Hi Bob. You wrote:
<It's been a long time since the days of everything being interlocked with wide switch and relay contacts. Everything from traffic and elevator controls to robotics, air traffic control and nuclear power has graduated to electronic protection and operation. It is possible to get much more reliability and safety out of electronic controls these days than the old electromechanical ways in many applications. Some applications such as motion detection or laser scan detection are impossible to implement otherwise. Some of the best machinery protection devices are proximity detectors. This doesn't mean its easy. You need lots of careful engineering work, robust designs and extensive testing and analysis.> You are quite right of course, but the point I allude to is... the final device which stops the motorised flailing blades is a contact! I know of no solid state devices that I would use to make a dangerous motion 'safe'. Many guarding systems use statistically sound methods with solid state sensors providing data to a machine guarding system, but the thing that stops the motor is necessarily a couple of bits of metal which are pulled apart in a contactor. I don't think that solid state devices, SCR's, Triacs, whatever, can be described as intrinsically safe, in the same way that MCB's can never be as failsafe as a simple fuse. i.e., fuses can only fail, but MCB's can get stuck, break, weld, catch fire, etc.etc. I use guarding systems which are intensely electronic, for instance the voting logic in Oil and Gas fire protection systems, but the thin part of the hourglass, the actual power control device, is always a couple of contactors, contacts in series, coils in parallel, with cross wired auxiliaries to detect a welded contact. These contacts are my primieval safety...When the guard is opened or the Emergency stop button hit, these contacts open, and the electron that can get across that air gap is named Houdini. Just a twopence worth.. Chris Dupres. Surrey, UK.
