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Safety monitors

Way back in the dark ages, actually the late 80s, and well before IEC61508, the Health and Safety Executive published a document set titled ‘Programmable electronic systems in safety related applications’ (PES). These provided guidance and best practise for safety-related systems and at the time were considered the bible for users and integrators of PLCs.

British Gas’s R&D division recognised the advantages that PLC technology could bring to its’ predominantly relay-based, critical applications but engineers were concerned about the failure modes of this relatively unproven technology. So they set about designing a fail-safe programmable device that would work alongside PLCs, allowing the flexibility that the new technology afforded whilst meeting the requirements of PES. With the assistance of two other companies the ‘Safety Monitor’ (SaM) was born. SaM was, uniquely, a sequence programmable watchdog. Designed to sit between the PLC I/O channels and final field devices, it was able to take executive shutdown action in the event of a sequence fault or interlock failure.

SaMs became the default method of protection for safety-related PLC solutions until the advent of cost-effective, SIL certified PLCs over a decade later.

Diagnostic facility
Some SaMs have now been in continuous service for over 28 years and are starting to fail due to component aging. For example, most of the electrolytic capacitors are well past their sell by dates and no longer able to ‘charge-pump’ efficiently. However, help is at hand. Some of our engineers were involved in the original BG development and have a unique knowledge and expertise in this product.

Utilising our dedicated SaM test rig and temperature controlled oven we are able to offer a diagnostic service. We can even test and reprogram EPROMs so ‘Truth Tables’ can be verified against our test standard.

With this in-house testing capability we are able to keep SaMs going for a little longer, at least while parts are still available. Please contact the office for further details of our fast turnaround and diagnostic structure.

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©Charter Tech Ltd