4 Protection settings
4.2 Overload pre-warning
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PXR Electronic trip unit
01/21 MN012005EN
www.eaton.com
4.2 Overload pre-warning
The overload pre-warning function warns of an overload tripping before it
occurs. The function is similar to the thermal motor image of the PKE motor-
protective circuit-breaker. With the digital NZM, however, this function is not
limited to the motor protection versions. The occurring overload is evaluated
and a value in the range from 0 % to 100 % is the output.
The value increases with the overload and decreases as soon as it is no
longer present. If the value reaches 100 %, the breaker trips. The speed of
the rising is determined by the height of the overload. If an increasing value
is detected, countermeasures can be initiated by the user. The function can
be read out via the communication connection, see here.
4.3 Thermal memory
In addition to the standard long-delay protection, the long-term memory
function (“thermal memory”) protects the load circuits against the effects of
repeated overloads. The thermal memory is enabled by default and can be
configured using the display and navigation buttons or the Power Xpert
Protection Manager configuration software.
Example
If a circuit breaker is closed immediately after an overload trip has occurred,
and the current again exceeds the threshold value of the overload protection
I
r
, the thermal memory will automatically reduce the trip time, as it is
assumed that the temperature in the load circuit is already higher than
normal due to the previous overload. Should an overload occur repeatedly,
the thermal memory will trip the circuit breaker at ever shorter intervals.
As soon as the load current falls back to within its normal range, the thermal
memory will start to reset. As such, the overload delay time of the next trip
will again correspond to the set value. When checking the tripping
characteristic, the thermal memory can be disabled in order to obtain
accurate test results. It is absolutely essential to reactivate the thermal
memory after the testing has been completed! This function enables the
circuit breaker to protect both downstream cables (outgoing cables) and
equipment as well as its own integrity against excessive heating in the event
of repeated overcurrents.
