electrostatic discharge
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Electrostatic discharge ( english discharge electrostatic shortly ESD ) are due to large potential differences resulting voltage breakdowns . These breakdowns (possibly visible as sparks ) cause a short, high electrical current and can lead to the ignition of flammable substances . Under unfavorable conditions, there is a risk of fire and explosion as well as danger to people from electric shock . Other undesirable consequences of electrostatic discharges are damage to electrical components in devices. Field effect transistors are particularly affected by this.
Cause of the potential difference is usually a charged by triboelectricity (triboelectric effect) or induction . Static electricity occurs, for example, when walking on a carpet. If the humidity is below 20%, a person can be charged up to 35,000 V. If the humidity is above 65%, the possible charge drops below 1,500 V.
Occurrence of electrostatic charges
Electrostatic charges are part of electrostatics and occur almost everywhere in our everyday life. Only from a certain strength of the electrostatic discharge (rule of thumb: approx. 2,000 V) can this be perceived by humans. The most common perception is feeling an electric shock when looking after a static charge, e.g. B. by walking on a synthetic fiber carpet or driving along with your hand on a plastic banister then a grounded body, z. B. a radiator is touched. Furthermore, flashes of a discharge can be perceived with the eye in a dark environment. This can be seen particularly well when taking off a synthetic fiber sweater in a completely dark room, for example. Many electrostatic discharges are below the human perceptibility threshold, but can occur e.g. B. be harmful to electronic components. Electrostatic charges can sometimes be made noticeable through light and insulating objects such as scraps of paper or hair.
Depending on the strength of the discharge, personal injuries and fires can occur. While electrostatic discharges on parts of the body usually only cause hazards due to the shock reaction, they can have serious consequences in explosion- protected areas . This applies to the handling of flammable liquids and gases (e.g. filling stations, gas systems) as well as dust-dry bulk goods ( dust explosions from flour, grain, coal mines).
The Technical Rule for Hazardous Substances TRGS 727 describes the methods of analyzing hazards due to electrostatic charges and regulates the measures to avoid ignition hazards in potentially explosive areas. In Appendix D of TRGS 727, dangers of electric shock from the discharge of static electricity are explained and measures are presented. For example, the ignition of gasoline vapors when refueling v. a. prevented by conductive tank hoses and adequately grounded tires, as well as electrical contact between the vehicle body and nozzle, which in the event of ignition, for. B. only parked by poorly grounded tires, but not pulled out.
Paper machines, loom trees , systems for film production and processing and flour mills are also at risk. Here, the charge separation of the manufactured film webs or the bulk material occurs similarly to a belt generator , which means that machine parts can also be charged with voltages that are dangerous for people. Flashovers can ignite dust and - with repeated discharges - also combustible materials.
Vehicles are charged by the friction of the rubber tires on the road. However, this effect is often overestimated - the tire rubber usually has sufficient conductivity to dissipate the charges. Discharges observed when getting out of the car are mostly caused by the friction of the clothing on the upholstery material of the car seats and lead to the driver being charged with respect to the body. They cannot therefore be prevented with a so-called anti-static tape on the stern.
In the case of vertically extended metallic objects isolated from earth, the electrostatic charge from the natural electrical field of the earth can assume considerable values. For example, contact of an earthed person with an earth-insulated transmitter mast can cause an electric shock (possibly even life-threatening) when the transmitter is out of operation and no thunderstorm is approaching.
Types of Electrostatic Discharge
Spark and lightning
The best known electrostatic discharge is lightning . Lightning can injure or kill people and animals, damage equipment, or cause fires and explosions, especially if flammable gases are present in the air.
In the broader, colloquial sense, lightning occurs whenever the electrical limit field strength is exceeded between two differently charged bodies and a spark discharge occurs between the bodies.
The hydrogen contained in the envelope of the Hindenburg airship was ignited by an electrostatic discharge on landing . The hull lining and the contents of the hull of the airship then burned.
Corona discharge
A corona discharge , also called Elmsfeuer, occurs as a result of high field strengths on tapering or at least not smooth surfaces of an electrode. At needle tips, the strong change in the normal vector leads to a high concentration of charge carriers, so that the free charge carriers can emerge from the electrode slowly - i.e. not in a flash. The curve of the surface causes a large change in the electrical potential gradient directly in the area of the needle tip.
