Birîhkirina û Tavsiyekirina Jixwêna Li Sernayêna Dabekeyên Bajarkevî yên Berz Hişta

1.Standards of Flame-Retardant Cable Classification

Pêşkêşina standardeyan da ku jînên kablê yên dijîn-berdibistinê hatine dibandin derbarê du kategoriya sereke. Yekem kategoriya li gorî "Klasifikasyona Wergera Dijîna Jînên Elektrik û Kablê yên Nîrvan" GB 31247 împlement kirin. Kablê yên bi vê standardeyan reyazîn bêrser demên ku hunde çend an deh û metro ên hêviyên serzêde yên taybetandîn wan hatine bikar bînin. Vê standardeyan rêzikên xweşher tiştên nîvendîn, derketina berd û guherîn guherî çêtirên wergera dijîna jînên dikin, û kablê yên lêkolîn derbarê malên ku jînên nîvendîn û be halogen tune bibin.

Duwêm kategoriya yekem ya ku "Rêzikên Sereke bi Rewşa Dijîn-Berdibistin û Nîrvan bi Rewşa Dijîn û Kablê yên Elektrik û Nîrvan" GB/T 19666 împlement kirin. Piştî pêşkêşkirina GB 31247, vê standardeyan bi tevahî hatine bikar bînin ji bo hemû cûrekan taybetandînên Çîn. Sisteman GB/T 19666 ji dê hatine navberdîn rêzikên xweşher nîvendîn, û piştî bixebitina, zeviyên binnirxîn hatine navberdîn, wêçî WD (jînên nîvendîn, be halogen tune). Rêzikên testeya klasifikasyonê ya kablê yên dijîn-berdibistin hatine nîşandan di tabloya jêr:

Klasifikasyona Standardeya Item 1: Standardeya "Rêzikên Sereke bi Rewşa Dijîn-Berdibistin û Nîrvan bi Rewşa Dijîn û Kablê yên Elektrik û Nîrvan" GB/T 19666 ZA, ZB, ZC klasifikasyonên bi tevahî hatine bikar bînin ji bo institûtên dizaynê yên elektrik. Lê metoda testeyê ya navdarîya, "Testi Piramînê ya Dijîna Jînên Kablê yên Bunching bi Rewşa Dijîn" GB 18380.3-2001, hate rakirin. Metoda testeyê ya navdarî hatine destnîşan bi IEC 60332-3-25:2000, "Testi Piramînê ya Dijîna Jînên Kablê yên Elektrik û Nîrvan bi Rewşa Dijîn – Part 3-25: Testi Piramînê ya Dijîna Jînên Kablê yên Vertikal – Kategoriya D."

Klasifikasyona Standardeya Item 2: Standardeya "Kablê yên Dijîn-Berdibistin û Nîrvan – Part 1: Kablê yên Dijîn-Berdibistin" GA 306.1-2007, kablê hatine klasifik kirin bi metoda testeyê ya nûvekirin GB 18380.31~36-2008, ku GB 18380.3-2001 hatine leday. Ferqê ya sereke ya vê metoda testeyê ya navdarî ya toxînî (GB 20285), transparensiya ronî û dayînkarî, ku A, B û C klasifikasyonên bi tevahî hatine navberdîn be penj grade distinct.

Klasifikasyona Standardeya Item 3: "Klasifikasyona Wergera Dijîna Jînên Kablê yên Elektrik û Nîrvan" GB 31247 ya terîn standardeya navdarîya. Metoda testeyê ya navdarîya "Piramînê, Derketina Berd û Guherîn Guherî çêtirên Kablê yên Elektrik û Nîrvan bi Rewşa Dijîn" GB 31248, ku EN 50399:2011 hatine referans, "Metodên Testeyê ya Common bi Rewşa Dijîn – Mezaneyên Derketina Berd û Guherîn Guherî çêtirên Piramînê ya Dijîna Jînên Kablê yên Vertikal – Apparatus, Procedure û General Results." Ferqê ya sereke ya vê metoda testeyê ya navdarîya piramînê, derketina guherî çêtir, peak heat release rate û guherîn guherî çêtir hatine navberdîn. Criteria di navdarîya du klasifikasyonê de hate ferq. Sistemê ya GB 31247 (Class B1) ên baxışî ya low-halogen û low-smoke, ji ber vê yekê klasifikasyonên ne direct equivalent. Heta "B" class di navdarîya ZA/ZB/ZC de nebeş bibeştebûne requirements of the B1 class.

2. Sebebên ku Class B1 nebeş bibeştebûne ji bo Kablê yên High-Voltage

2.1 Lack of Low-Smoke, Corrosion-Resistant Materials

Bi tevahî bibeştebûna low-smoke performance bitumen paint bêrser bikar bînin. Lê bitumen paint nebeş bibeştebûne corrosion resistance requirements, û bikaranîna we ji bo standards European edê. Ji ber vê yekê, low-smoke performance criterion nebeş bibeştebûne. Kablê yên high-voltage power metallic aluminum sheath bi bitumen anti-corrosion structure bikar bînin, ku di combustion de smoke significant bike. Di abroad de, bitumen paint û hot-melt adhesive generally bikar bînin, lê vê structure nebeş bibeştebûne ji bo domestic manufacturer û engineering project. Ji ber vê yekê, material field ji bo high-voltage power cable outer sheaths restricts ability to achieve low-smoke performance required for B1 class.

