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Discussion Starter · #1 ·
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.
 

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Depends on the cards and a bunch of other things. (especially 120vac verses 24dc)
Generally if a wire goes from the building to the field we use surge suppressors and relays. Some of our cards (24dc) detect broken wire/ load loss so isolation would disable that feature.
 

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Discussion Starter · #4 ·
I've heard some just say to put an interface relay on all discretes, fuses and surge suppression on all analogue. Seems a little excessive to me.

I'm slowly developing my opinions on what should be in our cabinets. We pretty much only use AC input cards, relay output cards, and analogue cards. Analogue that leaves the building gets surge suppression now, hopefully someday all analogue will be fused better. Relays make sense for outputs that are more than just a electronic signal. AC inputs I'm not sure about. Most of our inputs are simple feedbacks, ie power goes out from the plc cabinet through the aux contact of a motor starter and back to the input. I suppose something could short and surge the plc, but how likely is that really. Stuff that leaves the building I could see isolating with a relay.
 

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Fuses are good as long as they are the correct size especially on dc because the power supply will go into low voltage lock down before blowing a oversize fuse. I like to see ac output's fused as it saves pulling the card to replace a fuse but im not going to bother with relays for aux contacts.
 

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Bilge Rat
motors and controls.........
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When I build cabinets, digital outputs will always have some sort of an ice-cube relay between the card and whatever it's feeding. These are individually fused. The basic idea is if something goes wrong in the field, it'll either blow a fuse or worst case, it'll blow the relay but it won't blow the PLC card. I never take DOs outside the cabinet.

Digital inputs, I fuse the hot going out to the field individually regardless of voltage. I do not fuse the signal back, it just goes a terminal block then to the PLC card.

Analog out, no fusing at all, it goes from the PLC card to the instrument.

Analog inputs are individually fused on the 24DC+ side.
 

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Discussion Starter · #7 ·
When I build cabinets, digital outputs will always have some sort of an ice-cube relay between the card and whatever it's feeding. These are individually fused. The basic idea is if something goes wrong in the field, it'll either blow a fuse or worst case, it'll blow the relay but it won't blow the PLC card. I never take DOs outside the cabinet.

Digital inputs, I fuse the hot going out to the field individually regardless of voltage. I do not fuse the signal back, it just goes a terminal block then to the PLC card.

Analog out, no fusing at all, it goes from the PLC card to the instrument.

Analog inputs are individually fused on the 24DC+ side.
So you hit on something that I am going to start working on for our digital inputs. Right now there is a couple of fused hots in the panel then these get sent out in every direction to the field equipment. I want to go through and fuse every hot that goes out to the field. Right now if you have to work on something there is no way to kill this foreign power at a specific piece of equipment without shutting down half the inputs to the plc. Flip out fuse holders would make it so much easier.

So do you use isolation relays even when you have a relay output card?
 

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Bilge Rat
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So you hit on something that I am going to start working on for our digital inputs. Right now there is a couple of fused hots in the panel then these get sent out in every direction to the field equipment. I want to go through and fuse every hot that goes out to the field. Right now if you have to work on something there is no way to kill this foreign power at a specific piece of equipment without shutting down half the inputs to the plc. Flip out fuse holders would make it so much easier.

So do you use isolation relays even when you have a relay output card?
The flip-out fuse holders are the kind use. They make it easy to kill the power to an instrument.

The relays in the output cards are easy to burn up and hard, if not impossible, to replace. The ice-cube ones will handle much more power (like a size 5 starter.......lol) and a cinch to replace.
 

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Discussion Starter · #9 ·
Very valid point. In fact we have one relay output burned out in one of our cards because the contractors who installed it didn't realize the alarm horn load was to much for it. I bought a bunch of slim interface relays, rated at like 6 amps to try out. A lot of our relay outputs are running the coil of an ice cube in another cabinet, basically being used as enable commands for chemical feeders. Most of the rest are open or close commands to valve actuators.
 