Brush discharge
With an electrode radius of 5 mm to 50 mm, a so-called brush discharge occurs against a plate at an electrical field strength of the order of magnitude of 500 kV / m.
ESD in the field of electronics
Electrostatically sensitive components
The group of ESD-sensitive (ESDS, English electrostatic discharge sensitive) components includes almost all electrical, electronic and optoelectronic components. This category also includes numerous electromechanical components. The function of all such components can be impaired or destroyed by electrostatic discharges.
Electrostatic discharges can cause damage in microelectronic components because in relation to the component size, the energy of a static discharge in a semiconductor behaves like the energy of a lightning strike in a tree. This becomes clear when you see ESD destruction in a chip under a microscope, which has created a 'crater' there. Compared to lightning in nature, an electrostatic discharge has a much smaller amount of charge and thus a much smaller amount of stored electrical energy . However, the electrical power that acts during the discharge must be considered. Since the discharge duration can be in the very short time range from ps to ns and the damage area or impact area of the discharge is often in the range of 5 µm to 10 µm, a very high electrical power and a very high power density occur despite the relatively low electrical energy ( Power per area) in the component.
ESD is one of the most common causes of failure, particularly with integrated circuits based on semiconductors. Circuits from high frequency technology , diode lasers (GaAs semiconductors) as well as field effect transistors and light emitting diodes , which often only tolerate blocking voltages of 5 - 30 V, are particularly sensitive . Since discharges can only be felt from approx. 2,000 V, measures must be taken to reliably prevent the charges.
Not only external discharges, but also electrical fields generated by handling can destroy these components if the dielectric strength of their sometimes very high-resistance connections in the entrance area is exceeded. Internal voltage flashovers or voltage breakdowns lead to destruction or previous damage, which leads to immediate or later failure.
An evaluation by a manufacturer of electronic components has shown that around a quarter of the components marked as defective are damaged due to electrostatic discharge.
To test the ESD sensitivity, devices or systems are subjected to standardized discharges and checked for malfunction or failure. The ESD sensitivity is dealt with and examined in the context of electromagnetic compatibility (EMC). ESD resistance is an important issue in electronics production, industrial electronics, computer technology , telecommunications technology and automotive electronics .
To avoid ESD damage, all ESD-critical components (especially integrated circuits , light-emitting diodes , semiconductor lasers , Schottky diodes, MOSFETs and IGBTs ) and assemblies (e.g. computer components ) must be handled in an ESD-protected environment ( Electrostatic Protected Area , EPA), packed and stored. Such ESD workstations and ESD protected areas in the semiconductor manufacturing lead existing electrostatic charges in a controlled against earth and prevent the most by static electricity generated charges. This is done using electrically conductive work surfaces , antistatic tapes , appropriate furniture, clothing, shoes, floor coverings, ionized ambient air and earthing of all components.
Basic principles of ESD protection
Protection against electrostatic discharges is essentially aimed at this
- Avoidance of charging - to minimize unavoidable parasitic charges, e.g. B. by deriving and grounding the body
- Avoidance of rapid discharges - discharges can never be avoided, but precautions can be taken so that there are no rapid discharges and existing electrical charges slowly, e.g. B. over a large electrical resistance, can flow away.
Models
In order to test the durability of electronic components, different simulation models for ESD impulses have been introduced. These are roughly divided into 4 ESD simulation models :
- HBM - Human Body Model: The Human Body Model simulates the discharge of an electrostatically charged person when they touch a component. A current flow through the component between different connection pins is assumed as the current flow path.
- MM - Machine Model: The basic idea of the Machine Model is related to the Human Body Model, but it simulates the rapid discharge of an electrostatically charged machine when it comes into contact with a component. As with the previously mentioned human body model, a current flow through the component between different connection pins is assumed as the current flow path.
- CDM - Charged Device Model: The Charged Device Model differs fundamentally from the Human Body Model and the Machine Model. This model assumes that the entire component is electrically charged and is suddenly discharged against a low-resistance electrode. A current flow through the component is not assumed here.
- FCDM - Field Induced Charged Device Model.
Based on previous experience, the numerical values of the individual models cannot be converted between the models using a fixed factor. Due to the model, however, the numerical value for the human body model is greater than the numerical value for the machine model. In principle, however, the statement that the components are more robust, the larger the respective numerical value.
