Siemens 7SR242 Duobias, Technical Manual

Page: 347 / 382

©2015 Siemens Protection Devices Limited Chapter 7 Page 31 of 64
The chosen Metrosil ‘C’ value must;
1) Ensure negligible current flows through the Metrosil at relay operate voltage (Vs), and,
2) Limit over-voltages for operational and safety reasons i.e. 1.09C (I
Fint
) < 3kV
A ‘C’ value of 450 is generally acceptable where the relay operate voltage is less than 100V, a ‘C’ value
of 1000 is recommended for settings above 100V.
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
Fint
for the maximum fault clearance time. For a failed circuit
breaker condition the back up protection clearance time must be considered – typically a one second
rating is sufficient.
P
1SEC
K F
V T I
4