Master Control Relay

Master Control Relay can mean different things to different individuals. For the context of this discussion, I am speaking of a device that is in the logic rung of an E-stop switch and is a component that senses the opening (or closing) of the E-stop switch and subsequently opens the power to critical loads in a piece of machinery.

My question is, are their code requirements for a certain type of component to be used as an MCR? If so, what are they? If not, are there "commonly accepted" engineering standards as to what types of components may be used for this functionality?

Several vendors offer Master Control Relay components. What I would like to know is what, if anything, is unique about these parts as opposed to a "General Purpose" relay.

Thanks,

JS

the following post might be interesting ...

http://www.plctalk.net/qanda/showthread.php?p=354014&postcount=54

basic idea:

make sure whether we're discussing a Master Control RELAY - or a Master Control RESET ...

Welcome to the forum.

For older systems, the power on and e-stop controlled the MCR (master control relay).

Today, the E-stop is monitored by a safety control relay that you reset.
you then push a power on button to turn on the Power relay.

It's been a while since I have designed a system, but we always used the safety relay / power relay combination.

I would look up NFPA 79, electrical standard for industrial machinery.

regards,
james

First and foremost,

A PLC is not a safety device and should never be used as such.

The "MCR" you see in a program is no substitute for a hard-wired safety circuit.

What you "need" depends on the safety audit of the machine. If the machine poses no physical danger to a person, you could possibly get away with a single contact E-Stop and a general purpose relay. It goes up from there. Generally, I do at least a dual-coil safety relay, and if it's a life-and-death type of machine a fully-redundant contactor/safety relay setup with contact weld detection.

FactoryTalktotheHand said:

A PLC is not a safety device and should never be used as such.

Click to expand...

This is true, unless the PLC is specifically marketed and rated as a Safety PLC, and it is used in conjunction with IO that has been specifically designed for safety. These products are available from many vendors, and have been for quite a while.

You are 100% correct, though, that it all comes down to the Risk Assessment, to determine what could go wrong and how to fix it.

FactoryTalktotheHand,

Thanks for pointing that out !! I misread the original post.
I 1000% agree.

james

Bottom line. If you are here in the USA, nobody "has a gun to your head" forcing any specific safety regulations on you. YET. If you are in most of the rest of the world, you do, via IEC regulations. That is where the "Saftey Controllers" market is being driven from. Those IEC standards are in the process of being adopted here and it is coming, but there is intense lobbying going on about who will enforce them and how strict it will be. There's no good prediction I'm aware of as to when the hammer comes down.

So as it stands today in the USA, you are not bound by specific safety system requirements, but you ARE bound to making sure your machines are safe for the employees using or working on them. How you get there is up to you. OSHA is not a police force, but they ARE the Judge, Jury and Executioner if something happens and it turns out it could have been reasonably avoided. Because of this, smart employers are using the IEC safety requirements as the safety plan, knowing that when the IEC rules get here, they will slready be there.

How this relates; the old MCR system with e-stops will not cut it in the new world order. So if you want cheap and dirty, expect to have to throw it out and do it again in a few years.

jraef said:

How this relates; the old MCR system with e-stops will not cut it in the new world order. So if you want cheap and dirty, expect to have to throw it out and do it again in a few years.

Click to expand...

Things might have changed but existing systems used to be "grandfathered" in IEC territory so long as you didn't change anything. The standards I worked with only applied to new builds and modifications/upgrades.