8.9.3 Earthing of Screened Control Cables
Correct screening
The preferred method in most cases is to secure control
and serial communication cables with screening clamps
provided at both ends to ensure best possible high
frequency cable contact.
If the earth potential between the frequency converter and
the PLC 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: 16 mm
2
.
Min. 16 mm²
PE
FC
PE
PLC
130BB922.10
PE PE
- Equalizing cable
50/60Hz 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 100nF capacitor (keeping leads short).
100nF
FC
PE
PE
PLC
130BB609.11
Avoid EMC noise on serial communication
This terminal is connected to earth via an internal RC link.
Use twisted-pair cables to reduce interference between
conductors. The recommended method is shown below:
Min. 16 mm²
PE
FC
PE
FC
130BB923.10
PE PE
- Equalizing cable
69
68
61
69
68
61
Alternatively, the connection to terminal 61 can be
omitted:
Min. 16 mm²
PE
FC
PE
FC
130BB924.10
PE PE
- Equalizing cable
69
69
68
68
8.9.4
RFI Switch
Mains supply isolated from earth
If the frequency converter is supplied from an isolated
mains source ( IT mains, floating delta and grounded delta)
or TT/TN-S mains with grounded leg, the RFI switch is
recommended to be turned off (OFF)
1)
via 14-50 RFI Filter
on the drive and 14-50 RFI Filter on the filter. For further
reference, see IEC 364-3. In case optimum EMC
performance is needed, parallel motors are connected or
the motor cable length is above 25 m, it is recommended
to set 14-50 RFI Filter to [ON].
1)
Not available for 525-600/690V frequency converters in
frame sizes D, E and F.
In OFF, the internal RFI capacities (filter capacitors)
between the chassis and the intermediate circuit are cut
off to avoid damage to the intermediate circuit and to
reduce the earth capacity currents (according to IEC
61800-3).
Please also refer to the application note VLT on IT mains,
MN.90.CX.02. It is important to use isolation monitors that
are capable for use together with power electronics (IEC
61557-8).
Electrical Installation FC 300 Design Guide
224 MG.33.BD.02 - VLT
®
is a registered Danfoss trademark
88
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