Mineral-insulated copper-clad cable is a variety of electrical cable made from copper conductors inside a copper sheath, insulated by inorganic magnesium oxide powder. The name is often abbreviated to MICC or MI cable, and colloquially known as pyro (because the original manufacturer and vendor for this product in the UK was a company called Pyrotenax). A similar product sheathed with metals other than copper is called mineral-insulated metal-sheathed (MIMS) cable.
Construction MI cable is made by placing copper rods inside a circular copper tube and filling the spaces with dry magnesium oxide powder. The overall assembly is then pressed between rollers to reduce its diameter (and increase its length). Up to seven conductors are often found in an MI cable, with up to 19 available from some manufacturers. Since MI cables use no organic material as insulation (except at the ends), they are more resistant to fires than plastic-insulated cables. MI cables are used in critical fire protection applications such as alarm circuits, fire pumps, and smoke control systems. In process industries handling flammable fluids MI cable is used where small fires would otherwise cause damage to control or power cables. MI cable is also highly resistant to ionizing radiation and so finds applications in instrumentation for nuclear reactors and nuclear physics apparatus. MI cables may be covered with a plastic sheath, coloured for identification purposes. The plastic sheath also provides additional corrosion protection for the copper sheath. The metal tube shields the conductors from electromagnetic interference. The metal sheath also physically protects the conductors, most importantly from accidental contact with other energised conductors.
History The first patent for MI cable was issued to the Swiss inventor Arnold Francois Borel in 1896. Initially the insulating mineral was described in the patent application as pulverised glass, silicious stones, or asbestos, in powdered form. Much development ensued by the French company Société Alsacienne de Construction Mécanique. Commercial production began in 1932 and much mineral-insulated cable was used on ships such as the Normandie and oil tankers, and in such critical applications as the Louvre museum. In 1937 a British company Pyrotenax, having purchased patent rights to the product from the French company, began production. During the Second World War much of the company's product was used in military equipment. The company floated on the stock exchange in 1954. Around 1947, the British Cable Makers' Association investigated the option of manufacturing a mineral-insulated cable that would compete with the Pyrotenax product. The manufacturers of the products "Bicalmin" and "Glomin" eventually merged with the Pyrotenax company. The Pyrotenax company introduced an aluminum sheathed version of its product in 1964. MI cable is now manufactured in several countries. Pyrotenax is now a brand name under nVent Electric (nVent) (formerly known as Pentair Thermal Management).
Purpose and use MI cables are used for power and control circuits of critical equipment, such as the following examples:
Nuclear reactors Exposure to dangerous gasses Air pressurisation systems for stairwells to enable building egress during a fire Hospital operating rooms Fire alarm systems Emergency power systems Emergency lighting systems Temperature measurement devices; RTDs and thermocouples. Critical process valves in the petrochemical industry Public buildings such as theatres, cinemas, hotels Transport hubs (railway stations, airports etc.) Mains supply cables within residential apartment blocks Tunnels and mines Electrical equipment in hazardous areas where flammable gases may be present e.g. oil refineries, petrol stations Areas where corrosive chemicals may be present e.g. factories Building plant rooms Hot areas e.g. power stations, foundries, and close to or even inside industrial furnaces, kilns and ovens MI cable fulfills the passive fire protection called circuit integrity, which is intended to provide operability of critical electrical circuits during a fire. It is subject to strict listing and approval use and compliance
Heating cable A similar-appearing product is mineral-insulated trace heating cable, in which the conductors are made of a high-resistance alloy. A heating cable is used to protect pipes from freezing or to maintain the temperature of process piping and vessels. An MI resistance heating cable may not be repairable if damaged. Most electric stove and oven heating elements are constructed in a similar manner.
Typical specifications
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