Where is static electricity going?

Static Electricity - An unpredictable and often underestimated hazard

One of our customers wanted to send a defective forklift battery back to the manufacturer. According to the manufacturer's instructions, the battery was covered with a plastic sheet and stored in the workshop overnight.

The battery suddenly exploded early the next morning. The shock wave shattered the pane of the window between the workshop and an adjacent office. Large splinters of glass were thrown across the room, some of which buried themselves in an office chair. Fortunately, no one was in the office at the time of the explosion.

Who was the culprit? We suspect that static electricity discharged in the plastic wrap and ignited the hydrogen that had accumulated under the cover overnight.

Stationary electrical charge

Static electricity is a stationary electrical charge that arises when there is an imbalance between the positive and negative charges inside or on the surface of a material.

In everyday life, our encounters with static electricity are usually harmless and even entertaining - for example, when our hair stands on end after we have taken off a wool sweater. However, static electricity can also be used.

In many industrial companies, for example, electrostatic smoke separators are used. These remove soot particles by electrostatically charging the exhaust gases and then conducting them through a metal grid with the opposite charge. Static electricity is also used in inkjet printers and copiers to transfer an image onto paper.

Static electricity is also known as risk. Under certain circumstances, it can generate sparks that can cause a fire or cause an explosion. For example, the explosion that destroyed the Hindenburg is said to be due to static electricity. As a result, many industrial operations and procedures must be carefully reviewed to minimize the risk of a catastrophic accident caused by static electricity.

The conditions have to be right

For static electricity to cause a fire or an explosion, four different conditions must be met.

First, there must be sufficient charging. Many common processes can lead to an electrostatic charge. For example, when plastic containers are stacked, when liquids or powders are pumped into containers or when conductive materials are fed through pipes or hoses. High pressure CO2, which is discharged in liquid form, can also become charged. In addition, certain materials can become charged that are otherwise not electrically conductive - such as plastic or damp wood. The risk that these materials can generate electrostatic energy is often overlooked.

The charging can also be influenced by the ambient conditions. In particular, lower air humidity contributes to better insulation of the neighboring materials and thus favors charging. This is one of the reasons that electrostatic incidents are more likely to occur in winter, when the air is cooler and drier.

Second, there must be fuel and oxygen. In addition to the ignitability / explosivity of the fuel source, their quantities naturally affect the extent and intensity of the fire or explosion.

Third, the static electricity that has already built up has to discharge again. A discharge takes place when differently charged materials interact in a certain way. There are different types of discharge with different hazard potential.

A corona discharge is a weak, local and continuous discharge that usually occurs on sharp edges or tips. The energies that occur are not sufficient to ignite explosive mixtures. The so-called tuft discharge is more energetic, involves larger surfaces and consists of a sequence of discrete events. The plasma channel emanating from the electrode widens diffusely (tufts). A brush discharge can ignite explosive gases, but usually not dust.

Spark discharges and sliding tuft discharges are significantly more dangerous. A spark discharge occurs between an isolated, charged object and the earth. A sliding tuft discharge is a large-area discharge of a high-energy charge brought about by special processes. Spark discharges and sliding tuft discharges can release a great deal of energy and are often the source of ignition for fires or explosions triggered by electrostatic energy.

If the electrostatic discharge releases sufficient ignition energy - the amount depends on the ignition point of the fuel source (s) - the conditions are finally met and a fire or an explosion occurs.

A fundamentally dangerous process can run for many years without incidents, because at any point in time (at least) one single condition was not met.

The treacherous

The unpredictability of electrostatic energy, and especially the impact of environmental factors, means that practical experience related to operational processes may be of little use when it comes to static electricity. A fundamentally dangerous process can run for many years without incidents because at any point in time (at least) one single condition was not met.

But then circumstances change: the air may be unusually dry. Or the object charges more than usual. And then suddenly there is an unexpected fire or explosion that nobody expected.

However, this is not to say that there are no practical steps companies can take to reduce this risk. But on the contrary!

The precautions are basically three main points:

  • The limitation of the charge
  • Enabling a safe equalization of the loads
  • The avoidance of ignition hazards.

Earthing and equipotential bonding are central components of the protection concept. In particular, cables, pipes and hoses must be earthed over their entire course and have potential equalization.

Grounding helps balance the potential imbalance between the negative and positive charges in objects and the earth. A separate system is required for this, which is not connected to the electrical earth, so that a backflow of charge is prevented.

Ground connections must also be checked regularly. We often find that these connections are broken. Such interruptions can easily occur during normal operation, but this is rarely checked.

So-called “bonding” is used to connect two or more objects to each other in order to equalize the electrical potential. A connection cable is also used in the event of an interruption in the ground connection, i.e. if the connection runs through painted parts, non-conductive seals or similar obstacles, for example.

For some areas of application, a flexible earthing and equipotential bonding system must be used, for example for filling certain liquids into portable containers. In such cases, we recommend the use of clamps and uninsulated cables. Clamps are used to provide a suitable connection and grounding in a rusty or dirty environment. The uninsulated cables ensure that cracks in the cable are detected immediately.

Non-conductive materials such as plastic, which can become statically charged, become conductive through the addition of carbon or steel particles and can therefore be earthed.

Finally, under certain circumstances, additional measures may be necessary to avoid ignition hazards. In potentially explosive areas, for example, special ventilation systems may be required to ensure that the ambient conditions do not exceed the explosion limits. In addition, it may be necessary to reduce the flow velocities in the lines to limit the risk of excessive static build-up.

The challenge with static electricity is that its formation and effects can be difficult to predict. Our customer also discovered this with the exploded battery. The above examples show only some of the possibilities how this hazard can develop in different situations. Fires and explosions caused by electrostatic energy often arise suddenly and in unexpected places.

Since static electricity is relatively unpredictable, the specific procedures need to be analyzed in detail in order to identify the potential hazards. In addition, regular checks of the protective measures, which also include earthing and equipotential bonding systems, are of crucial importance.

Frank Dörr is the Regional Head of the Property Risk Engineering Team at XL Catlin. He is based in Frankfurt and can be reached at [email protected].