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Entry type: FAQ Entry ID: 109742003, Entry date: 08/21/2017
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Bypassing safety functions with a key-operated switch in the sensor or actuator circuit

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This entry provides information on bypassing a safety function with a key-operated switch

QUESTION:

Bypassing the safety function should be implemented electrically at the highest SIL/PL level required for the plant section.
For example, is it possible to bypass the contacts of a position switch directly with a key-operated switch, or does a key-operated switch have to switch a safety relay, the contacts of which are then switched in parallel?



ANSWER:

Bypassing in the sensor circuit is only possible if certain requirements are met. It must be ensured that, instead of the safety function that is to be bypassed (e.g. the position switch), another safety function can take over (difficult to implement with wiring). It is recommended (because it is easier) to always switch over the safety functions at separate safety relays and to have parallel wiring in the actuator circuit.

It is not sufficient to bypass only one safety function. Another safety function must take over instead. If, for instance, the protective door is bypassed then the machine should only start up if an enabling button is pressed.
The safety functions that mutually replace each other go to separate safety relays and switching takes place back and forth between the two. These safety relays would be switched in parallel in the actuator circuit.

Bypassing could also take place in the sensor circuit. This would however mean that no other safety function would be implemented in place of the bypassed safety function.
Example:
The contacts of a protective door are bypassed with a key-operated switch. The door can be opened and the machine continues to run unhindered and without any reduction in speed. This means that the operator is exposed to danger without any protection. This must not actually happen, otherwise the risk assessment would not have stipulated that a door is required.
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