Insulation Materials and Dielectric Strength
Insulation Material
Typical Composition of the Standard Quality MgO and the High Purity Qualities MgO and AI_{2}O_{3}.
MgO (97%) (Standard) 
MgO (High Purity) 
AI_{2}O_{3} (High Purity) 

MgO  >97,0  >99,4  0,08 
AI_{2}O_{3}  0,15  0,019  99,8 
CaO  0,7  0,02  0,004 
Fe_{2}O_{3}  0,09  0,018  0,009 
5iO2_{2}  2,0  0,02  0,08 
B, Cd, S  >10 ppm  >10 ppm  >10 ppm 
C *  10 ppm  50 ppm  20 ppm 
*) can be reduced further if necessary (exposure to radiation) before use. 
Physical Properties of Insulation Materials
MgO  AI_{2}O_{3}  Unit  
Density (crystal)  3,65  3,98  g/cm3 
Density in MIC  3,0  2,9  g/cm3 
Melting Point  2800  3000  °C 
Specific Heat (20300°C)  1,03  0,95  J/gK 
Coefficient of Exp. 20200°C  11,3  6,55  10^{6}/K 
Coefficient of Exp. 20600°C  13,2  7,62  10^{6}/K 
Resistivity 20°C  5x10^{16}  1x10^{14}  Ω x m 
Resistivity 400°C  1x10^{13}  1x10^{12}  Ω x m 
Resistivity 800°C  5x10^{8}  2x10^{8}  Ω x m 
Dielectric Constant 20°C  5  9   
Knoop Hardness  3700  21000  N/mm² 
Modulus of Elasticity 20°C  3x10^{5}  3,6x10^{5}  N/mm² 
Insulation Resistance
The two figures show typical curves for the insulation resistance.
Insulation resistance of ISOMIL 
Insulation resistance of ISOMIL 
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 ISOMILH.
Dielectric strength as a
function of bending radiusmeasured
on ISOMILH
at
alternating voltage (50 Hz).
Measurements have shown that the dielectric strength is virtually temperature independent up to 800°C.