Thank you Gerry M!
I can't believe I'm reading this, one-size-fits-all does NOT work!!! This is why everyone
has more than one kind of output card for their PLCs. Your contractor doesn't seam to
understand the difference between different types of outputs (*)and only considers max load
and max number or points.
Interposing relays have their function. using them for few selected loads is part of the job
(it's part of design process and involves reading specs and sizing...).
but putting blindly one relay on every single output is rather extreme. I call this
waste of resources and it's not just overkill, it could be very poor design too...
1) Easier to replace one failed relay rather than PLC output card
I don't think so - it's not easier! It may be cheaper but only if:
a) relays ware already installed
b) failure is on relay or relay base (and not on the card for example)
c) you can identify failure fast enough (production downtime is ticking bomb costing much more than new card)
(sorry for splitting hairs)
2) Less stress on the PLC output card
If card is designed to handle the load, WHO CARES? There is no guarantee that relay will not be
overloaded either, shall we all start using PLC card+interposing relay+interposing contactor+bunch of
paralleled contactors - on each point? Maybe everyone should be afraid to sit on the chair or use stars without fall protection gear?
3) Can divert the output to a different relay in case of failure on no IO re-mapping.
someone still lives in the past and prefers costly wiring change versus simple program change?
sounds like someone who doesn't get along with technology, maybe a brainstorming session on why the
PLCs ware invented in the first place? (Hint: it's not that there was shortage of relays...)
4) Different voltages possible on outputs by using different relays and supplies
nothing that different output cards would not handle, in fact there are relay output cards,
some have each output isolated and can handle bigger loads.
5) Can use high density cards (more outputs because can use lower voltage versions)
So... you cannot use high density cards unless you add bunch of relays? How is that?
i've been and still use number of different platforms and some of the output cards allow as little as 100mA
per point (plus some additional limitations such as max group current, max % of energized outputs etc.)
none has failed yet... (i did have few cards with dead output but so far every single one was
result of poor quality control - bad out of box and exchanged since new and under warranty)
Btw. there are also high density output cards with protection so you don't have to sweat
for each short made by contractors...
Sure there are heavier loads one can encounter (some valve types take more than 2A etc.)
This is what designer should look into and decide what kind of output is needed, and maybe
add relay or contactor - when needed.
I agree on disadvantages:
1) More wiring - more possible points of failure
2) Higher initial cost
3) Take lot's of space (even the TB style)
4) Relays are slow and don't produce clean signal, they have a contact bounce - see (*) and makes them useless
in so many applications (anything high speed, packaging, triggering cameras etc.)
Another note: The only saving on rack based system when using high density versus regular cards is the
rack space (used number of slots), overal panel space saving is usually only marginal (you still need
to put those TBs somewhere), there is no money or time saving (TBs and cables also cost, labor for
32 point is still same regardless if they are on one or two cards etc.). This is one of the reasons
other I/O systems have emerged.