Entry type: FAQ, Entry ID: 40349715, Entry date: 03/13/2015
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What safety level can be reached when combining a contactor with a circuit-breaker for fail-safe switching?

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QUESTION:

What safety level can be reached when combining a contactor with a circuit-breaker for fail-safe switching?

 

 

ANSWER:

Since circuit-breakers do not offer any diagnostic functions, i.e. mirror contacts, they cannot be used as a second functional channel in a safety function. Additionally it would be impractical to trip the circuit-breaker with every fail-safe switching, since it needs to reset manually.
However, it is possible to use a circuit-breaker as an “output of the test equipment”. In combination with a power contactor this conforms to Cat. 2 according to ISO 13849-1 und thus the safety function can reach up to PL d. According to IEC 62061 a hardware fault tolerance (HFT) of 0 can be assumed und thus the safety function can reach up to SILCL 2.
Cat. 2 is fulfilled by monitoring the power contactor by a safety evaluation unit and executing a fault reaction within sufficient time in case the power contactors fails (welding of the contacts). In case of a hardware failure, the circuit-breaker is tripped by the means of an undervoltage release.
The architecture is thus a single-channel architecture with a specified fault reaction.
In addition, the power contactor and the circuit-breaker are well-tried components in accordance with ISO 13849-2.

The diagnostic coverage (DC) can be assumed to be 90 %-99 %.

Reason
Due to the mirror contacts of the power contactor, its diagnostics capability can be assumed to be 99 %.
Now it has to be considered that this diagnostics capability alone can trigger or even prevent the fault reaction.
It is absolutely necessary that this fact be taken into account.
An additional worst case consideration should thus be made by reducing the diagnostic coverage from originally 99 % to 90 % and increasing the associated dangerous failure rate accordingly.
Table E.1 of ISO 13849-1 lists additional measures that recommend the use of a 90 % diagnostic coverage.
The failure rate of the power contactor is calculated based on the operating cycles. As an example, it is assumed that the safety function is demanded once per week (or 52 times per year). In a worst case consideration, the same demand mode is assumed for the circuit-breaker.

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