[ RadSafe ] Interlock question
Bob.Westerdale at ametek.com
Wed Oct 29 15:50:07 CDT 2014
I've had to deal with this situation,,, we had developed an XRF instrument
driven by an FPGA ( Field Programmable Gate Array).
The whole safety network was redundant ( 2 switches on virtually
everything) but the Customer feared that the FPGA's
failure modes ( stuck bits, memory holes, etc) were too risky to accept.
We had to duplicate the logic with external ( ie. non-FPGA)
gates which ran in parallel with the PFGA logic. This effectively gave
us 4 separate decision making paths for verifying the
X-Ray shutter was closed or the door secured. ( etc.) Each path had to be
in agreement. It would be difficult to design a
digital control system that monitored/tested every decision making element
in a complex instrument without adding more unreliability
in the process!.
I haven't come across an EU standard that specifically prohibits
semiconductor devices in mission-critical
safety circuits. I'd suggest that the next revision of the N43-2 Safety
Standard ( XRD and XRF systems) include some
basic logic/architecture guidelines. I doubt we're going back to relays
and knife switch disconnects anytime soon!
From: Ted de Castro <tdc at xrayted.com>
To: "The International Radiation Protection (Health Physics) Mailing
List" <radsafe at health.phys.iit.edu>,
Date: 10/27/2014 09:11 PM
Subject: Re: [ RadSafe ] Interlock question
Sent by: radsafe-bounces at health.phys.iit.edu
Specifically - analytical x-ray machine ....
On 10/27/2014 2:27 PM, Ted de Castro wrote:
> 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.
> Ted de Castro
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