PVC-sheathed MICC cable. Conductor cross section area is 1.5 mm ; overall diameter is 7.2 mm.Mineral-insulated cables at a panel board
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.
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.[1] 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.[2]
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 (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:
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 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.
Properties and comparison with other wiring systems
The construction of MI cable makes it mechanically robust and resistant to impact.[5]Copper sheathing is waterproof and resistant to ultraviolet light and many corrosive elements. MI cable is approved for use in areas with hazardous concentrations of flammable substances, being unlikely to initiate an explosion even during circuit fault conditions. MI cable is smokeless, non-toxic and will not support combustion. The cable meets and exceeds BS 5839-1 making it fire-rated surpassing 950°c for over three hours with simultaneous mechanical stress and water spray as well without failure.
MI cable is primarily used for high-temperature environments or safety-critical signal and power systems; however, it can additionally be used within a tenanted area, carrying electricity supplied and billed to the landlord. For example, for a communal extract system or antenna booster it provides a supply cable that cannot easily be 'tapped' into to obtain free energy.
The finished cable assembly can be bent to follow the shapes of buildings or bent around obstacles, allowing for a neat appearance when exposed.
Since the inorganic insulation does not degrade with (moderate) heating, the finished cable assembly can be allowed to rise to higher temperatures than plastic-insulated cables; the limits to temperature rise may be only due to possible contact of the sheath with people or structures or the physical melting point of copper. This may also allow a smaller cross-section cable to be used in particular applications.
An additional advantage of Mi cable is the ability to use the copper shield as a neutral or earth in particular situations
Due to oxidation, the copper cladding darkens with age. However, where MICC cables with a bare copper sheath are installed in damp locations, particularly where lime mortar has been used, the water and lime combine to create an electrolytic action with the bare copper. Similarly, electrolytic action may also be caused by installing bare-sheath MICC cables on new oak. The reaction causes the copper to be eaten away, making a hole in the sheath of the cable and letting in water, causing a breakdown of the insulation which causes short circuits. The copper sheath material is typically resistant to most chemicals but can be severely damaged by ammonia-bearing compounds and urine. A pinhole in the copper sheathing will allow moisture into the insulation, and eventual failure of the circuit. A PVC over jacket or sheaths of other metals may be required where such chemical damage is expected. When MI cable is embedded in concrete as in floor heating cable it is susceptible to physical damage by concrete workers working the concrete into the pour. If the coating is damaged pin holes in the copper jacket may develop causing premature failure of the system.
While the length of the MI cable is very tough, at some point, each run of cabling terminates at a splice or within electrical equipment. These terminations are vulnerable to fire, moisture, or mechanical impact. MICC is not suitable for use where it will be subject to vibration or flexing, for example, connection to heavy or movable machinery. Vibration can cause cracking in the cladding and cores, leading to failure.
During installation MI cable must not be bent repeatedly as this will cause work hardening and cracks in the cladding and cores. A minimum bend radius must be observed and the cable must be supported at regular intervals. The magnesium oxide insulation is hygroscopic so MICC cable must be protected from moisture until it has been terminated. Termination requires stripping back the copper cladding and attaching a compression gland fitting. Individual conductors are insulated with plastic sleeves. A sealing tape, insulating putty or an epoxy resin is then poured into the compression gland fitting to provide a watertight seal. If a termination is faulty due to workmanship or damage then the magnesium oxide will absorb moisture and lose its insulating properties. Installation of MI cable takes more time than installation of a PVC-sheathed armoured cable of the same conductor size.[6] Installation of MICC is therefore a costly task.
MI cable is only manufactured with ratings up to 1000 volts.
The magnesium oxide insulation has a high affinity for moisture. Moisture introduced into the cable can cause electrical leakage from the internal conductors to the metal sheath. Moisture absorbed at a cut end of the cable may be driven off by heating the cable. If the MI cable jacket has been damaged the magnesium oxide will wick moisture into the cable and it will lose its insulating properties causing shorts to the copper cladding, and thence to earth. It is often necessary to remove 0.5 to 2 metres (1.6 to 6.6ft) of the MI cable and splice in a new section to accomplish the repair. Depending on the size and number of conductors, a single termination can be a large undertaking to repair.[6]
Alternatives
Circuit integrity for conventional plastic-insulated cables requires additional measures to obtain a fire-resistance rating or to lower the flammability and smoke contributions to a minimum degree acceptable for certain types of construction. Sprayed-on coatings or flexible wraps cover the plastic insulation to protect it from flame and reduce its flame spreading ability. However, since these coatings reduce the heat dissipation of the cables, often they must be rated for less current after application of fire-resistant coatings. This is called current capacity derating. It can be tested through the use of IEEE 848 Standard Procedure for the Determination of the Ampacity Derating of Fire-Protected Cables.
