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Mineral insulated cable is named for what is inside it. In place of a polymer, the space between the conductors and the outer sheath is filled with a compacted mineral, magnesium oxide, and the outer sheath is a drawn metal tube. The name describes the construction, and nearly every property the cable is bought for follows from that one difference.
Three words carry three decisions, and a fourth term that is often attached to the name means something else.
| Term | What it says | What it does not say |
|---|---|---|
| Mineral | The insulation is a mineral, magnesium oxide, compacted into the space around the conductors | It does not promise the flexibility of a polymer-insulated cable |
| Insulated | The conductors are separated from the sheath by that mineral along the whole length | It does not state the temperature the cable will survive, which the materials decide |
| Cable | It carries a signal or a current, and it is supplied in lengths | It does not mean power circuits are the only duty it is made for |
| "Armored", where the term is attached | The metal sheath sits outside the insulation and takes the mechanical load | It does not describe a separate armour layer, because the metal sheath is already the outer layer |
The distinction matters when a specification is written, because two of those terms are used loosely on catalogue pages for the same product, and a buyer who reads "armored" as a second layer will expect a construction the cable does not have.
The construction has three parts. Each is chosen separately, and each is written on a different line of the order.
| Part | Material options | The line it decides |
|---|---|---|
| Sheath | SS304, SS316, SS321 or Incoloy 800 | The temperature ceiling, and the resistance to whatever is in the atmosphere at the measuring point |
| Insulation | Compacted magnesium oxide | Nothing the buyer chooses. It is the reason the cable is inorganic, and therefore the reason for the fire and moisture behaviour |
| Conductor | Thermocouple alloys for Types K, J, T, N, E, S and B, copper, or a nickel-chromium resistance alloy | The signal the instrument reads, or the current the circuit carries |
Because the sheath and the insulator are both inorganic, the cable holds its circuit through a fire without an added fire-rated layer, and it does not depend on a jacket staying intact to keep moisture away from the conductor. The metal sheath does a second job at the same time: it screens the millivolt signal a thermocouple produces from the electrical noise of the plant around it.
An MI cable is specified in one short line, and the line works differently depending on the duty. On a thermocouple cable the size given is the sheath outside diameter; on a power or heating cable the number is the conductor cross-section. Reading one as the other is the most common mistake made from a catalogue page.
| Field | Thermocouple cable | Power or heating cable |
|---|---|---|
| Conductor | The type, with both legs identified: for example a Type T pair is written TP and TN, and a Type N pair NP and NN | Copper, or a nickel-chromium resistance alloy |
| Sheath | SS304, SS316, SS321 or Incoloy 800 | Copper or stainless, depending on the circuit |
| Size | The sheath outside diameter: 0.5 mm, 1.5 mm, 3.2 mm or 12.7 mm | The conductor cross-section, written as cores by area, for example 4 x 70 mm |
| Cores | One or two | Two and above |
A typical thermocouple line therefore reads as a type, a sheath and a diameter together, such as a Type J pair in a 3.2 mm SS321 sheath, or a single-core Type K pair in a stainless sheath. The diameter is doing two jobs at once: a narrower sheath reacts faster to a change at the tip, and a wider one survives more mechanical abuse before the insulation is reached.
The two legs of a pair are written separately because they are different alloys. A cable ordered with the legs the wrong way round reverses the polarity at the instrument, and the reading arrives inverted, which is a fault that looks like a calibration problem and is not one.
