3.3.11 Earthing of Screened/Armoured Control Cables
In general control cables must be screened/armoured, 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 communication must be at-
tached with cable clamps at both ends to ensure maximum
possible electrical contact.
2.
Incorrect earthing
Do not use twisted screen ends that are plaited together (pig-
tails), as these increase screen impedance at higher frequen-
cies.
3.
Protection with respect to earth potential between PLC
and VLT
If the earth potential between the VLT frequency converter and
the PLC (etc.) is different, electric noise may occur that will dis-
turb the whole system. 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 prob-
lem is resolved by attaching one end of the screen to the earth
via a 100 nF capacitor (short pin length).
3 Installation
VLT
®
Decentral FCD 300 Design Guide
44
MG.90.S1.02 - VLT
®
is a registered Danfoss trademark
3
