10
3 Protection setting description
PXR tRiP units foR PoweR Defense molDeD case ciRcuit bReakeRs
MN012007EN June 2018 www.eaton.com
3.1.2 Long delay thermal memory
In addition to the standard long delay protection, a long time memory (LTM) function is
supported. This protects load circuits from the effects of repeated overload conditions.
LTM is enabled from the factory but can be reconfigured using the UI or by using PXPM
software.
As an example, if a circuit breaker is closed soon after a long delay trip, and the current
again exceeds the long delay setting (I
r
), the LTM automatically reduces the time to trip to
allow for the fact that the load conductor temperature is already higher than normal because
of the prior overload condition. When the load current returns to normal, below pickup, the
LTM will begin to reset (after about ten minutes it will have reset fully) so the next long
delay trip time will again correspond to cold start on the curve. In certain applications and
when doing repetitive field testing, it may be desirable to disable the LTM function.
3.2 Short delay pickup and time settings
Settings for short delay pickup (SDPU or
I
sd
) are expressed as multiples of the long delay
pickup current setting (
I
r
).
The short delay time (t
sd
) is selected in conjunction with one of two short delay slopes, flat,
or I
2
t. 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 reverts to a flat response.
The optional zone selective interlocking (ZSI) feature may affect the tripping times for the
short delay protective function. Please refer to the section on ZSI.
3.3 Instantaneous pickup setting
The instantaneous (
I
i
) setting is expressed as multiples of the circuit breaker frame rating
(
I
n
). The instantaneous protection trips the breaker with no intentional time delay.
3.4 Ground fault settings
When the PXR 20, 20D or 25 trip unit includes ground fault protection features, the distribu-
tion 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. To ensure correct ground fault equipment
performance and compliance, you must conduct the field testing required to comply with
country or regional requirements.
3.4.1 Ground fault pickup
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. Three modes of operation are selectable
from the front of the trip unit.
•
The ground detection may be turned off by selecting “OFF”.
•
The ground fault detection pickup level with an alarm only action can be used by selecting
“Alarm”. Multiple levels of pickup are available depending on the trip unit style.
•
The ground fault detection pickup level with an action of trip may also be used by selected
“Trip”. If a ground fault causes the circuit breaker to trip.
3.4.2 Ground fault time
The PXR trip unit provides selection for two different ground fault slopes: a fixed time (flat)
or I
2
t response. The slope should be chosen to match coordination needs. The I
2
t slope
response provides a longer time delay for coordination of currents below 1.0 x I
n
frame.
After 1.0x the response reverts to a fixed time (flat) response. The time delay and slope are
selected using PXPM or the user interface (UI).
