Quote Originally Posted by EddyCurrent View Post
It's about meeting certain standards;

"PNOZ safety relays comply with international standards and are approved by BG and international certification bodies. They provide maximum safety up to and including Category 4 in accordance with EN 954-1, Performance Level "e" in accordance with prEN ISO 13849-1 and SIL 3 in accordance with IEC 62061. "

Manufacturers have to demonstrate their product meets these standards and this can be a costly and lengthy process.

These things matter under scenarios like, legislation, reliability, safety, compensation, prosecution, etc. So if these is an accident for example, you may need to demonstrate that you or your company have exercised 'Due Care' by using components that comply with the required safety category.

These devices are also called 'Safety Monitoring Relays' there are basically 4 safety categories, it's up to the owner to work out which category is applicable to their particular application, this information is used;
http://www.pilz.com/en-GB/knowhow/st...articles/00238

You then select a safety relay that meets the required category and also has the required electrical characteristics for the application.

Further reading would also be this, http://en.wikipedia.org/wiki/Safety_Integrity_Level
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Thanks for the references Eddy, I had a look at them, but I gave up trying to understand those sort of documents when I retired in 1999.
They may be relevant for a corporate engineer, but I am just a hobby player.
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That's all well and good, but how do they get to be £25 on eBay (if they haven't fallen off the back of a stores shelf ).
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I'm not an employee and I don't need to have duty of care over some moron who is bound for self destruction by his own stupidity. All I need is that if I have to bang E-stop the damn machine stops PDQ, and if it can't account for all the electricity going in through the live and coming out through the neutral, I also need it to isolate itself until the leak is fixed (within 30 milliseconds, preferably a lot less, if it is more than 30 milliamps going through me). There may be other requirements yet to be determined.
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As a silly aside ;- Pilz is German for Mushroom.
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Thanks anyway for the input, but it is far too heavy for my butterfly brain.