Transmission risks
Many of these requirements are laid down in BS 8519: 2010, and of specific importance is the fundamental cable selection. This process requires cable sizing for voltage drop and current carrying capacity under normal operating conditions and also under fire conditions. Certainly, if the fire rated cable is exposed to fire temperatures, it is still expected to maintain its electrical integrity in order to ensure that connected life safety and firefighting equipment will continue working. Chapter 13 of BS 8519: 2010 does recognise
that volt drop and impedance of the cables’ copper conductors will increase with temperature, and it does recommend getting advice from the cable manufacturer for cable ratings based on the load being fed. Perhaps unfortunately, however, it does not specifically highlight that these ratings are not those listed in the standard tables of the BS 7671 wiring regulations. This has been a common error for many designers and installers. BS 8519: 2010 also suggests that the ‘normal
maximum ambient temperature likely to be experienced under fire conditions’ is 840°C. This contrasts starkly with the fire resistance requirement for every other fire rated component, system, structure penetration and enclosure in buildings. Approved Document B of the Building Regulations defines fire resistance as resistance to fire when tested to BS 476 Parts 20 to 24, and this requires three specific provisions: load bearing capacity, integrity and insulation.
If we look at the time temperature protocol of BS 476 Parts 20 to 24, we see that for a fire resistance of: • 30 minutes, the maximum resistance temperature is 842°C
• 60 minutes, the maximum resistance temperature is 945°C
• 90 minutes, the maximum resistance temperature is 1,006°C
• 120 minutes, the maximum resistance temperature is 1,049°C
During a fire, it can be reasonably expected that the protected circuits (fire rated cables) might be exposed to the same fire temperatures as everything else in the building. This exposes an anomaly, because fire rated power cables are generally only tested to 842°C (BS 8434-2 does test small cables up to 2.5mm2 at 930°C). In turn, this means that we have no idea whether the fire rated cables, tested and certified for use, of 4mm2
and above (power
cables) can provide reliable performance for any fire exceeding 30 minutes, because we do not test and are not required to test to these higher temperatures. Interestingly, BS 8519: 2010 specifically
requires that junction boxes and joints for these cables are tested to BS EN 1366-5, and this standard specifically requires testing to the same time temperature protocol as BS 476 Parts 20 to 24 (EN 1363- 1 or ISO 834-1), which appears somewhat contradictory.
FOCUS
www.frmjournal.com OCTOBER 2019
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