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. A similar product sheathed with metals other than
copper is called mineral insulated metal sheathed cable.
MI cable is made by placing copper rods inside a circular copper tube and
filling the intervening spaces with dry magnesium oxide powder. The overall
assembly is then pressed between rollers to reduce its diameter. 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, 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. The metal tube shields the conductors
from electromagnetic interference, but at low frequencies only electrostatic
radiation. The metal sheath also physically protects the conductors, most
importantly from accidental contact with other energised conductors.
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. 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.
About 1947 the British Cable Maker's 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 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 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 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 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
Advantages The metal sheath and solid filling of MI
cable makes it mechanically robust and resistant to impact; an MI cable may be
struck repeatedly with a hammer and still provide adequate insulation
resistance for a circuit. Copper sheathing is waterproof and resistant to
ultraviolet light and many corrosive elements. MI cable is approved by
electrical codes for use in areas with hazardous concentrations of flammable
gas in air; an MI cable will not allow propagation of an explosion inside the
copper tube, and the cable is unlikely to initiate an explosion even during
circuit fault conditions. Metal sheathing will not contribute fuel or
hazardous combustion products to a fire, and cannot propagate a fire along a
cable tray or within a building. The cable is inherently fire-rated without
additional coatings, and will survive designated fire tests representative of
actual fire conditions longer than the enclosing structure.
When 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. Although made from solid copper
elements, the finished cable assembly is still pliable due to the malleability of
copper. The cable can be bent to follow shapes of buildings or bent around
obstacles, allowing for a neat appearance when exposed.
Since the inorganic insulation does not degrade with 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. This may also
allow a smaller cross-section cable to be used in particular applications.
Due to oxidation, the copper cladding darkens with age and MICC is therefore
often used in historic buildings such as castles where it blends in with
stonework. 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
side of the cable and letting in water, causing a short-circuit between live,
neutral and earth. The appearance of green verdigris on the bare copper
sheath may be a sign this has occurred. Disadvantages
The termination points: 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.
Vibration: MICC is not suitable for use where it will be subject to vibration or
flexing, for example connection to heavy or movable machinery. Vibration will
crack the cladding and cores, leading to failure.
Labour Cost: 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. Depending on the size and number of
conductors, a single termination can take between 1 to 2 hours of labour.
Installation of a three-conductor MI cable takes about 65% more time than
installation of a PVC-sheathed armoured cable of the same conductor size.
Installation of MICC is therefore a costly task. Certain PTFE, silicone or
other polymer-insulated cables have been substituted in applications which
require similar properties in terms of flame spread, which use less labour to
terminate. MICC is still used in applications which are particularly
suited to its combination of properties. Voltage rating: MI cable is only
manufactured with ratings up to 1000 volts.
Moisture absorption: 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.
Corrosion: The copper sheath material is 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 snow melting cable it is subject to physical damage by concrete
workers working the concrete into the pour. If the 3-5mil coating is damaged
pin holes in the copper jacket develop causing premature failure of the snow
melting system. Repair: 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 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 take between one and two
hours of labour. 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.
See also Listing and approval use and compliance
Passive fire protection Circuit integrity
Fireproofing Cable tray
Copper wire and cable References
^ Robert M. Black, The History of Electric Wires and Cable, Peter
Peregrinus Ltd.London, 1983 ISBN 0-86341-001-4, pgs. 158-159
^ Fire Performance Test for Fire Survivable Cables.
.fire-cables.co.uk/fire_performance.html ^ a b R.S. Means Co, Electrical Cost
Data 22nd Annual Edition, 1999, ISBN 0-87629-504-9
External links https:www.wrexhammineralcables.com
Wrexham mineral cables the LARGEST MICC cable manufacturer in the UK