WCA November 2015

3.1 Material ageing is the new focus 3.1.1 Thermo-oxidation of polyolefins

The dispersion of carbon black is an integrated part of the extrusion process of the jacket, which has a high impact on the UV resistance. The proper management of the machine parameters is the critical key factor for best results. Carbon black is in EN50290 (“Communication cables. Common design rules and construction”) a mandatory requirement for communication cables for exposed outdoor use. 3.2 Basic points of the new requirements in 2007 The main basic point of the new version of the Pfg1169/2007.8 is the thermal endurance test according to IEC60216 “Electric insulating materials – Thermal endurance properties” (120°C/20,000h). In the application of this standard, it is assumed that an almost linear relationship exists between the logarithm of the time required to cause defined property change (less than 50 per cent elongation at break) and the reciprocal value of the corresponding absolute temperature. This test is to conduct at least three different temperatures. The highest temperature shall be selected to result an endpoint not less than 100h and the lowest temperature is to be selected for the expected result not before 5,000h. A straight line is drawn to connect the various recorded points. By extending the line until it intersects the 20,000h on the ordinate – axis (logarithm of time) it is possible to determine the temperature rating on the abscissa – axis (the reciprocal absolute temperature). Additional essential points are: • The used materials shall be halogen-free • The used conductors shall comply with IEC 60228 class 5 • The cables and wires have to comply with IEC60332-1-2 (vertical flame test) The result of this work was published by VDE as: • VDE-AR-E 2283-4 “Requirements for cables for PV systems” And by TUV as: • TUV 2Pfg1169/2007.8 “Requirements for cables for use in photovoltaic systems” 3.3 The specification of PV wires by UL In 2005 UL published the first edition of Outline 4703. The UL type ‘PV’ was created. This outline was based on UL854 (Service Entrance Cables). But in 2005, the NEC2005 (Article 690) was requiring USE, USE-2, UF and SE. As recently as 2008, the PV type was mentioned for the first time in the NEC2008. The required wires were in this edition USE-2 or PV. Mentionable is the acceptance of metric sizes of conductors in the UL outline 4703. In 2010 UL published the fourth edition of UL outline 4703, which is the relevant version until today. In this edition is the reference standard UL 44 “Thermoset- insulated wires and cables”.

One of the elementary chemical laws is the Arrhenius law. This law describes the correlation of temperature and process speed. The thermal ageing of polymer is nothing else than a chemical process, and every chemical process is dependent on the process temperature. Increasing the temperature by 10°C accelerates the process by a factor of two. This also works backwards. Decreasing the temperature slows down the ageing process by a factor of 0.5. The specified temperature rating of a cable should be in combination to a specified time period. Without a time indication, the temperature rating is useless. The standard temperature rating in the European cable industry is xxx°C at 20,000h. The PV industry standard period of use for PV modules is 25 years. These are roughly 150,000h. The assumed ambient temperature is 90°C, ie the minimum temperature rating shall be 90°C/150,000h. Normalised to the industrial standard time of 20,000h, the new temperature rating shall be 120°C/20,000h. 3.1.2 Photo-oxidation Sunlight contains a high amount of ultraviolet radiation. The ultraviolet radiation that is absorbed by a polymer material will result in its degradation. The energy may be sufficient to cause the breakdown of the unstable polymer and, after a period of time, changes its components. Polymer materials which are to be exposed to UV for long periods of time should be made from polymer compounds that are appropriately stabilised for such environmental conditions. The basic polyolefin polymers have limited outdoor life. However, most polyolefin, coloured (non-black) solar cables manufactured today contain an ultraviolet stabilisation package which is satisfactory for limited time of 5–10 years. But for prolonged outdoor service life, polyolefins should be formulated with a minimum of 2.5 per cent finely dispersed carbon black. Implementing carbon black in polyolefins greatly increases the UV resistance. Carbon black acts as a UV absorbent and screens the polyolefin from damaging ultraviolet radiation. Until now there has been no known physical or chemical interrelationship applicable to extrapolate an accelerated weathering test up to the lifetime of cables. The conducted test durations in standards UL and TUV are 720h whose results cannot be extrapolated based on a mathematical formula. These tests provide only compar- able results, but no real statement about the real lifetime. As has been demonstrated through over four decades of outdoor experience with polyethylene jacketed communication cables, the addition of 2.5 per cent finely dispersed carbon black results in more than 25 years of protection against UV.

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Wire & Cable ASIA – November/December 2015