Which Type of Limit Switch

[EddyCurrent]

I understand the pricing policy. If you are selling to a large industry customer you have to provide support and have product liability. They have to be at such a price that they can just swap out any problematic unit. Also the employer has to have employee liability and that would probably preclude buying from an 'unreliable ' source.

I wouldn't mind if anyone could tell me the modular functionality of such a unit, and what qualifies it to have the title 'safety relay'.

I'll probably include one in my next build.

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