Mineral-insulated cable (MI/MICC) — fire-resistant cable without organic insulation

The guide on fire-resistant (functiebehoud) cable covers the PH and E classification that demonstrates a cable for fire alarm, evacuation, or sprinkler pump circuits keeps working electrically during a fire. This article covers a specific cable technology that achieves that result in a fundamentally different way from the usual, organically insulated fire-resistant cable: mineral- insulated cable (MI, in practice often referred to by the historical abbreviation MICC, for mineral-insulated copper-clad).

Construction: nothing organic to burn

An MI cable consists of copper conductors surrounded by compacted magnesium oxide powder (MgO) as the insulating material, enclosed in a seamless, drawn copper sheath. Unlike virtually any other cable construction, this build contains no organic material at all: no plastic insulation, no plastic outer sheath (apart from an optional thin, non-load-bearing low-smoke halogen-free (LSZH) overall sheath applied for aesthetic or corrosion reasons). Because there is simply nothing combustible present in the core of the cable, the fire resistance of an MI cable is not the result of a special fire-resistant compound passing a test standard — as with the PH/E-tested, organically insulated fire-resistant cable from the related guide — but an intrinsic property of the material itself.

Standard and voltage rating

MI cable and its terminations are standardized in IEC 60702-1 (the cable itself) and IEC 60702-2 (the termination components), typically rated at 500 V or 750 V, suitable for ordinary power and lighting circuits with a limited number of conductors per cable.

Fire resistance: functioning well above 1000 °C

An MI cable can briefly withstand temperatures well above 1000 °C while remaining electrically functional — the copper conductors and sheath only melt at a temperature considerably higher than a realistic fire scenario produces, and the magnesium oxide itself is non-combustible and does not degrade at these temperatures. Because no organic material is present, the cable itself does not burn, produces no smoke, and releases no toxic combustion gases — a relevant difference from some organically insulated fire-resistant cables, which do pass the PH or E test but can still show some smoke or gas generation from the outer sheath during that test.

The hygroscopic pitfall: careful termination is essential

The compacted magnesium oxide is hygroscopic: it absorbs moisture from the surrounding air as soon as a cut cable end is left unprotected. Moisture absorption quickly reduces the cable's insulation resistance and, in the case of severe contamination, can cause permanent damage to the insulation. An MI cable must therefore always be terminated with a purpose-designed seal — typically a pot seal (a sleeve filled with a sealing compound) or a suitable crimp or screw-type coupling — immediately after cutting, and preferably as soon as possible after the cable end is exposed. An insulation resistance measurement immediately after termination is the usual check that the seal has actually been made moisture-tight.

The copper sheath as a current-carrying or earthing element

The copper outer sheath of an MI cable is itself a conductor and, in some designs, is deliberately used as a return conductor or as a protective conductor (CPC), comparable to the role that the metal armour of an armoured cable can fulfil (see the guide on armoured cable as a protective conductor) — with the difference that for MI cable the entire, seamless sheath performs the conducting function rather than a braided or spirally wound armour layer.

Note: whether the sheath may actually be used as a CPC in a specific installation, and which cross-section requirements apply, follows from the installation design and the manufacturer's product documentation; this article covers the principle, not a ready-made design rule.

Practical relevance

MI cable is applied in practice where a very high level of fire resistance is required and where the relatively higher material and labour costs (specialist termination with pot seals) are outweighed by the benefit of an intrinsic, non-organic fire behaviour — for example for a sprinkler pump supply, fire alarm control panel wiring in critical buildings, or wiring from a central battery for emergency lighting (see the related guide on central battery systems). When assessing an existing MI installation, it is important to check that previously made terminations are still moisture-tight — an aged or damaged pot seal is one of the most common causes of a declining insulation resistance with this cable technology.

Common mistakes

Leaving a cut MI cable end unprotected, even briefly — the hygroscopic magnesium oxide immediately starts absorbing moisture from the surrounding air, measurably lowering the insulation resistance.
Using a standard crimp coupling intended for plastic-insulated cable instead of the pot seal or manufacturer-specific seal intended for MI cable — this does not provide sufficient moisture tightness for the hygroscopic insulation.
Confusing the PH/E class of an organically insulated fire-resistant cable with the intrinsic fire resistance of MI cable — both can satisfy a comparable functional requirement, but through a fundamentally different mechanism, with different installation and termination requirements.
Not performing an insulation resistance measurement immediately after termination — this is the usual, quick check that a pot seal has actually been made moisture-tight.