11
3. Protection functions
Power XPert® release triP unit for MagnuM PXr circuit breakers
MN013015EN January 2022 www.eaton.com
already higher than normal because of the prior overload
condition. Each time the overload condition is repeated,
the LTM causes the circuit breaker to trip in a progressively
shorter time. When the load current returns to normal, the
LTM begins to reset (after ten minutes it will have reset
fully) so the next long delay trip time will again correspond to
the setting value. During sequential field testing, it may be
desirable to disable the LTM function.
3.3 Short circuit settings
Settings for short delay pickup (SDPU or I
sd
) are expressed as
multiples ranging from 1.5 to 10 times the long delay pickup
current setting (I
r
).
The short delay time (t
sd
) settings range from 0.05 seconds
to 0.5 seconds. For both a fixed time (flat) or I
2
t response,
the I
2
t response curve will provide a longer time delay
for currents below eight times I
r
as compared with a flat
response curve. For currents greater than eight times I
r
, the
I
2
t response flattens out to the flat response.
The optional zone selective interlocking (ZSI) feature may
affect the tripping times for the short delay protective func-
tion. More information can be found in Section 3.9 - Zone
Selective Interlocking (ZSI).
If a short delay causes the circuit breaker to trip, the
SHORT indicator will be illuminated, and the short delay trip
message will be displayed.
3.4 Instantaneous protection setting
The instantaneous (I
i
) setting is expressed as multiples
ranging from 2 to 15 of the (I
n
) value or can be set to OFF.
The instantaneous protection trips the breaker with no inten-
tional time delay.
The instantaneous (INST) indicator shows this cause of trip.
Figure 9. Ground fault examples.
3.5 High instantaneous (withstand) trip
The PXR trip unit provides a factory set function that will
trip the circuit breaker at currents above the short time
withstand rating of the circuit breaker frame. The function is
always active on breakers with interrupting capacity higher
than the short time withstand rating and reacts to the peak
current level.
3.6 Ground fault settings
When the PXR 20/25 trip unit includes ground fault protec-
tion features, the distribution system characteristics (such
as system grounding, number of sources, and number and
location of ground points) must be considered along with the
manner and location in which the circuit breaker is applied
to the system (see Figure 9). To ensure correct ground fault
equipment performance and compliance, you must conduct
the field testing required to comply with country or regional
requirements.
The PXR trip unit provides flexibility in detecting and acting
on ground currents. A ground fault alarm can provide an
early warning of a ground fault condition and a ground fault
trip can provide protection under these conditions. Settings
options are the same for alarm or trip and can be set using
display (Main -> Edit Settings -> Protection settings) or using
PXPM.
On a PXR trip unit with ground fault protection, there are
three modes of operation which are selectable:
•
Ground fault protection may be turned OFF;
•
Ground fault protection with an Alarm only action; or
•
Ground fault protection with a Trip action.
The PXR 20/25 trip unit provides for two options for sensing
to detect ground fault currents: residual and source ground/
zero sequence. The option is selected using the display and
navigation buttons or by using PXPM configuration software.
