Electrostatic charge can occur in almost every body. The electronic energy that can build up in a human body is 10 to 30 milli-Joule. The size of the electrostatic field can reach up to 20 kV. With a current drain of up to 100 A/ns, the short-term discharge currents can have up to 30 A. Charging caused by friction such as running on carpets can cause damage to sensitive components. Electrostatic discharges generate voltages of several 10,000 V by friction electricity.
For protecting ESD-sensitive components, so-called Electrostatic Protected Areas (EPA) can be set up at the workstations. Conductive and grounded table and floor coverings, so-called ground potential reference surfaces, and grounded tools are at these workplaces. The workers' clothes and shoes are made of antistatic material. In addition, antistatic wrist straps can be connected to ground potential via a grounding strap. The VDU work regulation includes measurements to reduce electrostatic discharges and protect health. In addition to the static discharge, there is also the inductively induced interference, the electromagnetic interference (EMI).
Voltages that cause damages
ESD class 1
Sensitive against ESD voltages between 0 V and 1.9 kV
ESD class 2
Sensitive against ESD voltages between 2 kV and 3.9 kV
ESD class 3
Sensitive against ESD voltages between 4 kV and 15.9 kV
ESD class insensitive
Sensitive against ESD voltages above 16.0 kV
A shoe is antistatic if the measured volume resistivity is directed against the flow of current through a material and is in the range between 100-kilo ohms and 1-giga ohms. If the resistance is lower, it is conductive according to the directive EN ISO 20345, and if the resistance is greater than it is insulating. Employees should wear anti-static footwear to reduce electrostatic charge and ensure drainage. For protecting people, and the electronics, this is necessary when the risk of electric shock from equipment or live parts is not excluded. According to EN ISO 20345, the lower limit of the volume resistance for ESD is 100-kilo ohms and the upper limit is 35 mega ohms. That means that a shoe with ESD is also antistatic, but an antistatic shoe has not automatically ESD. The labeling must be made separately from the CE mark, as this is a product protection standard. If the shoes or clothes meet the standard, they have the ESD label. An identification with SI indicates antistatic.
ESD in TBH filter and extraction systems
All TBH filter and extraction systems are made of high-quality sheet steel and are accordingly conductive. For this reason, all TBH systems are classified in the protection class I for electrical equipment. Devices of the protection class I must always be earthed with low resistance in order to prevent damage from the operator / maintenance personnel in the event of a fault. In the test field, earthing is ensured by a 100% initial test in accordance with DIN EN 61010 (thereafter repeat test in accordance with DIN VDE701 / 702). The limit value is <0.3 ohms (with cable lengths <5m). Equipotential bonding between the system parts ensures that the system cannot charge in the airflow. This provides additional security.
The demand from ESD environments for a discharge of electrostatic discharges is based on a high-resistance resistor according to IEC 61340-5-1 <35 MegaOhm, in order to keep the leakage currents low. This protects sensitive electronic components from overvoltage. The protection of humans has the highest priority. Therefore, the user must not come in contact with the extraction system.
Detection elements can be grounded using their ground connection with high resistance to the system. Thus, a touch is not a problem. The system should be placed under a worktable so as not to be in the normal working environment. During maintenance work, attention must be drawn to any critical electronic components in the environment or to remove the system from the ESD area during maintenance.