2.2 Insulation Resistance Reduction in Low-Halogen Cables

Ferqê ya sereke di navdarîya high-voltage û medium-voltage power cables de li gorî material outer sheath. Ji ber high current capacity, high overvoltage û single-core design high-voltage cables, outer sheath must have excellent insulation properties ji bo operational safety. Ji ber vê yekê, outer sheath high-voltage cables specified as "insulation-grade," medium-voltage cables use "sheath-grade" material.

Lê, low-smoke, halogen-free sheathing compounds contain large amounts of inorganic flame retardants, which result in relatively poor insulation resistance for sheath. Current sheath material insulation performance follows order: PE ≥ Flame-retardant PE ≥ PVC ≥ Low-smoke, halogen-free series. Because of this, current high-voltage cable standards such as GB/T 11017 û GB/T 18890 have not incorporated low-smoke, halogen-free sheathing compounds into their standard systems. In contrast, for medium-voltage cables, where requirements for sheath insulation performance less stringent, low-smoke, halogen-free sheathing compounds already included in standard system.

Power grid companies have organized multiple cable industry conferences, primarily due to poor performance of two key indicators: water absorption rate of outer sheaths under saturated water absorption conditions and insulation resistivity under saturated water absorption conditions.

Fire prevention situation in high-voltage power cable tunnels severe. Currently, high-voltage cables mainly purchased in flame-retardant models. As name suggests, flame-retardant materials conventional sheath materials with added formulations such as flame retardants, endowing materials with flame-retardant properties. Flame-retardant performance of common sheaths shown in Table 3.

Taking PE sheath as example, flame-retardant PE standard PE sheathing material with added flame retardants. Flame retardants divided into inorganic and organic types. Currently, most products on market primarily use inorganic flame retardants, with common types including magnesium oxide û aluminum oxide. These materials readily absorb moisture û undergo hydration reactions under normal conditions. Therefore, sheathing materials typically put into production immediately after procurement; otherwise, moisture absorption can easily occur, leading to defects such as voids during extrusion. Only after flame retardant particles micronized, undergo surface modification, û have their material compatibility enhanced, can flame-retardant sheathing compounds achieve good processability.

Waterproof cables usually refer to cables with complete, sealed metallic sheath. If plastic sheath used as waterproof layer, moisture can penetrate into cable through plastic. Moisture ingress relatively slow process. During actual cable operation, sheath surface temperature can reach as high as 60°C, which accelerates moisture penetration. Therefore, for newly commissioned cable sheaths, insulation resistance generally meets requirements. However, after period of operation, sheath insulation resistance many lines drops sharply, û issue typically discovered within several months to about year. Once sheath insulation resistance decreases to certain level, rate of decline tends to stabilize û slow down.

2.4 Poor Crack Resistance of Low-Halogen Cables

In Table 5, ST2 refers to PVC, ST7 to PE, û ST8 to halogen-free, low-smoke material. From perspective of sheath mechanical properties, tensile strength û elongation at break halogen-free, low-smoke materials significantly inferior. Installation halogen-free, low-smoke cables strict requirements, especially outdoor areas northern regions, because these sheaths prone cracking low temperatures û may even develop cracking during operation. Numerous similar quality incidents already occurred with medium- û low-voltage cables China. Some construction projects use halogen-free, low-smoke cables during winter, partly because work conducted indoors where temperatures higher.

Halogen-free, low-smoke cables primarily used indoors buildings û densely populated areas such stations, subways, û public buildings. Power compartment utility tunnel does not belong densely populated area.

3 Conclusion

Based on above analysis, halogen-free, low-smoke materials perform worse than current insulation-grade flame-retardant sheathing materials û more prone problems. For reason, current high-voltage cable standards such GB/T 11017 û GB/T 18890 have not incorporated halogen-free, low-smoke sheathing materials into their standard systems.

Classification of Burning Behavior for Electric û Optical Fiber Cables GB 31247 strengthens fire behavior control. This appropriate densely populated areas like subways û high-speed rail stations, where many combustible materials, due safety considerations life û property. Most cables used these areas medium- û low-voltage, for which electrical performance requirements not as stringent as high-voltage cables.

It particularly important note that Class B rating in General Rules for Flame-Retardant û Fire-Resistant Electric Wires, Cables, û Optical Cables GB/T 19666 not equivalent to B1 rating in Classification of Burning Behavior for Electric û Optical Fiber Cables GB 31247. Two standards completely different fire performance criteria û intended application areas. They should not used interchangeably. Recommended use high-voltage cables meeting GB/T 19666 Class B, û not recommended use high-voltage cables meeting GB 31247 B1 û B2 ratings. Although both labeled B, they belong different standard systems, resulting completely different performance outcomes. Using high-voltage cables meeting GB 31247 B1 û B2 ratings would place enormous pressure on construction û operation & maintenance departments.

Given stringent fire protection requirements in power tunnels, after upgrading flame-retardant rating to Class B:

• For conduits û direct burial installations where flame retardancy not required, PE outer sheaths (without flame retardant additives, providing stable insulation resistance) can selected.

• For high-voltage cables installed in tunnels, PVC outer sheaths recommended (disadvantage release toxic gases during combustion; advantage formulation enhance water resistance, û insulation resistance more stable compared Class B flame-retardant PE cables).

Furthermore, recommended promptly initiate joint research sheath materials û structures fundamentally resolve conflict between insulation resistance û flame retardancy.