Large Fire Cable BTTRZ 4x95mm²+1x70mm² Mineral Insulated 0.6/1kV Fire Resistant

Cable Designation & Meaning

Construction: Flexible mineral-insulated fire-resistant cable, corrugated copper tube sheath, round profile (5-core)

Note: BTTRZ employs Class 5 fine-stranded flexible copper conductors. Each core is insulated with fire-resistant mica tape plus XLPE (or ceramic silicone rubber). Cores are assembled with mineral fillers and enclosed in a continuously welded corrugated copper tube sheath, which serves simultaneously as armour, water barrier, and PE conductor. The 4+1 configuration is designed for TN-S three-phase five-wire systems; the PE conductor is 70 mm². Per GB/T 16895 / IEC 60364, for phase conductor S > 35 mm², PE ≥ S/2. Since phase = 95 mm², S/2 = 47.5 mm², thus PE = 70 mm² exceeds the minimum requirement​ and is fully compliant (many projects specify PE = phase or PE = S×0.5–0.7 for added safety margin).

BTTRZ 4×95 + 1×70 mm² — Cable Specification & Engineering Data 

1. Product Overview & Rating Summary. BTTRZ 4×95+1×70 is a flexible mineral-insulated fire-resistant power cable rated 0.6/1 kV (Um = 1.2 kV) at 50 Hz, built around four 95 mm² tinned-copper Class 5 flexible phase conductors and one 70 mm² neutral/earth conductor. Unlike rigid magnesium-oxide mineral-insulated cables, BTTRZ uses mica-tape wrapped insulation enclosed in a continuously corrugated copper tube, then jacketed in a low-smoke zero-halogen (LSZH) outer sheath. The cable maintains circuit integrity for at least 90 minutes at 950 °C open-flame exposure (GB/T 19216.21 / IEC 60331-21). Finished diameter is about 44 mm and nominal mass about 3200 kg/km (≈ 320 kg per 100 m), supplied on 500 m or 1000 m wooden-steel drums. With four 95 mm² cores loaded, the cable carries about 270 A in free air at 25 °C (engineering estimate), making it the standard fire-rated main feeder for large fire pumps, emergency main switchboard risers and critical life-safety power circuits.

2. Layer-by-Layer Construction (Conductor to Outer Jacket).

3. Electrical Characteristics. Each 95 mm² phase conductor is held to a maximum DC resistance of 0.193 Ω/km at 20 °C, rising to about 0.252 Ω/km at the 90 °C operating point. The 70 mm² neutral/earth conductor holds R ≤ 0.268 Ω/km at 20 °C. Every length survives the routine 3.5 kV AC power-frequency proof held for five minutes. Finished-cable insulation resistance remains above 36.7 MΩ·km (mica-rated, very high), and typical figures include per-core capacitance ≈ 0.60 μF/km and 50 Hz AC impedance ≈ 0.193 Ω/km. With four 95 mm² cores energised, reference loading is about 270 A in free air at 25 °C.

Insulation Data

Electrical Parameters

Reference values: 95 mm² at 250 A, 3-phase 400 V → max balanced load ≈ 173.2 kW. Voltage drop at 250 A ≈ 2.0 V/km per core (≈0.5% per 100 m).

Temperature Characteristics

4. Thermal Response & Flame Behaviour. Mica insulation sustains a 90 °C continuous conductor temperature, with short-circuit capability up to 250 °C for ≤ 5 s. The cable is rated for service ambients between −15 and +70 °C. The defining property is fire resistance: under GB/T 19216.21 / IEC 60331-21, the cable maintains circuit integrity at rated voltage for at least 90 minutes when directly exposed to a 950 °C open flame. The corrugated copper sheath provides both a fire barrier and a mechanical shield against flame penetration. The LSZH outer jacket emits low smoke and no acid gas (halogen-free), meeting the requirements for tunnels, metro systems, hospitals and high-rise public buildings.

