1. Introduction

Flame-proof wires and cables are a general term for electrical wires and cables with fire-resistant properties, typically categorized into two types: flame-retardant cables and fire-resistant wires. From the perspective of fire safety and firefighting rescue operations, the requirements for the fire performance of wires and cables are steadily increasing, for example:

Flame retardancy — inhibiting and slowing the spread of flames along wires and cables, preventing fires from escalating.

Fire resistance—maintaining operational capability for a specified duration under fire conditions, ensuring the integrity of the circuit.

Halogen-free — The materials used to construct wires and cables contain no halogens, resulting in lower corrosivity of their combustion products.

Low halogen – Materials used to make wires and cables may contain halogens, but at lower levels.

Low smoke — When wires and cables burn, they produce less smoke and dust, meaning their light transmittance is higher.

Low-toxicity (LOW toxicity) — The gases produced when wire and cable materials burn have low toxicity.

China's research and development of flame-retardant and fire-resistant wires and cables began in 1982. After five years—by 1987—many cable manufacturers had already started production, earning recognition from users. Addressing the then-existing confusion over naming and classification of combustion characteristics, the author proposed categorizing fireproof wires and cables based on their corresponding combustion test methods. Specifically, the first letters of the Chinese Pinyin system were adopted as prefixes for the standardized models of ordinary wires and cables. Among these, the model designation "ZR" for flame-retardant cables and "NH" for fire-resistant cables have been consistently used ever since.

With numerous new advancements achieved in the development and research of fire-resistant wires and cables, the original models have become inadequate. Moreover, as more manufacturers enter the market, companies have begun tailoring their own enterprise standards by selecting specific features from various models, inadvertently creating further confusion. Most notably, while fire-resistant wires and cables share identical combustion characteristics, they are still labeled and designated with different names and model numbers. Industry stakeholders and end-users alike are strongly urging a change to address this issue, calling for a unified system of naming and classification. To meet this need, this article proposes a systematic approach for developing standardized models of fire-resistant wires and cables, which can serve as a valuable reference for future enterprise or industry-wide standards.

2. Principles for Compiling Fire-Resistant Wire and Cable Models

(1) Each model corresponds to a specific combustion characteristic, with corresponding test methods and defined criteria for evaluation—represented by the first letter of the Chinese Pinyin—whilst keeping it as simple as possible.

(2) The model is indicated by adding a prefix before the standard wire and cable model number.

(3) The inherent combustion characteristics of standard wires and cables are not assigned a separate model number.

(4) Leaves room for further development and expansion.

Currently, there are still some differences between China's national standards and the IEC international standards, as certain standards were issued by the IEC after they had already been adopted in China. However, following the principle of adopting international standards on an equivalent basis, these discrepancies will eventually be eliminated.

3. Instructions for Model and Usage

3.1 About Flame Retardancy (ZURAN)

According to IEC 332, flame retardancy is categorized into single-wire and bundled types. Over a decade ago, China’s wire and cable industry primarily relied on halogen-containing products such as polyvinyl chloride and chloroprene rubber. Since passing the single-wire vertical burning test was already a fundamental requirement in product standards, there was no need to designate specific models for single-wire flame retardancy. Instead, Class C bundled flame retardancy became the baseline standard for flame-retardant wires and cables, while Classes B and A were reserved for cases where users specifically required even higher levels of fire resistance. As a result, using "ZR"—the initial letters of "ZURAN" (meaning "flame retardant")—as a model designation quickly gained popularity.

