Dangers of Electrostatic Discharge

Martin Migliori, CFPHS; Joseph Lisowski, CFPHS

Dangers of Electrostatic Discharge

Electrostatic charges pose a severe threat to equipment and team members, and with environmentally friendly fluids and more efficient filtration creating higher fluid velocities, we must be aware of the dangers to prevent disaster from occurring.

By understanding how this phenomenon occurs, how it can be detected, and what hazards it presents, maintenance and engineering personnel can mitigate the risks before there are serious, or even deadly, consequences.

What is an Electrostatic Charge?

An electrostatic charge occurs when there is friction between two bodies of different potentials. Remember rubbing your feet on the carpet as a kid and then zapping your sibling? That was an electrostatic charge at work. This same phenomenon also occurs in hydraulic systems.

In a hydraulic system, you may have materials with two different electron energies, such as your hydraulic fluid and filter media. As your hydraulic fluid travels at a high velocity through the system, there will be a transfer of charge to the hydraulic fluid. This charge is carried by the fast-moving hydraulic fluid until it reaches a threshold value and discharges, most likely in the form of a spark.

Why Does Electrostatic Charge Occur?

Base fluids are classified according to their manufacturing process and level of refinement.

  • Group I, II, and III: Mineral oils
  • Group IV: Polyalphaolefins
  • Group V: Esters and other stocks not covered in Groups I-IV

Group I is less refined and has lower oxidative stability and viscosity index characteristics than Group II or III, but has higher conductivity due to additive packages, which contain zinc, a heavy metal, and ash, a combustion residue.

With the adoption of more environmentally friendly fluids and the desire to reduce the levels of additives, heavy metals, and ash in the oil, more companies are switching to Group II and III base oils. This results in facilities using fluids with low levels of conductivity, and when combined with higher fluid velocities and increased filter efficiencies, the result is an increase in electrostatic charge.

Dangers of Electrostatic Charge

There are dangerous consequences when an electric current is introduced to a hydraulic system.

  1. Sparking Within the Tank Downstream of the Filter: When in operation, the headspace in the hydraulic system’s tank is filled with air and atomized hydraulic fluid. When a spark is introduced, the fluid combusts, and tank pressure increases drastically. This results in blown tank seals, which turn components into deadly projectiles, or hydraulic fluid spraying out of the system, potentially starting a fire if a heat source is nearby.
  2. Filter Element Damage: Releasing the electrostatic charge causes localized temperatures of over 300˚C, resulting in burn holes on the filter, which allow contaminants to enter the fluid. This contaminated fluid will flow freely through the system and cause component damage, leading to costly downtime and repairs.
  3. Possible Hose Damage: Hydraulic hoses are designed with a metal layer and fitting, and as the charge tries to reach the ground, it will travel through these metal components, resulting in hose damage, reduced hose life, and possible hose failure.
  4. Breakdown of Hydraulic Fluid: During electrostatic discharge, temperatures can exceed 300˚C and cause the oil to varnish, leading to stuck valves, clogged filters, and damaged pumps and motors. You will need to completely change out the fluids and clean the entire system to restore it, which can be an extremely expensive process.  

Electrostatic Charge Detection

To determine if electrostatic discharge can occur in your system, you must know:

  • How the system operates.
    • Gather information such as the system fluid type, the flow rate, and the operating temperatures.
    • An electrostatic discharge will not normally occur once the operating fluid rises above 15.5˚C. If you cold-start or operate in cold weather, you are more susceptible to electrostatic charges.
  • What type of fluid and additive packages are being used.
    • Your supplier can provide this information.
  • The flow rate of the system.
    • This will help determine the velocity in the conductors.

High system velocity, operating temperatures below 15.5˚C, and the use of Group II or III fluids with low conductivity are all characteristics to look for. If your system matches these conditions, you need to investigate further.

Start by listening for clicking sounds in your filter housing, which indicates the current is discharging in the filter. Then listen for clicking in the reservoir; if occurring, this represents a very serious problem that must be addressed immediately.

If you don’t hear clicking, but your system still matches the characteristics, there are other signs to look for.

First, inspect the filter for burn marks. Do this by removing the filter and cleaning it using a solvent. Then, with a magnifying glass, inspect it for any indication of burn marks. If you don’t feel confident performing this, consult your filter supplier, who may be able to perform the inspection for you.

Next, if valves are sticking or varnish is occurring, assume there is an issue. While it could be another problem, there’s a good chance it’s electrostatic discharge.

Finally, perform an oil analysis on your system, and have a qualified individual translate the results. This will reveal if your oil is aging rapidly. If you perform routine oil analyses, this anomaly will be easier to detect.

How to Mitigate and Prevent Electrostatic Discharge

Hydraulic fluids must travel through piping at a high velocity, meaning some type of electrostatic charge will always exist; they require active mitigation to prevent irreversible machine damage and serious bodily injury. To mitigate electrostatic discharge:

  • Ensure the filter housing is grounded.
    • This provides the charge with a path to the ground, away from the reservoir.
  • Where practical, ensure the filter is sized for the maximum flow.
    • This reduces the hydraulic load and electrostatic charge buildup on the filter.
  • Use a stat-free, or anti-static, filter element.
    • These are designed to eliminate charge buildup in the fluid and minimize charge generation.
  • Ensure the system is designed properly.
    • Incorrectly sized connectors or conductors increase fluid velocity and increase charge buildup.
  • Ensure the system’s velocity is appropriate for the conductor.
  • Have the proper reservoir size.
    • An improperly sized reservoir allows for charged fluid to pass through the pump and into the system. This leads to greater charge buildup and eventual electrostatic discharge. This is extremely dangerous and should be avoided at all costs.
  • Educate all team members on the system and its function.
    • Knowing system functions and electrostatic charge warning signs increases the success rate of mitigation efforts.


Electrostatic discharge presents a serious threat that affects worker safety, equipment safety, and equipment reliability. The best tools to combat this issue are education and mitigation. Once you understand how your system should work, you can create and implement an effective, actionable plan to mitigate the risk and protect your critical assets.

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