Earthing Of Screened/Armoured Control Cables - Danfoss VLT MICRO Manual

Earthing of screened/armoured control cables

In general control cables must be screened/ar-
moured, and the screen must be connected to
the unit's metal cabinet with a cable clamp at
each end. The drawing below shows the
correct way to perform the earthing, and what
to do when in doubt.
Correct earthing
1.
Control cables and cables for serial
communication must be attached with cable
clamps at both ends to ensure maximum
possible electrical contact.
Incorrect earthing
2.
Do not use twisted screen ends that are plaited
together (pigtails), as these increase screen
impedance at higher frequencies.
3. Protection with respect to earth
potential between PLC and VLT
If the earth potential between the VLT adjustable
frequency drive and the PLC (etc.) is different,
electric noise may occur that will disturb the
whole system. This problem can be solved by
fitting an equalising cable, to be placed next to
the control cable. Minimum cable cross-section:
2
16 mm .
4. In the event of a 50/60 Hz earth
If very long control cables are used, 50/60 Hz
earth loops can arise, and these can interfere
with the whole system. This problem is resolved
by attaching one end of the screen to the earth
via a 100 nF capacitor (short pin length).
loop
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VLT MICRO