Thread: Which Type of Limit Switch
Now sorted, Many thanks, once again to all who have helped me with there advice and wisdom.
The contactor I was referring to seems to be built into the safety relay. Then there is also an e-stop input and presumably other safety measures built in. I looked at the PILZ website for some information, but just got boggled with detail. They quote £360 for the PNOZ. I'd want it in gold for that money !
Last edited by cropwell; 23-07-2014 at 07:27 PM.
23-07-2014 #46Spelling mistakes are not intentional, I only seem to see them some time after I've posted
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.
Looking at the one I have, the key thing about it seems to be duplication. There are effectively two relays which can be wired in parallel or, ideally, driven via a double-pole e-stop switch. Then the relay n/o contacts are in series so either relay dropping out will open the circuit. It's a bit more complicated than that, of course, depending on which model relay you get, but that's the heart of it and how it can reasonably claim to be a "safety" relay. I guess that build quality is better than a cheap relay from Maplin as well.
I'll only take issue with one thing, I find 'cheap' and 'Maplin' don't sit easily in the same sentence.
"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;
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
Last edited by EddyCurrent; 24-07-2014 at 12:34 PM.Spelling mistakes are not intentional, I only seem to see them some time after I've posted
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