Category-3 UTP at Elevated Temperature

Both TIA/EIA and ISO/IEC specifications for horizontal cabling apply at a nominal inside-building operating temperature of 20 °C. When operating at elevated temperatures, the increased resistance of copper exaggerates the skin-effect losses. For category 5e and 6 cables, whose losses are dominated by the skin effect, the attenuation in dB scales directly in proportion to the increase in resistance of the base metal (becoming larger at elevated temperatures ). TIA/EIA 568-B specifies a de-rating factor for categories 5e and 6 of 0.4% insertion loss (in dB) for each degree above 20 °C.

The performance of category 3 cable suffers more noticeably at high temperatures, because its de-rating factor depends on both skin effect and dielectric properties. Because the dielectric properties are not well specified, your performance will vary. TIA/EIA 568-B recommends a de-rating factor of 1.5% for each degree above 20 °C for category 3 cabling and also recommends that it not be used in hot environments.

Plenum-rated category 3 cables are made from plastic materials with a higher glass-transition temperature than ordinary PVC-insulated category 3 cables, rendering their electrical properties more stable at elevated temperatures. According to ISO 8802.3 clause 14 (Ethernet 10BASE-T), "The loss of PVC- insulated cable exhibits significant temperature dependence. At temperatures greater than 40 °C (104 ° F) it may be necessary to use a less temperature-dependent cable, such as most plenum-rated cables."

What this means to you is simple: Never use PVC-insulated category 3 cables in an uncooled attic space .

Typical (not worst-case) attenuation measurements taken by Synoptics (later Bay Networks, later part of Nortel) were presented to the token-ring committee IEEE 802.5 in March 1993. These measurements are reproduced in Figure 8.27.

Figure 8.27. The performance of PVC-insulated category 3 cable (non-plenum rated) varies dramatically with temperature.

POINT TO REMEMBER

• Never use PVC-insulated category 3 cables in an uncooled attic space.

 

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