Master Control Relay...Code Requirement?

Just a quick question...I know that a hardwired master control relay should be present and you should never rely on one programmed into a PLC. What I would like to know is if there is a code requirement for this or is it just good practice? Thank you for any insight you can provide.

Depending on the equipment and what you are expecting this to accomplish for you it may not be near enough.

Quite a few years back a master control relay circuit was used like an e-stop chain and when the MCR was disabled it would kill all the outputs from a PLC. If this is what you are asking then yes there are codes that require a safety circuit for most manufacturing equipment and a MCR is no longer good enough to meet codes (NEC, ANSI, NFPA, etc. for the US).

I guess what I am asking is where in any code (NEC, ANSI, NFPA, etc.) does it say "you must not rely on a programmed MCR in a PLC, you must have a hardwired MCR/e-stop".

Yes, there is. Unfortunately I am a little rusty in that area and no longer have the specific codes and sections memorized. Maybe someone else can throw those out for you. If I have some time this afternoon I will dig through some older projects and list the most common you should be familiar with.

If you google machine safety you should come up with some good hits for the required codes. Many of the machine safety equipment sites have references to required codes.

Darren

Here is a quick list I found on google.

1. ANSI B11.TR3- 2000, Risk Assessment and Risk Reduction
2. ANSI B11.TR4-2004, Selection of Programmable Electronic Systems (PES/PLC) for Machine Tools
3. ANSI Z244.1-2003, Control of Hazardous Energy
4. ANSI/PMMI B155.1-2006, Safety Requirements for Packaging Machinery
5. NFPA 79-2007, Electrical Standard for Industrial Machinery
6. SEMI S2, Safety Guidelines for Semiconductor Manufacturing Equipment

There are also some brief references in the NEC, but mostly it will point you back to one of the above standards. Those mainly apply to typical discrete manufacturing systems. If you are in some type of process industry there may be codes that apply to your specific process that may be much more in depth than what is listed above. Also note that that is not all there is, it is a quick list and a start.

Darren

Thanks dash, I really appreciate it!

dash said:

There are also some brief references in the NEC, but mostly it will point you back to one of the above standards.

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To the best of my knowledge, the NEC does not allow itself to reference other standards.

Just on the basis of UL 508A I know that is not correct. You can find references to 508A in the motors section, circuit protection, transformers, etc. UL 508A also references back to the NEC.

It also references some other codes in the hazardous area wiring (including other NFPA codes).

Maybe that was true in the past, but that is not so in the current form.

Darren

dash said:

You can find references to 508A in the motors section, circuit protection, transformers, etc.

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When you have a minute, could you please give me a reference? I'd like to see one.

FPNs (Fine Print Notes) or references in brackets are for informational purposes only and are not requirements per 90.5(C).