Function
15
NGRM7xx_D00292_07_M_XXEN/02.2022
3.2 Functional description
NGRM7… monitors NGR resistance R
NGR
, neutral voltage U
NGR
and current I
NGR
. NGR re-
sistance is monitored using an active and a passive procedure:
active The device generates an active test pulse and measures R
NGR
even
if the installation is de-energized.
passive Only effective when installation is energized: The resistance R
NGR
is
determined when I
NGR
or U
NGR
exceeds an internal threshold. The
device measures the existing current and voltage and calculates
R
NGR
.
In the case of the "auto" method, monitoring switches automatically between "active"
and "passive" when the measured current or voltage value exceeds or falls below the
internal threshold. The threshold is 15 % of the nominal value and can be adjusted by
Bender if required.
A shorted or open NGR is reliably detected in an energized as well as a de-energized in-
stallation with the active measurement method.
When the "passive" method is selected, no switching of the monitoring takes place. No
monitoring of the NGR occurs while the installation is de-energized.
The measurement method can be selected as a set point or via the configurable digital
input I1 if the NGR method "external" has been selected (for software versions from July
2021).
The NGR-fault relay switches from the operating state (selectable as fail-safe or non-fail-
safe) to the alarm state when the measured resistance R
NGR
is outside of the configured
thresholds.
A ground fault is signalled via the corresponding ground-fault relay when I
NGR
or U
NGR
exceeds the selectable thresholds. After the adjustable time delay has elapsed, the trip
relay operates.
A connection to installations ranging from 400 V…25 kV is possible via the appropriate
CD-series coupling device.
I
NGR
is measured with (universal) measuring current transformers with a 5 A or 50 mA
secondary rating. The ratio of the used measuring current transformer can be set inter-
nally for best measurement performance of I
NGR
.
The phase-voltage monitoring function can be used to indicate which phase has the
ground fault. Direct coupling is possible up to a system voltage of 690 V. For higher volt-
ages, use potential transformers (PT). The ratio is an NGRM7… setting.
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