Fire Performance (BTTRZ — Mineral Insulated)

5. Installation Practice & Handling. Bends must not fall below 15 times the actual outer diameter — about 660 mm for this size. The corrugated copper sheath must be earthed at both ends to provide protective-earth continuity. Special sealing glands and termination kits are required at both ends to keep moisture out of the hygroscopic mica layer — improperly terminated ends are the most common failure point on mineral-insulated cables. Reels held below 0 °C in storage must be warmed indoors before uncoiling. The 270 A reference requires derating whenever ambient temperature, grouping or burial depth deviates from reference conditions. At about 320 kg per 100 m, the cable requires drum jacks, rated pulling gear and at least four fitters.

Mechanical & Installation Parameters

6. Typical Service Applications. This 4×95+1×70 BTTRZ cable is the fire-rated main feeder for large emergency and safety-critical circuits: main fire-pump and sprinkler-pump feeds up to about 68 kW, main smoke-exhaust and pressurisation fan feeds up to about 22 kW, emergency main switchboard risers, fire-alarm system main power loops, emergency elevator main power supply, hospital emergency main switchboard feeders, metro and tunnel emergency power main distribution, high-rise building fire-fighting main risers, data-centre UPS main emergency feeders, oil and gas platform fire-resistant power cables, stadium and large-venue emergency main distribution, industrial furnace and oven emergency main power, and any circuit where continued operation during a 90-minute fire event is mandated by building code. Approved installation methods include heavy-duty cable tray, direct tray in fire-protected zones, and fire-rated conduit.

7. Testing & Quality Assurance Regime. Routine tests on every length include conductor DC resistance verification (95 mm² ≤ 0.193; 70 mm² ≤ 0.268 Ω/km) at 20 °C, the 3.5 kV AC / 5 min withstand, insulation resistance ≥ 36.7 MΩ·km, conductor continuity across all five cores, corrugated copper sheath electrical continuity, and dimensional inspection covering mica ≈ 0.7 mm, Cu sheath ≈ 0.5 mm, LSZH jacket ≥ 2.2 mm and overall diameter ≈ 44 mm. Sample and type tests comprise the 950 °C / 90 min fire-resistance test (GB/T 19216.21), vertical flame spread GB/T 18380.34 class A, smoke density GB/T 17651, halogen acid gas content GB/T 17650, sheath moisture-tightness / water-immersion leak test, bending at ≥ 15 × OD, short-circuit to 250 °C, and voltage re-test after fire exposure. A factory test certificate accompanies each drum.

Cable Parameters

Cable Test Parameters

Cable Cross Section Structure

Cable Construction Description 

8. Standards & Conformity. Design and production are governed by GB/T 19216.21 / IEC 60331-21 for fire-resistance (950 °C / 90 min), GB/T 19666-2019 for flame-retardant and fire-resistant cable classification, and GB/T 12706.1-2020 for low-voltage power cables at 0.6/1 kV. Conductors satisfy GB/T 3956 Class 5. General test methods follow GB/T 19216 series and GB 5023. Surface marking complies with GB/T 6995. The LSZH jacket meets halogen-free requirements per GB/T 17650. Cables can also be supplied to IEC 60331, BS 5839, BS 7621 or NEK 606 requirements on request.

9. Packaging, Storage & Logistics. Standard delivery is 500 m or 1000 m per wooden-steel drum, with bespoke cut lengths available on request. Each drum carries type, voltage, size, supplier identity, production year and sequential metre marking per GB/T 6995. Storage should be in dry, covered premises at −15 to +40 °C, shielded from direct sunlight, moisture, oils and sharp objects; drum end seals must remain intact until installation. Because mica is hygroscopic, any drum whose end seal has been damaged must be re-tested for insulation resistance before use. During transit, drums must be secured against rolling and impact.

