VLT
®
FCD Series
■
Earthing of screened/armoured control cables
In general control cables must be screened/ar-
moured, and the screen must be connected to the
unit’s metal cabinet with a cable clamp at each end.
The drawing below shows the correct way to perform
the earthing, and what to do when in doubt.
1.
Correct earthing
Control cables and cables for serial communica-
tion must be attached with cable clamps at both
ends to ensure maximum possible electrical con-
tact.
2.
Incorrect earthing
Do not use twisted screen ends that are plaited
together (pigtails), as these increase screen
impedance at higher frequencies.
3.
Protection with respect to earth potential be-
tween PLC and VLT
If the earth potential between the VLT frequency
converter and the PLC (etc.) is different, electric
noise may occur that will disturb the whole sys-
tem. This problem can be solved by fitting an
equalising cable, to be placed next to the control
cable. Minimum cable cross-section: 16 mm
2
.
4.
In the event of a 50/60 Hz earth loop
If very long control cables are used, 50/60 Hz
earth loops can arise, and these can interfere
with the whole system. This problem is resolved
by attaching one end of the screen to the earth
via a 100 nF capacitor (short pin length).
MG.04.A1.02 - VLT is a registered Danfoss trade mark
36