The current situation is different. Polyolefins such as polyethylene, polypropylene, ethylene-propylene, and natural styrene-butadiene rubber are highly flammable materials. When their flame-retardant products pass the single vertical burning test, they are assigned a specific rating to clearly distinguish them from their original flammability characteristics. This rating is denoted by the letter "Z." For bundled combustion tests, since there are distinctions among Flame Retardant Class A, Class B, Class C, and Class D, these are further differentiated using the designations ZA, ZB, ZC, and ZD, respectively. Notably, Flame Retardant Class D represents a new IEC proposal specifically tailored for wires and cables with an outer diameter of 12 mm or less. The total non-metallic material volume used in its test specimens is only one-third that of Class C—specifically, 0.5 liters per meter. However, the requirement for the fire exposure duration remains at 20 minutes, and the char height must still stay below 2.5 meters, identical to Class C standards. It’s important to note that this proposal is currently awaiting approval through a formal vote.

Additionally, IEC 332-3 specifies that when conducting Class A tests, if the samples—particularly those with a single conductor exceeding 35 mm²—are too large to fit in the standard ladder arrangement, they may be placed at the rear instead, indicated by AF/R. Alternatively, wider ladders (0.8 meters wide) can be used entirely at the front of the ladder assembly (designated as AF), with both burners applied simultaneously. However, if larger-sized cables can still be neatly arranged on the standard ladder without obstruction, only a single burner is used for testing, and this scenario is still recorded as AF. To simplify the designation system, all such cases are uniformly labeled as ZA.

3.2 Regarding Fire Resistance (NAIHUO)

According to China’s national standard GB 12666.6-90, fire-resistant cables are classified into two levels: Class A and Class B. Class A cables are designed to withstand fire at temperatures ranging from 950°C to 1000°C, while Class B cables can endure fires at temperatures between 750°C and 800°C. Previously, the NH model was used to denote fire-resistant wires and cables, which, according to the IEC 331-1970 standard, corresponded to China’s Class B rating. However, since some customers now specifically request Class A fire-resistant cables, the model designation has been expanded to NA and NB for clearer distinction. Notably, in the recently published IEC standard IEC 60331-1999, the prescribed fire exposure temperature remains at 750°C to 800°C, though proposals to raise the test temperature are still under consideration. Therefore, if a new national standard is adopted that aligns closely with this IEC standard, the distinction between Class A and Class B fire-resistant cables will no longer be necessary. At that time, the cable model designations could be simplified accordingly. However, should the IEC eventually introduce further categorization based on different test temperatures in the future, our existing model system would still have room for expansion and adaptation.

Notably, the UK has the most diverse requirements for fire-resistant cables, resulting in a wide variety of models. Given the UK's influence on IEC, the organization is already considering raising test temperatures, introducing water spraying, and incorporating mechanical impact tests. Therefore, models designated with "C" will include additional impact testing, while those marked with "S" will feature enhanced water-spraying capabilities—both options reserved for future use.

Note: Fire resistance, water-spray fire resistance, and impact fire tests are conducted separately. For instance, a cable model passing the 950°C for 3 hours fire resistance test and the 750°C impact test would be designated as CY. Meanwhile, a cable model meeting the requirements for 650°C for 3 hours fire resistance, 950°C for 20 minutes fire resistance, 650°C water-spray fire resistance, and 650°C impact fire resistance would be labeled ASWX—and so forth. The highest-level model among these is CwZ.

3.3 Regarding Low-Halogen (DILU), Halogen-Free (WULU), Low-Smoke (DIYAN), and Low-Flammability (DIDU) materials

Regarding low-halogen (DILU), it is represented by the letter D. The HCI content is determined using the IEC60754-1:1994 method (national standard GB/T 17650.1-1998). Although the standard does not specify a particular limit, it is recommended that HCI ≤ 100 mg/g.