Protective structures within electronic components
To dissipate electrical charges, protective circuits such as ggNMOS are built into the integrated circuits in the case of external connections . These work within assemblies or at their connections. It should be noted that these protective circuits can only absorb a maximum amount of energy per discharge. If this amount of energy is exceeded, the circuit, including the actual circuit function, can be irreversibly damaged. In line with the general trend towards the downsizing of the structures of the semiconductor components, the protective structures within the components, which ensure ESD protection, are also being reduced in size.
Protective structures on assemblies through additional components for ESD protection
In order to increase the robustness of assemblies in the area of customer interfaces, special protective components can be installed on the electrical lines on the assembly in the entrance area, whose only task is ESD protection or EMC protection. These components then support the derivation of voltages on the lines with respect to the reference potential on the assembly.
ESD protection through ESD protection zones
Maximum permissible static charges
Working with electrostatically sensitive components, such as electronic components, requires special precautionary measures. Measures in electronics against static discharges and electrical fields are described in DIN EN 61340-5-1. The associated user manual contains specific design information, which, however, does not contain any additional normative specifications.
In the industrial environment, an ESD protection zone (EPA = Electrostatic Protected Area ) is set up to process components that are at risk of electrostatic discharge . According to the state of the art, the voltage level of the electrostatic charge within ESD protection zones should not exceed the limit value of 100 V. In order to guarantee this permanently, various structural and administrative preparations have to be made.
As a further requirement, electrical field strengths of 10 kV / m must not be exceeded in ESD zones. This numerical value sounds very high at first, but in practice it means a lot of effort. For example, electrostatically charged plastic bodies emit electrical fields. If a component comes into the effective area of this field, it can either be damaged by the direct field effect or it can be electrostatically charged and damaged when it comes into contact with a non-charged component or with a hard-grounded work surface.
ESD-compliant floors
As an elementary requirement, the floor of these ESD protection zones must have sufficient conductivity with respect to the reference potential PE. In practice, floors with a leakage resistance of 1 MΩ have proven effective. According to the state of the art, a resistance of 1 GΩ may be used in ESD protection zones if a walking test can demonstrate that the maximum charge on employees is not greater than 100 V.
Depending on the design of the ESD flooring, the conductive layer can be applied in the form of panels, rolls, coatings or paintwork. Today, mostly coatings or rolled goods are widely used as ESD flooring.
ESD-compatible shoes and safety shoes
To dissipate the electrostatic charge via the floor to the earth potential, people in ESD protection zones must wear conductive shoes. The total resistance of the human - earth potential system should not exceed a resistance value of 35 MΩ. The limit value is the series connection of the following partial resistances: floor, transition resistance floor-footwear, footwear, human body resistance and transition resistance human-component. In practice, the resistance of the shoes is often in the single-digit MΩ range. The body resistance value of a person is usually significantly lower than the other resistance values and is included in the calculation with a value of a few kΩ. The human-shoe transition resistance and the human-component transition resistance depend on various factors, including skin moisture, and can vary over a larger range.
ESD-compliant protective gloves
In the past there was no separate standard for protective gloves and therefore no explicit antistatic limit values or specifications. In future there will be EN 16350 ( protective gloves against electrostatic risks ). This specifies a maximum resistance value of 10 8 ohms. The minimum insulation protection is 10 5 Ohm. ESD-compatible (dissipative) gloves should therefore have a volume resistance of 10 5 to 10 8 ohms and refer to the EN 16350 standard or the EN 1149-1 test method.
ESD-compatible outerwear
Special conductive protective clothing must be worn so that people in the ESD protection zones do not become unacceptably charged by movement or friction on other bodies. Depending on the design and requirements, this is a pure cotton fabric or a special fabric with different base materials and the addition of special conductive yarn fibers, which have a high electrical conductivity. So that the clothing can fulfill its protective function, it must be worn tightly and closed. For example, this clothing can consist of a long work coat. When wearing ESD protective clothing, make sure that the items of clothing underneath are completely covered, otherwise the protective effect of the ESD protective clothing can be canceled again. The ESD protective clothing essentially fulfills two tasks:
- It is not or only weakly chargeable.
- It conducts the electrical charges that are applied to ESD protective clothing, for example (contact with charged surfaces or skin contact of clothing on people).
