Use of interposing relay

Sounds like a job specification, and the logic is that if work needs to be done on the connected device (downstream of the relay) it is easy field work to pull the interposing relay rather than having to force the PLC output to zero. Also makes testing simpler in many cases.
 
Maybe the current rating of the plc relay is not enough?
Valve solenoids and other devices sometimes have pretty high initial current draw.
 
Why there is a requirement to connect interposing relay when we are using relay output type PLC?

There isn't a requirement as such.

We only use interposing relays on transistor outputs, on relay outputs (which we use very little of) we use direct to end device but only when the current rating of the end device is < that of the PLC output relay
 
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.
 
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.
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.

I built a sortation system that used relay outputs to drive pilot lights, and as part of the design, they wanted a manual override for set-up and maintenance. The sortation loops were complex so I made the lights flash when someone tried to override the PLC control and anything manually, because they would lose safety interlocking etc. So by flashing the lights, I was trying to make it "uncomfortable" for an operator who thought they knew better than the program I had worked out with the owner as to how the system needed to function.

6 months later I get a call from the owner that the PLC had failed. Long troubleshooting story short, most of the relay contacts had welded on the output cards. I replaced them and the operator came back in, put on his sunglasses and started up the process in manual with all 48 red pilot lights flashing on and off at him! He had been running it that way since the day I left the site after commissioning it.
 
I only started using external relays on most PLC relay output cards only after taking a new job where most of our work was in the fresh & waste water treatment world.

Prior to that most of my work was in the factory automation world and we used AC or DC solid state output cards due to the # of cycles & the faster response times of these modules. Of course, interposing relays would be used for high current loads or mixed voltage loads.

It’s also important to note that some relay output cards have built in suppression across the contacts and measuring them with a meter will not be 0 ohms. This caused us problems on a factory test once when the result was expected to be 0 and we had to fail each one. This was in the pharmaceutical world where.they were very anal about the test results. You just couldn’t overlook it. What a PIA!!
 
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.
 
The terminal block relays by wiedmuller and others are pretty inexpensive now ($5 a pop or so?) so I always use AC/DC outputs and the terminal block relays. As mentioned, much easier to replace on the end-user, lasts longer, and the cost difference isn't that great anymore.
 
These are/were either ControlLogix or Micrologix relay output modules. It was about 6 years ago. It was show on their witing diagram.
 
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.
 
I'm not aware of a relay output module that does not provide surge suppression, usually with an MOV.

The signed, sealed, and approved testing document said there should 0V when relay output was off but due to the leakage thru the MOV or whatever, there was ghost voltage present (using a Fluke 87).

This was a big pharma application so all of the 0V tests were going to fail. I used a Wiggy instead and the ghost voltage magically disappeared! Luckily the test document didn’t specify what type of voltmeter had to be used.

Changing the test procedures would have meant about a two-three week delay.
 
A little harsh, but true.

See pdf note off leakage spec.
Just one example.

I have to use Honeywell HC900 PLCs on occasion. I do 24VDC systems.
The HC 900 is HORRIBLE with leakage on output AND input modules, even the "relay out module".
I have to use relays on inputs (prox switches) and outputs (LED lights, coils okay). Leakage current is obvious on Outputs to LED lights, they are dim when off.
 

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