Three other constructions do part of the same work, and the choice between them turns on the installation as much as on the temperature.
| Option | Where it wins | Where it loses |
|---|---|---|
| Mineral insulated cable | Fire, heat, moisture and mechanical abuse, with a signal or a supply carried through the same run | A larger bend radius than a flexible cable, and an end that has to be terminated properly to stay dry |
| Fire-rated polymer cable | Cheaper over long runs, and it routes like an ordinary cable | The fire performance comes from added layers and has to be demonstrated by test; it is still an organic construction, with the temperature limits that follow |
| Sheathed thermocouple assembly | A finished measuring point that arrives with its fittings and head ready to install | Built around one point, and less easy to adapt on site |
| PTFE or glass-insulated thermocouple wire | Light, inexpensive, and quick to route inside a panel | Not for the hot end, and no protection against mechanical damage |
None of the four is better in general. Each is the answer to a different installation question, and the metal-sheathed cable is the one to reach for when the run has to keep working while everything around it does not.
Four limits are worth knowing before the cable is ordered, because each one is cheaper to design around than to correct on site.
| Limit | Why | What to do about it |
|---|---|---|
| It does not bend to a tight radius | The sheath is a drawn metal tube and the insulation is compacted powder, so the construction has a minimum bend radius | Plan the route with that radius, and order the length the run needs |
| It does not tolerate a breached sheath | Magnesium oxide is hygroscopic, so an open or damaged end takes up moisture and the insulation resistance falls | Keep the ends sealed until the cable is terminated, and inspect the sheath before it is installed |
| It does not repair a measuring point | The cable carries the signal; it cannot correct an immersion depth or a location that is wrong | Set the diameter from the response time the loop needs, and the tip position from the process |
| The sheath sets an atmosphere limit, not only a temperature | Sulfur attacks nickel-rich alloys, and a reducing atmosphere affects a Type K pair | Confirm the atmosphere at the point, because the temperature figure alone does not settle the choice |
Mineral insulated. The insulation is compacted magnesium oxide, the conductors are the thermocouple alloys, copper or a resistance alloy, and the outer layer is a drawn metal sheath in stainless steel or a nickel alloy. There is no polymer anywhere in the construction.
No. Mineral insulated is a construction, and a thermocouple cable is one use of it. The same construction carries copper for power and a resistance alloy for heating, which is why the designation reads differently for each duty.
Yes. The end has to be sealed against moisture, using the gland or pot seal that suits the sheath and the environment. The construction is what makes the cable survive fire and heat; the termination is what keeps it dry, and a poor one is the usual reason a sound cable fails in service.
It can be bent, but not to the radius a flexible cable allows, and it can be cut to length on site. The mineral insulation is compacted powder and not a solid, so a tight bend or a crushed sheath damages the insulation around the conductor even when the cable still looks intact from outside.
Almost always moisture that reaches the magnesium oxide, through a termination that was not sealed or a sheath that was breached during installation. The second cause sits in the environment and not in the cable: a nickel-rich sheath in a sulfur-bearing gas, or an unprotected Type K pair in a reducing atmosphere, both fail earlier than the temperature rating suggests.
Two things in opposition. A narrow sheath reaches the process temperature faster and takes up less room; a wider sheath survives more mechanical load and lasts longer in a hostile atmosphere. The diameter also caps the working temperature, because a thin sheath oxidises through sooner.
Mineral insulated cable is a construction, not a single product, and the name states it: a metal sheath, a compacted mineral insulator and a conductor chosen for the duty. What varies between orders is the conductor, the sheath and the diameter, and those three decide the temperature, the atmosphere the cable survives and how fast it responds.
DLX Alloy has produced nickel and nickel alloy products at Changzhou, Jiangsu since 2002, in a 12,000 m² factory certified to ISO 9001 with SGS audits. MI cable is supplied in the thermocouple types above with SS304, SS316, SS321 and Incoloy 800 sheaths, from 0.5 mm to 12.7 mm: send the duty, the atmosphere and the temperature for a confirmed quotation.
The figures quoted are nominal and are not acceptance limits. There is no single temperature range for mineral insulated cable: a Type K pair in an Inconel 600 sheath at 8.0 mm is rated to 1,150 °C, the same pair in a stainless sheath is rated lower, and both fall again as the diameter narrows. The working figure is the lowest of the three, and it is confirmed against the application on each enquiry and not read from one cell of a table.
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