Safety relais

If you have a safety circuit with e.g. K1&K2 controlled by the safety-relais. Do you use the NO-contacts of K1&K2 to kill the power of the control-circuits or do you use them to kill the power of the power-circuits?

In my opinion killing the power of the power-circuit should be safer but then you also need more powerfull relays. For safety-reasons this should be no issue. But I never saw this in the schematics what I could lay my hands on. Maybe it is also dependant of the safety-cat. Can somebody comment on how you do this?

I always use K1 & K2 to kill both the control power (the feed to the output cards) and the power feeding all the motor contactors. If you only kill the control power, you could have a case of a contactor welded closed and still feeding power to a motor/pump/etc. If you cut the power to the motor contactor, it wouldn't matter if it welded or not.

side note on that in some cases you need a controlled stop on certain types of equipment the machine should have a risk assessment done to describe what category you require for safety ie cat 1 or cat 3 in cat one a weld can cause a problem on the cat 3 standard you would use multiple NO contacts on each senser and pushbutton so that a single weld does not prevent a proper shutdown. I'll do a search and find the link I've posted pevious with an excelent article on the seimens website

It is application dependant so evaluate machine design etc to determine risks and select proper category but when you kill something (actuator etc.), i always kill motive power, period.

Killing power instantly may be a problem in some cases since motion need to be stopped before power is removed. There are solutions for that (in both hardwired safety and safety plcs). For example one can introduce delay (Safety!) for killing supply power but pulse trains to Drives are stopped immediately.

Safety integrated from Siemens is just Siemens name for same thing that others also have (like AB for example) and provide means to save on those big contactors (equivalent is at least partially integrated into drive design).