Danfoss FC 300 Design Manual page 192

8 Electrical Installation
8.9.3 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 clamp at 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 fit-
ted with cable clamps at both ends to ensure the best possible
electrical contact.
b. Wrong earthing
Do not use twisted cable ends (pigtails). They increase the
screen impedance at high frequencies.
c. Protection with respect to earth potential between PLC
and
If the earth potential between the frequency converter and the
PLC (etc.) is different, electric noise may occur that will disturb
the entire system. 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
8
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 currents between two frequency
converters 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 interference
between the conductors.
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)
is needed, parallel motors are connected or the motor cable length is above 25 m, it is recommended to set par. 14-50
1) Not available for 525-600/690 V 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
power electronics (IEC 61557-8).
192
1) via par. 14-50 RFI Filter
VLT on IT mains, MN.90.CX.02.
It is important to use isolation monitors that are capable for use together with
MG.33.BC.02 - VLT
. For further reference, see IEC 364-3. In case optimum EMC performance
® is a registered Danfoss trademark
FC 300 Design Guide
RFI Filter to [ON].