Isolating relays on PLC output?

[brucechase]

Almost NEVER use them.

Like other, I have over 125 Ab SLC PLC systems installed here since the late 1990s with thousands of I/O. The only time I've used relays in on drives as the start signal.

All motor starters on 120VAC card (going to MCCs). All other outputs are 24VDC going to solenoid valves, lights, and alarms. I've replaced very few cards in these systems (most being input cards).

Using a relay on every output adds wiring, wasted space, additional failure points, and a substatial cost.

 

[Calistodwt]

Don't use unecessary relays on PLC outputs especially when using safety PLC's with rating of SIL 3 and better. What is the point in applying high integrity / reliabilty PLC's and negating all of that by adding a relay on the output

 

[Binaural]

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Why do I get the sneaky feeling you're a spammer?

On topic: Modern I/O modules are very well protected. Unless you need an interposing relay (contactors, VSD signals etc) then why put them in? Extra drawing, more parts to wire, more parts to fail. I'd guess that far from improving plant reliability, relays would reduce overall reliability by introducing a part nearly guaranteed to be less reliable than the original output. Sure, changing a relay is quick, but it's not much longer to swap out an I/O card if that is your criteria for most big brands (undo one screw, pull out terminal block, pull out I/O card, reverse process with new I/O card).

 

[rdrast]

On new gear, I don't use any isolation relays unless absolutely necessary (large loads, surge loads, the oddball different voltage loads, etc).

On Retrofits and upgrades, like the current one I'm working on, EVERY output has an interposing relay. In general, I try to move everything to 24VDC control. This machine is all 120VAC control. The inputs aren't bad to rewire, but changing every output device is not feasible right now, so.... interposing relays.

Like so many things in this business, it is almost entirely situational, except for large loads. I always isolate them.

For less then a dozen, or wildly mixed voltages, I'll use slim line relays like these. Where I have a lot of outputs, I prefer to use this kind.

 

[504bloke]

Same as RDRast for me

Also try to use diagnostic modules and electronic fused modules where possible as daba mentions

 

[rstech]

Everything that has been said is true and good advice, except...

I only use interposing relays when the load is too great for the relays on the output module. I like to eliminate wiring, and extra points of failure (which the extra wiring itself quite often is - as well as the extra relay).

The other situation is when number of cycles will be too great for a relay and then I use an interposing SSR connected to a transister output.

I'm sure that there are folks who have a larger installed base than I do, but I have thousands of points installed this way and I can't remember the last time I had any problem.

Ditto for me. Only use them when I absolutely have to and I've rarely had a problem in 14 years.

 

[[email protected]]

Can you use a analog output to drive a 120 vac relay?

 

[bce123]

can you use a analog output to drive a 120 vac relay?

have you been binge drinking?

 

[[email protected]]

seriously, I haven't wire up a relay, I figure you must be able to take a 4-20 ma signal a drive a 120vac relay

 

[bce123]

seriously, I haven't wire up a relay, I figure you must be able to take a 4-20 ma signal a drive a 120vac relay

can you explain why you would want to use a analog output to do this?

 

[bce123]

heres how you could do it!
http://www.automationdirect.com/adc/Shopping/Catalog/Process_Controls/Signal_Conditioners/884116

 

[Rube]

seriously, I haven't wire up a relay, I figure you must be able to take a 4-20 ma signal a drive a 120vac relay

With that logic, do you also assume that you can send an analog card 120VAC?? Here's a hint-----------Not more than once.

In all seroiusness, do you need to operate a relay and only have analog outputs available? Or what?

 

[[email protected]]

Thanks, I see,

 

[Plcwill]

I use interposing relays for loads that go above the current rating of the output point or interfacing with VFDs. I also use a interposing with manual overriding feature for troubleshooting my connected load.

 

[[email protected]]

Tell me why, use can use a 24vac to drive a relay that closes a contact for 120 vac, There is no need to send high voltage to the small switch - the whole point is to use small wires and
switches to control the relay, after all. Why do you need 120 volts just to simply close a relay?