E-Stop Circuits and Risk Assessment

On safety grounds, it is not permissible to rely on software for personnel protection where either electrical or mechanical hazards exist.

• For example, a technician is delegated to change a pump motor. On breaking the coupling, what is there to prevent the motor turning while spanners are in the mechanism? On opening the motor terminal cover, what is there to prevent the terminals becoming live while being unscrewed?

So there has to be a way of locking-off the power sources to enable this to happen, and a way of controlling it under Permit To Work arrangements to assure a high degree of safety (which is a low level of risk). Lock-off isolators adjacent to the rotating device are best practice: both the permit signatority and the technician can apply padlocks should local procedures require it. Then, even if the PLC says "run", the motor will remain stopped.

E-stops must break the auxiliary supply to the starting contactor when pressed. Local standards will determine whether or not these buttons are to have key-turning releases.

Does that help?

Yes, I just finished a system design for category 1 and category 3 emergency stop and safety relay systems, and the information available is a little vague and varies from application to application.

One of the main standards is EN 60204-1 (European). As I didn't have access to the standard at the time most of my design data came from safety relay company literature and data sheets (which show example solutions and different ways for achieving the different category levels of safety). I am not sure if the NFPA guidelines are a copy of the EN 60204 guidelines.

Try the websites of Duelco, Pilz, Omron and Sick (possibly) for literature and data sheets which give design examples.

Alternatively try a web search, the answer regarding NFPA categories is there somewhere.

FYI, yes, that is NFPA 79. Thank you.

Dear Guest,

Check out the link beow. Not sure if it is what you are looking for but it might provide some additional information.

http://www.schneider-electric.co.uk/internet/pws/literature.nsf/luAllByID/JRIS-6RWJS4/$file/How_to_use_BSEN60204-1_2006_Safety_of_Machinery_SE5941_full.pdf/

Regards

Mr. W.A Snow

NFPA 79, "Electrica Standard for Industrial Machinery". If this section doesn't have what you need, it has references to other areas of the NFPA that should. As a note, PLC, DCS type systems are not allowed to function as an emergency stop, unless it is rated as such. These are dedicated systems, manufactured to function in this capacity. This is covered in the NFPA. Just remember, the NEC is the minimum required. Be safe!

Thank you. I do not have NFPA 79. Is it possible to have me that part copied & send me! Thanks again.

Sorry, can't do that, but it is available thru the NFPA, non-member price: $39.50.

http://www.nfpa.org/catalog

Guest,

I concur with all the previous posts. There are PLCs that are rated and approved for such applications, but I have never worked with them.

One addition--in designing your automatic controls it is very wise to have the PLC system monitor the status of the e-stop circuit(s). This can be by inputs from auxiliary NO or NC contacts at the individual e-stop locations or by auxiliary contacts on the e-stop relays or master control relay. The rule being followed here is that you always want the PLC logic to know and be able to respond to all conditions in the equipment it operates. That way it can provide supervisory notification (such as a graphical operator interface showing what e-stops have been activated so operators/workers can respond appropriately). Another approach I frequently use is to have all e-stops be with an illuminated operator that is on a PLC output; programmed so the activated one is blinking and all others are illuminated steadily (this alerts workers everywhere that the system has an e-stop activated and pinpoints the exact one for appropriate worker response).

--JMM