Understanding Oil's Dielectricity

"Can you explain the relative dielectricity of oil?"

The dielectricity of a lubricating oil is a property that typically is not of great concern. However, in some applications, it can be critical and one of the most important factors in selecting the right fluid.  

Generally, lubricants are not thought of as a means to conduct or store electricity, as current shouldn’t be moving through most oil-filled systems. For example, consider a splash-lubricated gearbox.

This mechanical device should not encounter an electrical charge unless something drastic occurs. Of course, just because a lubricating oil was not designed to conduct or store electricity does not mean you shouldn’t monitor its ability to do so.

The field of dielectrics is broad and can be quite complex. The following information is intended to provide a working understanding of the field as it applies to lubrication. To begin, let’s define what a dielectric is.

A dielectric is a medium or substance that transmits electric force without conduction – an insulator. In this case, we are talking about a lubricating oil. All lubricating oils are dielectrics to varying degrees.

A capacitor is a classic example of using a dielectric. A capacitor stores an electric charge which can then be discharged later. Lubricating oils can do the same thing. Since the oil is often non-conductive, the charge builds up to the point where it can be discharged on a machine part. This phenomenon has been known to produce electrical arcs and audible popping sounds.

Not all dielectrics are created equal. Each has distinct abilities to conduct and store electricity. To compare different materials, you must look at the dielectric constant of the material. This is also called the relative permittivity.

The relative permittivity of a substance describes how the speed of an electric charge in the material compares to that of a vacuum. The more conductive the substance, the higher the relative permittivity or dielectric constant.

By definition, the value for a vacuum is one. Most mineral lubricating oils have values that range from 2.1 to 2.4 but can vary depending on the additive concentration and temperature.

Synthetic lubricating oils can have significantly higher values, which means they are more conductive. True conductors would have an infinite relative permittivity, as they do not store a charge and electricity flows through them quickly.

Determining a lubricant’s dielectricity involves measuring the speed of the electrical field. Many instruments can do this, including real-time monitoring systems that can be attached to a machine in the field. As with most oil analysis tests, this test becomes most valuable when a baseline is established and changes are monitored over time.

When the oil becomes contaminated with polar or conductive materials (water, dirt, etc.), the dielectric constant climbs. The same can be said when the oil begins to degrade and produces oxidation byproducts such as sludge.

Therefore, monitoring a lubricant’s dielectricity can provide information on the health of the oil as well as the state of contamination. It typically is used only as a screening test. However, an increase in this value should trigger more in-depth testing to help you understand what is occurring inside your oil.