Insulating Liquids

 

Property Transformer OilCable Oil Capacitor OilAskarels Silicone Oils
Breakdown strength (20 C, 2.5mm sphere gap)150 300 200 200-250 300-400
Relative Permittivity (50Hz)2.2-2.3 2.3-2.6 2.1 4.8 2-73
Loss Tangent (50Hz) .001 .002 0.25E-3 0.60E-3 .001
Loss Tangent (1 kHz) .0005 .0001 0.10E-3 0.50E-3 0.1E-3
Resistivity (Ohm -cm) 1e12-1e13 1e12-1e13 1e13-1e14 2e12 3e14
Specific Gravity at 20 C0.89 0.93 0.88-0.89 1.4 1.0-1.1
Viscosity at 20 C (cStokes)30 30 30 100-150 10-1000
Refractive Index 1.4820 1.4700 1.4740 1.6000 1.5000-1.6000
Saponification 0.01 0.01 0.01 <0.01 <0.01
Thermal Expansion 7e-4/deg 7e-4 7e-4 7e-4 5e-4
Max permissible Water content (ppm) 5050 50 <30
negligble
<30
negligible

Pure Liquids

Pure liquids often have much higher breakdown strengths than commercial liquids. For instance, the addition of 0.01% water to insulating oil reduces its breakdown strength to 20% of the "dry" value. Compare, for example, the breakdown for Transformer Oil is usually taken as 150 kV/cm (see above table), but when highly purified, it is almost 8 times that, or 1000 kV/cm.

Liquid

Max Breakdown Strength
MV/cm

Hexane1.1-1.3
Benzene1.1
Transformer Oil1.0
Silicone1.0-1.2
Liquid Oxygen2.4
Liquid Nitrogen1.6-1.9
Liquid Hydrogen1.0
Liquid Helium0.7
Liquid Argon1.10-1.42