This might be a very very simple question, but I'm unsure how to interpret the rules.

I have a circuit which switches 230V using relays. The circuit is a motor controller for a motor control box attached to Modbus RTU over RS485. I may not be able to achieve the required creepage distance to separate the low-voltage parts of the circuit from the 230V parts in accordance with the rules for SELV ( http://en.wikipedia.org/wiki/Extra-low_voltage ) . This means the motor controller may possibly be rated only FELV.

Does this mean that anything else connected to the same MODBUS bus is now also only FELV? (Since, potentially, if the motor controller fails it could in principle put 230V out on the modbus wires)

If the whole system is to be considered FELV, would it still be okay to use CAT6 cable to implement the MODBUS bus? I believe CAT6 cable can probably withstand 230V easily, but it isn't usually rated for this I think?

  • 2
    \$\begingroup\$ Not sure about how your system is considered FELV or not, but wouldn't it be possible to isolate the RS485 with an optical isolator (or something) so the rest of the system is SELV? \$\endgroup\$
    – Dejvid_no1
    May 26, 2015 at 21:35

1 Answer 1


The first solution I would try is routing an isolation slot under your relays to boost the board creepage distance. This by itself may be enough to bring your motor controller to a SELV level of safety; however, if the problem is the relays themselves (are they too small, or have poor isolation ratings?) then using an isolator on the RS-485/ModBus connection (in between the UART and the RS-485 transceiver for traditional approaches, or integrated into the RS-485 transceiver if you can't wedge it in any other way).

If those don't apply in your case, then I would say no -- most communications-type cables are not rated for mains voltages. (They are a type CL or CM in UL parlance, vs. say NM mains wiring.) A control network like this would likely be considered a Class 2 circuit (at least over here in the US, your electrical code probably has a passage equivalent in function to NEC Article 725, just written differently), and those have strict energy limits, similar to the concept of SELV where you are, in order to maintain their Class-2-ness (which is what allows them to be wired the way they are). Lose that, and you downgrade to Class 1, which allows for higher energy, but requires mains-type wire and must be isolated from Class 2 and Class 3 circuits. This is analogous to FELV in your case -- the circuit may run at low voltages, but you can't guarantee that the voltage will always be limited in case something goes wrong.

  • \$\begingroup\$ I don't really need SELV. I'm just curious how the classifications work. Also I'm unsure if CAT6 cable is allowed for signals which could carry 230V during faults. \$\endgroup\$
    – avl_sweden
    May 27, 2015 at 15:18
  • \$\begingroup\$ Sorry for the late accept. Your response makes a lot of sense. \$\endgroup\$
    – avl_sweden
    Jun 12, 2015 at 19:27

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