8. Other informations
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8.14 General measures when using electronic control systems
So that even complicated regulatory tasks can be presented to the user in a manner
which is clear and simple and ensures high measurement accuracy, today's elec-
tronic control systems make increasing use of microprocessors. However, the ben-
efits of these systems are countered by the disadvantage that increased measure-
ment accuracy is accompanied by sensitivity to interference. In order to minimise the
effect which interference may have on the regulator the user also must take account
of a number of points when installing a new regulator.
Assistance here is provided by standard DIN VDE 0843 on the electromagnetic com-
patibility (EMC) of measurement, control and regulatory devices in industrial process
technology. The following table shows, for example, the maximum interference lev-
els to which (according to the standard), an appliance may be exposed.
Degree of
severity
Environment
class
Test voltage
Power supply
Test voltage
Signal/control line
1
well-protected
environment
0.5 kV
0.25 kV
2
protected
environment
1.0 kV
0.5 kV
3
typical industrial
environment
2.0 kV
1.0 kV
4
industrial environment
with very high
interference level
4.0 kV
2.0 kV
As the values given in the table are maximum values, operational values should
remain well below them. However, in practice this is possible only with difficulty, as
even a normal contactor without interference suppression produces interference
pulses of up to 3.0 kV. For this reason we recommend that the following principles
be taken into account during installation:
a. Try to eliminate all sources of interference by carrying out interference suppres-
sion and minimising the interference level. Radio interference suppression is re-
quired under VDE 0875 and confirmed by VDE 0874. In principle the interference
must be eliminated at source. The nearer the interference suppresser is to the
source of interference the greater its effect.
Interference spreads through wires or by electromagnetic radiation. It is usually
the former which interferes most seriously with regulation systems.
Possible interference sources (to name a few) include:
bouncing contacts when switching loads
switching off inductive loads (contactors, motors, solenoid valves, etc.)
unsatisfactory routing of wires, too small cross-sections
loose contacts
rhythmically changing power stages (power converters)
power breakers
high-frequency generators