Regarding halogen-free (WULU), which refers to low corrosivity and is denoted by "W": This is determined according to IEC 60754-2:1991 (revised in 1997). China has adopted this standard equivalently as GB/T 17650.2-1998. The specified criteria are a pH ≥ 4.3 and an electrical conductivity (r) ≤ 10 µS/mm. However, many countries and companies internationally have set their own halogen-free standards, specifying HCI content no higher than 5 mg/g—a practice that some in China have begun to emulate. Yet, this approach is inappropriate. First, IEC 60754-1 explicitly states that this method is unsuitable for materials with HCI levels below 5 mg/g, meaning it cannot reliably confirm whether a material is truly "halogen-free." Second, when the HCI content exceeds 2 mg/g, the pH of its aqueous solution drops below 4.3, thereby failing to meet the requirements outlined in IEC 60754-2. Moreover, some argue that while IEC sets the pH threshold at ≥ 4.3, Germany’s standard specifies a lower limit of ≥ 3.5, suggesting that IEC’s criteria are stricter. However, this perception is merely superficial. In reality, both standards achieve identical results in practice. If the opportunity arises, the author will elaborate further in a separate article.

Regarding low smoke (DIYAN), it is also denoted by the letter "D." Although this "D" overlaps with the one used for low halogen materials, it has become a conventional practice and does not lead to confusion when combined with other model designations. The international standard for low-smoke materials requires a light transmittance of ≥60%. It is important to note that in China, wire and cable products made from PVC-based so-called low-halogen, low-smoke materials often fail to meet this minimum light transmittance requirement, and thus should not be labeled as "low smoke." Unless specified otherwise in the product standards—such as by setting a lower light transmittance threshold and clearly stating that this figure falls below either the international or national standards—users could easily be misled.

Regarding low toxicity (DIDU), the letter "D" can no longer be used; instead, "U" should be employed. The relevant standard, IEC, is still under consideration. Currently, the British Naval Engineering Standard NES713 is widely adopted, using the Toxicity Index (TI) to indicate toxicity levels—for instance, specifying that the toxicity index of insulation materials must be below a certain threshold.

3. The toxicity index of the jacket material is less than 5. Some domestic manufacturers claim to offer halogen-free, low-smoke, and non-toxic cables; however, the term "non-toxic" used here is inaccurate—it should correctly be described as "low-toxicity." This is because, when halogen-free, low-smoke materials burn, they produce toxic CO gas. Moreover, if the materials contain phosphorus (P), nitrogen (N), or sulfur (S), the amount of harmful gases generated will be even greater.

3.4 Regarding the Combination of Models

When a certain wire or cable exhibits multiple combustion characteristics, according to international practice, the specific model and designation should follow this order (omitting any items that do not apply):

Halogen-free (low-halogen), low-smoke, low-toxicity, flame-retardant, and fire-resistant

For example:

(1) Flame-retardant (A)-class polyvinyl chloride insulated and polyvinyl chloride sheathed power cable, model: ZA-VV

(2) Low-halogen flame-retardant (Class B) PVC-insulated, PVC-sheathed power cable, model: DZB-VV

(3) Low-halogen, low-smoke, flame-retardant (Class C) PVC-insulated, PVC-sheathed power cable, model: DDZC—VV

(4) Halogen-free, low-smoke, flame-retardant (Class A) cross-linked polyethylene-insulated power cable with steel tape armor and polyolefin sheath, model: WDZA—YJY23 (where the "3" in the outer sheath indicates the polyolefin material).

(5) Halogen-free, low-smoke, flame-retardant (Class B) cross-linked polyethylene-insulated polyolefin-sheathed power cable, model: WDZB-YJY

(Polyolefin grades use Y instead of X, E, O, or P, according to GB/T 13849-1993)

(6) Halogen-free, low-smoke, fire-resistant (Class A) cross-linked polyethylene-insulated polyolefin-sheathed power cable, model: WDNA-YJY

(7) Fire-resistant (Class B) polyvinyl chloride-insulated, polyvinyl chloride-sheathed power cable, model: NB—VV

3.5 Issues That Must Be Noted

The above outlined a method for determining the appropriate model and designation based on the combustion characteristics of wires and cables. Below are some issues that have emerged recently, along with explanations on how to address them:

(1) Do not blindly apply flame-retardant coatings or materials simply because they claim to meet flame-retardant standards—there are various methods to achieve this. For instance, using an inorganic-based paste filler (commonly referred to as "oxygen-isolating layer" or "fire-isolating layer") or coating the cable with a flame-retardant tape based on inorganic flame retardants (often called "high-flame-retardant tape" or "oxygen/fire-isolating tape") can play a crucial role in addressing the flame-retardancy issues of cables made from highly flammable materials like cross-linked polyethylene insulation. In some cases, these methods can even deliver A-class flame-retardant performance.

