Category 5 Safety Circuits


Thread Starter


I have heard from a client a reference to a Category 5 safety requirement, but I personally have no knowledge of it. Anyone give me any direction on this one?


Hakan Ozevin

I think category 5 implies any control device that will prevent "catastrophic results". Control systems in nuclear plants will fall in this category.
go to "": and get a copy of their free Guide to Machinery Safety 6th Edition this lists the main standards relating to EU Supply of Machinery (Safety) Regulations. The chapter on Risk Assessment is very good. The book lists some 59 standards(EN numbers) supporting the Machinery Directive.

Good luck

Trask, Robert

There is a concept called "Category 4", which is the most stringent. It involves a concept called 'control reliability' which requires complete redundancy AND self checking (of contacts) of the safety circuit.

In reality all safety systems in the US that involve hazardous motion and are not mechanically protected should be 'category 4'. Europeans make
distinctions between various levels of danger - the US does not.

FWIW I have only experienced one client that went through all the steps necessary to design, build, and pay for a true 'control reliable' safety
circuit. It is not cheap. And it is sneakingly complex from a design standpoint. The good news is that it WILL provide safety even under failure
conditions. You must design systems under the assumption that any one component WILL fail at any given time.

It has become more of a focus of OSHA in recent years. Especially with respect to presses.

I have never heard of 'Category 5' WRT safety - are you sure you're not talking about Ethernet...

Robert Trask, PE
MindSpan eSciences
Wilminton, NC USA

Bouchard, James \[CPCCA\]

We routinely require the design of the guarding and e-stop circuits on our machines to be Category 3. Most of them are built in Europe so it is not as much of a problem. It is more costly than the simple relay circuits we used to use. It is also a corporate standard and is required for all new equipment.

James Bouchard
Johnson & Johnson Products Montr=E9al

Bogumil Eichstaedt

Do you mean AK 5?
German Standard DIN V 19250 specifies Anforderungsklasse (AK) 5 which is roughly equivalent to SIL3 in the new IEC 61508 or to Steuerungskategorie 4 of EN 954-1 (that is, probability of failure to act on demand is 1 failure in 1000 to 10000 calls for low demand mode of operation or probability of dangerous failure is 10-8 to 10-7 per hour of mission in high demand mode of operation).

Bogumil Eichstaedt
You should look at the latest NFPA 79 2002.

any one not using catagory 3 safety is in violation.

BTW GM now requires a term called catagory 3.5. it is primarialy a catagory 3 system but brings in some of the requirements of catagory 4.