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MG.60.C4.02 – VLT is a registered Danfoss trademark
VLT
®
6000 HVAC
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Earthing of screened/armoured control
cables
Generally speaking, control cables must be
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 be done if in
doubt.
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
Do not use twisted cable ends (pigtails), since
these increase the screen impedance at high
frequencies.
Protection with respect to earth potential
between PLC and VLT
If the earth potential between the VLT frequency
converter and the PLC (etc.) is different, electric
noise may occur that will disturb the whole
system. This problem can be solved by fitting an
equalizing cable, to be placed next to the control
cable. Minimum cable cross-section: 16 mm
2
.
For 50/60 Hz earth loops
If very long control cables are used, 50/60 Hz
earth loops may occur that will disturb the whole
system.
This problem can be solved by connecting one
end of the screen to earth via a 100nF condenser
(keeping leads short).
Cables for serial communication
Low-frequency noise currents between two VLT
frequency converters can be eliminated by
connecting one end of the screen to terminal 61.
This terminal is connected to earth via an internal
RC link. It is recommended to use twisted-pair
cables to reduce the differential mode
interference between the conductors.