According to the current state of the art, ESD-compliant clothing has mostly been made from pure cotton. With increasing sensitivity of the components, cotton is getting closer and closer to the limit, so that new ESD garments made of a special fabric with conductive fibers are becoming more and more popular.
ESD-compatible work surfaces
To ensure that no unacceptably high charges arise in the ESD protection zones, the work surfaces, e.g. B. be sufficiently dissipative from tables, shelves, etc. The standard regards an upper limit value of 1 GΩ as the limit value. When it comes to work surfaces, it should always be borne in mind that hard-earthed metallic arrangements are often not optimal, as they allow very fast electrostatic discharges and hardly reduce the discharge current.
Earthing of persons during seated activities in ESD protection zones
When sitting in a chair, there is an increased risk of electrostatic charging, even if these are displayed in ESD protection zones and are made of a static dissipative fabric. A wrist grounding strap must also be worn for seated activities, as the ability of people to conduct electricity via the ESD shoes and ESD floor system is no longer sufficiently guaranteed due to the insufficient contact force.
Facilities and equipment within the ESD protection zones
The principle applies: "What is not there cannot be charged". Specifically, this means that everything that is required within the ESD protection zones must first be checked for necessity and secondly checked for behavior in accordance with ESD. Basically, ESD-compliant products that are certified in accordance with DIN EN 61340-5-1 are urgently recommended. If this is not possible, always use equipment and resources that are earthed or electrostatically dissipative. The conductivity can be determined by measuring the resistance. But this alone is usually not enough. In addition, all facilities and equipment should be charged with an insulating friction partner and the maximum voltage of the static electricity should be determined. In addition, the self-discharge of the equipment and equipment (how quickly the charge drops to a non-critical value again) must be taken into account.
Tools
In ESD protection zones, all tools that come into contact with electrostatically sensitive components should be largely conductive. For example, the plastic handles of tools can cause electrostatic potential differences, which can damage sensitive components. Metal tools can sometimes be critical. For example, in the area of pointed tools, e.g. B. tweezers , come to the concentration of electrical charge carriers. Due to the high electrical conductivity of the tool, rapid discharges can occur even with low charges. These can then lead to ESD damage.
Tools are now available commercially which use electrostatically conductive materials for the handles instead of highly insulating plastic materials. The conductivity leads to equipotential bonding between the person and the tool. When components or assemblies are touched, a defined, slow charge equalization occurs, which prevents ESD damage. These tools are used, for example, in electronics production or in customer service when work on assemblies has to be carried out.
In general, however, it must be pointed out that this type of tool cannot be used in environments in which open voltages are used or in which live parts can accidentally be touched. For this purpose, protective insulating tools must be used in accordance with the VDE regulations.
ionization
By ionized air to electrostatic charges on bodies build accelerates from. For this purpose, an ionizer can be used, which emits ionized air in a targeted manner onto more chargeable devices and equipment or onto particularly endangered components. By means of ionization, electrical charges can be given off both to an insulator (for example a highly insulating plastic part) and to an electrically insulated conductor (for example a metal body which is held by highly insulating plastic parts). However, ionization is not a means of improving insufficient ESD protection. Ionization can be used specifically at particularly critical points in order to minimize individual, locally limited risks in a workplace. Furthermore, when using ionization, the health effects on people in the vicinity must be given special consideration.
Packaging of components and finished products
In addition to processing the electrostatically sensitive components, safe transport of the components is also required. Therefore, packaging for ESD-sensitive components made of electrically conductive materials, e.g. B. electrostatically dissipative plastics. Some electronic components are destroyed by being transported in a plastic bag.
Packaging for ESD-sensitive components must consist of conductive (electrostatic dissipative) plastics. There are foils, fillers and foams that are dissipative or metallized with fillers. Often the sensitive connections of the components are connected with a short-circuit bridge for transport. ESD-sensitive semiconductor components that are not properly packaged (ESD-protected) should be sent back to the supplier, as they are not fail-safe - even if they initially function.
