10.2 Specialised Cables: MICC & Fire-Performance Cables

10.2 Specialised Cables: MICC & Fire-Performance Cables

Life-safety systems—including automatic fire detection, emergency escape lighting, sprinkler booster pumps, and smoke evacuation extractors—must maintain circuit integrity during severe building fires. Standard thermoplastic (PVC) cables ignite and decompose rapidly at temperatures between 200°C and 300°C, releasing dense toxic fumes and causing catastrophic short circuits. To keep power and signalling available during a fire, fire-resistant wiring systems are specified by the relevant codes of practice, notably BS 5839-1 for fire detection and alarm systems and BS 5266-1 for emergency lighting.

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Mineral Insulated Metal Sheathed (MICC / Pyro) Cable

Manufactured in the United Kingdom to BS EN 60702, Mineral Insulated Metal Sheathed (MICC) cable—frequently referred to by the trade name Pyro—represents the most durable wiring system in electrotechnical engineering.

  ┌──────────────────────────────────────────────────────────┐
  │               Seamless Copper Outer Sheath               │
  │  ┌────────────────────────────────────────────────────┐  │
  │  │        Compacted Magnesium Oxide (MgO) Powder      │  │
  │  │  ┌──────────────┐                ┌──────────────┐  │  │
  │  │  │ Solid Copper │                │ Solid Copper │  │  │
  │  │  │Conductor (L) │                │Conductor (N) │  │  │
  │  │  └──────────────┘                └──────────────┘  │  │
  │  └────────────────────────────────────────────────────┘  │
  └──────────────────────────────────────────────────────────┘

Construction and Physical Metallurgy

1. Conductors: High-conductivity solid plain annealed copper conductors (Class 1).
2. Insulation: Highly compressed, pure Magnesium Oxide (MgO) mineral powder. Magnesium oxide is an inorganic refractory ceramic with exceptional dielectric strength and high thermal conductivity.
3. Outer Sheath: A continuous, seamless, malleable copper tube drawn down tightly over the compressed powder. The copper sheath acts as an exceptionally low-resistance integral Circuit Protective Conductor (CPC).
4. Polymer Oversheath (Optional): An extruded outer polymer/LSF jacket applied over the bare copper sheath. Oversheaths are available in several colours: red is commonly used to identify fire alarm circuits, while white and other colours suit neat surface runs in decorated or historic buildings.

Exceptional Engineering Properties

• Incombustibility and Extreme Temperature Withstand: Because it contains zero organic or plastic materials, MICC cannot ignite, sustain combustion, or propagate flame. Copper melts at 1083°C, while magnesium oxide melts at 2800°C. MICC can operate continuously at temperatures exceeding 250°C and maintain circuit integrity throughout full-scale structural building fires exceeding 1000°C.
• Zero Smoke and Toxic Emission: Produces zero smoke, halogens, or toxic fumes, eliminating asphyxiation risks along escape corridors.
• Mechanical Robustness: The cable is completely malleable and ductile. When struck by a hammer, crushed by falling masonry, or bent around tight radii, the copper conductors, compressed powder, and outer sheath deform proportionally without insulation rupture, preserving electrical isolation.
• Imperviousness: The seamless copper sheath is 100% impermeable to liquid water, steam, hydrocarbons, industrial solvents, and compressed gases.
• Electromagnetic Compatibility (EMC): The continuous solid copper sheath provides an absolute Faraday shield against radio frequency interference (RFI) and electromagnetic interference (EMI).

The Critical Vulnerability: Hygroscopic Nature of MgO

Despite its extraordinary physical strength, MICC cable possesses one critical vulnerability: magnesium oxide is intensely hygroscopic.

When a cable end is cut and left exposed to ambient air, the dry compressed powder greedily absorbs atmospheric moisture. Moisture travels rapidly along the cable core by capillary action. As water molecules penetrate the porous ceramic, the insulation resistance plummets from several hundred megohms (> 500 MΩ) down to zero ohms (0 Ω), converting the mineral insulation into a conductive slurry that causes immediate phase-to-earth dead shorts.

Drying Out Moist Cable (The Sweating Procedure)

If moisture penetrates an unsealed MICC cable end, the installer must sweat out the moisture using an LPG blowtorch or gas flame:

1. Apply heat to the copper sheath starting approximately 150 mm to 300 mm back from the exposed cut end.
2. Maintain gentle, circular flame movement to heat the copper without melting the conductor copper or annealing it excessively.
3. Slowly advance the flame along the sheath toward the open cut end, driving vaporized moisture out into the air as a visible wisp of steam.
4. Allow the cable to cool slightly, and immediately apply a temporary seal or terminate with a permanent brass pot.

