Hardware manual PCD1.M2220-C15 │ Document 27-640 – Version ENG03 │ 2020-03-25
Saia-Burgess Controls AG
Installation regulations and relay contacts
Appendix
A-5
A
A.3.3
Switching inductive loads
Due to the physical properties of the inductance, noiseless shutdown of the induct-
ance is not possible. These interferences must be minimised as much as possible.
Regardless of whether the Saia PCD
®
is immune to these interferences, other
devices may still be impaired.
It should also be noted that, as part of EU standard harmonisation, the EMC
standards have been valid since 1996 (EMC Directive 89/336/EC). For this rea-
son, two principles can be observed:
●
THE SUPPRESSION OF INDUCTIVE LOADS IS ABSOLUTELY NECESSARY!
●
INTERFERENCE SHOULD BE ELIMINATED AT THE INTERFERENCE
SOURCE IF POSSIBLE!
The relay contacts on the existing module are wired. It is nevertheless recom-
mended to install a suppressor on the load.
(Often available as standard components for standardised gates and valves).
When switching DC voltage, it is highly recommended to install a freewheeling
diode over the load. This is also the case even if theoretically a resistive load is
switched. There is always an inductive share in practice (connection cable,
resistor coil, etc.). Please note that the shutdown time is extended here.
(Ta approx. L/RL * √ (RL * IL/0.7).
The transistor output modules are recommended for DC voltage.
A.3.4
Specifications of the relay manufacturer
about the dimensioning of the RC elements
Contact protection circuits:
The purpose of the contact protection circuits is the suppression of the switching
arcs (“switching sparks”) and thus achieving a longer service life of the contact
pieces. Each protection circuit may exhibit advantages and disadvantages.
For arc quenching using RC elements, see the figure opposite.
When shutting down load circuits with inductive components (e.g. relay coils and
magnet windings), an overvoltage (self-induction voltage) can arise which may
be many times higher than the operating voltage and jeopardise the isolation on
the load circuit due to the current interruption at the switching contacts. The initial
sparks that arise from this lead to rapid wear on the relay contacts. For this rea-
son, the contact protection circuit is particularly important for inductive load cir-
cuits. The values for the RC combination can also be determined from the diagram
opposite, however the overvoltage (e.g. to be measured with an oscilloscope)
arising from the current interruption for the voltage U is to be
applied. The current must be calculated based on this voltage and the known re-
sistance at which this was measured.
Only anti-interference capacitors in accordance with VDE 0565 T1 class X2 may
be used in suppressors. These capacitors have a high switching capability and
are designed for particularly high switching overvoltages. Direct operation on the
mains voltage is also possible.
