PLC's and Safety Systems

[bpic2881]

I am currently studying for my bachelours degree and I am compelting a unit of PLC's. I have an assigment around Siemens PLC's with also some knwoledge questions, one which I am stuck with. could anyone help as I can find any straight answers on the internet. the question is,

[FONT=&quot]Research and explain why an Emergency Stop function cannot be connected to an ordinary PLC system. Discuss options for implementing an emergency[/FONT][FONT=&quot] system.[/FONT]

I understand the whole differnce between the contacts being normally and open and closed being reversed when it goes thrgouh the PLC but assuming that is not the answer.

THanks!!

Beth x

 

[jtashaffer]

The main thing to remember is a e-stop should be had wired and be a input in the plc.

 

[iant]

I am currently studying for my bachelours degree and I am compelting a unit of PLC's. I have an assigment around Siemens PLC's with also some knwoledge questions, one which I am stuck with. could anyone help as I can find any straight answers on the internet. the question is,

[FONT=&quot]Research and explain why an Emergency Stop function cannot be connected to an ordinary PLC system. Discuss options for implementing an emergency[/FONT][FONT=&quot] system.[/FONT]

I understand the whole differnce between the contacts being normally and open and closed being reversed when it goes thrgouh the PLC but assuming that is not the answer.

THanks!!

Beth x

you can search this site for this information.

however go to www.pilz.com
there is plenty of info there

with any safety device - ask "what happens IF?"

a 'hard wired' emergencey stop
1. if it is wired normally open and the wire breaks
2. if it is only wired to a plc input and the plc input card fails
3. if the plc output welds and stays on.

these arre some questions for you.

 

[iant]

The main thing to remember is a e-stop should be had wired and be a input in the plc.

Not to sure what you mean.
Hard wired - independant of the PLC - Yes.
with an aditional input to the PLC for internal control - yes.

Hard wired into a "SAFETY PLC" - yes - but
these are specifically designed for safety control.
Siemens have this available in their S7-315's and above
BUT - again - the software is additional cost and you need
specific Safety input and output cards


EDIT

Also the processor speed needs to be taken into account with the machine actual stopping time.
CAT. 4 level safety

 

[Ken Moore]

Ordinary PLC systems can and will be programmed by ordinary people, who may or may not understand the safety requirements of an e-stop and they may alter the function of the e-stop with out knowing so.

 

[jtashaffer]

I mean the e-stop should be hard wired. You need to come out of your transformer to your e-stop and then to your other components. And on the other contacts from your e-stop to your input.

 

[L D[AR2P#0.0]]

... could anyone help as I can find any straight answers on the internet.
[FONT=&quot][/FONT]

Here's the first relevant entry I found when I googled "Emergency Stop"

http://www.google.co.uk/url?sa=t&rc...sg=AFQjCNG-wou7ch4JzIiKQSuckIptSL_MTw&cad=rja

 

[Doug_Adam]

OK, here goes
1, Siemens has safety rated PLCs that can form part of a safety system. These are a small selection of their entire range.
2, Ordinary PLCs can be used as either part of, or form the core of, your safety or emergency shutdown systems. However, to do this requires a lot of design effort making it impractical for most applications. From a practical point of view, you should avoid using a non safety rated PLC in safety shutdown systems.

Generally, for the UK, there are 5 performance levels for safety shutdown. These are a, b, c, d and e, with e being the highest.
Which level to use depends on how dangerous your machine is, taking into account likely injuries that could be sustained, how often operators or persons are exposed to the hazard and what the likelyhood or escaping the hazard is without a safety shutdown system in place.
Ordinary PLCs can be used up to performance level d, as long as you have all the correct information to calculate reliability and you have taken correct steps to prevent tampering with the code and also have implemented error checking. For some performance levels, the use of two PLCs cross checking each other may be required.

More information, including sample circuits, can be found in this report:
http://www.dguv.de/ifa/en/pub/rep/rep07/bgia0208/index.jsp

Overall, from the point of view of getting jobs done quickly and being certain you get it right, using either a safety rated PLC or safety relays in an independent circuit, is usually the best way.

 

[iraiam]

An ordinary PLC is not suitable for an E-stop because once the input goes into the PLC, it is UN-monitored software. Safety PLCs have redundant processors, circuitry to monitor and test the outputs for proper function, see document below for more details. It is not Siemens but is good info for your question.

http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/safety-wp002_-en-e.pdf

 

[iant]

I mean the e-stop should be hard wired. You need to come out of your transformer to your e-stop and then to your other components. And on the other contacts from your e-stop to your input.

This is good but you appear to be missing the safety relay
what you are mentioning is a CAT. 1 or less safety level.

 

[leitmotif]

I am currently studying for my bachelours degree and I am compelting a unit of PLC's. I have an assigment around Siemens PLC's with also some knwoledge questions, one which I am stuck with. could anyone help as I can find any straight answers on the internet. the question is,

[FONT=&quot]Research and explain why an Emergency Stop function cannot be connected to an ordinary PLC system. Discuss options for implementing an emergency[/FONT][FONT=&quot] system.[/FONT]

I understand the whole differnce between the contacts being normally and open and closed being reversed when it goes thrgouh the PLC but assuming that is not the answer.

THanks!! Beth x

Your assignment says research so I am not going to give the answers. There are several good referances here. It is a complicated question with more complicated answers. A lot of the answers depend on the type of machine and process and the risk to humans -you will find risk defined in Cats 1 thru 4(??)

There are cases where the PLC must remain operational to ensure safe shutdown of machine or process. So you cannot just shut off the PLC nor disconnect all outputs.

Do a bit of research, get confused (GUARANTEED), and come back with more questions. I promise with good questions you will get more help than you will know what to do with.

I took a look at
http://www.hs-compliance.com/uploaded/documents/THE%20EMERGENCY%20STOP.pdf
already recommended - and think it is a good overview.

Dan Bentler