Photovoltaic DC Cable Insulation Under EN 50618 and 1500 V System Voltages

For photovoltaic direct-current circuits, WMC-1901B is extruded as insulation on tinned copper conductors from 1.5 mm² to 16 mm², with insulation wall thickness between 0.7 mm and 1.2 mm for 1.5 kV DC service. The application requires simultaneous long-term wet insulation resistance, UV resistance, and flame retardance. The product is evaluated within the compliance framework shown in Table 1. Acceptance is not based solely on material type; the finished cable must retain tensile strength and elongation after long-term aging and after xenon-arc exposure, while hot-set performance must remain within thermoset limits after crosslinking.

Formulation addition ratio for PV-grade insulation includes 100 parts WMC-1901B, 5–7 parts silane catalyst masterbatch where not pre-dispersed, 2.5–3.0 parts high-dispersion carbon black masterbatch, and 0.3–0.8 parts antioxidant masterbatch. Some halogen-free constructions incorporate 3–5 parts metal-hydrate flame retardant masterbatch to meet flame propagation limits under IEC 60332-1-2. Dispersion of the carbon black phase is critical because undispersed agglomerates above 10 µm create water-tree initiation sites and reduce wet insulation resistance.

Extrusion is run on a L/D 25:1–30:1 single-screw line with a mixing screw and melt temperature controlled at 160–180 °C. Conductor preheating at 80–110 °C is applied before the extruder head; die draw ratio is kept between 1.05 and 1.15 to prevent shrink-back in subsequent thermal cycling. The moisture cure step is conducted at 80 °C and 85% relative humidity for 8–16 h, after which the cable is subjected to oven conditioning to remove surface moisture. Terminal product types include H1Z2Z2-K 1.5 kV DC solar cables, TÜV-certified PV1-F cables, and UL 4703 USE-2/PV wire used in module interconnections and string transmission.

What Limits Thin-Wall Extrusion of Silane XLPE at 0.35 mm Insulation Thickness?

Automotive primary wire applications using WMC-1901B require insulation wall thickness as low as 0.20–0.35 mm over conductors from 0.13 mm² to 1.5 mm². The critical processing limit is the narrow melt-temperature window between 165 °C and 175 °C; deviations greater than ±5 °C produce either melt fracture and microvoids at the lower boundary or die drool and premature scorch at the upper boundary. In high-speed thin-wall extrusion, melt viscosity at the die land must remain stable at shear rates from 100 s⁻¹ to 300 s⁻¹. If the compound is run above 180 °C, silanol condensation initiates prematurely and forms gel particles visible as surface protrusions; below 160 °C, poor melt homogeneity creates a rough inner wall and eccentricity drift.

Relevant compliance standards include ISO 6722-1:2011 for single-core cables, LV 112-1 for German automotive low-voltage cable, and SAE J1128 for low-tension primary cable. Heat aging is carried out at Class C or Class D temperatures depending on final vehicle installation zone; short-term aging at 150 °C for 240 h is commonly used as a screening condition for XLPE insulation before full class qualification. Scrape abrasion resistance, cold impact at -40 °C, and pressure resistance are tested on the insulated conductor, not on pressed plaques.

Formula addition ratio is 100 parts WMC-1901B, 5 parts catalyst masterbatch where needed, 0.5–1.0 parts external process aid, 0.3–0.6 parts antioxidant masterbatch, and, when flame retardance is required, 2–5 parts halogen-free flame retardant masterbatch. The process aid must be selected from silicone or fluoropolymer chemistries; amine-based additives are avoided because they accelerate premature crosslinking and reduce the induction time window on the screw. At relative humidity above 60%, the compound is pre-dried at 60 °C for 2–4 h before feeding to prevent surface porosity.

Downstream production is performed on a L/D 30:1 single-screw extruder equipped with a grooved feed section and a barrier screw or Maddock mixing section. Screen packs of 80/120 mesh are placed at the breaker plate to remove contaminants. The die draw ratio is maintained at 1.05–1.15, and the cooling trough temperature is held at 15–25 °C to freeze dimensions without causing inner stress. Crosslinking is completed in a hot-water or steam chamber at 80 °C for 6–10 h. Published data for WMC-1901B at line speeds above 500 m/min on 0.13 mm² conductors is limited; validation trials on 30:1 L/D lines are required before fixed production parameters can be certified. Terminal product types include FLRY-B, AVSS, and TWP automotive primary wire with 0.2–0.35 mm XLPE insulation, used in engine compartment harnesses, door harnesses, and battery management wiring.

When Building Wire Insulation Must Satisfy UL 44 and BS 7889 Simultaneously

Building wire constructions use WMC-1901B as the crosslinked insulation layer for conductors from AWG 14 to AWG 4/0, with insulation thickness between 0.76 mm and 1.52 mm. Compliance is evaluated under UL 44:2018, UL 1581, CSA C22.2 No. 38, and BS 7889:2012. The material must pass long-term insulation resistance under wet conditions, heat deformation at rated temperature, cold bend at -25 °C or lower, and flame tests such as VW-1 or FT-1 depending on the construction. Unlike thermoplastic building wire, the XLPE layer must retain dimensional stability after short-circuit current impulses and after thermal overload cycles.

Table 2 summarises the starting-point addition ratios across four separate application groups. These ratios are compounding starting points derived from industrial silane XLPE practice; each lot is adjusted after hot-set and tensile retention testing on the production line.

Extrusion is performed on a single-screw line with L/D 24:1–30:1, a compression ratio of 1.5:1–2.0:1, and barrel temperatures from 140 °C at the feed zone to 185 °C at the head. Melt temperature is kept between 160 °C and 185 °C; die draw ratio is set at 1.05–1.20 to minimise insulation shrink-back after cutting. Cure is performed in low-pressure steam at 85 °C for 4–8 h or in hot water at 80 °C for 8–12 h. Terminal product types include XHHW-2, RHH/RHW-2, and BS 7889 building wires rated 600 V to 1000 V, installed in conduit, raceway, and direct-burial service where thermoset insulation is specified.

Industrial Control Cable: UL 1277 Type TC Constructions

For industrial control and instrumentation circuits, WMC-1901B is applied as the insulation layer on multi-conductor constructions from AWG 22 to AWG 10, with insulation thickness from 0.30 mm to 0.80 mm. Compliance is assessed under UL 1277 for Type TC control cable, IEEE 383 for flame propagation in nuclear and industrial systems, and IEC 60092-353 where marine control circuits are specified. The insulation must survive vertical tray flame exposure, oil immersion at 75 °C for 4 h, and cold bending at -40 °C without cracking. Insulation resistance is measured after 14 days in water at 75 °C to detect water-treeing potential.

Formula addition ratio uses 100 parts WMC-1901B, 5 parts catalyst masterbatch if separate, 0.3–0.8 parts antioxidant masterbatch, and 2.5–3.0 parts carbon black masterbatch only when the cable is exposed to UV from outdoor tray or duct installation. No halogenated flame retardants are introduced unless the control cable must match a specific customer flame specification; standard silane XLPE insulation is used without inorganic hydrate filler to retain clean signal transmission. Process parameters include extruder barrel temperatures from 140 °C to 185 °C, melt temperature 160–180 °C, and die draw ratio 1.05–1.15. The insulated cores are twisted and cabled, then the entire assembly is moisture-cured at 80 °C for 8–12 h before jacketing. Terminal product types include UL 1277 Type TC control cables, 600 V power-limited tray cables, and marine control cables used in conveyor systems, water treatment plants, and continuous process instrumentation.