In this regular column, “Dr. Zeus” Tom Brooks, Managing Director of Zzeus Training and Chairman of the Financial Supervision Authority (FSA), answers your questions about fire safety compliance. In this month’s issue, he takes a look at the correct cutting distances for fire-resistant cables.
Q: What are the correct cutting distances for fire resistant cables? My manager has asked us to cut at a distance of 1 metre, but I am concerned that this may compromise the integrity of the circuit during a fire. Can you explain the correct spacing and the possible consequences of incorrect spacing?
When specifying appropriate cutting distances for fire-resistant cables, BS 5839-1:2017 provides basic guidance for the installation and support of alarm cables, particularly in Sections 26.2(f) and 26.2(g), and Clause 37, which deals with installation and manufacturing practices.
An important aspect of the standard is the focus on ensuring the integrity of the cable circuit during a fire, which is vital for the safety of both building occupants and emergency services.
To prevent premature failure, the standard explicitly prohibits the use of plastic clips and ties as the primary means of support, which can melt at high temperatures. Instead, she advises following recommendations provided by cable manufacturers for safe installation.
The tragic events of the Harrow Court fire in Stevenage on 2nd February 2005 are a stark reminder of the consequences of incorrect cable installations.
Fallen cables, supported by non-fireproof fittings, played a significant role in the deaths of two firefighters. This tragedy underscores the critical need for fire-resistant cable supports, ensuring cables remain securely fixed during a fire. Subclause 37.2(b) of BS 5839-1:2017 states that fasteners must be safe and in accordance with the manufacturer’s instructions. It also specifies that suspended ceilings should not be relied upon to support cables, as they may fail in fire conditions.
Cable manufacturers generally recommend installation intervals of 300 mm for horizontal runs and 400 mm for vertical installations, especially for cables with a diameter between 8 mm and 15 mm. These intervals are important to maintain circuit integrity, as demonstrated by testing under BS 8434-2, which validates similar installation arrangements for fire-rated cables.
In certain circumstances, such as vertical cables falling into hard-to-reach areas, slightly extended installation times may be considered acceptable following comprehensive risk assessments by system designers and installers.
An example of this is the vertical drop of cables from the ceiling or floor to a device inside a suspended ceiling. In these cases, the cable industry allows some flexibility, allowing cables to be dropped vertically without fixing to structural elements, as long as the following conditions are met:
1. The maximum permissible vertical cable drop without fixing is 1 meter.
2. Anchors should be positioned as close as practicable to the vertical drop to restrict cable movement and prevent slack.
3. Any spare cable loops intended for future re-termination must be securely fastened to avoid kinking.
4. The manufacturer’s minimum bend radius must be followed, especially in cable loops, at device entry points, or where the cable direction changes after the last installation.
5. The diameter of the ring should generally not exceed 150 mm, unless the manufacturer’s recommendations specify otherwise.
6. Excess horizontal cable should be kept to an absolute minimum.
Finally, BS 7671 contains a general requirement that wiring systems should be supported in a way that prevents premature failure in the event of a fire. This ruling also reinforces the ban on the use of plastic clamps and ties for fire-resistant cables.
For further guidance on BS 7671, the IET On-Site Guide is a valuable resource, providing detailed advice on compliance and best practice for electrical installations, ensuring safety and reliability during fire events.
Do you have a question you want answered? Email your inquiries to: Tom@Zzeus.org.uk
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