Apprentice, on placement at a small plant. We have a test rig built by a local panel shop last year, a pump and a small VFD. The E-stop is a red 22 mm pushbutton, flat, an 800FP I think, one NC block into a 440R safety relay. Press it, the relay drops out, the pump stops.
An auditor came through last week and flagged it as "not an emergency stop device". My supervisor says add a second NC block and it'll be fine.
I checked the button ten times with the relay, it drops every time, and looked up ISO 13850 on the college portal, where I only got the abstract.
It's NC, so if a wire breaks it fails safe. Photo of the button attached, but what's the difference between that and a "real" E-stop, or am I missing something obvious?
One question off your photo. When you hit that button, does it latch and stay in until you twist it out, or does it spring back the moment you let go?
Springs back. The relay latches out though, so a press stops it and it stays stopped until someone presses reset. Head is flat, red, no yellow ring behind it.
Three things wrong and NC isn't one of them.
The block. A standard NC opens because a spring pushes the contacts apart. If they weld or the spring breaks, you press and nothing opens and nothing tells you. An E-stop block is positive opening, the plunger forces the contacts apart, there's a symbol on the block for it.
The latch. An E-stop stays in mechanically until you twist it out. Yours relies on the relay to remember. If the relay's latch ever fails on, the button springing back means nothing.
The head. Mushroom, red, yellow background, because a panicking person hits it with a palm. Flat 22 mm is a fingertip button.
An 800FM mushroom with positive opening NC blocks. Two standard blocks are still two standard blocks.
So the relay latching isn't the same as the button latching, because they fail differently? And positive opening means forced apart, standard means spring. Not sure I've got that right, but if I do, adding a second block like my supervisor said doesn't fix any of the three?
you've got it. Second block on a fingertip button is two of the wrong thing.
Replaced it this morning with an 800FM mushroom E-stop operator, same 22 mm cutout, twist to release, on a yellow legend plate, with a two NC positive opening block wired to both channels of the 440R. Tested, drops the relay, stays latched until twisted, symbol on the block. Auditor signed it off on his return visit.
So the panel shop fitted a standard momentary pushbutton where an E-stop device was needed, and a second block was never going to fix that. Proper operator, positive opening block, done.
The panel shop told my supervisor they've built forty of these rigs. Thanks edj and pat_k.
Forty rigs with a red stop button. Somebody should tell them.