MIG-OS-OS1_MIG0603OS1_MIG0603OSI Modular Impulse Generator
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1.1.2 "CWG Combination Wave Generator"
Introduction, "Combination wave test"
Long wires to sensors or supplies are very often connected to the inputs and outputs of industrial electronic
equipment. The most frequent damages are overvoltages, caused either by switching actions in the
equipment itself or by atmospheric discharges such as lightnings. To avoid the destruction of electronic
equipment by overvoltages, protection elements and circuits are placed at the inputs and outputs of the
equipment ports.
Consumer electronic devices such as antenna ports on television set, telephones, faxes, can also be
influenced by atmospheric discharges. Mostly the disturbances are tolerable because of their single event.
To protect such equipment from damages, protection elements and circuits are installed. Tests must be
carried out to determine whether these protective circuits are really effective.
The following aspects of surge testing electronic systems are relevant:
•Testing for failure modes that involve flashover are influenced by the surge current that would flow after
flashover.
•The surge let-through of a protective device depends on the applied voltage front.
•The response of a crowbar-type device, subjected to an intended current test, will be influenced by the
voltage front applied by the generator, that senses a high-impedance test piece, until operation of the
crowbar.
Therefore the generator must be capable to generate a waveform 1,2/50 µs or a current waveform of 8/20
µs at clamping status of the protection circuit. Traditionally, the 1,2/50 µs voltage waveform was used for
testing the basic impulse level of insulation, which is approximately an open circuit until the insulation fails.
The 8/20 µs current waveform was used to inject large currents into surge protective devices. Since both
the open circuit voltage and short circuit current are different aspects of the same phenomenon, such as an
overstress caused by lightning, it was necessary to combine them to a single waveform when the load is
not known in advance.
The combination wave generator has been defined first for Electro Magnetic Compatibility tests up to 4 kV
in the document IEC 61000-4-5 or IEEE 587. EMC test must be carried out on powered equipment. There
are several reasons for performing powered test:
•From a standpoint of a good practice, it is best to perform laboratory tests in a manner that most closely
simulates the actual service environment.
•It is the applied ac that furnishes the energy following the surge, that can establish sustained arcing
faults, tracking on insulation, destruction of printed wiring, and so on
•The application of normal ac power generally rises the EUT to an initial level of stress. Without power
current following a surge-induced flashover, the resulting defect might not be detected.
The loading by the EUT might cause appreciable discrepancy between the preset nominal voltage open-
circuit voltage or short circuit current and the actual voltage across or current in the load. This effect is the
reason why surge parameters are not specified with the EUT connected.
Equations for standards waveform:
1,2/50 µs Wave V t AVp tt
( ) ( exp )exp( )=−
−
112
ττ
8/20 µs Wave It AIpt t
( ) exp( )=
3
τ
