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Protect relay contacts effectively
Selection criteria for protective circuits of inductive loads
In our selection tables we specified the maximum
recommended switching currents for inductive loads
without protective circuits. If you want to increase
the service life of the contacts, you must equip the
relay contacts with an effective protective circuit.
The protective circuit on the coil side of a relay module can,
for example, be implemented with an integrated or addition-
ally pluggable freewheeling diode. However, this only
protects the controlling periphery from the voltage peaks
that occur in the coil of the relay module. The relay contact is
usually not sufficiently protected against the voltage peaks
of the inductive load to be switched, although with optimum
dimensioning almost the same values for switching capacity
or switching cycles can be achieved as with resistive load.
The largest reduction factor for the service life of a relay
contact is the arc generated during switching off inductive
loads. It is caused during the switching process by the
energy stored in the coil and can destroy the contact
through material evaporation and material migration.
With DC voltage and standing arc, the relay can even fail
during the first switching cycle. Voltage peaks caused by
electric arcs can reach values up to several 1,000 volts.
A protective circuits must be used to suppress
the formation of electric arcs.
In the following, we will explain the correct installation
of the protective circuit and the effectiveness of the
most common types of protective circuit. There are
various ways to install an effective protective circuits. For
example, the protective circuit can be mounted either
parallel to the relay contact or parallel to the load.
However, the protective measure should always apply
directly to the source of the fault. Therefore, the protective
circuit of the load is preferable to the circuit of the contact.
Advantages of a protective circuit at the load:
• When the contact is open, the load is still
galvanically isolated from the operating voltage
• The switch-off peaks of the load cannot be coupled
into the control lines running in parallel
A.10 2737920000
Selection guide
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Selection criteria for protective circuits of inductive loads