The basics of device circuit breakers | Influencing factors in the application
currents near the nominal current . If the flowing current is detected as a fault , the shutdown occurs quickly . The cable resistance that would have acted to excessively limit the tripping current is suddenly no longer a factor ( Fig . 19 ).
In the case of electronic overcurrent protection , currents and voltages are measured and analyzed , fault currents are detected and shut down accordingly . This places the required fault current in the range of 1.2 to 2.0 times the nominal current , and not up to 15 times , as is the case with mechanical circuit breakers . This enables much more precise protection . However , in the case of long cables , the voltage drop is much more significant , since it is of little benefit that the current arrives at the load if only a slight residual voltage is still present . For that reason , the voltage drop must also be taken into consideration during configuration .
Cable calculation To ensure that the protective device shuts down safely in the event of a short circuit or overload current , it is a good idea to calculate the maximum usable cable length just to be on the safe side ( Fig . 20 and 21 ).
The following data is necessary for the calculation : R max Maximum total resistance U Nominal voltage UCB Voltage drop for device circuit breaker I CB Rated current for device circuit breaker xI Tripping factor in accordance with the current characteristic / nominal current multiplier RL max Maximum cable resistance R CB1A 1 A internal resistance for device circuit breakers L max Maximum cable length A Conductor cross section ρ Specific cable resistance Rho , ( Cu 0.01786 )
Values for sample calculation |
U |
= 24 V DC |
U CB
|
= 0.14 V DC |
xI |
= 15 |
> from the M1 characteristic curve |
|
= 2 |
> from the E characteristic curve |
I CB
|
= 1 A |
|
R CB1A = 1.1 |
> from table of |
typical internal |
resistances , |
Section 4.3 |
ρ |
= 0.01786 > copper |
A |
= 1.5 mm2 > assumed |
U 24 V
R max
= = = 1,6 Ω I CB • xI 1 A • 15
R Lmax
= R max - R CB1A
= 1,6 Ω - 1,1 Ω = 0,5 Ω
R Lmax • A 0,5 Ω • 1,5 mm 2
L max
= = = 42 m ρ
0,01786 Ω • • mm2 m
Fig . 20 : Calculation in three steps : 1 . Total resistance of the circuit 2 . Maximum cable resistance 3 . Maximum cable length
U - U CB 24 V - 0,14 V
R Lmax
= = = 11,93 Ω I CB • xI 1 A • 2
R Lmax • A 11,93 Ω • 1,5 mm 2
L max
= = = 1001 m ρ
0,01786 Ω • • mm2 m
Fig . 21 : Cable calculation for electronic circuit breakers
The value calculated is only theoretically possible . In practice , the voltage drop across the line must be taken into consideration .
20 PhoENix CoNtACt