Emergency Stop Circuits and Equipment

[mikek2]

This is related to industrial installations. Let's say we have multiple pieces of equipment for a process. This process equipment has an E-stop circuit associated with it. Are there special requirements related to the equipment used in that E-stop circuit?

Does it need to be designed such that a single point of failure will not prevent the system from working? For example, if we use a relay or contactor to interrupt power, the contacts could have welded shut and then the E-stop would not do anything.

Do the components of that system need to be monitored such that a failure of a component would be detected?

 

[websparky]

Re: Emergency Stop Circuits and Equipment

Hi Mike,

There are a few options depending on the power ciruits that need to be disconnected.

IMO,
Some of the safest methods is to use "shunt trip" breakers if possible or use heavy duty multi-pole contactors. The contactors are often more adaptable to multiple pieces of equipment that need to be "stopped" at the same time.

There are several ways to help insure the contacts do not "weld" shut. First, make sure the contact rating is more than adequate for the potential amp draw under "locked rotor" conditions. Second, if at all possible, selective coordination for fuse sizing and rating of the loads.

NFPA 79 is the standard for industrial machinery if you can get your hands on a copy. If you will be doing this more than once, I would invest in a copy.

 

[mikek2]

Re: Emergency Stop Circuits and Equipment

Thanks for the response. NFPA documents are available at their web site: http://www.nfpa.org/codesonline/ I downloaded NFPA 79 from their and have referred to it. It is geared to "Industrial Machinery" but it is written more for someone supplying a piece of equipment. In my case we have numerous pieces of equipment from different sources being run from a control/starter panel.

One method of E-stopping is to have it kill the main power to the panel as you described. Another is to have a "relay", kill the outputs from the PLC and prevent any motors from running this way. In both cases, it gets down to a contact on an E-stop button and a contact in a relay or contactor.

Companies like AB also make special safety devices. AB has the Minotaur system. It provides monitoring of all the safety devices attached to it and somehow detects and alarms failures. Some European Standards and ANSI standards like the Robotic one require this type of system. I cannot really find anything specific in US standards. OSHA and NEC say almost nothing on E-stops.

Somewhere I had read about not having single point of failure prevent the operation of the system. For example, if you used a relay or contactor to kill power and the contacts welded shut, the e-stop would fail and you have the potential for major injury. I do not know if any of this is required in the US by anything other than "Good Engineering Practices". It seems the system in the US is kept intentionally vague to keep more lawyers in business.

 

[richwaskowitz]

Re: Emergency Stop Circuits and Equipment

Mikek2, what you need to look at is control reliability, "A method of ensuring the integrity of the performance of guards, devices or control systems". We are a controls systems integrator and are in the same boat that you are. Some of our customers are requiring devices and techniques be used in the name of safety that actaully come from European standards, not North American and there can be a lot of confusion in applying all the devices that are out there. A lot of good information that can help you is available from Pilz Industrial Electronics at http://pilsusa,com. There is an execllent slide presentation there that sites OSHA 1910.217 (b) (13) on "Control Reliability", ANSI B11.19-1990 Section 2.12 "Definitions of Control Reliability" and ANSI B11.19-1990 Section 5.5.1 also on Control Reliability.

I do not claim to be an expert in this area (yet) but I would offer to assist you in any way I can if you would like to contact me directly, or we can continue here to keep the topic going if there are others interested.

We have done a lot of research in this area and have develeoped and implemented many systems using these standards.

 

[mikek2]

Re: Emergency Stop Circuits and Equipment

Thanks for the reply. I am still trying to get a copy of ANSI B11.19, I have not read that yet. I will check out the Pilz site also. I wish the American standards were a little more defined than just the references from the OSHA General Duty Clause, 1910.212. What are "all applicable standards?"

Our safety department does not have much proficiency in these areas. I am trying to get them involved in doing a risk analysis, then I can use the RIA 15.06 Robotic Standard or the EN method to determine how I need to do my E stop circuity. Following the EN standards of 1 being least harmful and 4 being extremely dangerous, most of our situations are probably 1 and may not require more than we are presently doing. 2 and 3 get into redundancy and monitoring and we do none of that.

I do not know if anyone else is reading these posts or if they are interested. I have put them on some other boards too and have not gotten much response. I appreciate any information you might have. I am going down some other routes too. I would rather address the issue before we have a problem than wait and have a scandal occur when something happens. I probably will not be able to fix all existing systems but at least I can get new ones to come in right.

 

[mikek2]

Re: Emergency Stop Circuits and Equipment

I think the web site mentioned above should be www.pilzusa.com/english/. I think you go to Products the Safe Control Technology. Maybe Rich can better define the path to the information.

 

[arcblast]

Re: Emergency Stop Circuits and Equipment

I whole heartedly agree with the folks here recomending PILZ units.....we use them here at work and we go above and beyond when it comes to e-stops....PILZ are easy to troublshoot and use...they cost but what is a Human life worth ?

Good product and great protection !!!

Mike

 

[richwaskowitz]

Re: Emergency Stop Circuits and Equipment

Sorry for teasing you. I went to that website and I can't even find that presentation myself!!!

I have Pilz helping me find it again. I will give you the full url when I get it.

There is also very good information in the following catalogs:

Allen-Bradley ("You can always buy better, but you can't pay more) "Safety Products publication S115-CA001A-EN-P May 2003 pages 1-1 through 1-49.(AB.COM)

Banner "Machine Safety Products 2001" P/N 20005 (www.bannerengineering.com)