17 years ago
cannot find a definative answer. I was trained that an E-Stop circuit on
a machine should always be hard-wired with redundant failsafes (i.e.
safety relays, etc) and this makes perfect sense, as you do not want the
E-stop circuit to fail, if possible.
Lately though, we have been seeing machines where all the E-Stop circuits
are ran through the PLC. There is *no* physical hard-wired E-Stop
circuit, even the E-stop button is just wired to inputs on the PLC.
IMHO, this is bad practice. I have already seen an instance where this
design failed in practical application.
In these designs, the PLC is responsible for shutting down everything
else, which works fine, as long as the PLC is actually RUNNING. When the
PLC 'locks up', or has other glitches (RAM problems, etc) this could lead
to bad things.
My question is: Is it mandated anywhere (OSHA, etc) that an E-stop
circuit should be hard-wired?
I was hoping that someone here could provide a link to information that
specifically addresses this question.
Thanks in advance.