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I've found Weidmueller SAK-R terminals, or equivalent, are handy to field wiring on DI, AI, and AO devices. Usually I've done a dedicated hot to half a DI card, then a jumper to all the other hots, and knife terminals to and from the device. AI's get a fuse, knife terminal(s), and a regular terminal for the shield. AO's get 2 knife terminals and a regular terminal. Makes for the ability to easily isolate whatever device, plus troubleshoot.
DO's get relays depending on voltage and load. A regular solenoid gets a fuse and a knife terminal. Motor coils get a relay, with the same terminals.
 

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Voltage is obvious, I have a rule that all I/O will be relay rated not Triac.

If the I/O is Triac then you need relays on everything.

This is a presonal observation just had to much trouble with Triac's especailly after and during the Monsoons we get twice a year.
 

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Discussion Starter · #12 ·
OK so here is another question that's sort of an off shoot of this. Say you have a running feedback that is an input to a plc. Do you send a hot out from the plc to the aux contact, then bring your signal back to the plc card. Or do you take power from inside the mcc bucket (assume each bucket has its own control transformer) through the aux contact and on to the plc input, via a relay probably. One way you can shut the motor breaker off and everything is dead, the other way requires way less wires because you can use a common hot out to the field panel. Which one is better.
 

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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.
 

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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.
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...
 

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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...
Oh, and it’s a real joy doing what you described when there’s an irate production manager & foreman breathing down your neck asking how much longer until it’s fixed.
👌👌
 

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I worked for a brief time at a place that made precast concrete stepping stones for HD.
The boss and I had a disagreement after lunch on the first day. Screaming at me about anything is not the way to find a soft spot in my heart. I got pissed on the 3 day and walked off. I had to go to the break area to get my cooler and as I walked by the PLC while the boss is screaming at god, he turned away and I hit the reset button and every thing started booting back up. He looked at me and said what did you do? I smiled still heading for the door "that's why I deserve more money than you get" I was pretty sure there was a intermittant extra ground that was mucking up the process. I was leaving for a new job where the starting salary was 8 bucks an hour more.

One other thing about PLC panels I believe we all can agree on, excellent grounding, <5 ohms and surge protection with a front panel that has a lock.
 

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Bilge Rat
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OK so here is another question that's sort of an off shoot of this. Say you have a running feedback that is an input to a plc. Do you send a hot out from the plc to the aux contact, then bring your signal back to the plc card. Or do you take power from inside the mcc bucket (assume each bucket has its own control transformer) through the aux contact and on to the plc input, via a relay probably. One way you can shut the motor breaker off and everything is dead, the other way requires way less wires because you can use a common hot out to the field panel. Which one is better.
It depends on how the bucket is wired, if it has separable terminals then go ahead and run the PLC power to the bucket. When you pull the terminals apart, you disconnect the PLC power from the aux contact.

If there are no separable terminals, if the digital input is 24DC, obviously, you can't get shocked but grounding the 24 power might do bad things.........depending on how it's fused.

the DI is 120AC, often it's run to the aux contact and if you're not careful, you'll get bit. The proper way is to run 120 from the aux contact to the PLC cabinet and use a relay for the DI. The reason is you're not expecting more than one source of power in a starter but most PLC cabinets have multiple sources.
 

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It depends on how the bucket is wired, if it has separable terminals then go ahead and run the PLC power to the bucket. When you pull the terminals apart, you disconnect the PLC power from the aux contact.

If there are no separable terminals, if the digital input is 24DC, obviously, you can't get shocked but grounding the 24 power might do bad things.........depending on how it's fused.

the DI is 120AC, often it's run to the aux contact and if you're not careful, you'll get bit. The proper way is to run 120 from the aux contact to the PLC cabinet and use a relay for the DI. The reason is you're not expecting more than one source of power in a starter but most PLC cabinets have multiple sources.
I hate multiple sources...Not for my safety, I consider myself pretty thorough and safe in a panel, but for others who can't recognize. Safety starts with engineering, and there are usually ways to keep things safe....

Sent from my SM-S908U using Tapatalk
 
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