Insulating Liquids


PropertyTransformer OilCable OilCapacitor OilAskarelsSilicone Oils
Breakdown strength (20 C, 2.5mm sphere gap) kV/cm 150300200200-250300-400
Relative Permittivity (50Hz)2.2-2.32.3-
Loss Tangent (50Hz).001.0020.25E-30.60E-3.001
Loss Tangent (1 kHz).0005.00010.10E-30.50E-30.1E-3
Resistivity (Ohm -cm)1e12-1e131e12-1e131e13-1e142e123e14
Specific Gravity at 20 C0.890.930.88-0.891.41.0-1.1
Viscosity at 20 C (cStokes)303030100-15010-1000
Refractive Index1.48201.47001.47401.60001.5000-1.6000
Thermal Expansion7e-4/deg7e-47e-47e-45e-4
Max permissible Water content (ppm)505050<30

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
Transformer Oil1.0
Liquid Oxygen2.4
Liquid Nitrogen1.6-1.9
Liquid Hydrogen1.0
Liquid Helium0.7
Liquid Argon1.10-1.42

Sources for Transformer Oil

Current pricing for Shell Diala AX in Southern California (June 2007) is $7.95/gallon for a 5 gallon pail. This is from L.T. (Lee) Sawyer, a Shell jobber.

Copyright 1998, Jim Lux / filename here / revised date / Back to HV Home / Back to home page / Mail to Jim
Revised 19 June 2007, added price for Diala AX