Siemens 7SR51 Device Manual page 358

Protection and Automation Functions
5.29 87GH Restricted Earth-Fault Protection – High-Impedance
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Notwithstanding the above calculation SPDL recommend that a Metrosil is always fitted in the high‑impedance
relay circuit.
The use of non-linear resistors manufactured by Metrosil is recommended. The operate characteristic is
defined by:
Voltage characteristic:
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For DC or instantaneous values.
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For applied sinusoidal voltages.
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For applied sinusoidal currents.
Where: C and β are Metrosil constants.
When supplied as discrete components 7XG14 Metrosils can be specified as single or 3 phase, with a diameter
of 75 mm or 150 mm and have constant C values of 450, 900 or 1000.
Metrosils of diameter 75 mm have a thermal rating of up to 8 kJ. Where a higher thermal rating is required
Metrosils of 150 mm diameter should be used.
The optional component box includes both 3 phase resistors and 3 phase Metrosils. Metrosils in the compo-
nent box have a C value of 1000.
The chosen Metrosil C value must:
•
Ensure negligible current flows through the Metrosil at relay operate voltage (V
•
Limit overvoltages for operational and safety reasons i.e. 1.09C (I
A C value of 450 is generally acceptable where the relay operate voltage is less than 100 V, a C value of 1000
is recommended for settings above 100 V.
Metrosil short time power rating must:
Be sufficient to dissipate the heat created by the flow of maximum secondary internal fault current. The
Metrosil is chosen so that it can withstand I
circuit‑breaker condition the back up protection clearance time must be considered, typically a 1 second rating
is sufficient.
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358
for the maximum fault clearance time. For a failed
Fint
Reyrolle 7SR5, Overcurrent Protection Device, Device Manual
), and,
set
β) < 3 kV
Fint
C53000-G7040-C014-1, Edition 11.2019