Filtration without any static charging of the system

Causes and consequences of electrostatic discharge

Over the past years, the requirements for hydraulic systems have increased drastically. Smaller hydraulic components, measuring and control systems with sensitive electronic analysis units, higher cycle change factors or simply only higher recirculation quantities and the corresponding increasing flow rates are increasingly requested by customers, and provided by RT Filtertechnik GmbH.

The oil suppliers respond in this context by placing increasingly refined and zinc- and ash-free hydraulic oil products on the market. The advantages of these oils are, in addition to the additives, significantly longer service lives and less frequent change intervals, as is already the standard in the daily use of modern motor vehicles.

The problem with these oils is that due to their production method and additives, they only have a low electrical conductivity.

When the oil flows through the filter element, the filter layers and the oil are charged electrostatically and electrostatic discharge may occur in the filter or in the system.

These discharge processes in the element burn holes into the filter material, and the hydraulic system can no longer be maintained sufficiently. In addition, during the discharge processes, electromagnetic waves are emitted that can disturb or damage the measuring sensors in the system. In addition, the discharge processes accelerate oil aging and oil sludge develops. It may be deposited on the seats of moving valve components, on gaps in the system at screwed fittings or in the tank, and damage these components on a lasting basis. Costly maintenance and cleaning or even the complete failure of the hydraulic system could be the result. In the most unfavourable case, electrostatic discharge may occur in the tank or outside the system. An explosion or deflagration in the tank or even personal injury would be the result. The following figure shows an air filter that has been burnt completely in the tank, due to deflagration caused by electrostatic discharge in the tank.

How do charges develop in the hydraulic system?

Due to its fibrous structure, the filter element has a very large contact area with the oil. Tribo-electrical effects result in opposite poled charges in the filter medium and in the oil.

When the oil starts to flow, charge carriers are also moved along, due to the low conductivity of the oil. The electrical voltage in the hydraulic system increases. If the voltage gets too high in the oil, sudden electrostatic discharge occurs.


These effects are increased by:

  • Low conductivity of the carrier oil
  • High flow rates
  • Unfavourable material combinations
  • Low temperatures

A distinction can be made between three types of electrostatic charge in the hydraulic system:

1.      Conductive filter element:

with wire mesh support and metal end caps, conductive path to the housing

No charges are registered in the element itself, as the charges can flow out via the metallic cage of the filter element. However the pressure fluid will be severely charged. Uncontrolled discharge processes in the system are thus possible at undefined points, which could damage the hydraulic system or components on a lasting basis.

2.      Non-conductive filter element:

without wire mesh support and plastic caps, no conductive path to the element 

There will be an excessive amount of charges in the element itself. The charge carriers are discharged in an uncontrolled manner to the potential. It is possible that sparks penetrate the filtration medium and damage it on a lasting basis, which would at least worsen the oil cleanliness. The filtration performance is reduced significantly. In this case, the fluid is charged less severely.

3.      Stat-Free® (SFREE) filter element 

The filter media are adjusted in such a way that they have very close electrical potentials. Much fewer charge carriers are released in the system, and there is thus no discharge in the element or in the hydraulic system.

The new filter media are available for all common fineness grades of 3µm, 5µm, 10µm, 12-15µm and 20µm and can naturally also be assembled as combination or 2-stage filter elements.

Stat Free® filter elements provide the best filter performance with a long service life of your hydraulic system and the protection of the additionally installed electronic measuring and control units. They are the best alternative for preventing failures or damage due to electrostatic charge.