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Topic Title: e/stop through a plc
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Created On: 23 September 2010 11:42 PM
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 23 September 2010 11:42 PM
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aiden251181

Posts: 10
Joined: 16 September 2010

hi,
i was just wondering if there are any regs on e/stopping a motor through a plc? can you do this or do you have to use an e/stop monitoring relay?
 24 September 2010 09:09 AM
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mike.mcclory

Posts: 30
Joined: 26 April 2006

I would strongly advise against it. Even safety rated components in a PLC are still controlled by non-safety rated controllers.

Personally, I would use the PLC (or drive controller) to bring the motor to a controlled stop and a seperate safety system to then remove power (either interrupting the power to the drive controller or power lines to the motor) to make it safe. This could be a simple E-stop relay or a more complex system like a Pilz PNOZmulti type programmable controller.
 24 September 2010 09:44 AM
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jpittwood

Posts: 62
Joined: 19 August 2005

Aiden,

This is not as simple as just choosing a product.

First thing that is required is a risk evaluation. This should take into account all of the potentially dangerous occurances. It may be much wider than just immediate personal safety looking environmental & other risks.

From this you will identify who is exposed to which risks. If possible the risks shoud be reduced.

Part of the risk assesment should indicate the categories of safety related parts of the control system. If this is according to a EN 954-1 the categoies are B, 1, 2, 3 & 4.

When you have this information you can decide how to control the equipment in an emergancy. If you need redundacy in the system and how you will prove the system.

All of this should be documented, remember if someone is hurt or killed by the equipment YOU may end up explaining to a Judge or Coroner how you decided on the particular control system.

As Mike has said there Safety devices in the market but just because you have used one of these it does not mean you have a safe system.

This may seem long winded but there are several pieces of legislation that you have to take into account. If you want to get an idea of these go to the HSE web site. Schneider's Preventa Safety Catalogue, section 6, will give you an idea of the legislation & the process.

Hope this has helped

-------------------------

John Pittwood IEng FIET
------------------------
 25 September 2010 04:20 PM
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chartassuk

Posts: 57
Joined: 25 July 2008

Originally posted by: aiden251181

hi,

i was just wondering if there are any regs on e/stopping a motor through a plc? can you do this or do you have to use an e/stop monitoring relay?


The regulations:

Machinery Directive 2006/42/EC

Guidance documents include:

EN12100 - safety of machinery: basic concepts
EN14121 - safety of machinery: risk assessment
EN13849 - safety of machinery: safety related parts of control systems
EN62061 - safety of machinery: functional safety of electrical, electronic and programmable electronic control systems

So, dependant on category of risk it is highly likely that implementing a safety circuit through an automation controller is only allowable if the controller has a verified safety rating commensurate with the risk assessed.
Again, if a hard wired safety circuit is to be used then it must be verified as above.
 25 September 2010 05:42 PM
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aiden251181

Posts: 10
Joined: 16 September 2010

cheers guys,
i was looking into the pilz units anyway but just needed to know if it was within the regs to do it through a plc. i think i'll have a look get the h&s dept to risk asses it with me and look from there

cheers
 28 October 2010 12:24 AM
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CanoeBoy

Posts: 7
Joined: 28 October 2010

As everyone states, its not simple, there are risks and standards

But in answer to the OP question

estop through a plc

Thats a definate NO

however can an EStop be wired though a safety PLC then thats a YES
 29 October 2010 05:13 PM
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Iangreaney

Posts: 6
Joined: 08 April 2010

I have ALWAYS used a Safety Relay (PILZ) on any Machine or Control System etc.

This is to ensure that the Circuit is physically isolated in the event of an E-stop. I ALSO use a Aux contact on the Safety Relay as an Input for the PLC, which I then use (in software) to bring to a safe stop the Machine.

This approach ensures "belt & braces", and the Machine will be waiting for the removal of the hazard, and a "Control On" signal from the Safety Relay when you reset it allowing the Software to resume control from a safe start-up.

I have come across (usually Jap Machinery) that rely only on Software, and quite often they would become "confused" and lose their sequence after an e-stop.

So in summary! use both, good practice in my book.

Regards,

Ian
 11 November 2010 08:13 PM
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CanoeBoy

Posts: 7
Joined: 28 October 2010

Nothing wrong with Safety Monitor (Pilz, Dold, Siemens etc) with an Aux fed to a PLC for monitoring

Never come across the Jap stuff you talk of, but a properly done Siemens S7 Safety PLC running Profisafe or ASisafe is an excellent solution and far cheaper and beneficial on a large scale project. All the Profisafe and ASisafe jobs i have done have never "got confused"...

There is a big difference between a PLC and a Safety PLC
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