7.4 Trip circuit supervision
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The external resistance value is calculated using Equation
7.4.2-4.
Equation 7.4.2-4
coi l
K
K
R
I
UU
R
min1
min1min
R = (104.5 – 96)/0.0048 - 242 = 1529
By selecting the next smaller standard size we get 1.5 k.
The power rating for the external resistor is calculated using
Equation 7.4.2-5. This equation includes a 100 % safety margin
to limit the maximum temperature of the resistor, because
modern resistors are extremely hot at their rated maximum
power.
Equation 7.4.2-5
P = 2*0.0061^2x1500 = 0.11 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 7.4.2-3) for this short time.
P = 121^2/1500 = 9.8 W
A 1 W resistor should be selected to withstand this short time
peak power. However, if the trip relay can be closed for longer
time than a few seconds, a 20 W resistor should be used.
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