Machinery Safety 101
2 June, 2021

Manual reset using an HMI

by Doug Nix
in Complementary Protective Measures, Control Functions, Control Reliability, Emergency Stop, Functional Safety, Inherently Safe Design, ISO 13849 /
1 Comment
A Siemens HMI displaying control functions and data.
This entry is part 2 of 2 in the series Under­stand­ing Safety Functions

Ques­tion: Can a safety-related stop func­tion, for example, be reset via a graph­ic­al object rep­res­ent­ing a reset but­ton on an HMI? The short answer: No, with an excep­tion. Read on if you’d like… 

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8 May, 2021

What is risk assessment?

by Doug Nix
in Canada, EU European Union, Hierarchy of Controls, International, Risk Assessment, Standards, USA
This entry is part 11 of 10 in the series Risk Assess­ment

Risk assess­ment began as a dis­cip­line in the late 1960s, with some of the earli­est form­al papers pub­lished in the early 1970s. The early research­ers were part of the US military… 

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14 April, 2021

What exactly is PELV?

by Doug Nix
in Electrical Safety, Inherently Safe Design
Photo of the interior of an electrical control cabinet showing the components and wireways on the backplane.

What exactly is PELV? Elec­tric­al design­ers run into all kinds of spe­cial­ized ter­min­o­logy as part of their work. IEC is notori­ous for cre­at­ing spe­cial­ized terms that are not famil­i­ar to… 

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23 May, 2019

Force and injury — How hard is too hard? ISO/TR 21260 will help

by Doug Nix
in Hierarchy of Controls, Inherently Safe Design, Robotics, Standards /
5 Comments
ISO/TS 15066 body model
This entry is part 5 of 5 in the series Hier­archy of Controls

Force rep­res­ents the mech­an­ic­al energy that causes injury to the human body. ISO/TC 199 has been work­ing on answer­ing that ques­tion since 2012.

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22 May, 2019

Instructions for Use – the New ISO 20607

by Doug Nix
in Canada, CE Mark, EU European Union, General, Hierarchy of Controls, Information for Use, International, Robotics, Standards, USA

Instruc­tions are one of the basic items that users expect to get when they pur­chase a product, and yet these import­ant doc­u­ments are often poorly writ­ten, badly trans­lated, and incom­plete. Key… 

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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 /
4 Comments
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… 

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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
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.… 

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2 January, 2019

Do-It-Yourself Safety Labels, Signs and Tags

by Doug Nix
in Hazard Warnings
Safety label on a roller conveyor
This entry is part 1 of 5 in the series Hier­archy of Controls

One of the great chal­lenges that all product design­ers face is the sourcing of appro­pri­ate product safety labels. There are many sources for off-the-shelf labels includ­ing some of the biggest… 

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28 August, 2018

Emergency Stop Pull-Cords

by Doug Nix
in Complementary Protective Measures, Emergency Stop /
2 Comments
This entry is part 15 of 16 in the series Emer­gency Stop

This art­icle was updated 2020-06-13, adding links to the Rock­well Auto­ma­tion and Schmersal pull-cord data, and on 2019-04-26, adding spe­cif­ic details related to IEC 60947 – 5‑5 [7]. Addi­tion­al vendor links and… 

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3 July, 2018

Trapped Key Interlocking

by Doug Nix
in Control Functions, Functional Safety, Hazardous Energy Control Procedures, Interlocks, Interlocks /
6 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… 

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