2.12 Inrush Current Blocking Function (2ND Harm Blocking)
Inrush current is a regularly occurring phenomenon and should not be considered a fault as we do not
want the protection device to issue a trip command whenever a machine is switched on. However, this
presents a problem as the device should always trip on an internal fault. The problem is that typical
internal faults may produce overcurrents which are not necessarily greater than the inrush current.
Furthermore, faults tend to manifest themselves on switch on due to the high inrush currents. Facility
exists to apply a restraining method based on the harmonic content of the inrush current. The
mechanism by which this is achieved is called second harmonic blocking.
2.12.1 Second Harmonic Blocking Implementation
Second harmonic blocking can be applied to the following overcurrent protection types:
• Phase Overcurrent protection
• Earth Fault protection
Second harmonic blocking is implemented in the GROUP (n) SYSTEM CONFIG column, where (n) is
the number of the setting group.
Second harmonic blocking is applicable to all stages of each of the elements.
The function works by identifying and measuring the inrush currents present at switch on. It does this
by comparing the value of the second harmonic current components to the value of the fundamental
component. If this ratio exceeds the set thresholds then the blocking signal is generated. The
threshold is defined by the 2ndHarm Thresh setting.
We only want the function to block the protection if the fundamental current component is within the
normal range. If this exceeds the normal range then this is indicative of a fault, which must be
protected. For this reason there is another settable trigger I> lift 2H, which when exceeded, stops the
2nd harmonic blocking function.
Each overcurrent protection element has a I>(n) 2H Blocking setting with which the type of blocking is
defined.
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