Danfoss VLT Series Design Manual page 121

VLT ® HVAC Drive Design Guide
5.7.3. Grounding of Shielded/Armored Control Cables
Generally speaking, control cables must be braided and shielded/armored, and the shield must be
connected by means of a cable clampat both ends to the metal cabinet of the unit.
The drawing below indicates how correct grounding is carried out and what to do if in doubt.
a.
b.
c.
d.
e.
Correct grounding
Control cables and cables for serial
communication must be fitted with
cable clamps at both ends to ensure
the best possible electrical contact.1
Wrong grounding
Do not use twisted cable ends (pig-
tails). They increase the shield impe-
dance at high frequencies.1
Protection with respect
ground potential between the
PLC and VLT
If the ground potential between the
adjustable frequency drive and the
PLC (etc.) is different, electric noise
may occur that will disturb the entire
system. Solve this problem by fitting
an equalizing cable next to the con-
trol cable. Minimum cable cross-sec-
tion: 0.025 in 2 [16 mm 2 ].1
For 50/60 Hz ground loops
If very long control cables are used,
50/60 Hz ground loops may occur.
Solve this problem by connecting
one end of the shield to ground via a
100nF capacitor (keeping
short).1
Cables for serial communica-
tion
Eliminate low-frequency noise cur-
rents between two adjustable fre-
quency drives by connecting one end
of the shield to terminal 61. This ter-
minal is grounded via an internal RC
link. Use twisted-pair cables to re-
duce the differential mode interfer-
ence between the conductors.1
MG.11.B4.22 - VLT ® is a registered Danfoss trademark.
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5. How to Install
5
121