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PR512/P (50-51-50N-51N) UNIT
Protection against earth fault is possible if the
release is connected to three phase CTs or to two
phase CTs and to the external toroidal trans-
former.
When the release is self-supplied, the Io> and
Io>> protection functions are only activated if the
primary current exceeds the value of 0.2 x In (In is
the rated current of the phase CTs).
For higher performances, a 24 V dc auxiliary
power supply unit must be provided.
Earth fault external toroid
The value of the rated homopolar fault current (In)
varies according to whether the external or
internal toroid is used:
– with an internal toroid: In = rated current of the
– with an external toroid: In = rated current of the
The solution with the external toroid allows any
homopolar current to be controlled.
The toroid characteristics are described on page 8.
16
Protection functions
The PR512/P (50-51-50N-51N) unit
carries out all the functions of the
PR512/P (50-51) unit as well as the
following protection functions:
– 51N protection against earth fault:
• 0.2 < I < 2 In (rms)
• 2 < I < 20 In (peak);
– 50N protection against instantane-
ous or delayed earth fault (opera-
tion on peak value).
The homopolar current is calculated
as the vectorial sum of the three
phase currents. This sum is done by
the toroidal current transformer
inside the release or by means of an
external toroidal current transformer
connected to the special input.
phase CT
external toroidal current transformer.
Protection against earth fault (51N)
This function makes four different families of
protection curves available, defined as follows
(IEC255-3):
– definite time
– normal inverse time
– very inverse time
– extremely inverse time.
The threshold value of this protection is indicated
by Io>, whereas the relative trip time is indicated
by to>.
Start of timing is signalled by the ALARM LED
lighting up, whereas circuit-breaker opening is
signalled on the front by the magnetic flag Io>
Io>>, turned to the yellow position. To reset
signalling, the FLAG RESET pushbutton on the
front of the release must be pressed, ensuring one
of the following conditions:
a) 24 Vdc auxiliary voltage present (display lit);
b) primary circulating current higher than 0.2 x In
(display lit with indication of the circulating
current);
c) application of the TT2 unit to the TEST connec-
tor on the front of the release (optional acces-
sory).
Selection of threshold Io> value
Setting the Io> threshold is carried out using the
Dip Switches (see figure on page 17).
The sum of the values selected is the fraction of In
corresponding to Io>.
16 threshold values are available defined from 0.1
to 1 x In with steps of 0.05 x In.
The protection can be excluded by positioning the
first Dip Switch in the OFF position.
Remember that the Io> function with an internal
toroid is disabled when the fault current is higher
than 3 x In; with an external toroid no disablement
is foreseen.
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