Reader Question: Multiple E-​Stops and Resets

This entry is part 7 of 13 in the series Emergency Stop

Control Panel with Emergency Stop Button.I had an inter­est­ing ques­tion come in from a read­er today that is rel­ev­ant to many situations:

When you have mul­tiple E-​Stop but­tons I have often got­ten into an argu­ment that says you can have a reset beside each one. I was taught that you were required to have a single point of reset. Who is correct?”

— Michael Barb, Sr. Electrical Engineer

The Short Answer

There is noth­ing in the EU, US or Canadian reg­u­la­tions that would for­bid hav­ing mul­tiple reset but­tons. However, you must under­stand the over­lap­ping require­ments for emer­gency stop and pre­ven­tion of unex­pec­ted start-up.

The Long Answer:

First I need to define two dif­fer­ent types of reset for clarity:

  1. Emergency Stop Device Reset: Each e-​stop device, i.e. but­ton, pull cord, foot switch, etc., is required to latch in the activ­ated state and must be indi­vidu­ally reset. Resetting the e-​stop device is NOT per­mit­ted to re-​start the machinery, only to per­mit restart­ing. (NFPA 79, CSA Z432, ISO 14118).
  2. Restarting the machine is a sep­ar­ate delib­er­ate action from reset­ting the emer­gency stop device(s).

ANSI B11-​2008 provides some dir­ect guid­ance on this topic:

7.2.2 Zones

A machine or an assembly of machines may be divided into sev­er­al con­trol zones (e.g., for emer­gency stop­ping, stop­ping as a res­ult of safe­guard­ing devices, start-​up, isol­a­tion or energy dis­sip­a­tion). The machine and con­trols in dif­fer­ent zones shall be defined and iden­ti­fied. Controls for machines in zones can be loc­al for each machine, across sev­er­al machines in a zone, or glob­ally for machines across zones. The con­trol require­ments shall be based on the oper­a­tion­al require­ments and on the risk assessment.The inter­faces between zones, includ­ing syn­chron­iz­a­tion and inde­pend­ent oper­a­tion, shall be designed such that no func­tion in one zone cre­ates a hazard(s) /​ haz­ard­ous situ­ation in anoth­er zone.

CSA Z432-​04 has sim­il­ar wording:

When zones can be determ­ined, their delim­it­a­tions shall be evid­ent (includ­ing the effect of the asso­ci­ated emer­gency stop device). This shall also apply to the effect of isol­a­tion and energy dissipation.

Let’s take a case with a single e-​stop but­ton first. The same require­ments apply for all e-​stop devices. The require­ments include:

  1. Button must be in ‘easy-​reach’ of the nor­mal oper­at­or pos­i­tion. I con­sider ‘easy-​reach’ to be the range I can touch while sit­ting or stand­ing at the nor­mal oper­at­or pos­i­tion. This pos­i­tion is not neces­sar­ily in front of the con­trol pan­el. This is the pos­i­tion where the oper­at­or is expec­ted to be while car­ry­ing out the tasks expec­ted of them when the machine is oper­at­ing. This is the require­ment that drives hav­ing mul­tiple but­tons in most cases.
  2. E-​stop devices can­not be loc­ated so that the oper­at­or must reach over or past a haz­ard to activ­ate them.
  3. The but­ton must latch in the oper­ated position.
  4. The but­ton must be robust enough to handle the mech­an­ic­al and elec­tric­al stresses that will be placed on it when used. i.e. rugged but­tons are required.
  5. When the e-​stop device is reset – i.e returned to the ‘RUN’ pos­i­tion – the machine is NOT per­mit­ted to restart. It is only PERMITTED to restart. It must be restar­ted through anoth­er delib­er­ate action, like press­ing a ‘Power On’ button.

So what do you do with the ‘POWER ON’ or safety cir­cuit reset but­ton? The first ques­tion to ask is: ‘What hap­pens when I reset this cir­cuit, apply­ing power to the con­trol circuits?”

Case A: If it is impossible to see the entire machine from the loc­a­tion of the reset but­ton, then I would recom­mend a single reset but­ton loc­ated at the HMI or main con­sole. The oper­at­or must check to make sure the machine is clear before re-​applying power. Where the machine is too big to be com­pletely vis­ible from the main oper­at­or con­sole, then I would also recommend:

  • warn­ing horn, 
  • warn­ing lights, and 
  • a start-​up delay that is long enough to allow a per­son to get clear of the machine before it starts moving.

Case B: If the machine is simply ‘enabled’ at this point, but no motion occurs, then mul­tiple ‘reset’ or ‘power on’ but­tons may be accept­able, depend­ing on the out­come of the risk assess­ment and start/​stop ana­lys­is. Having said that, the oper­at­or will likely have to return to a main con­sole to reset the machine and restart oper­a­tion, and chances are there is only one HMI screen on the machine, so there may not be any advant­age to hav­ing mul­tiple reset buttons.

