Horrifying incidents such as the Bhopal Gas tragedy, Chernobyl, and the Challenger and Columbia space shuttle disasters are extreme cases of failure that only seek to reinforce the urgent need to make programmable safety systems a vital part of control and operations. Particle accelerators do not pose anywhere near the same level of hazard. That doesn’t mean that the importance of stringent systems meant to protect personnel from the hazards typically encountered at accelerators such as radiation and electrical hazards can be overlooked. To help design such programmable safety systems, performance goals and requirements should be quantified with the help of reliability engineering and compared favourably with an acceptable risk level. The right question to ask is not whether something is safe, but whether it is safe enough.
With availability to quantify the performance of safety systems, a manager may suggest an availability of 99.9% believing this to be safe enough. In the real world though, this could mean the loss of 16,000 mails lost every hour, 22,000 checks being deducted from the wrong account, or even drinking unsafe water for an hour every month! While these examples are certainly extreme cases, they show the importance of developing a performance level that is acceptable for any and every process.
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- Overt failure – Many safety system failure types have been identified where the designer should be rightly concerned and actively try to prevent if possible. The overt failure of safety systems leads to a fail-safe, revealed action. In the case of a particle accelerator, this could be in the form of a coil failure of an energized relay opening that results in the critical power supply switching off. As these failures cause an accelerator to enter into a safe shutdown mode, this is the first priority of the system designer. Unfortunately, these system failures can be extremely costly as they directly impact the availability of the accelerator.
- Covert failure – Covert failures receive less attention than overt failures but are far more dangerous. Covert failures may not even be found until an atypical situation arises or demand is placed on the system as these failures are well hidden. Covert failures remain within the system and are only revealed when some sort of response is needed from the system. It can only be hoped that covert failures are discovered during the course of system testing as opposed to an actual system need where the system does not respond. A fault can statistically occur at any moment between two tests, and the more frequently a system is tested, the higher the availability of the covert system. That is the reason why frequent testing is absolute vital to identify covert failures. Systems relying on an electromechanical relay logic required manual testing at six monthly intervals. Modern systems are far more flexible and enable automated testing frequently
- Electromechanical relays – Initially, choosing a programmable safety system for controlling applications was quite easy as there were few options and the use of simple and reliable electromechanical relays was often mandated. Relays have indeed passed the test of time. Relay systems continue to function through most kinds of interference, are simple to document, have a minimal initial cost and are vital to safety system designers as they are 98% fail-safe with easily understood failure modes. Electromechanical relays are also inflexible and this is an advantage as errors are introduced when changes are made and not when there is proper testing or documentation. However, the system inflexibility leads to some changes being unable to be implemented because of the difficulty or time required to do so
- Hardwired solid state controllers – A few of the deficiencies linked to relay based systems can be solved by way of hardwired solid state systems. These systems comprise electronic logic devices placed in a particular configuration. Hardwired systems have an inherent advantage when compared to relay systems in terms of weight and size sensitive applications and they allow the easy development of redundant systems that consume low power. Solid state controllers permit on-line input and output circuit testing both automatically and manually and therefore increase covert availability during frequent tests
A number of technologies exist today for designers to solve complicated safety problems. That is why examining availability requirements should be started from scratch with careful consideration given to common, overt and covert mode failures. It is equally vital to appropriately select the technology best suited to the specific application. Some of the companies actively involved in the programmable safety systems market are Omron and integrated Control Systems Ltd.