An electrical insulator is a material in which electric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors and conductors—conduct electric current more easily. The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors. The most common examples are non-metals.
A wire is a flexible, round, bar of metal.
An electrical cable is an assembly of one or more wires running side by side or bundled, which is used as an electrical conductor to carry electric current.
Magnesium oxide (MgO), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ ions and O2− ions held together by ionic bonding. Magnesium hydroxide forms in the presence of water (MgO + H2O → Mg(OH)2), but it can be reversed by heating it to remove moisture.
Electrical wiring in North America follows the regulations and standards applicable at the installation location. It is also designed to provide proper function, and is also influenced by history and traditions of the location installation.
Electrical wiring is an electrical installation of cabling and associated devices such as switches, distribution boards, sockets, and light fittings in a structure.
Electrical wiring in the United Kingdom is commonly understood to be an electrical installation for operation by end users within domestic, commercial, industrial, and other buildings, and also in special installations and locations, such as marinas or caravan parks. It does not normally cover the transmission or distribution of electricity to them.
Pyro comes from the Greek word πῦρ (pyr), meaning fire. It may refer to:
A power cable is an electrical cable, an assembly of one or more electrical conductors, usually held together with an overall sheath. The assembly is used for transmission of electrical power. Power cables may be installed as permanent wiring within buildings, buried in the ground, run overhead, or exposed. Power cables that are bundled inside thermoplastic sheathing and that are intended to be run inside a building are known as NM-B.
Fireproofing is rendering something resistant to fire, or incombustible; or material for use in making anything fire-proof. It is a passive fire protection measure. "Fireproof" or "fireproofing" can be used as a noun, verb or adjective; it may be hyphenated ("fire-proof").
A thermoplastic-sheathed cable (TPS) consists of a toughened outer sheath of polyvinyl chloride (PVC) thermoplastic, covering one or more individual annealed copper conductors, themselves insulated with PVC. This type of wiring is commonly used for residential and light commercial construction in many countries. The flat version of the cable, with two insulated conductors and an uninsulated earth conductor, is referred to as twin and earth. In mainland Europe, a round equivalent is more common.
Circuit integrity is how little can a fire affect an electrical circuit's operation. It is a form of fire-resistance rating. Circuit integrity is achieved via passive fire protection means, which are subject to listing and approval use and compliance. Alternatively, cable construction and materials can achieve fire-resistance ratings on their own such as mineral-insulated copper-clad cable, or MI cable.
Building insulation materials are the building materials that form the thermal envelope of a building or otherwise reduce heat transfer.
Twin and earth cable is a colloquial name in the UK, Australia, New Zealand and other countries for a type of flat sheathed fixed mains electricity cable, containing two insulated current-carrying conductors and an Earth connector. In Australia and New Zealand this type of cable is referred to as 'Flat TPS', as well as "Twin and Earth" or "Twin with Earth".
Tri-rated cable is a high temperature, flame retardant electrical wire designed for use inside electrical equipment.
A high-voltage cable is a cable used for electric power transmission at high voltage. A cable includes a conductor and insulation. Cables are considered to be fully insulated. This means that they have a fully rated insulation system that will consist of insulation, semi-con layers, and a metallic shield. This is in contrast to an overhead line, which may include insulation but not fully rated for operating voltage. High-voltage cables of differing types have a variety of applications in instruments, ignition systems, and alternating current (AC) and direct current (DC) power transmission. In all applications, the insulation of the cable must not deteriorate due to the high-voltage stress, ozone produced by electric discharges in air, or tracking. The cable system must prevent contact of the high-voltage conductor with other objects or persons, and must contain and control leakage current. Cable joints and terminals must be designed to control the high-voltage stress to prevent the breakdown of the insulation.
Copper has been used in electrical wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 created further demand for copper wire as an electrical conductor.
Plastic is the generic name for a family of synthetic materials derived from petrochemicals. It is often product of two or more components.
Telecommunications power cable, as described in Telcordia GR-347 & GR-347, consist of a stranded copper conductor used in AC/DC circuits up to 600 V that are insulated with non-halogen, limited smoke, polyolefin materials that are heat-resistant, moisture-resistant, and flame-retardant. These cables are provided as either Class B (standard) or Class I (flexible) products.
References
↑ Black, Robert M. The History of Electric Wires and Cable. Peter Peregrinus Ltd, London. pp.158–9. ISBN0-86341-001-4
↑ Countess of Carnarvon. Lady Catherine and the Real Downton Abbey. Hodder & Stoughton Ltd, London. pp.164–5. ISBN978-1-444-76210 5
1 2 R.S. Means Co, Electrical Cost Data 22nd Annual Edition, 1999, ISBN0-87629-504-9
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