VEGA VEGAPULS 45 VBUS - Manual (Page 31)

VEGAPULS 42, 44 and 45 – VBUS 31
Electrical connection
Earthing on both ends (on the signal
conditioning instrument via the potential
separation capacitor) Earthing on the sensor end
Screening magnetic low-frequency high-frequency ground currents
fields electrical electrical and superimposed
fields fields potential currents
none – – – –
one side – ++ – –
two sides + + ++ ++
++ good protection against electromagnetic pollution
+
protection against electromagnetic pollution
– no protection against electromagnetic pollution
c
Note:
λ
(Lambda) =– – –
f
Example: Interfering frequency approx. 100 kHz
l > ––
λ
7
l < ––
λ
7
l cable length
c light velocity (300000 km/s)
f interfering frequency
λ
wavelength
This means that interference can be avoided by earthing on both ends (better on one end) in
case of an interfering frequency of 100 kHz up to a cable length of approx. 4000 m.
Screening of the sensor cable
Very often, the "electromagnetic pollution"
caused by electronic actuators, energy
cables and transmitting stations is so
considerable that the sensor cable must be
screened.
We recommend the use of screening to avoid
future interferences. 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
connect the cable shield on one earth side
(e.g. in the switching cabinet) via a capacitor
(e.g. 10 nF; 1500 V) to the earth potential.
Use a very low-resistance earth connection
(foundation, plate or mains earth).
m
–
s
1 c 1 3 • 10 9
l < – • – = – • ––––––––– = 4285 m
7 f 7 100 • 10 3
1
–
s
e.g. 10 nF,
1500 V AC
VEGAMET 515V
VEGAMET 515V