BTTZ-500V/750V
BTTZ Mineral Insulated Copper-Sheathed Fire Resistant Cable
Magnesium oxide inside a copper tube: no organic material in the cable to burn.
BTTZ is the construction every other fire-resistant cable gets compared against. A solid copper conductor sits inside a drawn copper tube with no longitudinal seam, and the space between them is filled with magnesium oxide powder compressed to roughly 80% relative density. Nothing in the cable is organic. Magnesium oxide melts above 2,800 °C, so the insulation path from conductor to sheath survives flame temperatures that reduce a polymer cable to ash and copper itself does not melt until 1,083 °C.
The tube is drawn down over the assembly, which is what makes BTTZ rigid: a 4 × 25 mm² cable arrives on a drum but will not turn tight corners, and every cut end has to be sealed because magnesium oxide absorbs moisture and loses insulation resistance. Voltcore supplies BTTZ in 500 V and 750 V grades with factory-fitted brass glands, pot seals and crimp terminals, so the crew terminating a riser does not compound seals on site.
Specification
| Item | Value |
|---|---|
| Rated voltage U0/U | 500 V light duty, 750 V heavy duty |
| Conductor | Plain copper, Class 1 solid to 6 mm², Class 2 stranded from 10 mm² |
| Insulation | Compressed magnesium oxide powder, minimum 1.0 mm radial |
| Sheath | Drawn copper tube with no longitudinal seam, annealed, 0.62 – 1.05 mm wall |
| Core count and sizes | 1 core 1.5 – 400 mm²; 2, 3, 4 cores 1.5 – 25 mm²; 7 and 12 cores 1.5 – 2.5 mm² |
| Continuous temperature | 250 °C at the conductor with matched high-temperature seals |
| Circuit integrity | BS 6387 C (950 °C, 180 min), W (650 °C plus water), Z (950 °C plus shock) |
| Combined-duty test | BS 8491 — fire, water spray and shock applied together, 120 min |
| Min bending radius | 6 × OD for a single formed bend; re-bending is not recommended |
| Termination | Brass compression gland, pot seal and crimp lug; seals mandatory at both ends |
| Reaction to fire | Zero halogen, zero smoke contribution — no organic sheath compound |
Layer-by-layer construction
Listed from the conductor outward. Read this alongside the specification table — the two together tell you whether the cable will survive its installation method.
- 1. ConductorPlain copper, Class 1 solid up to 6 mm² and Class 2 stranded from 10 mm², IEC 60228
- 2. InsulationMagnesium oxide powder filled under vibration to a minimum 1.0 mm radial thickness
- 3. Metallic sheathCopper tube drawn over the filled core with no longitudinal seam, annealed, 0.62 – 1.05 mm wall
- 4. Oversheath (optional)LSZH or PVC jacket over the copper tube for buried, wet or chemically aggressive routes (BTTVZ / BTTYZ)
Reference dimensions and weights
| Nominal cross-section (mm²) | Conductor structure (n × mm) | MgO radial thickness (mm) | Copper sheath wall (mm) | Approx. overall diameter (mm) | Approx. weight (kg/km) | Max. DC resistance at 20 °C (Ω/km) |
|---|---|---|---|---|---|---|
| 4 × 1.5 | 1 / 1.38 | 1.0 | 0.62 | 9.6 | 340 | 12.1 |
| 4 × 2.5 | 1 / 1.78 | 1.1 | 0.68 | 10.9 | 435 | 7.41 |
| 4 × 4 | 1 / 2.25 | 1.2 | 0.74 | 12.4 | 585 | 4.61 |
| 4 × 6 | 1 / 2.76 | 1.3 | 0.80 | 13.8 | 760 | 3.08 |
| 4 × 10 | 7 / 1.35 | 1.4 | 0.86 | 16.2 | 1090 | 1.83 |
| 4 × 16 | 7 / 1.70 | 1.5 | 0.95 | 18.7 | 1540 | 1.15 |
| 4 × 25 | 7 / 2.14 | 1.6 | 1.05 | 21.9 | 2220 | 0.727 |
Reference figures for preliminary sizing and drum planning. Overall diameter and weight vary with strand construction and sheath thickness tolerance; certified drawings are issued with each quotation.
