The function picks up and trips when the measured (IG) or calculated (IN) ground current
exceeds the set limit. The trip time characteristic for low stage 51N/G and high stage 50N/
G-1/2 can be selected to be either definite time (DT) or inverse definite minimum time
(IDMT). The instantaneous stage 50N/G-3 always trips with the DT characteristic.
In the DT mode, the function trips after a predefined trip time and resets when the fault
current disappears. The IDMT mode provides current-dependent timer characteristics.
The function contains a blocking functionality. It is possible to block function outputs,
timers or the function itself, if desired.
4.1.4.4
Operation principle
The function can be enabled and disabled with the
Operation
setting. The corresponding
parameter values are “Enable” and “Disable”.
The operation of 51N/50N and 51G/50G can be described by using a module diagram. All
the modules in the diagram are explained in the next sections.
A070437-ANSI V1 EN
Figure 79:
Functional module diagram
Level detector
The measured ground current is compared to the set
Pickup value
. If the measured value
exceeds the set
Pickup value
, the level detector sends an enable-signal to the timer module.
If the
ENA_MULT
input is active, the
Pickup value
setting is multiplied by the
Pickup
value Mult
setting.
Do not set the multiplier setting
Pickup value Mult
higher than necessary.
If the value is too high, the function may not trip at all during an inrush
followed by a fault, no matter how severe the fault is.
The pickup value multiplication is normally done when the inrush detection function
(INR) is connected to the
ENA_MULT
input.
Section 4
1MRS240050-IB C
Protection functions
162
REF615R
Technical Manual
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