The ESD-compliant design of packaging is described in DIN EN 61340-5-3.
| Items to be packaged | EPA | UPA | ||
|---|---|---|---|---|
| ESDS | Directly attached | Enveloping | Directly attached | Enveloping |
| electrostatically conductive ESD-C or dissipative ESD-D (see note 1) | electrostatically conductive ESD-C or dissipative ESD-D | as for within the EPA ESD-C or ESD-D and with shielding effect against electrostatic discharge ESD-S (see note 2) | ESD-S shielding effect against electrostatic discharge | |
Note 1: For battery-powered ESDS, the selection of the material or the design of the packaging should ensure that the battery will not discharge
Note 2: Shielding against electrostatic discharge is only required if the wrapping does not provide shielding against electrostatic discharge Terms: ESDS - Electrostatic-sensitive device (electrostatically sensitive component), EPA - Electrostatic Protected Area (electrostatically protected area), UPA - Unprotected Area (electrostatically non-protected area)
Packaging is usually divided into categories (S), (C), (D) and (F) based on its electrical conductivity.
ESD-C Conductive: Conductive; Resistance between 1kΩ and 1MΩ
ESD-D Dissipative: Dissipative; Resistance between 1MΩ and 1TΩ
ESD-S Shielding: shielding; Shielding against electrostatic discharges
ESD-F Electrostatic Field Shielding: Shielding against electrostatic fields
In addition to protection against electrostatic discharges, this packaging must also offer adequate protection against the influence of static fields. In addition to the known material classes for packaging, the protection category (F) was also introduced with the introduction of this standard. Materials and packaging materials in this category also offer the necessary protection against the static fields described.
Depending on the type of design, several layers of packaging can be used. The ESD-compliant design must have at least the inner layer that touches the components directly.
In addition to ESD protection, this packaging must also adequately protect the packaged contents against mechanical and climatic influences.
ESD protection outside of ESD protection zones
An ESD protection zone is not available everywhere where electrostatically sensitive components are handled. Let's think, for example, of a service assignment in the electronics area for an end customer. In this case, however, adequate ESD protective measures can also be taken. For example, a wrist grounding strap, which is connected to the ground potential, can prevent the person from being charged in this case. There are also conductive mats that can also be connected to the earth potential and thus enable components and assemblies to be stored safely. For work in the electronics area, ESD-compatible, conductive tools are also available, which differ from commercially available, insulating tools due to their sufficient intrinsic conductivity.
Classification of materials
Shielding protection category
The protective effect of materials in the shielding category is ensured for metals by the high electrical conductivity of the material. This category has the highest conductivity. Packaging in this category is identified by the letter (S) in conjunction with the ESD protection symbol. According to the standard, the surface resistance of the materials is below 100 Ω.
Protection category conductive
In the case of plastics, the protection category conductive is created through the use of graphite particles which are introduced into the plastic matrix. This category has a conductivity which is lower than the conductivity of the shielding category but higher than the conductivity of the static dissipative category. Packaging in this category is identified by the letter (C) in connection with the ESD protection symbol. According to the standard, the surface resistance of the materials is in the range between 100 Ω and 100 kΩ.
Static dissipative protection category
The materials of protective packaging of the protection category static dissipative have a higher electrical resistance than the packaging of the category "conductive". The conductivity can be increased by the introduction of metal ions, e.g. B. copper ions, or by applying an antistatic to the surface. These materials are also referred to as electrically conductive. Packaging in this category is identified by the letter (D) in connection with the ESD protection symbol. According to the standard, the surface resistance of the materials is in the range between 100 kΩ and 100 GΩ.
Category isolator
All materials with a surface resistance greater than 100 GΩ are classified as electrical insulators from an ESD point of view and no longer have the electrical conductivity required for ESD protection. In addition to the consideration from the ESD point of view, it must be added that the materials in this category also conduct electrical current for physical reasons, even if the surface resistance is greater than the limit value according to the standard. From an ESD point of view, the dissipative effect of isolators is no longer sufficient and, for this reason, isolators must not be used.
Measurement
Electrostatic charges
In order to record possible risks from electrostatic charges, you have to rely on measuring devices, since people can only perceive discharges from a voltage value of around 2,000 V. Electrostatic charges and their electric field strength can be measured with an electric field meter.
