Machinery Safety 101
28 October, 2019

Using Form C relays in safety circuits

by Doug Nix
in Control Functions, Control Reliability, Functional Safety /
No Comments

I recently had a dis­cus­sion with a col­league who wanted to know if it was OK for a design to include a form C relay in an e‑stop cir­cuit. You might recall that e‑stop func­tions are required to meet at least PLc/SIL1 require­ments [1], [2]. It’s import­ant to remem­ber that PLc/SIL1 can be met using Cat­egory 1, 2, or 3 archi­tec­tures.…

20 April, 2019

Can Emergency Stop be used as an “on/off” control?

by Doug Nix
in Complementary Protective Measures, Control Functions, Emergency Stop, Functional Safety /
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Emergency Stop Button
This entry is part 16 of 16 in the series Emer­gency Stop

Every couple of months I get an email ask­ing me if there is any reas­on why e‑stop func­tions can­’t be used as the primary power con­trol (on/off but­ton) for machinery. Fol­low­ing a recent exchange, I thought I would share the reas­ons for why this is such a bad idea. The short answer The short answer is an unequi­voc­al NO. Don’t…

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4 April, 2019

Introduction to Functional Safety Seminars

by Doug Nix
in Complementary Protective Measures, Control Functions, Emergency Stop, Events, Free Talks, Functional Safety, Guards and Guarding, Interlocks, Interlocks, ISO 13849, Seminars, Standards, Training /
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Man training a group of people, pointing to Functional Safety topics on the whiteboard

If you are inter­ested in func­tion­al safety, and I know many read­ers are based on the stat­ist­ics I see for my oth­er func­tion­al safety-related posts, I think you will be inter­ested in this. I am col­lab­or­at­ing with the IEEE Product Safety Engin­eer­ing Soci­ety’s Vir­tu­al Chapter to provide a series of three 35 minute sem­inars dis­cuss­ing the fun­da­ment­als of func­tion­al safety. The…

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3 July, 2018

Trapped Key Interlocking

by Doug Nix
in Control Functions, Functional Safety, Hazardous Energy Control Procedures, Interlocks, Interlocks /
5 Comments
This is a trapped key interlock on the door of an electrical switchgear cabinet. To open the door the key must be inserted and turned to withdraw a bolt that holds the door closed. With the bolt withdrawn, the key is held in the lock. The upstream switching device is held open by another interlock using the same key; since the key can only be in one of the two locks, it prevents accidentally closing the upstream switch while the cabinet is open for maintenance. The interlock is attached to the door with one-way screws to discourage casual removal of the lock, which would defeat the system.
This entry is part 3 of 7 in the series Guards and Guard­ing

Many machine design­ers think of inter­locks as exclus­ively elec­tric­al devices; a switch is attached to a mov­able mech­an­ic­al guard, and the switch is con­nec­ted to the con­trol sys­tem. Trapped Key Inter­lock­ing is a way to inter­lock guards that is equally effect­ive, and often more appro­pri­ate in severe envir­on­ment­al con­di­tions. Copy­right secured by Digi­prove © 2018Acknow­ledge­ments: As cited.Some Rights ReservedOri­gin­al con­tent here is pub­lished under…

16 May, 2018

Q & A: Category 2 and Testing Intervals

by Doug Nix
in Control Functions, Functional Safety, Interlocks, ISO 13849, Q & A /
2 Comments
Logical block diagram for ISO 13849-1 Category 2 architecture.
This entry is part 1 of 2 in the series Q&A

Dur­ing the Free Safety Talks that we did with Schmersal Canada and Frank­lin Empire, we had a “hot ques­tion” come up regard­ing Cat­egory 2 archi­tec­ture and the test­ing inter­val require­ment. The defin­i­tion of Cat­egory 2 in 6.2.5 does not include any men­tion of a min­im­um test­ing rate, but 4.5.4 “Sim­pli­fied pro­ced­ure for estim­at­ing the quan­ti­fi­able aspects of PL”…