Engineering characteristics
- Magnesium oxide contains no organic material, so the cable cannot add fuel or smoke to a fire.
- BS 6387 category C: circuit holds rated voltage at 950 °C for 180 minutes with the copper sheath acting as the earth path.
- Category W adds water spray after fire exposure at 650 °C and Z adds mechanical shock at 950 °C at 30 s intervals.
- Conductor rated 250 °C continuous, so ambient derating in boiler houses and plant rooms is rarely needed.
- The drawn copper tube provides a fault-current path of the same conductance class as the phase conductors and resists impact and rodents.
- Overall diameter is typically 25–30% smaller than an equivalent polymer fire-resistant cable, which matters in congested risers.
- Halogen acid gas and smoke emission are both zero, because there is no sheath compound to decompose.
Typical applications
- Fire pump, smoke fan and evacuation lift feeders where 950 °C for 3 h is written into the specification
- Emergency power and lighting circuits in road and rail tunnels
- High-temperature zones in steel, glass and cement plants
- Operating theatre and intensive care essential-services circuits
- Safety-related cabling in conventional and nuclear power plant
- Heritage buildings where a small-diameter surface-run cable is accepted
Ordering options
- BTTVZ / BTTYZ — LSZH or PVC oversheath for buried, wet or chemically aggressive routes
- Single-core 1.5 – 400 mm² for high-current feeder runs
- Factory-fitted terminations: brass gland, pot seal, crimp lug, heat-shrink sleeve
- Plain or tinned copper sheath, with red or orange oversheath for fire-system identification
- Pre-cut lengths with sealed and capped ends, tagged per riser and floor level
Standards manufactured to
- BS 6387 (C/W/Z)
- BS 8491
- IEC 60702-1
- BS 6207
- GB/T 13033.1
- IEC 60331-21
Manufactured to other standards on request. Send the standard number with your RFQ and we will quote against it rather than against a near-equivalent.
Price and availability for BTTZ Mineral Insulated Copper Sheathed Cable
Include cross-section, core count, voltage grade, route length and installation method — those five determine price more than anything else on the page.
- Engineer-reviewed before pricing
- Reference drawings issued with the quotation
- Free sample up to free sample up to 5 m
BTTZ Mineral Insulated Copper Sheathed Cable — frequently asked questions
Is BTTZ really mineral insulated, or is the term used loosely?
BTTZ is true mineral insulation: the only materials between conductor and earth are magnesium oxide powder and a copper tube, both inorganic. Flexible products sold as mineral insulated, such as BBTRZ, use mica tape and a polymer sheath; they can pass the same BS 6387 categories but are not MgO insulated. The test report will name which construction was tested.
Why is BTTZ supplied in short lengths and why can it not be re-bent?
The copper tube work-hardens as it is drawn. Bending tighter than 6 × OD, or re-forming a section already bent, can crack the tube and let moisture reach the magnesium oxide. Voltcore supplies standard drums of 100 m or 200 m in smaller sizes, cuts to drawing, and pre-forms bends before dispatch where the route is known.
What happens if a cut end is left unsealed overnight?
Magnesium oxide is hygroscopic: an open end can fall from hundreds of MΩ to below 1 MΩ within hours in humid air. Every cut end must be sealed immediately with compound and a pot seal, and ends left overnight must be capped. Voltcore ships sealing compound with the cable and fits seals in the factory when terminations are ordered together.
How is the copper sheath terminated for earthing?
The sheath is the circuit protective conductor. Fit a brass compression gland and pot seal, terminate the tube into the gland body, then bond the gland to the earth terminal with a conductor sized to the phase. Voltcore supplies matched glands, seals and lugs in one kit so the earth path continues through the termination instead of stopping at the tube.