Thought you might be interested in a new article from EAO on e-stops. It's fairly comprehensive and covers the basic background of what they are but also selection options such as design, mounting footprint, parts (actuator, sealing, front plate...), international standards and more. Here's the link: http://www.newark.com/pdfs/techartic...opSwitches.pdf as well as the main e-stop page http://www.newark.com/emergency-stop.