Switching Inductive Loads; Relay Manufacturer's Information On Rc Unit Dimensioning - SBC PCD1 Series Manual

Saia-Burgess Controls AG
A.4.3 Switching inductive loads
Because of the physical properties of inductive loads, it is not possible to disconnect
inductance without interference. This interference must be minimized as far as pos-
sible. Although the Saia PCD
which may be susceptible.
It should be noted here that, as part of the harmonization of standards throughout the
EU, EMC standards have applied since 1996 (EMC Directive 89 / 3 36 / E G). Two firm
principles can therefore be stated:
● PROTECTION AGAINST INTERFERENCE FROM INDUCTIVE LOADS IS
MANDATORY
● INTERFERENCE SHOULD BE ELIMINATED AS CLOSE AS POSSIBLE TO ITS
SOURCE
Relay contacts on the present module have been wired. However, it is still recom-
mended that a protection circuit should be fitted at the load.
(Often available as normal components on standardized contactors and valves).
When switching direct voltage it is urgently recommended that a recovery diode is fit-
ted above the load. This should even take place when, theoretically, an Ohmic load is
switched. In practice, there will always be a proportion which is inductive (connection
cable, resistance coil, etc.). In this case it should be noted that the switch-off time will
be longer.
(Approximate Ta . L / R L * √ (RL * IL / 0 ,7).
For direct voltage, transistor output modules are recommended.
A.4.4 Relay manufacturer's information on RC unit dimensioning
Wiring contact protection:
The purpose of contact protection wiring is to suppress switch arcing ("sparks") and
thereby prolong the lifetime of the contacts. All protection wiring has disadvantages
as well as advantages. For the cancellation of arcing by means of an RC unit, see
adjacent diagram.
When switching off load circuits with inductive components (e.g. relay coils and
magnet coils) the interruption of current results in overvoltage (standard inductance)
at the switching contacts. This may amount to many times the operating voltage and
so threaten the insulation of the load circuit. The resultant breaking spark leads to
rapid wear of the relay contacts. For this reason contact protection wiring is particu-
larly important with inductive load circuits. Values for the RC combination can also
be determined from the adjacent diagram. However, for voltage U it is necessary to
use the overvoltage arising from the interruption of current (e.g. measurable with an
oscillograph). Current must be calculated from this voltage and the known resistance,
against which the voltage was measured.
Screening units should only use anti-interference capacitors that comply with VDE
0565 T1 class X2. These capacitors are switchproof and designed for particularly
high switching surges. They can also run directly on mains voltage.
Manual Manual PCD 1 / PCD 2 Series │ D ocument 26 / 737 EN22 │ 2 013-11-26
Installation direction and relays contact protection
is immune to this interference, there are other devices
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Appendix
A
A-8