Specifications

6.8.3 Grounding of Screened Control
Cables
Correct screening
The preferred method in most cases is to secure control
and cables with screening clamps provided at both ends
to ensure best possible high frequency cable contact.
If the ground potential between the frequency converter
and the PLC is different, electric noise may occur that
disturbs the entire system. Solve this problem by fitting an
equalising cable next to the control cable. Minimum cable
cross section: 16 mm
2
.
1
2
PE
FC
PE
PLC
130BB922.12
PE PE
<10 mm
Illustration 6.65 Control Cable with Equalising Cable
1
Min. 16 mm
2
2 Equalizing cable
Table 6.40 Legend to Illustration 6.65
50/60 Hz ground loops
With very long control cables, ground loops may occur. To
eliminate ground loops, connect one end of the screen-to-
ground with a 100 nF capacitor (keeping leads short).
100nF
FC
PE
PE
PLC
<10 mm
130BB609.12
Illustration 6.66 Screen-to-ground Connected to a 100 nF
Capacitor
Avoid EMC noise on serial communication
This terminal is connected to ground via an internal RC
link. Use twisted-pair cables to reduce interference
between conductors.
PE
FC
PE
FC
130BB923.12
PE PE
69
68
61
69
68
61
1
2
<10 mm
Illustration 6.67 Twisted-pair Cables
1
Min. 16 mm
2
2 Equalizing cable
Table 6.41 Legend to Illustration 6.67
Alternatively, the connection to terminal 61 can be
omitted:
PE
FC
PE
FC
130BB924.12
PE PE
69
69
68
68
1
2
<10 mm
Illustration 6.68 Terminal 61 not Connected
1
Min. 16 mm
2
2 Equalizing cable
Table 6.42 Legend to Illustration 6.68
6.8.4
RFI Switch
Mains supply isolated from ground
If the frequency converter is supplied from an isolated
mains source ( IT mains, floating delta) or TT/TN-S mains
with grounded leg (grounded delta), turn off the RFI
switch via 14-50 RFI Filter.
In OFF, the internal capacitors between the chassis
(ground), the input RFI filter and the intermediate circuit
are cut off. As the RFI switch is turned off, the frequency
converter is not be able to meet optimum EMC
performance.
By opening the RFI filter switch, the ground leakage
currents are also reduced, but not the high-frequency
leakage currents caused by the switching of the inverter. It
is important to use isolation monitors that are capable for
use with power electronics (IEC61557-8). E.g. Deif type SIM-
Q, Bender type IRDH 275/375 or similar.
Also refer to the application note VLT on IT mains.
NOTICE
If the RFI switch is not turned off, and the frequency
converter is running on isolated grids, ground faults can
potentially lead to charge-up of the intermediate circuit
and cause DC capacitor damage or result in reduced
product life.
Electrical Installation Design Guide
MG11BC02 Danfoss A/S © Rev. 06/2014 All rights reserved. 119
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