Mineral-insulated, metal-sheathed (Type MI) cable is an ultra-reliable, extremely robust wire. It is suitable for the most demanding applications that include instrumentation and power in hazardous locations, outdoor use for services, feeders and branch circuits, nuclear reactors, and extremely high ambient temperatures. Maximum circuit rating, depending on the manufacturer, is 1 kV.
Type MI cable consists of one or more (up to 19) copper conductors inside a continuous copper sheath, which provides excellent corrosion and mechanical protection. It also serves as an equipment-grounding conductor. (A stainless-steel sheathed variant requires an additional copper equipment-grounding conductor.)
The copper conductors inside the copper sheath are separated and insulated from one another by a dry magnesium oxide powder that is introduced during the manufacturing process.
Type MI cable is permitted in all locations except where the copper sheath can be damaged either mechanically or by corrosion. Specifically, in underground runs, the cable must be protected from physical damage. And there’s a need for additional protection when the cable sees conditions that are destructive or corrosive to the copper sheath.
Type MI cable resembles flexible copper water pipe in that it can be carefully bent around obstacles and formed to make angle turns, as long as the minimum-bending radius is observed. According to NEC article 332, which covers this interesting cable, the radius of the inner edge of any bend is to be not less than five times the external diameter of the metallic sheath for cable not more than 0.75 in. in external diameter. For larger cable up to and including one inch in external diameter, this ratio must not be less than ten times the external diameter.
The highly compressed refractory magnesium oxide powder is effective at what it does. But it becomes conductive if moist, and there will be line-to-line or line-to-ground short circuits. For this reason, the installation will fail if the copper sheath is corroded or otherwise damaged. Even a pinhole break in the outer copper sheath will compromise the circuit.
By the same token terminations require special materials and labor-intensive workmanship. The copper sheath is stripped back and a compression gland fitting is attached. Plastic sleeves are used to insulate the individual copper conductors. Then, an epoxy resin is mixed up and poured into the compression gland so as to realize a water-tight seal. If this procedure is not done to perfection, water will infiltrate and cause the circuit to fail.
Prior to installation, in storage, cut ends of the cable must be sealed to keep out moisture. As long as the integrity of the outer sheath and terminations is maintained, Type MI cable may be used in nuclear reactors, hospital operating rooms, high temperature areas, and most critical applications.
Where not exposed to touch and not PVC sheathed, Type MI cable may be operated at a continuous temperature of 250 °C. It will withstand an intermittent temperature of close to the melting point of copper, which is 1,083 °C. The refractory magnesium oxide powder is not damaged by that amount of heat.
Finally, never use Type MI cable to supply an electric motor. Vibration would eventually crack the outer copper sheath.
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Where to use it, and where not to use it appeared first on Test & Measurement Tips.
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