  Sweating Out Moisture from MICC Cable:
  [Solid Copper Sheath] ──► [Flame Applied 300 mm back] ──► [Advance Flame]
                        ──► [Steam Driven Out Forward] ──► [Exposed Cut End]

Step-by-Step MICC Termination Sequence

Terminating an MICC cable requires specialized trade tools and methodical craftsmanship to ensure a permanent, hermetic, moisture-proof seal:

1. Gland Assembly: Slide the brass compression gland nut, compression olive (compression ring), and gland body over the cable before stripping.
2. Ringing and Stripping: Adjust the cutting blade of an MICC ringing tool to score a neat, square line around the copper sheath circumference at the exact stripping depth. Using an MICC rotary stripper (or hand-held joist stripper), rotate the tool clockwise around the cable. The hardened blade peels the copper sheath away as a continuous coiled shaving, exposing the conductors and compacted powder.
3. De-powdering: Wipe the conductors clean with a dry rag. Scrape away approximately 2 mm of loose powder from inside the sheath rim to provide a clean seating shoulder for the seal.
4. Screwing on the Brass Pot: Push an internally threaded brass pot over the conductors. The leading edge of the pot features a tapered, self-tapping thread. Using a pot-wrench or screw-pot tool, rotate the pot clockwise with firm forward pressure. The pot cuts its own matching threads directly into the soft outer copper sheath until the sheath edge seats flush against the interior shoulder of the pot.
5. Injecting Cold Compound: Knead a two-part cold sealing compound (mastic or epoxy resin). Force the compound firmly into the open mouth of the brass pot from one side using a potting tool or thumbs. Packing from one side prevents trapping air pockets or moisture bubbles against the powder face.
6. Fitting the Insulating Sleeving Disc: Thread flexible silicone or neoprene insulating sleeves over each exposed copper conductor. Push the pre-formed insulating disc down over the conductors and seat it squarely into the rim of the brass pot.
7. Pot Crimping: Place a brass pot crimping tool over the assembly. Tighten the tool to compress and crimp the upper brass rim of the pot down over the perimeter of the insulating disc, permanently locking the compound and conductors in a hermetic mechanical seal.
8. Immediate Insulation Testing: Connect an insulation resistance tester set to 500 V DC or 1000 V DC between conductors, and between each conductor and the copper sheath. The reading must be verified immediately. A healthy seal typically yields an insulation resistance reading well above 100 MΩ. Any reading below 1.0 MΩ indicates trapped moisture or air, requiring the pot to be cut off and the seal remade.

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Modern Polymeric Fire-Resistant Cables (BS 7629-1 / FP200 Gold)

While MICC provides the ultimate fire endurance, its specialized tooling and labor-intensive termination led to the development of lightweight, easily terminated polymeric fire-performance cables, the most widely recognized being Prysmian FP200 Gold (manufactured to BS 7629-1).

  ┌──────────────────────────────────────────────────────────┐
  │          Tough LSOH Thermoplastic Outer Sheath           │
  │  ┌────────────────────────────────────────────────────┐  │
  │  │        Laminated Aluminium Screen Tape             │  │
  │  │  ┌──────────────┐   ┌────────────┐   ┌──────────┐  │  │
  │  │  │Silicone Cores│   │ Drain Wire │   │ Silicone │  │  │
  │  │  │ (Phase Con.) │   │(Tinned Cu) │   │ (Neutral)│  │  │
  │  │  └──────────────┘   └────────────┘   └──────────┘  │  │
  │  └────────────────────────────────────────────────────┘  │
  └──────────────────────────────────────────────────────────┘

Construction and Fire-Resisting Mechanism

1. Conductors: Solid plain annealed copper conductors (Class 1).
2. Insulation: A hard, damage-resistant fire-resistant insulation (Prysmian's Insudite, classified as type EI5) designed to keep the conductors separated and the circuit working for the rated time in a fire. Some other fire-resistant cables use silicone-based insulation that forms a protective ceramic layer when it burns.
3. Electrostatic Screen: An aluminium/polyester laminated metallic screening tape folded longitudinally over the insulated cores, providing 100% electrostatic screening against electromagnetic noise.
4. Circuit Protective Conductor (drain wire): An uninsulated tinned copper conductor in continuous contact with the aluminium screen, so the screen is earthed automatically. Sleeve it green-and-yellow and connect it to the earth terminal at every termination.
5. Outer Sheath: A robust, white or red Low Smoke Zero Halogen (LSOH) thermoplastic jacket that emits virtually no toxic halogens (such as hydrogen chloride) and minimal smoke when burned.

Standard vs. Enhanced Fire Performance (BS 5839-1)

British Standard BS 5839-1 (Fire detection and fire alarm systems for buildings) categorizes fire-resisting cables into two distinct performance tiers based on fire survival duration:

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Conductor Identification to BS 7671 Table 51

In 2004, the UK harmonized its cable conductor colour code with European CENELEC standard HD 308 S2, incorporated into BS 7671 Table 51 (fully mandatory since 31 March 2006).

Harmonised vs. Pre-Harmonised Conductor Colours

The Danger of Mixed Colours and Regulation 514.14.1

The introduction of harmonized colours introduced a critical trade hazard: in older UK installations, Black was Neutral, but in modern harmonised three-phase wiring, Black is Line 2 (Phase 2). Furthermore, Blue was formerly Line 3, but is now Neutral!

To prevent electricians from mistakenly connecting a 400 V phase conductor to a neutral bar or touch-accessible equipment, BS 7671 Regulation 514.14.1 mandates:

Where an addition or alteration is made to an installation such that wiring to two different versions of BS 7671 coexists (pre-harmonised and harmonised colours), an indelible, clearly readable caution notice must be permanently affixed at or near the appropriate distribution board or consumer unit.

  ┌─────────────────────────────────────────────────────────────────┐
  │                            CAUTION                              │
  │  This installation has wiring colours to two versions of        │
  │  BS 7671. Great care should be taken before undertaking         │
  │  extension, alteration or repair that all conductors are       │
  │  correctly identified.                                          │
  └─────────────────────────────────────────────────────────────────┘