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What is the definition of ANSI CLPU 50N/51N - Earth fault cold load pick-up/blocking?

The Cold Load Pick-Up I0 or CLPU 50N/51N function avoids nuisance tripping of the earth fault protection (ANSI 50N/51N) during energization after a long outage.
Depending on the installation characteristics, such operations can actually generate transient inrush currents. 
If the residual current measurement is based on the sum of the 3 phase CTs, the aperiodic component of these transient currents can result in 
saturation of the phase CTs. This can lead to measurement of an incorrect residual current likely to exceed the protection set points.

These transient currents are essentially due to:
  • the power transformer magnetizing currents,
  •  the motor starting currents.
In principle, the protection settings should be defined so as to avoid tripping due to these transient currents. 
However, if these settings result in inadequate sensitivity levels or delays that are too long, the CLPU 50N/51N function is used to increase or 
inhibit set points temporarily after energization.
If the residual current is measured by a correctly installed CT, there is less risk of measuring an incorrect residual current. In this case, there is no need to use the 
CLPU 50N/51N function.
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