Leakage resistance
A leakage resistor reduces electrical charges in a controlled manner . For this purpose, resistors are used which, depending on the task, are connected, for example, parallel to live lines or from them to earth.
Capacitors
A bleeder ( English bleed = bleed as bleeder resistor called) is in electrical capacitors as power capacitors or decoupling capacitors in line filters indispensable. It is used to discharge the capacitor to harmless electrical voltages within a certain time after it has been switched off or if it is not used .
Bleeder resistors are dimensioned in such a way that they impair the function of the capacitors as little as possible, but after switching off the device (and possibly opening the housing) ensure that they are discharged to a voltage of <50 volts within usually a maximum of 5 minutes. Touchable connectors (caused by X-capacitors of line filters or capacitor power supplies) require a discharge time of 5 s to <60 V or, for voltages below 15 kV, less than 45 µC stored charge (VDE 0140–1). This value is maintained at a mains voltage of 230 V with an X capacitor of 100 nF, which has no leakage resistance. In the case of voltages above 15 kV, the stored energy that may be released upon contact may not exceed 350 mJ (DIN VDE 0105-100).
They are a safety measure to prevent electric shock . Furthermore, they prevent the recharging of discharged capacitors due to any charges still present in their dielectric ( dielectric absorption ). If these charges migrate to the electrodes, capacitors can otherwise assume dangerous voltages again even after discharge.
Bleeder resistors can be built directly into the capacitors or located in the connection box or on the connection terminals. Capacitors with built-in bleeder resistors are usually marked with a resistor symbol on the housing. Bleeder resistors must have sufficient dielectric strength and power loss in order not to fail in continuous operation.
Power capacitors for operation on high voltage with bleeder resistance should be discharged before being touched and then the contacts should be provided with a low-resistance short circuit. Large capacitors are discharged via a protective resistor with sufficient voltage and pulse strength to avoid arcing faults . Power capacitors that do not have such a bleeder resistor must be stored short-circuited.
Soft ground
If the term " leakage resistance" is used in the context of ESD measures ( electrostatic discharge ), it is an ohmic resistance to divert uncertain potentials and charges of isolated circuit parts or components to earth . Such potentials or charges can arise from parasitic capacitances against active network conductors, from static electricity or interference fields. Direct hard grounding via a conductor would also dissipate the charges, but is often undesirable. Hard grounding possibly causes ground loops ( hum ), generates high current peaks in the event of interference pulses and is avoided in ESD-protected workplaces in order to protect people from additional electric shocks if they touch live parts.
The size of these resistors depends on the intended use and is on the order of about 100 ohms to> 1 GOhm. Such resistors are often bridged with interference suppression capacitors in order, for example, to preserve the shielding effect of housings.
Components and materials can also have soft ESD-dissipating properties. These are ESD packaging, types of rubber for conveyor belts, tires or special floors and work surfaces as well as ESD spray and cleaner. See also ESD protection .
literature
- Dieter Nührmann: work book electronics . Franzis, 1981, ISBN 3-7723-6543-4 .
Individual evidence
- ↑ a b https://www.sifa-sibe.de/sicherheit/arbeitsschutzorganisation/erste-hilfe-und-notfallmasshaben/ Falk Florschütz: First aid and emergency measures in the event of electrical accidents , Dr. Curt Haefner-Verlag GmbH, May 2018, accessed on July 13, 2018