5.7.3. Earthing of Screened/Armoured Control Cables
Generally speaking, control cables must be braided screened/armoured and the screen must be
connected by means of a cable clampat both ends to the metal cabinet of the unit.
The drawing below indicates how correct earthing is carried out and what to do if in doubt.
a.
Correct earthing
Control cables and cables for serial
communication must be fitted with
cable clamps at both ends to ensure
the best possible electrical contact.
b.
Wrong
earthing
Do not use twisted cable ends (pig-
tails). They increase the screen im-
pedance at high frequencies.
c.
Protection with respect to earth
potential between PLC and VLT
If the earth potential between the
frequency converter and the PLC
(etc.) is different, electric noise may
occur that will disturb the entire sys-
tem. Solve this problem by fitting an
equalising cable, next to the control
cable. Minimum cable cross-section:
16 mm
2
.
d.
For 50/60 Hz earth loops
If very long control cables are used,
50/60 Hz earth loops may occur.
Solve this problem by connecting
one end of the screen to earth via a
100nF capacitor (keeping leads
short).
e.
Cables for
serial communication
Eliminate low-frequency noise cur-
rents between two frequency con-
verters by connecting one end of the
screen to terminal 61. This terminal
is connected to earth via an internal
RC link. Use twisted-pair cables to
reduce the differential mode inter-
ference between the conductors.
VLT
®
HVAC Drive Design Guide
5. How to Install
MG.11.B2.02 - VLT
®
is a registered Danfoss trademark
109
5
