E-Stop circuit

I always assumed E-stop buttons had to be maintained? We have 3 OEM filler machines, one of which I worked on the E-stop circuit recently and noticed the switch was momentary, which drops out the MCR.....The system DOES require a "control power" pushbutton be pressed to re-activate the MCR. I just always assumed that an estop should be maintained?


Greg

What yoou describe is common - Dropping out MCR or 1CR.
The drawback is power is lost to drives and position/direction.

Only used on very simple machines.

Rod (The CNC dude)

Like Rob said, it is very common and provided device type and the rest of the safety system fit the application, it easily meets the OSHA guidelines.

Basically, OSHA wants an estop pushbutton to use a positive mode of operation (meaning that pushing the button physically forces the contacts open rather than relying on a spring) and that the machine will not immediately restart on its own once the pushbutton is released.

There is certainly a lot more than this to making a machine safety system right, but as far as the buttons themselves go, whether latching or momentary, either can be acceptable.

Steve

OSHA? Not sure they are the governing authority overall. NEC and NFPA 79 are more applciable.
NFPA 79 Chapter 13.2 Emergency stop devices. 13.2.4 The emergency stop actuator will be either a momentary or self-latching type.

Chapter 9.6.3 Emergency stop. ---Reset shall not initiate a restart
Category 0 shall only have hardwired electromechanical components.

Chapter 9.5 Stop functions
There are 3 categories of stops as follows:
Category 0: stopping by immediate removal of power
Category 1: controlled stop then power removed
Category 2: controlled stop with power left available.

Actually OSHA probably is the definitive authority.
NFPA79 is not a mandatory standard (although I believe it should be)
NFPA70 (the NEC) is applicable to building/premises wiring and field manufactured panels but not to pre-manufactured equipment.

Yes, the general assumption is E-Stops are maintained but as Ron quoted this is not a requirement.

GregPLC said:

The system DOES require a "control power" pushbutton be pressed to re-activate the MCR. I just always assumed that an estop should be maintained?

Click to expand...

I didn't like it the first time I saw it, either. When you've got multiple branches in your control power, I suppose it's easier to add on contacts to a relay circuit than it is to add them to a pushbutton.

AK

Re: Re: E-Stop circuit

akreel said:

I didn't like it the first time I saw it, either. When you've got multiple branches in your control power, I suppose it's easier to add on contacts to a relay circuit than it is to add them to a pushbutton.

AK

Click to expand...

I have almost never seen system power run through the actual e-stop button. Always through a (safety) relay attached to the e-stop.

To add to the answers to Greg's question. You don't need a maintained estop, howerver it is nice to help accidental restart. We still use the separate start button. To comply with the above standard.

As far as OSHA the new legislation in Alberta requires full compliance. Thanfully they have given everyone a leeway time in order to meet those standards. The company I currently am working with has been working diligently to meet those standards.

I am fully aware of a secondary input for restarts.....Just wasn't sure I liked the momentary E-stop button....Thanks for the feedback.


Greg

I would appreciate it if someone could tell me which OSHA standard references requirements pertaining to E-Stops.

BTW I dont like the use of momentary either, I prefer maintained preferably lit when latched in.

That is what I typically use also Ron, thanks again for the input.


Greg

Which of course comes back to a post I started a few weeks ago.... Does the e-stop light come on when it's pressed, or when it's not pressed The two schools of thinking there are:

1. lit when pressed, indicates a stop, off, or emergency situation.

2. lit when not pressed: easier to find!


I typically draw them up as lit when pressed.....

Again using NFPA as a reference the light should indicate an action...ie EStop button is illuminated then that button has been pushed. I prefer this method because if you have multiple estops in series the light tells you which one was pushed.

I had to redo a packing conveyance line once because they had pull rope and pushbutton estops, the line would be shutdown and they couldnt get it started because they didnt know or forgot about the estop. I illuminated the buttons plus put in stacklights in each zone to determine if an estop was latched. Looking 100 feet or more down a packing line a single illuminated red light is easy to see.

An estop PB should be RED and very distinguishable, the illumination should not be used for the color. This is also in NFPA 79.

Agreed.....we have several looooong packaging lines also....The stacklights do work out nicely for this. It's friday night, I have to go try have a life (outside of controls) for a few hours...Have a good weekend!

This brings up many confusing historical incidents.
I worked at Boeing and provided health and safety support to Facilities. We had numerical control on many many machines.
My first contact with PLCs was when I was asked to approve a drawing for a brake press. It had all the normal electrical stuff (fuses main contactor, lite shield etc etc)
BUT it had this box that said A B part number so and so. I went to the engineer and asked about it and was told it was a PLC made by Allen Bradley. He was a good controls engineer, so I trusted him.

My next confusion was when the machine tool operators and NC mechanics wanted disconnects for each of the drive motors. Their argument was that the drive motor could start up when they were changing out a tool because the programming was relied on to keep the motor stopped. The engineers talked about an E stop BUT that was not a switch that would interrupt power to a motor INSTEAD it was in the program. The engineers were concerned that if a disconnect were opened the computer would lose track of position. I tended to agree with the operators and mechanics for three reasons
1. I was on a submarine (Thresher clsss) We had auto controls BUT in every instance we had local override
2. This was shortly after the Washington State ferries computer controls were blamed for several very expensive incidents of the the ferries crashing the docks.
3. I did not understand much about PLCs at the time, and the engineers were unable to give me the education and knowledge to understand all this.

I asked for training on the whole issue but was turned down by my management.

Anyway here I am needing to learn about PLCs for different reasons and I still do not understand nor trust this E stop in the program. I do trust a switch or disconnect that physically interrupts the conductors -- I better -- now I haver returned to being an electrician and rely on them to keep me alive. I do not trust control circuits either - I use the main disconnect and lockout tagout and if I have to bonds and grounds.

There was also a mention of OSHA requirements. I have NOT kept up to date on this. I think what you will find in OSHA (or your state program) requirements is a referance to NEC. In many cases OSHA referred to NEC ASME ANSI etc etc, which is a good thing because those groupt update their requirements and recommendations more frequently.

SO to conclude
QUESTION ONE
WHAT IS THE DIFFERENCE IN RELIABILITY AND SAFETY for an E stop that is in the wiring control circuit versus one in the power circuit (ie a disconnect) and one that is "in the program". I have my own feelings / answers but want to hear yours.

QUESTION TWO
Is there a standard color code for indication lights on MCCs, motor starters etc etc. I walk into a MCC room and see all these red lights (indicating run). In the Navy red meant alarm or failure, white was power on, green was OK (green board was OK to dive), blue was for breaker shut, etc etc.

Thanks for help

Dan