EuroWire Sept 2015

Technical article

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”. 3.3.1 The differences to TUV 1169/2007.8 The significant differences between UL and TUV are: • Halogenated compounds are permitted in UL4703 • The required flaming test UL1581-1060 is more demanding than IEC60332-1 • No differentiation between DC and AC in UL4703 • 1,000V (or 2,000V) is permitted, which is more future-orientated • Aluminium wires are permitted in UL4703 • No differentiation U0/U in UL4703

Carbon black is in EN50290 (“Communi- cation 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.

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. 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.

GND

▲ ▲ Figure 2 : Definition of U 0 /U

4 New challenge for the cable industry 4.1 TUV and UL approved cables 2006–2013 In 2006 the module manufacturers started to think globally. The new marketing requirement was to manufacture one type of photovoltaic module with all relevant approvals to sell them on all markets. The challenge was to create a cable which could combine the opposed specifications of UL (PV / USE-2) and TUV 1169. Particularly the following discrepancies had to be overcome. • Halogen-free compounds are highly filled with flame retardant minerals. The physical properties required by UL are a challenge for this kind of compound • Passing the flame test required by UL is easy for halogenated compounds but difficult for halogen-free compounds • The long-term stability test of UL is a real challenge for filled compounds because the flame-retardant additives are hygroscopic However, it was possible to comply with all these requirements. 4.1.1 The first solution The outstanding characteristics of this design are: • Three-layer extrusion in one pass (implicated by increasing pricing pressure in the PV industry)

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September 2015