Safety PLC's

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Thread Starter

Simon Walker

I am considering changing the hard wired burner circuits on a gas fired oven for a safety PLC such as GuardPLC 2000 but yet have to do a cost analysis.

Has anybody had reasonable expierience with these PLC's? Are they cost effective and do they meet safety standards for purging and proofing. Are they worth it yet?
 
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darcy oldfield

I sat in on a CSA meeting and one point towards using the Safety PLC was that E-Stops still had to be hard wired. Also if you are in Canada you have to get a Profesional Engineer to buy off on the final machine. The other point to this is that if anyone was to change the program in the Safety PLC you would then need a Profesion Engineer to buy off on this system again since you changed the safety.

Make sure that you get people which will back the system besides Allen Bradley or the distributor. You need the thoughts from the people which might fine you if something goes wrong. I work alot with PLCs and safety circuit and I don't like these PLCs yet because I would hate to be the first guy to use them they are still to new to the North American market.
 
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Michael Griffin

Darcy Oldfield wrote:
<clip>
>I sat in on a CSA meeting and one point towards using the Safety PLC was
>that E-Stops still had to be hard wired.
>
>Also if you are in Canada you have to get a Profesional Engineer to buy
>off on the final machine. The other point to this is that if anyone was
>to change the program in the Safety PLC you would then need a Profesion
>Engineer to buy off on this system again since you changed the safety.
>
>Make sure that you get people which will back the system besides Allen
>Bradley or the distributor. You need the thoughts from the people which
>might fine you if something goes wrong. I work alot with PLCs and safety
>circuit and I don't like these PLCs yet because I would hate to be the
>first guy to use them they are still to new to the North American market.
<clip>

This is a bit off the original topic, but Jokab is coming out with a
"safety PLC" which is about the size of a safety relay (it looks similar to
their RT6). There is nothing on their web site yet, but a brochure came in
the mail which mentioned it. It has 16 I/O and 4 safety outputs.

We are interested in it because to properly implement the sort of
safety circuit we prefer in typical machines requires several conventional
safety relays. This device would combine all these functions in one package
which would be more easily wired and maintained in most of our applications
(there would be the occasional exception of course). At the moment, we are
just thinking about the idea and have not seriously pursued it yet.
However, we would only consider using it if we could get the Jokab
supplier to write a certified program for us (to our spec), and provide
these as a pre-programmed device with a part number stamped on them. We
wouldn't consider doing our own programming, or even allowing the
programming tools into our plant.

The most likely cause of failure in most PLC controlled systems is
program bugs. However, I think that reliable programs can be written which
perform specific safety functions. That would allow this sort of "nano
safety PLC" to be used for a variety of common applications which get rather
messy when you try to do it with a collection of conventional safety relays.

You would want to separate the safety system from the control system
in most applications because the safety system isn't a safety system if just
anyone can modify the program. If a safety PLC is used for normal control,
then you have lost most of the advantages (quick and easy programming) of
using a PLC if everyone is locked out of it.

You are correct about the PSR (or section 7) requirement in the
Province of Ontario. For the sake of others who are reading, this needs to
be done not just by any P Eng, but by one who is qualified for it. There are
a number of companies who do this sort of work.


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Michael Griffin
London, Ont. Canada
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