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Entry type: FAQ Entry ID: 25520973, Entry date: 06/06/2007
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How do you increase the useful life of relay contacts in the event of overvoltage?

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Protective circuit for the fail-safe electronic module 1F-RO DC24V/AC24..230V/5A When inductive loads (e.g. contactor coils) are switched off, overvoltage occurs, increasing the burden on the relay contacts. A protective circuit is recommended in order to counteract this reduction in the useful life of the contacts. (Refer to the technical data in the manual for the ET200S – Fail-safe modules) ...

Protective circuit for the fail-safe electronic module 1F-RO DC24V/AC24..230V/5A
When inductive loads (e.g. contactor coils) are switched off, overvoltage occurs, increasing the burden on the relay contacts. A protective circuit is recommended in order to counteract this reduction in the useful life of the contacts. (Refer to the technical data in the manual for the ET200S – Fail-safe modules)

Wiring DC-operated coils
DC-operated coils can be wired with diodes or Z-diodes as shown in the figure below. DC-operated coils can also be wired with RC elements or varistors for protection.


Fig. 1
 
Wiring configurations with diodes / Z-diodes have the following properties:
  • Cut-off overvoltages can be completely avoided. Z-diodes have a higher cut-off voltage.
  • High cut-off delay (six to nine times higher than without protective circuits). A Z-diode is better for short cut-off times (two to six times).

Wiring AC-operated coils
AC-operated coils can be wired with varistors or RC elements as shown in the figure.


Fig. 2
 
Wiring configurations with a varistor have the following properties:
  • The amplitude of the cut-off overvoltage is limited but not dampened.
  • The steepness of the overvoltage remains identical.
  • The cut-off delay is low.

Wiring configurations with RC elements have the following properties:

  • The amplitude and the steepness of the cut-off overvoltage are reduced.
  • The cut-off delay is low.

You can find tips on choosing wiring configurations in the system manual on low-voltage switchgear and controlgear. Overvoltage limiters are available as accessories for the SIRIUS system in the catalog for industrial low-voltage switchgear and controlgear.

There is also further information available about this in the Applications & Tools Entry 24500985.

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Safety Integrated

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