FOCUS
Transmission risks
Number of cores with or without earth
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Conductor size (mm2
)
BS 7671 and IEC 60364-5 method F current ratings for cables in trefoil in free air at:
10 16 25 35 50 70 95
120 150 185 240 300 400 500 630
3 and 4 2.5 3 and 4 4 3 and 4 6 3 and 4 10 3 and 4 16 3 and 4 25 3 and 4 35 3 and 4 50 3 and 4 70
Figure 1 Effects of temperature
Why is the test temperature for fire rated cables so important? Fire temperatures have adverse effects on electric cables and wiring systems. These include decreasing insulation resistance of dielectrics, material degradation of polymer based insulation and sheath materials which impact fixings and supports (especially for vertical installations). There are also fundamental changes to the
cables’ copper conductors, for example increasing conductor resistance, lowering of tensile strength, and oxidisation. Any current passing through a conductor will increase the temperature of the cable above
38 OCTOBER 2019
www.frmjournal.com
the surrounding ambient due to the electrical resistance of the conductor. If the ambient temperature is 30°C and the
conductor is required not to exceed 30°C, the cable will have a current rating (also referred to as ‘ampacity’) of zero amps. If the cable is in a fire (In the UK, we test fire rated power cables to 842°C, but with only a very small load and a three amp fuse), any significant current passing through the conductor will raise the temperature above the fire temperature of 842°C. This means that the conductor will be rather hotter than 842°C, and with the known reduction in insulation resistance performance of all dielectrics at these high temperatures, effectively these cables are not tested at the temperatures they are likely to operate at.
90oC 74
99
135 169 207 268 328 383 444 510 607 703 823 946
1088 32 42 54 75
100 127 158 192 246
Current rating for flexible fire rated cable in trefoil in a fire at:
950oC 1,000oC 49
66 88
108 141 171 216 253 298 347 412 491 581 704 848 20 26 34 47 66 88
108 141 171
30 40 53 65 85
107 130 153 180 209 249 296 350 425 511 12 16 20 28 40 53 65 85
107
Current rating for MICC cable in trefoil in a fire at:
950oC 53
71 94
117 148 182 226 266 313 361 434 491 639 - -
24 31 40 55 73 97 - - -
1,000oC 32
43 57 70 89
110 136 161 188 218 261 296 385 - -
14 19 24 33 44 58 - - -
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