Cable section/shielding for safety application

Hello,

Does anybody know if there is a normative or good practice guidelines for calculation wire cross section of cables used for safety applications?

For example, if I'm designing a SIL3 application with a safety PLC and want to connect an emergency push button that it is 50m away, how can I decide which cable to use (cross section, with or without shielding, etc.)?

Thank you very much!

using AC or DC ?

For hardwired Emergency Stop Push Buttons...

There are no Safety Standards that state a particular type, size or distance of cable may only be permitted between the input device and the monitoring device (Safety Relay or Safety PLC), in order to meet a particular Safety Integrity Level (SIL).

The impedance of the monitoring device's inputs determines how many input devices can be connected and how far away the input devices can be mounted. For example, a safety relay may have a maximum allowable input impedance of 500 ohms. When the input impedance is greater than 500 ohms, it will not be measured as a closed circuit, so it will not reset and switch on its outputs.

So if an installer chooses the wrong cable type for a longer distance installation, and the impedance is too high, then it fails safe.

So it is up to you to ensure that the input impedance remains below the maximum specification for the Safety PLC's inputs. The length, size and type of wire used affects input impedance. Even though there is usually very little load on safety input devices, voltage drops and increased impedance over longer distances can be an issue.

As for shielding, etc.; normal wiring guidelines and good practices should be followed for routing, fixing and segregating cabling, but nothing specific to attain SIL3. The monitoring device should have a high enough percentage Diagnostic Coverage (DC) to ensure the Performance Level (PL) is maintained.

But they are destinct - The applied Safety Integrity Level (SIL) is in relation to the dangerous failure rate of a Safety-Related Control System (SRCS), as a whole. The applied Performance Level (PL) is in relation to the probability of failure of the components of a Safety-Related Control System (SRCS). Diagnostic Coverage (DC) is part of the criteria for ensuring the Performance Level (PL) is maintained. Diagnostics monitors for wiring faults on a safety input device or safety logic solver. But it does go towards the overall SIL rating.

Even if you're getting into more specialized fieldbus safety protocols, such as PROFIsafe or CIP Safety over Ethernet media, there are no special safety cable requirements to maintain SIL and/or PL. Because they carry the safety data on the same bus i.e. same cable as the standard protocols, you only have to meet the same good practice standards for Ethernet media installations. Again, proper shielding, routing, segregation.

The SRCS, as a whole, should be designed to mitigate the likely failures within the system, which may include wiring faults. This is how the SIL is acquired, more so than using special "safety rated cable". i.e. the wiring does not have to be "faultless" to achieve SIL3, but a SIL3 SRCS should be able to detect and mitigate these wiring faults.

Regards,
George

Unless you are using safe inputs that use pulsed signals to verify everything is wired correctly, you shouldn't have to worry about shielding, and even then it usually isn't an issue. There are no specific standards on wire type because insufficient wiring that compromises the circuit should be detected by the safety system anyway.