Earthing Of Screened/Armoured Control Cables - Danfoss VLT AutomationDrive FC 300 Design Manual

FC 300 Design Guide

Earthing of Screened/Armoured Control Cables

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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 fitted with cable
clamps at both ends to ensure the best
possible electrical contact.
Wrong earthing
b.
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 VLT
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
d. For 50/60 Hz earth loops
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).
Cables for serial communication
e.
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.
108
How to Install
2 .
MG.33.B4.02 - VLT is a registered Danfoss trademark