CPD PROGRAMME | FIRE SAFETY Copper conductors Polyester-backed laminated aluminium tape or foil Low-smoke halogen-free thermoplastic CPC (earth) Figure 3: A simplied sketch of an example re-resistant polymeric cable. There are many variants of such cables that are designed to meet specic re-resistance requirements Figure 1: Grenfell Tower at 4.45am on 14 June 2017 (Source: Nathalie Oxford bit.ly/CJJan24CPD5) buildings but to all non-domestic premises, such as where people work, visit or stay, including workplaces, and the non-domestic parts of multi-occupied residential buildings (for example, communal corridors, stairways and plantrooms). The requirements do not apply within individual domestic premises. As seen in the fires discussed above, large, complex, and high-rise built environments that are more likely to be densely populated will have extended evacuation times. Smoke from inappropriately specified or inadequately manufactured cables will reduce the opportunities for successful escape from a fire. Circuits of safety-critical services need to function for extended periods, and fire plans are likely to rely on critical circuits continuing to perform to prevent potentially disastrous events, such as: fire alarm cable failure; sprinkler system not activating; smoke extract fans and smoke louvre power supply failure; and emergency lighting and signage failing to remain illuminated. Copyright: London Fire Brigade Figure 2: Fireghters emerge through the smoke as it billows from the re at Kings Cross Underground station (Source: London Fire Brigade bit.ly/CJJan24CPD4) Fire-resistant cables provide extended periods of circuit integrity where uninterrupted functionality is crucial during a fire. They are designed to maintain their functionality and structural integrity for a specified period of time during a fire, and are constructed using materials that can withstand high temperatures when exposed to tested levels of flame, water and shock. They remain intact in harsh conditions, although they are not necessarily fireproof. The insulation and sheathing are made from materials that do not propagate flames or produce excessive smoke, and so are able to provide protection against fire. Performance will vary depending on the specific type and design of the cable. The two principal types of fire-resistant cables are polymeric cables and mineral insulated copper cables (MICC). Polymeric cables use synthetic materials such as polyethylene (PE), polyvinyl chloride (PVC), cross-linked polyethylene (XLPE), mica tapes and other polymers for insulation and sheathing, such as in the simplified sketch in Figure 3. The flexibility of these cables makes them relatively easier to handle and install. Polymeric cables can be designed with various material layers to ensure that they maintain circuit integrity for a specified duration during a fire under defined conditions. The temperature rating of polymeric cables varies based on the specific polymers used. The Institution of Engineering and Technology (IET) notes5 that there are many acronyms employed to represent the emissions performance of polymeric cable, and it is important not to confuse low smoke halogen free (LSHF) and low smoke and fume (LSF). PVC compounds are used during the manufacture of LSF cables, and while additional additives reduce the smoke emissions, they are not eliminated. There are no standards governing LSF cables, unlike LSHF, which are manufactured and tested to BS EN 61034,6 which considers the measurement of smoke density from burning cables, and BS EN 60754,7 which provides guidance on corrosive and acid gas emissions. In MICC, the conductors are surrounded by mineral insulation, commonly magnesium oxide (MgO), and the principal outer sheath is typically made of metal, such as copper (Cu) or an alloy, as illustrated in Figure 4 and Figure 5. They are rigid and less flexible compared with polymeric cables; however, the rigid construction, resulting from the highly compressed powdered mineral insulation, provides excellent mechanical strength. MICC are inherently fire-resistant because of the mineral insulation, and can withstand high temperatures and maintain circuit integrity during intense fires. This type of cable can typically safely carry an electrical load at temperatures in excess of 1,000C. Most cables installed as part of a permanent installation within domestic, residential and commercial buildings are subject to the Construction Products Regulation8 (CPR) that requires relevant cables to be CE-marked. Cable conformance includes reaction to fire and release of dangerous substances in normal operation, dismantling and recycling. All UK countries will accept9 the EUs CE mark as appropriate for cables until at least 2025. The supporting standard BS EN 5057510 covers the reaction to fire of cables in construction works on a scale of Aca (non-combustible, such as bare MICC) to Fca (no performance determined and likely to burn uncontrollably in a fire). In addition, there are classifications for smoke (s), flaming droplets (d), and acidity (a) as described in BS EN 13501-6,11 with each classification graded from 0 or 1 to 3. The BCA provides guidance12 on appropriate CPR ratings for example, a cable designated as Cca - s1, d2, a1 is likely to be suitable for installations where improved (as opposed to low-risk) fire performance of cable is required. CPR covers both reaction to fire and resistance to fire, but only the harmonised 44 January 2024 www.cibsejournal.com CIBSE Jan 24 pp43-46 CPD Module 228 WMC.indd 44 21/12/2023 13:57