6.10.1 Earthing of Screened/Armoured
Control Cables
Generally speaking, control cables must be braided
screened/armoured and the screen must be connected
with a cable clamp at both ends to the metal cabinet of
the unit.
Illustration 6.11
indicates how correct earthing is carried
out and what to do if in doubt.
a.
Correct earthing
Control cables and cables for serial communi-
cation 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 potential
between PLC and frequency converter
If the earth potential between the frequency
converter and the PLC (etc.) is different, electric
noise may occur that disturbs the entire system.
Solve this problem by fitting an equalising cable,
next to the control cable. Minimum cable cross-
section: 16 mm
2
.
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
100 nF 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 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.
PE
FC
PE
130BA051.11
PE
PE
FC
PE
PE
FC
100nF
PE
FC
69
FC
PE
PE
PE
68
61
68
69
FC
PE
a
b
c
d
e
PLC etc.
PLC etc.
PLC etc.
PLC etc.
Equalizing cable
Min. 16mm
2
Illustration 6.11
6.11 Residual Current Device
Use RCD relays, multiple protective earthing or earthing as
extra protection, if local safety regulations are complied
with.
If an earth fault appears, a DC content may develop in the
faulty current.
If RCD relays are used, local regulations must be observed.
Relays must be suitable for protection of 3-phase
equipment with a bridge rectifier and for a brief discharge
on power-up see section
3.4 Earth Leakage Current
for
further information.
How to Install
VLT
®
Micro Drive FC 51 Design Guide
MG02K202 - VLT
®
is a registered Danfoss trademark
55
6
6
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