MCR circuits required to be hardwired?

Regarding conveyor systems:

Our current design guidelines incorporate and Emergency Stop circuit to remove power to all output modules that control moving equipment. The PLC program is written to monitor the E. Stop circuit, among other things. An MCR is programmed, which must be reset using a START pushbutton following any stoppage, safety or other.

I was recently questioned by a new designer on whether or not it was wise to not use a hardwired MCR circuit. My interpretation of NFPA 79, 9.4.3.4.2 (2) would indicate that a programmable circuit is acceptable.

I read through a previous post in which there was much discussion of hard wired vs. programmable E. Stop circuit, but little discussion of the MCR circuit.

Any thoughts are appreciated.

I would use safety relays for all of the series pull stations, and anything classified as SAFETY. You need to start to categorize your safety elements, in order of importance.

In any system that I have been part of, all safety items are hardwired.

It is also standard practice for "safety purposes" to disable the output cards used in a process.

You might want to contact your safety rep at the co that you will be installing in. All companies must comply to basic standards, but there are also standards on top of standards....

Sorry, after re-reading your post, the MCR is not disabling your safety circuits, hopefully. Your Safety will be hardwired, and your MCR is completely compliant to be programmed.

There have been many circuits in my past projects that leave the MCR enabled, when an E-Stop has been generated, for example, Blow Moulding systems....You would not want the heaters to keep stopping and starting......

Sorry for the misleading post..

A word of warning, lets be very careful with our terminology here. Is the MCR functioning as an E-Stop? It was common not all that long ago to label the E-Stop relay as an MCR and that practice may still be in use among many machine builders.

If the MCR performs any kind of Emergency Stop function it should be hardwired or if a PLC is used it should be a safety PLC. A standard PLC should alway have a hardwired emergency stop if there is any kind of safety issues involved.

One problem that can occur with PLCs and hard wired e-stop circuits is how do you want the PLC to behave if an e-stop is operated? It is worth spending some time in deciding what you want the machine to do when the e-stop is operated and after a e-stop reset. Do you want it to start from where it stopped or carry out a recovery operation or go back to its datum?

If you don't get this bit right then it can cause machine damage and possibly create a hazard on the re-start.

OK, you just open a can of worms. But there are some guidlines from OSHA and other agensies.
First of all - E-stop circutry must be hardwired. No PLC actions are allowed via CPU logic in order to perform E-Stop actions. Normally you would wire your PLC output module's incoming power through MCR relay contact (MCR relay must be wired to E-stop circuit). You PLC still can perform the E-Stop logix, but it wouldn't be a primary anymore.

val_99 wrote:

OK, you just open a can of worms. But there are some guidlines from OSHA and other agensies

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Uh... can you give a reference for OSHA? I went through a E-Stop fire drill... on a conveyor system come to think of it... about six years ago and I couldn't find anything from OSHA that required an E-Stop. OSHA requires guarding and labeling; they actually don't have a lot to say about the electrical or controls system. At least, I didn't find anything specific at that time. Electrical is NFPA 71, and controls is NFPA 79. As well as the plethora of UL documents.

NFPA 79 is the most specific reference that I have (anybody have the CE requirements handy? I don't! ** Edit- thanks Mordred!). NFPA 79, Chapters 9-10 deal with Stop and E-Stop.

Chapter 9 requires that all machines have a Category 0 stop; a Category 0 stop is "an uncontrolled Stop that immediately removes power to the machine actuators." A Category 1 stop is "a controlled stop with power to the machine actuators available to achieve the stop then remove power when the stop is achieved."

Per NFPA 79

9.2.5.4.1.3, "The emergency stop shall function as either a Category 0 or Category 1 stop."

9.2.5.4.1.4, "Where a Category 0 stop is used for the emergency stop function, it shall have only hardwired electromechanical components.

Exception: An electronic (hardware or software) system as well as the communication network or link that complies with both 9.4.3 and 11.3.4 and is listed for Category 0 emergency stop function shall be permitted. The final removal of power shall be accomplished by means of electromechanical components."

9.2.5.4.1.5, "Where a Category 0 or a Category 1 stop is used for the emergency stop function, final removal of power to the machine actuators shall be ensured and shall be by means of electromechanical components. Where relays are used to accomplish a Category 0 emergency stop function, they shall be nonretentive relays."

Finally, when you read through NFPA 79, 9.2.5.3 requires that the machine have Category 0 stop. This is NOT called the "E-stop." E-Stop may be either Category 0 or Category 1, and if provided must be provided in accordance with the requirements of Paragraph 9.2.5.4, which I reproduced (in part) above.

Confused yet? <g!> I used to work with one of the guys who WROTE the last release of NFPA 79, and he would never give me a straight answer either.

Of course, I'm still providing an E-Stop. It's always hardwired- I've never had occasion to use any of the red controllers, although they are now permitted at least by NFPA 79. It always uses a safety relay that in turn uses a manual reset. If I have final elements like servos, motors on VFDs, etc. their line side power is wired through contactors (i.e., ahead of the drive or controller) that are opened when the safety relay opens.

This is a good topic- what other references do you guys use for safety functions?

Recently I was looking at a system using devicenet where they had a number of safety devices connected via communication. I questioned the safety factor of this because anyone working with computer controller devices knows the hazards that can occour with any of the million reasons for failure.

I've never adapted using computer controlls for safety, all devices hard wired however their status monitored by the PLC for HMI and fault logging.

In some cases id use the MCR but special application only.

http://www.automation.siemens.com/cd/safety/index_76.htm



This site has a copy some excellent manuals covering this subject in thorough detail and includes a list of agencies and references to other regulations also covers some of the reguklations from other countries.

its "Safety integrated"published by Seimens edition 5

the supplement is also on this site

Thanks for the replies so far.

It seems there is much confusion between E. Stop and MCR circuits.

In my way of thinking, the Emergency Stop circuit is used to prevent personel and equipment damage. The MCR is used to intentionally enable machine operation.

The hard wired emergency stop circuit and ESCR (E. Stop Control Relay) disables moving equipment and disables the programmed MCR circuit. (via loss of E stop input).

The E. Stop circuit must first be reset manually, which activates the ESCR, then the MCR circuit (software) may be enabled via the Start PB.

According to everything I have found, this is sufficient.

cisco_kid wrote:

The E. Stop circuit must first be reset manually, which activates the ESCR, then the MCR circuit (software) may be enabled via the Start PB.

According to everything I have found, this is sufficient.

Click to expand...

Yup. I think that's the way that most of us do it.