There are 7 common uses for relays:As the title says, what's everyone's opinion on when it is worth using interface/isolation relays on PLC I/O. Non of our stuff has anything isolated unless it was actually required to drive the load.
An octal relay does take up quite a bit of space. The square style just isn’t much better. But have you looked at the slim interface relays made for the task?In my experience, using relays on discrete outputs (DO) is very common in water treatment plants for isolation. Sometimes also on discrete inputs (DI) but not as often. The consultants always specified 10 amp relays which are relatively huge and take up ALOT of panel space. I get the need for isolation due to lightening or few of mixing up voltage sources but extra panel space, increased material costs & labor costs to wire them are the downside.
Relays were rarely used in the industrial/machine control jobs I worked on due to the need for fast response time and the large number of cycles per day. Relays would quickly wear out. Neither of these is much of a problem in the water industry.
Most of the time contact failures are one of three things.We have a few Zone 2 panels that have relays in them and they are constantly burning out. I got tired of tossing them, so I tried moving the wiring down a set of contacts. On a 14 pin relay with only one set of contacts being used, it was easy to track what was going on. The odd time it has been a coil failure, but mostly just contacts. Then the odd person gets in the mix and changes the relay and doesn't return the contacts to the first set. So it is far from a perfect system, that's for sure...
Look at Omrons web site.The relays I'm referring to are:
They are controlling a 24 vdc solenoid on a slide valve on a screw compressor. They are programmed to load slowly and unload slightly faster than loading. I'll see if I can get a pic of the solenoids.
As mentioned, they're controlling a 24 vdc solenoid. They draw 1.37 amps.Look at Omrons web site.
330 VA max, silver alloy, “3 A” but it’s 8 A @ 110 VAC, only 1.5 A @ 24 VDC. Everything else is pretty vague. 330 VA gets you up to the smaller contactors and similar loads. 3 A is not much and most DC power supplies are larger than 1.5 A. It’s easy to see why someone might mistakenly think a “3 A” relay is “3 A” but unable to drive a bigger contactor or switch a decent load without burning up,
They don’t give minimums on current but the endurance chart stops at 250 mA for 24 VDC and 400 mA @ 110 VAC, and as per previous post gold is what you want with DC.
What is the load and voltage? This relay can easily be trouble with DC or AC, both high or low current.
The sawmills I work at use the IDEC version of those. They burn out every couple years, but they definitely save on the PLC relay outputs, and they are easy to pop the little relay card thing out and and pop a new one in. They had some controls that ran directly through the output cards, and more than once the output cards have had to be replaced. They seem like cheap insurance, if annoying.All our process control stuff/SCADA is 100% interface relay. We've been using a ton of these lately https://www.automationdirect.com/ad...mechanical_relays/slim_interface_relays/52003
All the inputs get optocouplers too.
The machine control side is a little more varied.
Paul, thanks for the reply. Yes, I’m familiar with those slim terminal strip style relays. We were able to convince the consultants to use them when the load was appropriate. These guys wanted 4-pole 10-amp relays on every point, PLC inputs and outputs. The specs also said all field wiring had to go to terminals, not to he relay itself. There simply wasn’t enough room to install a control panel that big.An octal relay does take up quite a bit of space. The square style just isn’t much better. But have you looked at the slim interface relays made for the task?
1/4” wide each, just slightly larger than a 5 mm terminal block and it is laid out like a triple terminal block. Lots of vendors for these. The relays themselves are just PCB relays. The downside is only 6 A contacts. But that runs most everything.
If you must have 10 A then the best you can do is miniature relays. They are similar to the slime lines but usually use 2 terminals side by side. The one below is 15.8 mm (3/8”). Not picking on Finder, just that lately Automation Direct is out of stock on almost everything.
Anytime on-off type wiring comes from or goes to locations outside the plc enclosure I use a relay. There are DIN rail mount relays that are the same width as passive DIN terminal blocks so really don’t take up additional space. You never know what the next guy’s going to do. If he’s using 120 Vac for signaling (not unusual in my experience) you can have that voltage inside the enclosure when your power is off and isolated. Using the DIN connector relays makes it finger safe.As the title says, what's everyone's opinion on when it is worth using interface/isolation relays on PLC I/O. Non of our stuff has anything isolated unless it was actually required to drive the load.