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The Preventa XPSAF5130 module is designed, amongst other things, to monitor emergency stops via its two input loops S11 - S12 and S21 - S22.
In industrial applications, emergency stops may be located some distance away from the factory and the safety module which monitors them mustn't cost too much.
There are two different, yet complementary, solutions which take account of these geographical and cost constraints:
Several emergency stops can be wired in series on the same loop; this is permitted by the safety regulations but not for all the other safety devices such as sensors, etc.
The line resistance of both loops on the Preventa XPSAF5130 module must be less than 90 ohms.
To calculate the maximum length of the emergency stop monitoring loop, you need to know the linear resistance of the cable.
For a cable with maximum linear resistance of the bare copper core at 20°C of 13.3 ohms/km, the maximum length of the loop will be:
90 ohms/13.3 = 6.7 km
It is also necessary to consider the contact resistance of the emergency stop contact blocks.
The emergency stops could therefore be distributed over a distance of 5 to 6 km.