VEGA VEGAPULS 45 4 … 20 mA; HART compact sensor - Manual (Page 28)

28 VEGAPULS 42, 44 and 45 – 4 … 20 mA
Screening magnetic low-frequency high-frequency Ground currents
fields electrical electrical and superimposed
fields fields potential currents
none – – – –
one end – ++ – –
both ends + + ++ ++
++ good protection against electromagnetic pollution
+
protection against electromagnetic pollution
– no protection against electromagnetic pollution
c
Note:
λ (Lambda) =–––
f
Example: Interference frequency approx. 100 kHz
l > ––
λ
7
l < ––
λ
7
l cable length
c speed of light (300000 km/s)
f interference frequency
λ wave length
This means that with an interference frequency of 100 kHz up to a cable length of approx.
4000 m, earthing the screen on only one end provides better protection than earthing on both
ends. However, with a cable length of more than 4000 m, earthing on both ends usually pro-
duces better results.
m
–
s
1 c 1 3 • 10 9
l < – • – = – • ––––––––– = 4285 m
7 f 7 100 • 10 3
1
–
s
Wiring instructions
The signal cables should be wired close to
the ground potential. Wiring in well-grounded
metal cable channels is an effective protec-
tion against interference. Obviously, signal
cables should not be wired directly together
with high-energy cables, but should be sepa-
rated from them, e.g., with sheet metal strips
in the cable channels. Twisted cable (twisted
pair) is especially suitable for signal circuits,
as it compensates coupled voltage vectors.
The distance between the outbound and the
return conductor is just as important as the
distance to ground when it comes to interfer-
ence signal reception. For that reason, the
distance between the two conductors should
be as small as possible. Twisted pair cable
meets this requirement. These measures are
described by specialists with the expression
"wiring close to the structure". To wire the
signal cable still "closer to the structure", i.e.
even closer to the ground potential, the signal
cable can be provided with screening.
Earthing the screen only on one end is not
always effective, see the following table. The
table gives a simple overview of the effec-
tiveness of the different protective measures
against electromagnetic pollution. In practice,
earthing of both sides is often useful. This
allows the option of earthing only one end
later by simply disconnecting the other earth
contact points of the screen. Why? Actual
interference only appears in the operating
environment. Earthing only one end of the
screen prevents interference, e.g. in the
100 kHz range, better than earthing both
ends, especially if there is insufficient poten-
tial equalisation. However, you must make
sure that no ground equalisation currents
flow through the cable screening. Ground
equalisation currents can be avoided by
ground potential equalisation systems. When
earthing on both ends, it is possible to con-
nect the cable screen on one earth side (e.g.
in the switching cabinet) via a Y
C
capacitor 1) to
the earth potential. Use a very low-resistance
earth connection (foundation, plate or mains
earth).
Electrical connection
1) max. 10 nF, e.g. voltage resistance 1500 V,
ceramic