Static electricity can charge an isolator

Electrostatic discharge (ESD) in the laboratory and in operation

Who does not know that?! Just pick up the doorknob, open the window or greet your visitor with a handshake, and bzzz - you've got a “swab”. In the following article you will find out how this electrostatic discharge (ESD, abbreviation for "electrostatic discharge") comes about, what risk it entails, and how you can protect yourself from it.

In many everyday situations, we encounter phenomena that can be traced back to electrostatics: the crackling of a sweater when taking off, the hair sticking to the balloon when you rub it on the head, the sticking of foil to the hands when the packaging is opened. And also the already mentioned ESDs. What rarely takes on critical proportions at home may appear completely different in the laboratory or in the company. Sensitive electrical components can already be damaged by discharges that humans do not even feel. In contrast to this, noticeable discharges caused by spark formation can lead to a fire or explosion if work is carried out in appropriate environments. Not to be underestimated are shock reactions, which can lead to accidents even if inflammable substances are no longer present.

The charging of an object or a person can have different causes, whereby the so-called contact charging is the most common cause of charging processes in electrostatics. If two substances come into contact with each other, a charge transfer takes place at their common interface, so that a layer of negative charge is created on one surface, while a positive excess charge remains on the other surface. If this charge difference is not completely neutralized again by discharge during the subsequent separation of the two surfaces, the substances become carriers of charges with opposite polarity (triboelectric effect). When two objects rub against each other, this process is repeated many times, and a considerable amount of tension can be generated with suitable materials. In particular, insulating materials can lead to large charge differences, since the static electricity cannot flow away.

Static electricity can arise at all interfaces between solid and / or liquid phases. This also includes the conveyance of liquids or solids through chargeable hoses, in which an electrostatic charge is generated by internal friction within the medium and by its friction on the inner wall of the hose. Pure gases cannot be charged, but any solid particles or liquid droplets contained in the gas flow can.

Since electrostatic discharges are among the 13 possible ignition sources that can ignite explosive mixtures of gases, vapors, mists or dusts, there are a number of regulations and guidelines for their assessment and avoidance as well as for the protective measures to be taken (keyword: "ATEX guideline "). With the Technical Rules for Hazardous Substances (TRGS) 727 - "Avoidance of ignition hazards due to electrostatic charges" - the requirements of the Hazardous Substances Ordinance in potentially explosive areas are specified. The document, which is available free of charge, has a length of over 100 pages that we cannot fully reproduce at this point without going beyond the scope of a short information article. Therefore, we have to be content with general statements about avoiding electrostatics as a cause of fires in operating areas that are otherwise only rarely or briefly at risk, and refer to the relevant information sources for detailed studies (ATEX, TRGS, DIN EN 12115).

Nonetheless, even in the less critical laboratory and operating situations we have considered, the devil is in the details! Looking at it superficially, who would assume that containers such as barrels or canisters pose a risk of electrostatics when they are filled?

The best protective measure is and remains to prevent static electricity from developing in the first place by choosing the right material. That means dissipating any generated electrostatic charge rather than accumulating it first. The ability of a material or an object to meet this requirement can be read from its resistance or conductivity. In fact, there are various resistance values ​​(volume resistance, specific resistance, surface resistance, strip resistance, leakage resistance, etc.) that must be taken into account and according to which one categorizes. However, we would like to limit ourselves to basic information at this point and leave the details to TRGS 727.

Insulators, for example, have a high resistance and are therefore poorly conductive and are therefore unsuitable for preventing charges. When used as intended, there is nothing against the use of insulating materials in less critical work zones if one is aware of their properties. And that includes the ability to be a carrier of electrostatics.

In contrast to insulators, conductive and dissipative materials are formed, whereby conductive substances have a lower resistance than dissipative substances. Both groups are suitable for avoiding electrical charges. Very important: Electrical conductors or conductive objects can of course only play out their material safety advantages if they are also earthed. The mere use of these materials does not automatically guarantee that you will avoid dangerous electrostatic discharges.

At this point an important note: You should not assume that grounding takes place via people and the floor.

It is precisely this lack of grounding that leads to a mini-electric shock as soon as you touch a door knob in an electrostatically charged state (either your shoes have insulating soles or your floor covering itself is insulated). As a result, a “real” earthing device is the method of choice.

Even if the term “antistatic” is used differently in different places (the TRGS has dispensed with a definition for this reason), objects designated as “antistatic” are a good guide for the prevention of electrostatic discharges. This concerns z. B. antistatic packaging, tools or clothing.

When filling containers as well as when conveying through hoses, the charge increases with the flow rate. In order to reduce the electrostatic hazard potential, sufficiently low flow velocities should therefore be selected. In addition, when pouring into a canister, avoid strong splashing inside by entering close to the liquid level if possible or by filling it below the level. Gas bubbles or a second, immiscible phase also contribute to the generation of charge when liquids are transported through a hose and should therefore be avoided.

There are also cases when charging cannot be avoided. Then it is important to control the electrostatics in such a way that they cannot cause any damage, i.e. H. to ensure a discharge without sparking. It may seem contradictory at first, but high conductivity is no safer in this case. Dissipative ("less conductive") devices are better suited under these circumstances to discharge electrical currents without sparks. In fact, a static charge dissipates by itself over time, with air humidity being an important factor: high air humidity can considerably increase the conductivity of objects. Looking at the hygrometer and taking appropriate action could also help prevent ESD in the workplace.