Why there is a requirement to connect interposing relay when we are using relay output type PLC?
Been there, done that, I'll never go back. Relay contacts have a finite life and when they wear out, you have to replace the entire output card.Essentially the relay protects the output. It is cheaper and easier to pull one relay module from the socket and plug in a new one than replace an entire PLC card if use or overcurrent fries the contacts.
The use of relays also eliminates issues of mixing voltages on a PLC card if several outputs share a common.
He had been running it that way since the day I left the site after commissioning it.
Suppression on output? How did the MFG know what kind of load A/C, D/C or Comms(e.g. telephone) was going to be put on the relay? I would never buy a relay card with suppression of any kind.
I'm not aware of a relay output module that does not provide surge suppression, usually with an MOV.
Totally false.
A little harsh, but true.
See pdf note off leakage spec.
Just one example.