But don't label it with terms like "highly flame-retardant," "ultra-high flame-retardant," or "extra-high flame-retardant," and avoid using GZR model designations. This is because, regardless of the specific methods or techniques employed to achieve flame retardancy—whether through IEC 332-3 or GB 12666.5 testing standards and criteria—their models and designations should remain consistent. Currently, the highest flame-retardant rating available is ZA. It's important to note that you shouldn't introduce new names for flame-retardant cables, such as "oxygen-isolation layer cable" or "fire-barrier cable," as this could lead to confusion during international bidding processes.

(2) Avoid mixing up the terminology—when you refer to "halogen-free low smoke," while others call it "low smoke halogen-free," there’s essentially no difference, but the naming becomes inconsistent. Moreover, this inconsistency leads to model numbers that could be either WD or DW, further complicating matters. Internationally, the standard practice is to list halogens before smoke; for instance, France’s Alcatel company uses the term HALOGEN FREE LOW SMOKE CABLES (meaning "halogen-free low-smoke cables"), with the model code XLS, while the UK’s Delta company opts for ZERO HALOGEN LOW SMOKE CABLES (which should properly translate as "halogen-free low-smoke cables," not "zero halogen low-smoke cables," to avoid creating confusion). As you can see, sticking with "halogen-free low smoke" is clearly the better approach. Similarly, "low-halogen low-smoke" should never be referred to as "low-smoke low-halogen."

(3) "Flame retardant" should not be referred to as "difficult-to-burn," as this term aligns more closely with the internationally recognized term "FLAME RETARDANT," since "RETARDANT" implies delaying or preventing combustion. In Japanese, it’s called "nanen" (difficult-to-burn), and flame-retardant agents are referred to as "nanen-ki" (difficult-to-burn agents). To ensure consistency in terminology, "flame retardant" is the preferred term, and models should use the letter "Z" rather than "N."

(4) Mineral-insulated (MI) cables should no longer be referred to as "fire-resistant" cables. Mineral-insulated (MI) cables are a distinct type of cable, separate from organic-insulated cables, and inherently possess both flame-retardant and fire-resistant properties. Therefore, MI cables can be recommended for applications where fire protection is critical. However, there’s no need to continue labeling them as "fire-resistant cables," as this could easily lead to confusion with flame-retardant or fire-rated cables that use organic insulation. Additionally, introducing yet another specific model number is unnecessary. In China, MI cables are sometimes called "fire-resistant cables" due to the perception that they offer superior flame-retardant and fire-resistant performance. In reality, though, the testing methods and evaluation criteria used for all these types of cables remain identical.

(5) Regarding the flame-retardant rating of fire-resistant cables, according to the recently published British standards BS7629-1997 and BS6387-1994, the flame-retardant performance requirement for fire-resistant cables is verified through a single vertical burning test. Therefore, if China chooses to follow suit, it could also standardize the flame-retardant rating of fire-resistant cables as "single-strand flame retardant" when developing its own standards. In that case, the flame-retardant designation could be omitted entirely from the cable model numbers. Of course, if users specifically request a higher flame-retardant rating, the flame-retardant code can simply be added back into the model number as needed.

The combustion characteristics of cables are assessed collectively, without focusing on the individual properties of each component. This not only leaves room for creative thinking in cable design but also provides favorable conditions for simplifying models and terminology.

 

The above method for compiling fire-resistant wire and cable models builds upon previous practices, aligns with established domestic conventions, is simple and intuitive, easy to remember, and facilitates future additions or deletions of specific features. We hope it will be widely accepted—or that you’ll share your feedback.