In a 230 V AC system with an RS-485 Modbus interface, what happens if the PCB cannot maintain the creepage and clearance distances required for SELV isolation?
Specifically:
Does that mean the entire RS-485 network must now be treated as FELV (or otherwise non-SELV) because a mains fault could propagate onto the bus?
If so, what type of cable is typically used for the RS-485 connection? Shielded CAT5e/CAT6 is commonly used, but it isn’t generally certified for mains-fault conditions.
I know the preferred solution is to provide proper galvanic isolation (for example, using an isolated RS-485 transceiver or improving the PCB isolation). I’m more interested in how this situation is handled from a system-level safety and cabling perspective when full SELV isolation isn’t achievable.
Improving the PCB creepage first is definitely worth exploring. Isolation slots or a different layout are sometimes enough to achieve the required creepage without changing the overall architecture. If the limitation comes from the isolation rating of the relay or interface itself, then using an isolated RS-485 transceiver (or digital isolator plus transceiver) is generally the cleaner solution.
If the RS-485 interface is not galvanically isolated from the 230 V circuitry, then I would assume the bus can no longer be treated as a SELV circuit, since a single fault could place hazardous voltage onto the communication lines. In that case, the cabling and installation should be selected for the resulting safety classification rather than simply using standard data cable because it is commonly used for RS-485.
That’s one of the main reasons isolated RS-485 interfaces are so common in industrial equipment. They keep the field bus electrically separate from the mains-referenced circuitry and avoid turning the entire network into a non-SELV installation.
One thing worth adding to the isolation point: RS-485 is a multidrop bus, so if one node isn’t SELV and there’s no isolation between nodes, the entire bus may need to be treated as non-SELV because all devices share the same conductors.
On the cabling side, once the bus is treated as mains-referenced, standard Cat5e/Cat6 is generally not sufficient just because it’s commonly used for RS-485. The cable should be selected to meet the required insulation and installation requirements for the fault conditions and applicable safety standard.