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 | >99,4 | 0,08 |
| AI2O3 | 0,15 | 0,019 | 99,8 |
| CaO | 0,7 | 0,02 | 0,004 |
| Fe2O3 | 0,09 | 0,018 | 0,009 |
| 5iO22 | 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 | AI2O3 | 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 (20-300°C) | 1,03 | 0,95 | J/gK |
| Coefficient of Exp. 20-200°C | 11,3 | 6,55 | 10-6/K |
| Coefficient of Exp. 20-600°C | 13,2 | 7,62 | 10-6/K |
| Resistivity 20°C | 5x1016 | 1x1014 | Ω x m |
| Resistivity 400°C | 1x1013 | 1x1012 | Ω x m |
| Resistivity 800°C | 5x108 | 2x108 | Ω x m |
| Dielectric Constant 20°C | 5 | 9 | - |
| Knoop Hardness | 3700 | 21000 | N/mm² |
| Modulus of Elasticity 20°C | 3x105 | 3,6x105 | 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 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.
