DCS integrator has requested to know which I/Os needs isolating relays in DCS cabinets.

DCS descrete inputs/outputs are from/to
- Package panels located at same room
- ESD panels located at same room
- Switchgears located in substation room

Is there any normal practice in this regard? (Other I/Os are EEx'i'from/to field via galvanic isolator/barriers).

 

The only places you will actually need isolating relays is where the current draw of the load exceeds what a regular output can provide or you need dry contacts.

Having said that a lot of places seem to like interposing relays for some reason. There are entire plants done this way. Its also not unheard of for some plants to have all 24 VDC outputs and where a load requires some other voltage to supply an interposing relay.