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 clamp at 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.
Correct grounding
Control cables and cables for serial communi-
cation must be fitted with cable clamps at both
ends to ensure the best possible electrical
contact.
b.
Wrong grounding
Do not use twisted cable ends (pigtails). They
increase the shield impedance at high
frequencies.
c.
Protection with respect to ground potential
between PLC and Adjustable frequency drive
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 control cable.
Minimum cable cross-section: 0.025 in
2
[16 mm
2
].
d.
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 leads short).
e.
>Cables for serial communication
Eliminate low-frequency noise currents between
two adjustable frequency drives by connecting
one end of the shield to terminal 61. This
terminal is grounded via an internal RC link. Use
twisted-pair cables to reduce the differential
mode interference between the conductors.
Figure 5.39
5.8 Residual Current Device
Use RCD relays, multiple protective grounding or
grounding as extra protection, provided that local safety
regulations are complied with.
If a ground fault appears, a DC content may develop in the
faulty current.
If RCD relays are used, local regulations must be observed.
Relays must be suitable for protection of 3-phase
equipment with a bridge rectifier and for a brief discharge
on power-up see
2.11 Ground Leakage Current
for further
information.
How to Install
VLT
®
HVAC Drive Design Guide
MG11BB22 - VLT
®
is a registered Danfoss trademark
5-37
5
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