Interposing relays

[Phil Buchanan]

PBuchanan
Many companies deserve what they get. If you fire those guys above what are you going to find to replace them with for 10 bucks an hour.
A molding plant owner I know pays 700 bucks to get his new Porsche serviced but thinks 14 bucks too high for maintenance guys. In
large plants I see one skilled guy per shift and the rest "helpers". The plant manager thinks that is adequate.
So don't blame Bubba and Cletus all the time, maybe they were thrown into something.

Gas

Jsu0234m did not say they were ignorant or untrained he said they were lazy and if you take a look at any dictionary you will see those are very different terms.

Ignorance can be corrected and training can be provided but when you have people who are trained and have experience and know better but they are just too lazy yo do the job correctly or too lazy to walk back to the store room and get the correct size fuse and risk damaging sometimes multi million dollar equipment or run the risk of causing an accident then those people need to be fired period.

Any company that puts unskilled and unqualified people in their controls systems deserves what they get but Jsu0234m posted and it's sounded like it could not be corrected and it can be.

Myself I would not try to change a control system to compensate for people who don't know what they are doing because the company owner or manager want to be cheap. I will let him get what he deserves and lay in the bed he made.

You can find qualified people who are not lazy and take pride in their job I know because I have 42 of them but if you accept low grade people and low grade performance that's what you will always be stuck with.

 

[redbarron55]

Years ago we used interposing relays ans interlocking because we did not trust the PLC, but now that is not necessary.
If you are using DC outputs then the diode can be used if AC then a suppressor.
Fused indicating terminal strips are nice to show where an output is bad (when energized)
More parts increase the likelihood of failure by that much.
If the load is too great for the rated output then you need some interposing device, other wise probably not.
If you are having problems with solenoid could then the problem could be dirty solenoids.
If the solenoid does not completely seat when energized then the magnetic field is not closed and the current stays higher than otherwise increasing the load on the system.
Many times the maintenance personnel swap the coils when they burn out and never investigate the why did it burnout in the first place.
If you must use relays then I would offer the Automation direct CUBE RELAY 15A SPDT 12VAC COIL LED INDICATOR TEST PUSHBUTTON.
This relay has an LED to indicate coil power and a mechanical flag that shows that it is actually pulled in and a push button and mechanical latch too hold it on.
It is not as well sealed if used in a dusty location. however.
All in all unneeded relays are just another part waiting to fail. IMHO.

 

[FactoryTalktotheHand]

I personally think any kind of PLC output should have individual protection. Any solid-state output's load should always use an interposing relay with surge suppression. The main reason for this is twofold. The first is that when an output shuts off, there is still power in the coil of the load. That power needs to be dissipated somewhere, and if the output is not protected, that power will feed back onto the delicate triacs/diodes of a solid state output. Adding a surge suppressor absorbs that power so that it minimizes damage to the output channel during field collapse. The second reason is that although a solenoid may have an amp draw that is less than what the triac/diode can handle, it also has a lot of inrush, much more so than an interposing relay. The inrush of a solenoid won't blow a properly-sized and classed fuse, but it will still over time damage a solid state output.

Personally, interposing relays just make sense. You limit your output card's exposure to the outside, so you don't have things like moisture, dust, heat, vibration, or other environmental factors exerting themselves on wires directly connected to the PLC. Also, it is a lot easier to change a plug-in relay than it is an output card.

 

[dogleg43]

I believe the answer of whether to use relay's or not on PLC output modules is.....it depends.

On almost every factory automation project I've ever done for the automotive industry relays were not used. The load was wired directly to the output module unless it was a large load like a motor starter or heater. They required us to pick out the appropriately rated output module.

On factory automation with the rapid cycle time the relays either wear out or the response time is too slow. Either way they are just a point of failure.

On the municipal fresh water and waste water projects relays on everything seems to be the norm. I believe this is due to the need for isolation for various voltage loads and also lightening protection (although that doesn't insure protection from lightening).

On my latest WW project we're required to use relay output modules and wire each point to an interposing relay. There aren't many cycle time issues when it comes to turd-work so there isn't much danger of relays wearing out from too many cycles.

The spec's even called for every PLC input to have a relay in front of it with separate fusing for each field device. The relay contacts were to be wired to isolated PLC input cards. The got rid of the relays and we're wiring directly to the isolated input modules.

Chrysler used to require every PLC output be fused but I haven't done much work for them in a long time.