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Topic Title: Category 2 and Category 4 Safety Circuits
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Created On: 26 September 2017 03:32 PM
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 26 September 2017 03:32 PM
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Ainsley751

Posts: 2
Joined: 16 September 2016

Hi,

I was hoping to find some help to do with CAT 2 and 4 safety circuits.

I've been given a bunch of questions, and one of them is to do with these safety circuits and the differences, and a wiring diagram of a Cat 4 Safety circuit, with 3 stops included.

I kind of understand the categories, having gone over it once very quickly on a PLC course. I get that the higher the category, the higher redundancy options and less chances of a fault affecting the safety circuit, but can't get it onto the paper in a drawing?

No one on my shift pattern seems to know either, so wasn't sure if it was a badly put question or anything?

Also, when it mentions dual channel, does that mean the stops are separate circuits, or each stop contains multiple NC contacts?

I'm sure I've forgotten other info I'm struggling with but any help would be appreciated,

thanks!
 26 September 2017 03:58 PM
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OMS

Posts: 22435
Joined: 23 March 2004

This might get you started

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 27 September 2017 07:28 AM
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Mal415

Posts: 8
Joined: 20 September 2017

Cat4 requires input and output redundancy, all inputs faults are uniquely identified by logic and no single point of failure can cause a dangerous fault and components are rated for an appropriate MTTFd (when combined as a circuit).
Also a single Point of failure must be detected either directly (stopping the output circuit) or detected when the circuit is attempted to be reset (Preventing the reset).
e.g. if one of the two output contactors that are used in series (for dual output redundancy) welds closed (ON) under normal operation it is not expected to be detected as the system is currently running but when normal operation of the safety is activated the feedback contact to the safety controller saying the contactor is still closed (on) even though power to the coil is removed this would be detected and prevent the restart of the system.
You see this a lot as a normally closed feedback contacts from output contactors/relays in the reset branch of a safety relay.
 27 September 2017 07:10 PM
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paulskyrme

Posts: 1291
Joined: 12 February 2003

You also need to be aware of fault masking when you get to PLd & PLe.
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