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2.1.2 Establishing the Required Stabilising Resistor Value
The relay burden need not be considered as it is effectively negligible relative to the burden of the stabilising
resistor. The setting (operate) voltage (Vs) across the Relay and Stabilising Resistor at the Relay operating
current (Is):
V
S
R
STAB
I
S
Stabilising Resistor power rating must:
Be sufficient for continuous operation at the circuit operate voltage (Vs):
2
P
(I
)
x
CONT
s
Short time rated to withstand I
condition the back up protection clearance time is considered – typically a one second rating is sufficient.
2
V
Fint
P
1SEC
R
stab
Where I
is not known, the breaking capacity current of the Circuit Breaker can be used.
Fint
2.1.3 Limiting Circuit Over-Voltages (Metrosils)
Non-linear resistors are connected in parallel with the relay circuit to limit the peak voltage developed across the
high impedance components during internal faults to a 'safe' level below 3kV peak. Where a Metrosil is not
connected in circuit the peak voltage can be calculated from:
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:
Vrms
Irms
. 0
52
Vpeak
. 1
09
C
Where: C and
When supplied as discrete components 7XG14 Metrosils can be specified as single or three phase, with a
diameter of 75mm or 150mm and have constant 'C' values of 450, 900 or 1000.
Metrosils of diameter 75mm have a thermal rating of 8kJ. Where a higher thermal rating is required Metrosils of
150mm diameter should be used.
The optional component box includes both three phase resistors and three phase Metrosils. Metrosils in the
component box have a 'C' value of 1000.
©2013 Siemens Protection Devices Limited
R
stab
for the maximum fault clearance time. For a failed circuit breaker
Fint
4
V
(V
Where:
Fint
K
V
2
x
2
x V
Pk
V
C
.
I
For dc or instantaneous values.
1
2
For applied sinusoidal voltages
C
(
Irms
)
For applied sinusoidal currents
are Metrosil constants
Chapter 7) 7SR23 DAD Applications Guide
3
x
R
x
I
)
x
1.3
stab
Fint
I (
R
-
V
K x
Fint
stab
K
)
Chapter 7 Page 9 of 42
(4)
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