Hi everyone, old bagging machine, 1998, SLC 5/03 still running it and running fine. The E-stop is one NC contact on the mushroom wired through a plain 24VDC ice cube relay, K5 on the drawing, and K5 drops the main contactor. A parallel wire off the same contact goes to I:1/0 so the program knows.
The new safety coordinator has asked me to prove it meets the risk assessment, which says PLr d for the E-stop. I can prove it works, I pressed it. I can't prove what PL it is.
I pulled the original drawings, they still match the panel, and checked the button and K5 with a meter.
What does one NC contact through one relay actually get you, category and PL wise? I'd rather not rewire the whole panel if a part swap does it, any pointers?
Does anything monitor K5? Anything at all that checks it actually dropped when the button was pressed. And is it one contactor or two in series? In 1998 that relay was the whole safety circuit on half the machines built, I think I remember why, it was normal and nobody asked. That doesn't make it Category 3.
Nothing monitors K5. One contactor. If K5 welded, the first anyone would know is when somebody pressed the button and nothing happened.
One question first: was the PLr d from a fresh risk assessment of this machine, or copied from a template? It changes what you build.
Assuming it stands. Single channel, no monitoring, no diagnostics: that is Category B or Category 1 at best, if K5 and the contactor count as well-tried components. Category 1 caps you at PL c no matter how good the parts are. A bigger relay is still one relay. A second button is still one channel.
PL d needs a two channel monitored architecture, Category 3: both contact blocks on the mushroom into a safety relay, monitored reset, relay outputs through two contactors in series or one with a mirror contact in the feedback loop. Someone did a decent article on the category and PL side: https://plctr.com/introduction-to-functional-safety-in-plc-programming/
Fresh assessment, she did it herself last month. So it's the whole thing. The mushroom already has a second NC block on it, never wired. Contactor is a 100-C with a built in NC aux, is that a mirror contact or do I need a different one?
I would not assume the integral aux on a standard contactor is a mirror contact. If it is, the label carries the symbol, so look before you count on it. Two in series is what most people build for PL d anyway, and either way let your coordinator run the real part numbers through SISTEMA before you buy anything.
Rewired it over the weekend. 440R dual channel relay, both NC blocks on the mushroom, monitored reset, relay outputs through two 100-S contactors in series with their mirror contacts in the feedback loop. I:1/0 now gets a relay aux instead of the button contact, the SLC program didn't change. Coordinator ran SISTEMA, PL d, signed it.
The answer to my question was Category 1 at best and PL c at most, because nothing anywhere checked that K5 had actually dropped. The assessment wanted d, and d wanted the second channel and the monitoring. A bigger relay would have got me nowhere.
K5 was original from 1998 and never welded once. The new stuff had better last as long. Thanks pnina and louk.
A 1998 relay that never welded is luck, not a category.