Well put, Chris. On this same topic, I've been curious for many years now about the suitability of Hall Effect sensors in interlock circuits. Any comments?
Peter L. Tarver Nortel [email protected] >---------- >From: Chris Dupres[SMTP:[email protected]] >Sent: Monday, June 23, 1997 11:04 PM > >Hi Israel. > >You wrote: > >---snip--- >< The Interlock protects against severe mechanical and laser radiation >hazards. The interlock system comprises of circuitry implemented on PCB.> >---snip--- >< Components (Integrated circuits) that comprise the interlock system have >pin spacings of less than 1.2 millimeters.> >---snip--- > >Wow, you are using semiconductors for an interlock system! Maybe I >misunderstand what you are doing.... > >Surely an interlock system for a system with hazards as you describe >requires a level of intrinsic safety way ahead of board level electronics. >The failure analysis would list far too many modes of failure that would >allow your interlock to be compromised. > >An interlock designed for compliance to the Euro Machinery Directive, etc. >etc. would require a minimum of two independently powered relay contacts in >series, with cross connected contacts to detect a seized relay and prevent >re-energisation. Two 3mm airgaps in series (open relay contacts) are >orders of magnitude more reliable than anything laid down on a non >conducting plane like a pcb, when the degree of pollution on that material >is unpredictable under any conditions. > >Interlocks need to be intrinsically safe, i.e. will always fail OFF in the >event of a fault....Any fault, including a spider crawling across your pcb, >or electrolytically grown trees across your conductors in high humidity, or >a solder splash, or whatever, will cause your interlock to be negated. > >Semiconductors are best avoided like the safety plague they are in >interlocks. They fail in quite unpredicable ways, sometimes S/C, sometimes >O/C, and for a multitude of reasons. > >Safety is two bits of metal separated by a nice air gap. Safety is not a >PN junction a couple of microns thick that happens to be missing a few free >carriers, on a bit of insulating material that is the perfect vehicle for >conductive media such as water, dust, corrosion effects, dirt generally >etc.to accumulate on. > >I'll get down out of my tree now, interlocks and intrinsic safety is a bit >of a hobby horse with me. > >Have a good day. > >Chris Dupres >Surrey, UK. >
