[ RadSafe ] Interlock question

[Ted de Castro]

I am dealing with an x-ray machine interlock design.

Of course it will be redundant and failsafe and testable - but the 
question has come up regarding using semiconductor devices in the 
interlock circuit.

When I was the x-ray safety officer at a national laboratory I resolved 
that issue by simply not accepting semiconductor devices in interlock 
circuits - problem solved.

I maintained that when such included logic circuits that showing that 
its was failsafe with the failure of any single component could not be 
demonstrated - even disregarding issues with defining what constituted a 
"component".

Further testing requires that each component be isolated, exercised and 
tested - and I maintained that is simply not possible with logic 
circuits.  ie. - just opening the door and observing that the x-rays 
turn off is most definitely NOT a test!

Its not as simple anymore.

I have heard however that there is a euro standard that prohibits the 
use of semiconductor devices in interlock circuits.  So - I was hoping 
someone here might know IF that were in fact so - and if so shat that 
standard is.

Thanks

Ted de Castro