In addition, electrostatic charge can in some cases also be recognized by means of electrostatic attraction (see electrostatics ). However, this usually requires very high charges. However, it must be taken into account here that the mere sticking of objects together can also be based on adhesion .
electrostatic discharge
The electrostatic discharge could only be measured a few years ago with real-time oscilloscopes> 1 GHz, since the memory chips could not record the fast, one-time and never even discharge pulse (<1ns). With fast real-time oscilloscopes it was found that z. B. when discharging a person by the hand (person walks over carpet and almost a handle) there are actually two discharges: 1. The hand is first quickly discharged (small charge, fast) 2. Then the whole body is discharged (large charge, about ten times the duration). Since charge distribution and spark generation vary greatly in practice under real conditions, voltage, current and discharge times are always individual. These are simulated with ESD pistols, which depict various simulation models with inserts (human body model, machine model, etc.).
Surface resistance and leakage resistance
The surface resistance of materials and the leakage resistance of equipment with respect to the reference potential PE have a decisive influence on ESD protection. Charging occurs through friction on bodies. Materials that have a sufficiently low surface resistance ensure that, on the one hand, the level of the charging voltage is minimized and, on the other hand, the electrostatic charge can be reduced again. The leakage resistance of devices ensures that electrostatic charges can flow away against the reference potential earth and thus no inadmissibly high charges can arise.
Evaluation of measurement results
The evaluation of measurement results usually requires a lot of experience. As can be shown by practical tests, materials whose surface resistance is above the permissible limit value can be used in certain cases. This is the case when these substances are only slightly charged, for example due to friction, and the charge carriers are broken down again after a very short time.
ESD in other industrial environments
Industrial environment
Electrostatic charges can hinder the manufacture and processing of plastic (especially plastic film ), paper, textiles and glass. On the one hand, the transport of the material is hindered; on the other hand, undesired particles ( dust , lint , powder ) usually adhere to the material due to the electrical charge . That is why ionizers are used to discharge these materials, especially in fast industrial plants.
For example, production machines for electrically insulating continuous products and the handling of insulating bulk materials require special safety measures against electrostatic charging.
The permanent grounding of people working in ESD areas is achieved by means of grounding straps, ESD clothing, ESD gloves and antistatic safety shoes. Other measures include antistatic work surfaces, floor coverings or conductive covers on office furniture.
literature
- DIN EN 61340-5-1 Electrostatics - Part 5-1: Protection of electronic components against electrostatic phenomena - General requirements (IEC 61340-5-1)
- DIN EN 61340-5-1 Supplement 1 Electrostatics - Part 5-1: Protection of electronic components against electrostatic phenomena - User manual (IEC 61340-5-2)
- DIN EN 61340-5-3 Electrostatics - Part 5-3: Protection of electronic components against electrostatic phenomena - Properties and requirements for the classification of packaging used for components that are sensitive to electrostatic discharges (IEC 61340-5-3 )
- Technical rule for hazardous substances TRGS 727 Avoidance of ignition hazards due to electrostatic charges
- Hartmut Berndt: Electrostatics - VDE series standards understandable . 3. Edition. VDE Verlag GmbH, Berlin 2009, ISBN 978-3-8007-3049-0 .
Individual evidence
- ↑ Philip Havens: When a smartphone or tablet is "wiped out"
- ↑ Fire at the pump nozzle - the underestimated danger. The West, July 5, 2010.
- ↑ Niels Jonassen: Mr. Static Explosions and ESD. (No longer available online.) In: Compliance Engineering. 1999, archived from the original on July 8, 2011 ; Retrieved March 5, 2011 .
- ^ Kaiser, Kenneth L .: Electrostatic discharge . Taylor & Francis, Washington, DC 2006, ISBN 0-8493-7188-0 , pp. 2-73 .
- ↑ a b c d e f g h i j k DIN EN 61340-5-1 Electrostatics - Part 5-1: Protection of electronic components against electrostatic phenomena - General requirements (IEC 61340-5-1)
- ↑ Dipl.-Ing., MBA Eng. Matthias Päselt: We help to protect your products from ESD damage. Retrieved June 9, 2017 .
- ↑ DIN EN 61340-5-1 Supplement 1 Electrostatics - Part 5-1: Protection of electronic components against electrostatic phenomena - User manual
- ↑ An overview of the values that exist, depending on the set of rules or standards, is available on ESD standards overview PPE (PDF; 66 kB)
- ↑ a b DIN EN 61340-5-3 Electrostatics - Part 5-3: Protection of electronic components against electrostatic phenomena - Properties and requirements for the classification of packaging used for components that are sensitive to electrostatic discharges (IEC 61340- 5-3)