7.4 Trip circuit supervision
7 Applications
Technical description
242
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VM259.EN007
Example of dimensioning the external resistor R
:
U
aux
= 110 Vdc –20 % + 10%
Auxiliary voltage with tolerance
U
DI
= 18 Vdc
Threshold voltage of the digital input
I
DI
= 3 mA
Typical current needed to activate the digital input
including a 1 mA safety margin.
P
coil
= 50 W
Rated power of the open coil of the circuit breaker. If
this value is not known, 0
can be used for the R
coil
.
U
min
= U
aux
– 20 % = 88 V
U
max
= U
aux
+ 10 % = 121 V
R
coil
= U
2aux
/P = 242
.
The external resistance value is calculated using Equation
7.4.2-1
Equation 7.4.2-1
DI
coil
DI
DI
I
R
I
U
U
R
min
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 7.4.2-2 and Equation 7.4.2-3. The Equation 7.4.2-2 is
for the CB open situation including a 100 % safety margin to
limit the maximum temperature of the resistor.
Equation 7.4.2-2
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 7.4.2-3) for this short time.
