For circuits of associated apparatus which are connected to safety extra low-voltage circuits
(SELV) or protective extra low-voltage circuits (PELV) circuits, Um shall only be applied as a
‘common mode’ voltage, with the nominal operating voltage applied for the differential mode
signal between the circuit conductors. (Typical examples are RS-232, RS-485 or 4-20 mA
circuits). The certificate number for associated apparatus relying on SELV or PELV circuits
shall include the "X" suffix in accordance with the marking requirements of IEC 60079-0 and
the specific conditions of use listed on the certificate shall detail the precautions necessary.