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15 May, 2018

Five reasons you should attend our Free Safety Talks

by Doug Nix
in Events, Free Talks, Functional Safety, Hazardous Energy Control Procedures, Interlocks, Interlocks, ISO 13849, Lockout, News, Ontario, Pre-Start Health and Safety Reviews, Standards /
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Banner for the Free Safety Talks

Reas­on #1 – Free Safety Talks You can­’t argue with Free Stuff! Last week we partnered with Schmersal Canada and Frank­lin Empire to put on three days of Free Safety Talks. We had full houses in all three loc­a­tions, Wind­sor, Lon­don and Cam­bridge, with nearly 60 people par­ti­cip­at­ing. We had two great presenters who helped…

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15 May, 2018

Q & A: Can Safety PLCs be used for Lockout?

by Doug Nix
in Functional Safety, Hazardous Energy Control Procedures, ISO 13849, Lockout, Ontario, Q & A /
3 Comments
Disconnect Switch with Lock and Tag
This entry is part 2 of 2 in the series Q&A

The ques­tion of lock­out and the use of safety PLCs as a means to meet the lock­out require­ments comes up more and more fre­quently these days. Can Safety PLCs be used for lock­out? Safety pro­fes­sion­als don’t always agree on this con­tro­ver­sial top­ic! Dur­ing the Free Safety Talks that we did with Schmersal Canada and Frank­lin Empire, this…

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3 July, 2017

Safe Drive Control including Safe Torque Off (STO)

by Doug Nix
in Canada, CE Mark, Complementary Protective Measures, Control Functions, Control Reliability, Emergency Stop, EU European Union, Functional Safety, Guards and Guarding, Interlocks, Interlocks, ISO 13849, Ontario /
3 Comments
Graph illustrating STO Function
This entry is part 12 of 16 in the series Emer­gency Stop

Ed. Note: This art­icle was revised 25-Jul-17 to include inform­a­tion on safe stand­still. Safe Drive Con­trol includ­ing STO Motor drives are every­where. From DC vari­able speed drives and index­ing drives, through AC Vari­able Fre­quency drives, servo drives and step­per motor drives, the cap­ab­il­it­ies and the flex­ib­il­ity of these elec­tron­ic sys­tems has giv­en machine design­ers unpre­ced­en­ted…

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26 June, 2017

How to do a 13849 – 1 analysis: Complete Reference List

by Doug Nix
in Control Functions, Control Reliability, Emergency Stop, EU European Union, Functional Safety, Inherently Safe Design, Interlocks, Interlocks, ISO 13849 /
No Comments
This entry is part 8 of 9 in the series How to do a 13849 – 1 ana­lys­is

Post updated 2019-07-24. Ed. As prom­ised in pre­vi­ous posts, here is the com­plete ref­er­ence list for the series “How to do a 13849 – 1 ana­lys­is”! If you have any addi­tion­al resources you think read­ers would find help­ful, please add them in the com­ments. Copy­right secured by Digi­prove © 2017 – 2018Acknow­ledge­ments: As cited.Some Rights ReservedOri­gin­al con­tent here is pub­lished under these license terms: X License Type:Non-com­mer­cial, Attri­bu­tion,…

26 June, 2017

ISO 13849 – 1 Analysis — Part 8: Fault Exclusion

by Doug Nix
in Canada, CE Mark, Control Functions, Control Reliability, Emergency Stop, EU Robotics, Functional Safety, How to, Interlocks, International Robotics, ISO 13849, North American Robotics, Robotics, Standards /
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This entry is part 9 of 9 in the series How to do a 13849 – 1 ana­lys­is

Post updated 2019-07-24. Ed. Fault Con­sid­er­a­tion & Fault Exclu­sion ISO 13849 – 1, Chapter 7 [1, 7] dis­cusses the need for fault con­sid­er­a­tion and fault exclu­sion. Fault con­sid­er­a­tion is the pro­cess of examin­ing the com­pon­ents and sub-sys­tems used in the safety-related part of the con­trol sys­tem (SRP/CS) and mak­ing a list of all the faults that could occur in each…

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