Siemens SIRIUS 3RT - Using Long Control Cables

Configuration
6.14 Using long control cables
SIRIUS - SIRIUS 3RT Contactors/Contactor assemblies
158 Manual, 09/2017, A5E03656507120A/RS-AE/006
6.14
Using long control cables
Malfunctions caused by long control cables
If long control cables are required for the control circuits of contactors or relays, malfunctions
may occur during switching under certain conditions. As a result of these malfunctions, the
contactors may no longer be able to switch on or off.
Switching on
Due to the voltage drop in long control cables, the control voltage applied to the contactor
may fall below the threshold value at which the contactor switches on. This affects both DC-
and AC-operated contactors.
The following counter-measures can be taken here:
● Changed circuit topology to allow the use of shorter control cables
● Increased conductor cross-section.
● Increased control voltage.
● Use of a contactor whose solenoid coil has a lower closing power.
Calculation of the maximum cable length:
The maximum permissible simple cable length l
zul
can be roughly calculated using the
equations given below.
Table 6- 11 Calculating the cable length
For AC voltage
For DC voltage
U
S
Rated control voltage in V
R
SL
Ohmic resistance per conductor and km of the control cable in Ω/km
U
SL
Voltage drop on the control cable in %
S
ein
, P
ein
Closing power of the contactor in VA/W
cos ϕ
ein
Power factor of the contactor coil on switch-on
Note
A
maximum cable voltage drop of u
SL
= 5 % is permitted for SIRIUS contactors.