Hardware manual PCD7.LRxx-PG5 room controller │ Document 27-653; version ENG07 │ 2019-03-21
Saia-Burgess Controls AG
Installation regulations and relay contacts
Appendix
A-5
A
A.3.3
Information from the relay manufacturers on dimensioning
the RC element
Contact protection networks:
The purpose of contact protection networks is to suppress switch arcs (“switching
sparks”) and achieve a longer service life for the contacts. Each protective network
has its advantages and disadvantages. See the nearby figure for arc elimination
via RC network.
When switching off circuits with inductive components (e.g. relay coils and mag
-
netic coils), the interruption in the current at the switching contacts causes an
overvoltage (self-induction voltage), which can be much higher than the operating
voltage and can pose a threat to the insulation at the circuit. The resulting sparks
quickly lead to wear on the relay contacts. For this reason, contact protection net-
works are especially important for inductive circuits. The values for the RC combi-
nation can also be determined from the nearby diagram, except that the voltage U
should be replaced with the overvoltage that occurs when the current is interrupted
(measured with an oscillograph, for example).The current must be calculated from
this voltage and the known resistance where it was measured.
Only interference-suppression capacitors that conform to VDE 0565 T1 class X2
are permitted for use in interference-suppression networks. These capacitors are
switch-proof and designed for especially high switch overvoltages. Direct operation
on the mains voltage is also possible.
The resistors used must withstand high voltages (pulse strength). Low-value resis-
tors are especially susceptible to voltage flashovers at the helical cut for manufac
-
turing reasons. Carbon composition resistors are therefore particularly commonly
used for interference-suppression networks. Glass-coated wire resistors or cement
resistors with a large helical cut pitch are also suitable.
Dimensioning aid:
The value for C is taken directly from the current to be switched. The value for the
resistance R is found by placing a straight line through the corresponding points
on the I and U curves and reading off the resistance at the point of intersection
with the R curve.
