Just one comment... I once submitted a product with interlock being monitored by a microprocessor to UL. After they (UL) advised me that testing all the various states that the microprocessor could fail was going to be a rather expensive venture, I decided that was a BAD thing to do. But, I think Chris put yea old technical nail in the coffin on this one.
Regards, Doug Peter Tarver wrote: > > 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. > > -- ************************************************************ ------------------------------------------------------------ The comments and opinions stated herein are mine alone, and do not reflect those of my employer. ------------------------------------------------------------ ************************************************************
