In hydraulic systems, electrostatic discharge in hydraulic oil is not a widespread problem. But for reasons I'll explain in a moment, it may be on the increase. So it's definitely something you need to be aware of, because this is the cause of damage to the hydraulic filter element.
As you probably remember from school science experiments, electrostatic charge is generated whenever there is friction between two bodies moving relative to each other. No real surprise then, that electrostatic charge generation occurs in hydraulic systems as a result of friction between the fluid and system components.
A common symptom of electrostatic discharge in a hydraulic system is an audible clicking noise as charge repeatedly increases and then discharges to a surface of lower voltage through sparking. And this often occurs in a filter - resulting in burn marks and other damage to the hydraulic filter element.
While you may yet to have come across a 'clicking' hydraulic filter assembly, there are a couple of reasons why this problem may be becoming more common. The first is a growing trend towards the use of hydraulic oils with non-metallic additives. Hydraulic oils with zinc-based anti-wear additives have relatively high conductivity.
Hydraulic oils with good conductivity assist the dissipation of electrostatic charge as it moves around the system. Studies have shown that hydraulic oils with zinc-based, anti-wear additives rarely accumulate enough charge for harmful discharge to occur. Synthetic hydraulic oils and those with non-metallic anti-wear additives on the other hand, have much lower conductivity. This increases the potential for electrostatic charge accumulation and therefore, the likelihood of discharge.