00430060| Datasheet

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Selection Procedure

2 2.1

Selection procedure Overvoltage types and sources

Overvoltages are distinguished according to where they originate. 2.1.1 Internal overvoltages

Internal overvoltages are those overvoltages that originate in the actual system which is to be protected, e. g. through
I I I I I I

inductive load switching arcing direct coupling with higher voltage potential mutual inductive or capacitive interference between circuits electrostatic charge ESD.

With internal overvoltages the worst-case conditions can often be calculated or traced by a test circuit. This enables the choice of overvoltage protective devices to be optimized. 2.1.2 External overvoltages

External overvoltages are those overvoltages that affect the system which is to be protected from the outside, e. g. as a result of
I line interference I strong electromagnetic fields I lightning.

In most cases the waveform, amplitude and frequency of occurrence of these transients are not known or, if so, only very vaguely. And this, of course, makes it difficult to design the appropriate protective circuitry. There have been attempts to define the overvoltage vulnerability of typical supply systems (e. g. industrial, municipal, rural) so that the best possible protective device could be chosen for the purpose. But the scale of local differences makes such an approach subject to uncertainty. So, for reliable protection against transients, a certain degree of "overdesign" must be considered. Therefore the following figures for overvoltage in 230-V power lines can only be taken as rough guidelines:
I amplitude up to 6 kV I pulse duration 0,1 m s to 1 ms

Where varistors are operated directly on the line (i. e. without series resistor), normally the type series S20 should be chosen. In systems with high exposure to transients (industrial, mountain locations) block varistors are to be preferred. Requirements are stipulated in IEC 61000-4-X. Severity levels are specified in the respective product standards. Table 4 shows the selection of varistors for surge voltage loads according to IEC 61000-4-5 as an example.

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