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Switching of large AC loads
If large AC loads are switched, the relay can in principal be operated until the
specified maximum value of switching voltage, current, or power is reached.
However, when switching AC loads, the switching voltage has a much smaller
influence on the service life of the relay contact than the switching current.
The reason for this is that the arc that occurs when the relay is switched off
usually extinguish automatically at the next zero crossing of the load current.
In applications with inductive loads, an effective protective circuit should be
provided, as otherwise a significantly reduced service life can be expected.
Switching of large DC loads
Relays can only switch off relatively small direct currents because the zero crossing
for extinguishing the arc is missing here. The maximum direct current value is
also dependent on the switching voltage as well as on design conditions such as
contact gap and contact opening speed. Corresponding current and voltage values
are documented in load limit curves.
With undamped inductive DC loads, these values are lower because the energy
stored in the inductance can ignite an arc that carries the current through the open
contacts. The resulting arc significantly reduces the service life compared to an
resistive load.
An effective contact protection circuit can increase the service life of the contacts
by 5 to 10 times compared to inductive loads that are not or unfavorably protected.
Type 1N4007 freewheeling diodes are preferably suitable for this purpose.
Switching of utilization categories according to EN 60947
When selecting the relay, the maximum breaking capacity for AC loads and
the DC breaking values taken from the load limit curves provide only rough
reference values. In practice, however, this is not sufficient because real loads
in industrial applications predominantly have inductive or capacitive load
shares. Those variables can result in very different values for the service life.
To avoid these disadvantages, the contactor standard EN 60947 divides the
loads into different use categories, such as DC-13 or AC-15. The standard
is also partly applied to relays. However, users must be aware that these
values are only partially suitable for practical use since all DC-13 and AC-15
test loads are highly inductive and operated without a protective circuit.
More precise statements on switching capacity and service life can be given
based on specific application data. The more extensive the data collection,
the more accurately the service life can be estimated for the respective
applications and, if necessary, optimisation suggestions made. For critical
applications, the users should determine the service life values themselves.
A.32737920000
Selection guide
A
Basics for relay module selection