and potential losses.
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