EoW July 2011

technical article

Current Situation The relevant IEC test standards have remained largely unchanged over recent years but have been subject to ongoing refinement (5,6,7,8) . For example, IEC 332-3 has developed into a multipart standard with each part covering a different installation condition or time of exposure to the fire source. The principles established in the early development of these standards still serve cable manufacturers and users well and the IEC standards are adopted in many countries. Based on the use of these tests, one can find in the market various types of cables. Reduced flame propagation (RP) cables which when installed in vertical bunches in accordance with the recommended procedures do not propagate fire more than a limited distance from the source. Such cables have been well established in the market since the 1970s for cabling in areas where, because of installation conditions eg vertically mounted bunched cables, the risk of propagation of fire is high. They are tested to the various parts of IEC60332-3. Low smoke cables which have limits on smoke evolution when assessed in the 3m cube smoke chamber with perfor- mance limits chosen to give visibility over 10m distance.

They were introduced because cables containing conventional sheathing mate- rials based on PVC were seen to give off large amounts of dense smoke when affected by fire. Such cables have been manufactured since the 1970s, but it was not until advances were made in cable making material technology in the 1980s that cost effective designs became widely available on the market. They are tested to IEC61034-2. Generally cables of this type also combine the properties of low corrosive gas emission and are manufactured using “halogen free”materials. Low corrosive gas emission cables which have limits on acid and corrosive gas emission when assessed by burning samples of materials in a bench tube furnace. They were first introduced when users began to raise concern over the large amounts of acid gas emissions from burning reduced propagation PVC cables. This corrosive and irritant gas had been seen to have devastating effects on electrical panels and instrumentation exposed to cable fire effluent (9,10) . The acid gas emission test of IEC60754-1 and/or the indirect corrosivity test of IEC60754-2 may be used.

Products meeting the requirement of less than 0.5% acid gas emission when tested to IEC60754-1 are often referred to as “halogen free”. In some product standards, an additional test for fluorine content (IEC60684-2) is required. Cables having low emission of toxic gases are generally restricted to specific applications where end users have imposed such a requirement. Such cables are particularly found in the rail sector. Users have their own “toxic emission” standards ranging from a simple restriction on certain elemental groups to indices derived from an analysis of the gases evolved and weighted according to the known mammalian toxicity factors of the gases present. The lack of general use of toxicity testing for cables may be explained by early work carried out in the UK which concluded that “From this brief excursion into consequences it appears that the acidic gas concentrations likely to cause harm to people and plant are similar” (11,12) . Current work (13) continues to demonstrate the large contribution of HCl to the fire hazard. Some interesting new work studying the effect of important irritant gases on animal lungs (14) has also found that HCl and PVC smoke inhalation caused an acute effect with a rapid decline in the lung physiology parameters.

▼ ▼ Figure 2 : prEN50399 apparatus

▼ ▼ Figure 3 : Typical SPR and HHR curves

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EuroWire – July 2011