I discussed with industry colleagues the two types of fire-resistant cables currently available on the domestic market: traditional mineral insulated cable (BTTZ) using magnesium oxide insulation, and inorganic mineral insulated cable (YTTW) using a glass fiber and mica tape composite material.
Traditional mineral insulated cable (BTTZ) features a tightly integrated copper core, copper outer sheath, and insulation. It has been produced and used overseas for nearly 80 years, and in China for over 20 years. It has been widely used in major projects both domestically and internationally, with excellent results.
Inorganic mineral insulated cable, also known as flexible fire-resistant cable (YTTW), consists of a conductor made of multiple strands of copper wire. The insulation layer is made of multiple layers of mica tape, with a glass fiber cloth base. The outer layer is wrapped longitudinally with copper tape and welded together to form a sheath. The smooth sheath is then pressed into a spiral shape.
This article primarily uses domestic and international cable testing standards, analyzes test results, and determines the performance of two cables. The results are based on test methods (primarily fire resistance and other tests) and determine the superiority of the two cables.
1. Fire Resistance Comparison Test
The fire resistance test reflects the cable's ability to maintain power supply under fire conditions. To examine the fire resistance of mineral-insulated cables (BTTZ) and inorganic mineral-insulated cables (YTTW), comparative tests were conducted on 4×25 cable samples according to the fire test specifications of GB/T19216, IEC331, and BS6387. The results are as follows:
GB/T19216: 750°C for 90 minutes; both cable samples passed the test.
IEC331: 750°C for 90 minutes; both cable samples passed the test.
BS6387: 950°C for 3 hours; both cable samples passed the test.
650°C for 15 minutes with spraying; both cable samples passed the test.
950°C for 15 minutes, with shocks applied every 5 minutes. Both cable samples passed the test.
Observations of the samples during the test and the final test results indicate that, under sustained high-temperature combustion conditions, the mica used for insulation and fire resistance in inorganic mineral insulated cables (i.e., flexible fire-resistant cables) will shed as powder, and the glass fiber cloth will become hard and brittle. Due to the structural characteristics of YTTW cables, there are considerable gaps between the sheath and insulation, creating space for the shed mica powder. This can easily cause short circuits under external impact.
The insulation layer of this type of cable is not a dense magnesium oxide layer like that of BTTZ cables, so its explosion-proof performance is also poor. Combustible gases, gasoline, and steam can spread through the gaps between the cable sheath and insulation layer to connected electrical equipment or other areas requiring explosion-proofing. Therefore, WTTY cables are not suitable for critical locations, such as fire protection systems, based solely on their fire resistance.
II. Comparative Voltage Withstand Test
The YTTW cable was subjected to a voltage increase at 150V/s for 15 minutes each. It failed to reach 2500V and broke down at 1300V. Three hours later, the voltage was applied again, and it broke down at 20V. The cable did not recover its performance after the breakdown.
The BTTZ cable was subjected to a voltage increase at 150V/s for 15 minutes each, and it did not break down at 2500V. It broke down when the voltage was further increased to 3500V. Three hours later, the voltage was applied again at 150V/s for 15 minutes each, and it did not break down at 2500V. The cable recovered its performance after the breakdown.
The test shows that the BTTZ cable's voltage limit is almost twice that of the YTTW cable. If an overvoltage is accidentally generated during use, the cable may break down.
Damage to the insulation layer of a BTTZ cable occurs due to air ionization at the point of the breakdown, causing the magnesium oxide to partially melt. However, the composition of the magnesium oxide remains unchanged after melting. After a period of solidification and crystallization, it remains a dense magnesium oxide, allowing the cable to regain its original electrical performance. However, once a YTTW cable is broken down, its electrical performance cannot be restored and it must be scrapped.
III. Ampacity Comparison Test
Tests were conducted using YTTW and BTTZ cable samples of the same cross-section (4×25 mm).
Ampacity Comparison
The test results show that under identical conditions, the conductor and sheath temperatures of the BTTZ cable are approximately 6°C lower than those of the WTTY cable. Regarding the cable itself, the type of insulation material and its heat resistance and heat dissipation properties significantly influence the cable's ampacity.
It's clear that mica tape has poorer heat dissipation than magnesium oxide. According to GB/T 16895, for bare mineral insulated cables that are not intended for human contact with flammable materials, the current carrying capacity is 140A when the metal sheath temperature is 105°C and the ambient temperature is 30°C.
The above tests were conducted at an ambient temperature of 20°C. If the ambient temperature reaches 30°C, the cable sheath surface temperature will be even higher. If both cables are at the same ambient temperature and the sheath surface temperature reaches a certain value, the current carrying capacity of BTTZ cable will undoubtedly be much greater than that of WTTY cable.
IV. Comparison of Other Characteristics
YTTW cable, also known as flexible fire-resistant cable, is primarily designed for fixed installation and does not require frequent movement like some organic cables. Therefore, flexibility is not an essential feature of this type of cable.
Furthermore, due to its structure and size, the allowable bending radius of YTTW cable during installation is significantly larger than that of BTTZ cable. This makes it unsightly and wastes space, whether installed over bridges or along walls.
The outer diameter of YTTW cable is larger than that of BTTZ cable of the same cross-section, which can significantly complicate installation in limited space. Additional testing is currently underway and will be published later.
Note: This article uses selected test results to illustrate the true characteristics of the two cables for reference, avoiding misleading promotional materials and presumptuous articles. Exercise caution when selecting cables for critical applications to avoid creating potential safety hazards.
