PLC switching

[WaterSparky]

Im talking about modular systems in which the I/O can be added and changed with different (cards) rather the smaller unitary type. Also me talking about a fault, I mean you may get 415V/230V/110V flowing via a cable becoming severed and joining with a control cable all kinds of faults the possibilities are endless. I agree with the volt free contacts on relay output cards being isolated, but i was just saying if a large system leading to more often than not being transitorised outputs isolation would be good practise via interposing relay or opto-isolator (more commonly Analogue I/O)

 

[joel89]

joel,
Your post is somewhat confusing.
I have to assume you mean through, rather than threw?
Why should there be sparking at a PLC IO card when switching a contactor coil?
A large transformer will have more inrush current and back EMF than a contactor coil.
The back EMF from a contactor is easily suppressible if you are using good equipment the contactor manufacturer will have a stock snubber of flywheel diode etc. that you can just fit to eliminate the back EMF through the PLC.

well i was taught to not switch big coils i.e contactors, solenoid's etc directly off the plc due to inrush and arcing, so the panels i've built i have switched the "big coils" from small relays or a seperate power supply.... i do love this forum though all people do is moan other peoples posts

 

[netblindpaul]

joel,
Don't be a prat.
I genuinely could not understand your reasoning.
Your response is one of the reasons I post less and less on here.

I won't bother trying to help any more as you obviously aren't interested.

 

[Darkwood]

I must agree with netblindpaul here... I already expressed options of suppressed low inrush contactors etc.. one of the beauties of using a PLC is it can help reduce the number of timers, relays other control devices but starts to defeats the object if you end up operating all your contactors through a slave relay when its just a matter of picking the correct gear in the first place or fitting suppressors.

I probably would throw caution to frequent switching or pulsing of solenoids and other inductive loads direct from plc as this is best done through a slave relay so as to make a programmed maintenance replacement of the slave relay simple and easy and you don't sacrifice a whole plc just because you have a damaged output that should be for-seen.

@ joel -your post was strongly worded that its more or less a no no to run contactors direct of plc outputs but this just isn't the case as ive been doing it many yrs without issue, ts just a matter of knowing what the inrush will be and what you plc is capable of.

 

[i4Automation]

I always think its better to use a relay between the PLC and the load. It's easier to change a relay if anything goes wrong. The exception to this is if I'm building several units and need to keep the price down. Then you need to look at all the details outlined here and make a case by case decision.