ISOMIL - Mineral Insulated Cables
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Insulation Materials and Dielectric Strength

Insulation Material

Typical Composition of the Standard Quality MgO and the High Purity Qualities MgO and AI2O3.

  MgO (97%)
(Standard)
MgO
(High Purity)
AI2O3
(High Purity)
MgO>97,0>99,40,08
AI2O30,150,01999,8
CaO0,70,020,004
Fe2O30,090,0180,009
5iO222,00,020,08
B, Cd, S>10 ppm>10 ppm>10 ppm
C *10 ppm50 ppm20 ppm
*) can be reduced further if necessary (exposure to radiation) before use.


Physical Properties of Insulation Materials

  MgO AI2O3 Unit
Density (crystal) 3,653,98g/cm3
Density in MIC 3,02,9 g/cm3
Melting Point 28003000°C
Specific Heat (20-300°C) 1,030,95J/gK
Coefficient of Exp. 20-200°C 11,36,5510-6/K
Coefficient of Exp. 20-600°C 13,27,6210-6/K
Resistivity 20°C 5x1016 1x1014Ω x m
Resistivity 400°C 1x10131x1012Ω x m
Resistivity 800°C 5x1082x108Ω x m
Dielectric Constant 20°C 59-
Knoop Hardness 3700 21000 N/mm²
Modulus of Elasticity 20°C 3x1053,6x105N/mm²


Insulation Resistance

The two figures show typical curves for the insulation resistance.

Insulation resistance of ISOMIL
measured with 20V DC.

Insulation resistance of ISOMIL
measured with 500V ∼ AC / 50Hz.

For both mineral insulated thermocouples and mineral insulated heating cables the standards (e.g. DIN 43721, ASTM E 420/71, VDE 0284) lay down dc measurements. Since polarization currents are accompanied by a time dependent increase of the insulation resistance after applying dc voltage, the measurement value is read off after 60 ± 3s.

The displacement currents lead to considerably lower insulation resistances especially in the temperature range below 600°C. When considering an FI switch as a protective measure, the above must be given careful consideration when designing the circuitry in equipment with mineral insulated heating cables.



Dielectric Strength - Bending Radius

Mineral insulated cables can easily be wound into a spiral. The minimum bending radius varies according to the width of the sheath and is three to five fold the outer diameter. Bending causes a reduction in density in the insulation material under the outer edge of the sheath, this reduction increases the smaller the bending radius. The associated reduction in dielectric strength conductor/sheath is of significance especially for ISOMIL-H.

Dielectric strength as a function of bending radiusmeasured on ISOMIL-H
at alternating voltage (50 Hz).

Measurements have shown that the dielectric strength is virtually temperature independent up to 800°C.

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