10. Procurement Specification. State "BTTRZ 0.6/1 kV 4×95+1×70" together with the required length or drum arrangement, and specify whether the corrugated copper sheath is to be used as the protective earth (PE) or whether a separate green-yellow 70 mm² conductor is required. Always order the matching sealing gland and termination kit at both ends. Always request the factory test certificate, which must include the 950 °C / 90 min fire-resistance report, to confirm the actual diameter (≈ 44 mm), weight (≈ 3200 kg/km) and conductor resistances for the production batch.

11. Cable Industry FAQ.

• Q1: When should I choose 95 mm² over 70 mm² for fire-rated circuits? Choose 95 mm² when the emergency load carries up to about 270 A (vs about 220 A for 70 mm²), such as main fire pumps up to 68 kW, large smoke-exhaust fans up to 22 kW, or main emergency switchboard risers where voltage drop is critical. 70 mm² is sufficient for branch-level fire-rated main feeders.
• Q2: Why is the neutral/earth core only 70 mm² while phases are 95 mm²? In typical three-phase balanced loads the neutral carries little current, so it is derated to 70 mm². The corrugated copper sheath additionally provides a continuous protective-earth path, so the separate 70 mm² core serves primarily as a system neutral.
• Q3: What does "950 °C / 90 min" actually guarantee? During the GB/T 19216.21 / IEC 60331-21 test, the cable is laid in a test furnace, energised at rated voltage, and directly exposed to a 950 °C flame for 90 minutes. The cable must continue to carry the load without electrical breakdown. This is the rating that justifies its use on fire pumps, emergency lighting and escape-route circuits.
• Q4: Why is the corrugated copper sheath necessary? The copper sheath serves three purposes: (i) physical flame barrier protecting the mica layer; (ii) moisture barrier — mica is hygroscopic and must stay dry; (iii) continuous electrical path that can double as the protective earth (PE).
• Q5: Why is the jacket LSZH instead of PVC? In a fire, PVC releases dense smoke and hydrochloric acid gas, which are the dominant causes of death in fires. LSZH jacket releases low smoke and no acid gas — essential in tunnels, metros, hospitals and high-rise public buildings where evacuation is slow.
• Q6: Why must the termination kit be special? Mica tape is hygroscopic: if moisture enters the cable end, the insulation resistance collapses. Standard PVC cable glands are not air- and moisture-tight. The BTTRZ termination kit uses a sealed brass gland with epoxy sealing compound and a heat-shrink boot.
• Q7: Can the copper sheath replace the separate earth conductor? Yes — the corrugated copper sheath is electrically continuous and can be sized as the protective earth (PE), provided its cross-sectional area and fault-current capacity meet the installation code.
• Q8: How heavy is this cable compared to ordinary PVC? At ≈ 3200 kg/km it is roughly twice the mass of an ordinary RVV 4×95 flexible cord, because of the corrugated copper sheath and mica layers. This is normal for fire-rated cables and must be planned for in tray loading and pulling.
• Q9: Can BTTRZ be used outdoors or buried directly? Yes — the LSZH jacket is UV-resistant and the copper sheath is an effective moisture barrier, so BTTRZ can be installed outdoors, in trays or directly in buried trenches.
• Q10: Which tests confirm acceptability on delivery and before energisation? At delivery, verify the factory fire-resistance certificate (950 °C / 90 min), conductor resistance (95 ≤ 0.193; 70 ≤ 0.268 Ω/km), dimensions (≈ 44 mm), and the drum end-seal integrity. After installation, complete an insulation-resistance test, a 3.5 kV AC / 5 min withstand, and verify both ends of the corrugated copper sheath are properly earthed before energising.
• Q11: What service life can be expected? When installed, terminated and operated within the stated temperature, voltage and fire ratings, BTTRZ cable is designed for a service life of around 30–40 years in fixed installation. The two failure modes to watch are moisture ingress through improperly sealed terminations and mechanical damage to the corrugated sheath.