Danfoss FC 300 Design Manual page 98

FC 300 Design Guide
Earthing of Screened/Armoured Control Ca-
bles
Generally speaking, control cables must be braided screened/armoured and the screen must be connec-
ted 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 com-
munication must be fitted 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 po-
tential between PLC and VLT
If the earth potential between the fre-
quency 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
d. For 50/60 Hz earth loops
If very long control cables are used, 50/60
Hz earth loops may occur. Solve this prob-
lem 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 ter-
minal 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.
98
How to Install
2 .
MG.33.B7.02 - VLT is a registered Danfoss trademark