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PARALLEL PROTECTIONS
Typically are those parts that protects the device, from the Extra-tensions (overvoltage), generally theirs use is conditioned by fuse(or series protection) that comes mounted upstream. In fact the choice is dictated as well from the current that must be absorbed, maintaining the circuit protected downstream, as from the current limit in order to release the series protection device upstream. The parallel protections can be subdivided in Primary and Secondary.
The primary protections are the first that takes care of attenuating the disturbance, therefore being of power (must bear the maximum energy) but unfortunately they are NOT so fast. This allows the disturbance, to transfer the first peak (uS) of extra-tension to the circuit, that in some cases it reaches also thousands of volts. This must be take care of by the secondary protections.
This Application Note from ST (AN579) describes very well the concept of primary and secondary protection.
Further it comes represented the REAL answer of one primary protection:
As you can notice in the first 80uS we are still over the threshold of 230V. Ulterior testimony of the necessity of a secondary protection in the sensitive circuits.
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