Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 159)

5.3.4 Effect of long control lines
5.3.4.1 Voltage drop
In accordance with IEC 60947-4-1 and IEC 60947-5-1, the normal control voltage range of
power and control contactors lies between 85 … 110 % of the rated control voltage. Within
these limits contactors pull-in perfectly. Frequently contactors are offered with an extended
control voltage range, thus for example with contactors with electronic coil control. The technical
documentation of the devices used is definitive.
At small control voltages and with long control lines, the voltage drop across the lines to the
contactor (both out-going and return conductors should be considered!) can be so big that
pulling-in reliably is no longer guaranteed. In addition to burnt coils, another consequence of this
may be welding of the main contacts. It must therefore be ensured that taking into account
ƒ Supply voltage variations
ƒ Voltage drop at the control transformer at peak load (see Section
5.2.1.1 ) and
ƒ Voltage drop across the control lines
the minimum pull-in voltage is always guaranteed.
For the voltage drop across the control lines the following applies approximately:
A U
l S
u
C
R ⋅ ⋅
⋅ ⋅
=
κ
2
100
[%]
or for the maximum line length at a given permissible voltage drop
A
S
U u
l C R
⋅ ⋅ ⋅ ≈ κ
2
max 100
l Line length (supply and return line) [m]
l
max
maximum line length (feeding and return line) [m]
u
R
Percentage voltage drop [%]
U
C
Rated control voltage [V]
S Pickup power of the contactor [VA]
κ
Conductivity of the conductor material [m·
Ω -1
·mm
-1
] = 57 for copper
A Conductor cross section [mm
2
]
1
10
100
1000
10000
10 100 1000 S [VA]
l [m] 24V/0.75 mm2
24V/1 mm2
24V/1.5 mm2
24V/2.5 mm2
110V/0.75 mm2
110V/1 mm2
110V/1.5 mm2
110V/2.5 mm2
230V/0.75 mm2
230V/1 mm2
230V/1.5 mm2
230V/2.5 mm2
Fig. 5.3-5
Line lengths for a voltage drop of 5 % and copper conductors
l
Line length (feeding and return line)
S Apparent power of load
LVSAM-WP001A-EN-P - April 2009 5-7