7.2 Trip circuit supervision
7 Application
Technical description
172
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VM59.EN001
The external resistance value is calculated using Equation
7.2.1-1.
Equation 7.2.1-1
DI
COIL
DI
DI
MIN
I
R
I
U
U
R
R = (88 – 18 – 0.003*242)/0.003 = 23.1 k
(In practice the coil resistance has no effect.)
By selecting the next smaller standard size we get 22 k
.
The power rating for the external resistor is estimated using
Equation 7.2.1-2 and Equation 7.2.1-3. The Equation 7.2.1-2 is
for the CB open situation including a 100 % safety margin to
limit the maximum temperature of the resistor.
Equation 7.2.1-2
R
I
P
DI
2
2
P = 2*0.003^2x22000 = 0.40 W
Select the next bigger standard size, for example 0.5 W.
When the trip contacts are still closed and the CB is already
open, the resistor has to withstand much higher power
(Equation 7.2.1-3) for this short time.
Equation 7.2.1-3
R
U
P
MAX
2
P = 121^2/22000 = 0.67 W
A 0.5 W resistor will be enough for this short time peak power,
too. However, if the trip relay is closed for longer time than a
few seconds, a 1 W resistor should be used.
