VLT
®
6000 HVAC
56
MG.60.B1.02 - VLT is a registered Danfoss trade mark
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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 connec-
ting one end of the screen to terminal 61. This termi-
nal 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.