I would recom­mend doing two things to get a good handle on this: Conduct a detailed risk assess­ment and include all nor­mal oper­a­tions and all main­ten­ance oper­a­tions. Then con­duct a start/​stop ana­lys­is to look at all of the start­ing and stop­ping con­di­tions that you can reas­on­ably fore­see. Combine the res­ults of these two ana­lyses to find the start­ing and stop­ping con­di­tions with the highest risk, and then determ­ine if hav­ing mul­tiple reset but­tons will con­trib­ute to the risk or not. You may also want to look at the con­trol reli­ab­il­ity require­ments for the emer­gency stop sys­tem based on the out­come of the risk assess­ment and the start/​stop analysis.

In a case where there are mul­tiple emer­gency stop devices, loc­a­tions are import­ant. There must be one at each nor­mal work­sta­tion to meet the reg­u­lat­ory require­ments in most jur­is­dic­tions, and with­in ‘easy reach’. You may also want some inside the machine if it is pos­sible to gain full body access inside the machinery. i.e. inside a robot work cell. Make sure that the but­tons or oth­er devices are loc­ated so that a per­son exposed to the hazard(s) inside the machine is not required to reach over or past the haz­ard to get to the button.

Michael, I hope that settles the argument!

BSI Publishes New Guide to Machinery Safety

BSI pub­lishes a new guide on the applic­a­tion of 2006/​42/​EC and the PUWER regs. If you are UK based or export to the UK mar­ket you need this guide.

The British Standards Institute (BSI) recently pub­lished a new guide to machinery safety entitled: “BIP 2184:2009 — Risk Management of Machinery and Work Equipment”.
Download BSI Standards (British Standards Institution)

This guide, writ­ten by John Glover, a highly exper­i­enced and well respec­ted con­sult­ant in this area, cov­ers the applic­a­tion of the Machinery Directive and the Provision of Use of Work Equipment Regulations (PUWER). Aimed at machinery users, buy­ers, spe­cifiers, con­sult­ants, man­agers and engin­eers, this book provides insight and dir­ec­tion in the applic­a­tion of these import­ant requirements.

The guide will help you to under­stand how your respons­ib­il­it­ies have changed and will help you to meet the leg­al require­ments of the new Machinery Directive.

The guide also provides inform­a­tion on the applic­a­tion of risk man­age­ment tech­niques in the workplace.

If your organ­iz­a­tion is UK-​based or exports into the UK mar­ket, this is a must-​have guide to the cur­rent regulations.

Contents of Risk Management of Machinery and Work Equipment include:

  • Corporate risk management
  • Risk man­ager vs insur­ance manager
  • Health and safety and the law
  • The Supply of Machinery (Safety) Regulations 2008
  • The Provision and Use of Work Equipment Regulations 1998
  • The use of har­mon­ized standards
  • ISO 13849 – 1, Safety of machinery. Safety-​related parts of con­trol sys­tems. General prin­ciples for design
  • High-​risk environments
  • Why sys­tems fail
  • The costs of non-compliance
  • Bibliography
  • Index of ques­tions by topic

Get more inform­a­tion or pur­chase a copy in the BSI Shop.
Download BSI Standards (British Standards Institution)

Interlocked gate testing

Did you know that inter­locked gates require stop­ping per­form­ance testing? 

Machinery needs to be able to stop in the time it takes a per­son to open the guard and reach the haz­ard. If the dis­tance from the guard open­ing to the haz­ard is short enough that a per­son can reach the danger point before the haz­ard can be con­trolled, the guard is use­less. The res­ult­ing situ­ation may be worse

Did you know that inter­locked gates require stop­ping per­form­ance testing?

Machinery needs to be able to stop in the time it takes a per­son to open the guard and reach the haz­ard. If the dis­tance from the guard open­ing to the haz­ard is short enough that a per­son can reach the danger point before the haz­ard can be con­trolled, the guard is use­less. The res­ult­ing situ­ation may be worse than not hav­ing a guard because it’s pres­ence leads to a false sense of secur­ity in users.

Test the stop­ping time of guarded haz­ards and make sure that guards are far enough away from the danger zone to be effect­ive. For more on stop­ping per­form­ance require­ments, see CSA Z434, EN 999 (soon to be replaced by EN 13855:2010), and in the USA, 29 CFR 1910.217(h)(9)(v).

Download ISO Standards 
Download IEC stand­ards, International Electrotechnical Commission standards.
Download BSI Standards (British Standards Institution)
Download ANSI standards

Need help with stop­ping per­form­ance test­ing? Contact us!