Help me with a Safety issue ?

Drawing schematic for a new machine. The previous machines of this type were done by my predecessor.
He used the old fashioned way of wiring two Estops in series to an MCR. (like we all used to) then the MCR relay controls the control voltage..
He connected two Safety Relays to each of the VFD's STO outputs and placed one of the MCR contacts in the monitoring channel of the safety relay. (single channel/No fault monitoring)
So looking at the drawings, IF the MCR contacts welded closed, There is now way to detect that failure. or even if the NC contacts fell off the Estop button. No way to detect that.


Is this something common, Surely we do not still use this one line MCR stuff on dangerous machinery ?

I am redrawing the machine to include dual channel estops / safety relay. And feeding the control voltage through the safety relay. Isn't that more of a standard now than the old MCR relay ?

Yes, absolutely, as long as the control voltage current is within the safety relay's limits. Best to put a fuse in series with the safety relay contacts, you'll often find that requirement in their specifications.

Thank You,

The reason I'm puzzled is I replaced an engineer that became ill and his condition is fatal. They hired me and through me into this office to figure out what he had done for 30 yrs with No supervision or oversight. I have never met the man but I feel like I know him from exploring his files and work. I have always worked in field service and I am experiencing a learning curve as well. And I don't wish to say anything negative about the man but some of what I have seen really surprises me out of an experienced engineer.

as the old saying goes,

1. if it ain't broke, dont fix it.
2. why should i tey something new, my design has always worked in the past.

not trying to slam anyone, i have run across this before myself.

regards,
james

And I don't wish to say anything negative about the man but some of what I have seen really surprises me out of an experienced engineer.

Click to expand...

Also you need to take into account how much machine safety rules have changed in the past 30 to 50 years. 50 years ago many machines had relatively simple control circuits. Saftey emphasis was on training people to work safely around the machines. Now safety rules assume that no employee will be able to understand and react quickly to the machines, so the machines must be made to work with completely untrained and dumb dangerous employees.

milmat1 said:

Thank You,

The reason I'm puzzled is I replaced an engineer that became ill and his condition is fatal. They hired me and through me into this office to figure out what he had done for 30 yrs with No supervision or oversight. I have never met the man but I feel like I know him from exploring his files and work. I have always worked in field service and I am experiencing a learning curve as well. And I don't wish to say anything negative about the man but some of what I have seen really surprises me out of an experienced engineer.

Click to expand...

Just a note in your predecessor's defense, safety circuits are designed to satisfy a safety assessment preformed on a machine. A MCR or CRM circuit is still valid provided the machine's safety assessment indicates its okay. Granted, its rare because machines usually have some kind of hazardous motion (either to an employee or to the machine itself) but its not impossible.

About 4 years ago, I updated the safety circuits for a bunch of machines that were shipped to Chrysler in the late nineties and they were all 110vac MCR-CRM safety circuits with single channel gate plugs and e-stops; they were all to Chrysler's powertrain specifications when shipped and usually up-to-date with industry standards. It wasn't that long ago.

Any Problem resetting the safety relay from PLC ?

Is there any problem with my resetting the safety relay via a PLC output from the touch screen?

My Programmer is having a fit because I have drawn the machine this way. And he wants a hard button on the cabinet door.

Before I explain who makes these decisions, I want to be sure that there aren't any issues with this setup ??

Yes, resetting a safety circuit with a plc output is quite common provided it doesn't interfere with the integrity of the safety circuit.

Thank You !

My thought was we need a means of preventing a reset as well in case of a fault seen in the logic etc...As all the Motion and VFD's are on field buss back to the controller.. Even though each drive or VFD does have an STO circuit tied to the safety relay.

I have come from another industry and this brick industry is really a shock to my system. They are stuck so far in the past that it has me second guessing myself...

I dislike push buttons, and why have the expensive touch screen HMI and then mount PB's next to it....LOL

Thanks Again !

I dislike push buttons, and why have the expensive touch screen HMI and then mount PB's next to it

Click to expand...

To keep the HMI from developing dead spots on the touch screen because of often used buttons. Such as Start, Stop and Reset.

I will design in pushbuttons and switches with a touchscreen when it makes it better/easier for the operator. This is typically for functions they use fairly often, and if they have to wear gloves.

I have to tell you, in 30 years of field service I have never ever seen a e-stop push button contacts "weld" together. I have seen them fail to close, but never weld together. and with the design of the button, if they ever did weld together, hitting the button would open them. I like Lancie 1's answer because it is exactly what the new safety rules are about. Trying to make engineers responsible for Ignorance and stupidity and be responsible for other people's safety. Some day you will not be able to build a machine because, the safeties for the safeties for the safeties might not be enough.

and no matter what you do any, and I mean any safety system you build, some idiot will find a way to bypass it

When resetting Safety Relays
there are two things you need to ensure
1. You can not reset it from inside the Guard
2. You Should be able to SEE THE AREA before you reset it
Never reset it from a remote location

this might start you in the right direction
might be the wrong safety relay MFG though:beer: