1MAC309294-MB F Section 4
Protection functions
RER620 125
Technical Manual
4.1.3.2 Function block
Figure 55: Function block
4.1.3.3 Functionality
The ground-fault function 51N/50N or 51G/50G is used as non-directional ground-fault
protection for feeders.
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.
4.1.3.4 Operation principle
The function can be enabled and disabled with the Operation setting. The corresponding
parameter values are Enable and Disable.
The operation of non-directional ground-fault protection can be described by using a
module diagram. All the blocks in the diagram are explained in the next sections.
Figure 56: Functional module diagram. I
0
represents the residual current.
Level detector
The measured ground current is compared with the set Pickup value.and, if enabled, the
Pickup block value. If the measured value exceeds the set Pickup value, and is less than
the Pickup block 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.
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