SSI Autocruiser

Preventing electrostatic discharge in intralogistics

SSI Autocruiser
Plastic containers play a critical role in moving materials from A to B. However, most plastics are insulators, and do not readily conduct electricity. And that can create challenges – as this means they can acquire an electrostatic charge. In fact, the surface of a conventional plastic container can have a potential as high as 20,000 volts. This poses a great danger to electronic devices, as an electrostatic discharge (ESD) with even very low voltages can cause significant damage.

An electrostatic charge can be generated in a number of ways

Electrostatic charges are primarily caused by the triboelectric effect: when two objects are rubbed against another, they exchange electrically charged particles. These spread across the objects’ surfaces, lending them a static charge. When the objects are separated – for example, when one of two adjacent containers is removed – the particles are transferred at the last point of contact. However, if the materials have high electrical resistivity, the charge cannot easily dissipate; compounded by rapidly repeated movements, and the fast separation of the objects, an increasingly greater charge accumulates.

In addition, a charge can be created by electrostatic induction. If an object is exposed to an electrical field, the distribution of its charge is modified – that is, a positive charge can be induced in one area of the object and a negative charge in another. Electrostatic induction can arise from operating machinery and all types of electrical devices. The resulting electric potential can be many thousands of volts.

Electrostatic discharge: creating a spark

When two objects with different charges are brought close together, an electric field is generated, and electrons flow from one object to the other. Anyone who has experienced a static shock – an electrostatic discharge – has felt this phenomenon first-hand. However, people often do not notice ESD events, as we can only detect an electrostatic discharge of 3,000 volts or higher. In fact, if we are aware of the electric shock, the voltage is often in excess of 10,000 volts – and up to 30,000 in low humidity.

An electrostatic-discharge-sensitive (ESDS) device is particularly susceptible. A discharge of just five to 20 volts can damage the delicate read/write heads of hard disk drives; 100 volts can wipe out information on magnetic storage media. Even minor electrostatic discharges can cause malfunctions, shorten the life of a product, or lead to the failure of electronic devices.

ESD containers safeguard sensitive components

Packaging and containers made of conductive plastics play a key role in preventing ESD events. The desired degree of conductivity is achieved using a mix of plastic compounds and conductive materials; one of the most common additives is graphite. The plastics are classified by their electrical resistivity, and their function is assigned accordingly – to bleed off voltage, prevent electrostatic charge, and shield against electric fields, where and as needed. The type of ESD protection is defined by the combination of properties.

R-KLT container with ESD safe EPP foam insert
Materials with a resistivity of ≥ 1 x 102 Ω and < 1 x 105 (IEC 61340-5-1) are conductive materials (e.g. PPL 3; PPL represents conductive polypropylene). They enable electrical charges to bleed off quickly and completely. Dissipative materials (e.g. PPL 6) have greater electrical resistivity, between ≥ 1 x 105 Ω and < 1 X 1011 Ω (IEC 61340-5-1), meaning they are less conductive, and they may be temporarily penetrated by an electric field. Containers (R-/RL-KLT from PPL 6) that comply with VDA 4504 (ESD) must have a resistivity of ≥ 1 x 104 Ω and < 1 x 1010 Ω.

Where PPL 3 and PPL 6 ESD containers are deployed

ESD containers are primarily categorised according to their conductivity. Those made from conductive materials, such as PPL 3, are used to transport components that do not come into contact with the container surface. Their purpose is to provide electrostatic shielding and prevent ESD events. Sensitive items stored in these containers are placed in purpose-designed inserts.

Dissipative ESD containers (PPL 6) bleed off existing charges. Sensitive components are allowed to touch surfaces during transport. Because charges are redistributed only slowly, the risk of a spark discharge is minimised – making these containers suitable for the transport of highly flammable goods.

ESD containers as part of an ESD-protection system

ESD containers alone are not enough to provide complete ESD protection and prevent damage to sensitive electronic devices. Discharge prevention must be viewed as a system, with a number of safeguards that interact; combined, they provide a sound, end-to-end defence.

The manufacture and assembly of sensitive electrical components takes place in special-purpose, electrostatic protected areas (EPAs). The goal is to inhibit the build-up of electrostatic charge from people, machinery, tools, floors, containers [ok für Transportmittel?], shelving and racking, and conveyors throughout an EPA – and to enable existing charges to dissipate in as controlled a manner as possible. Against this background, ESD protection must be effectively implemented across an EPA. Where no charge is acquired, there is no discharge – and if an initial charge is present, it must be bled off completely.

Risks introduced by the human factor – antistatic and ESD equipment for staff

Machinery, conveyors, and storage and other equipment are relatively predictable systems with regard to ESD protection – standardisation ensures lasting, effective protection. People, on the other hand, are the main source of static charge. With this in mind, all personnel and workstations should be well equipped against ESD. In addition, all antistatic mechanisms must be applied correctly and without fail across the board.

Special earthing wrist bands, clothing and shoes, plus furniture and materials that comply with corresponding standards provide ESD protection – as do restricted physical access to EPAs and a system of ensuring a person is earthed before entering the area.

ESD protection – an indispensable safeguard

Electrostatic discharges are dangerous. ESD events have the potential to unleash a disaster – for example, a spark may have caused the Hindenburg zeppelin disaster and the explosion of a Pershing rocket in Ramstein. However, most accidents involving electrostatic discharge are not spectacular – just expensive. The financial damage, particularly in the electronic and microelectronic industries, runs into the millions of euros annually.

Rigorous implementation of available ESD-protection mechanisms across an enterprise and regular training of staff ensure comprehensive and lasting defences against ESD events. Additionally, all stakeholders must be aware of its importance; external service providers should also be included in these actions, and must deploy safeguards on their part. In this way, the potentially immense damage caused by ESD events in manufacture, logistics and intralogistics processes can be all but eliminated.

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