NFPA 79 - Operator Stations - using more than one

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john.mays@westrock.com

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Hello, I'm new to the forum and hope someone here can help with a question that has been debated in our plant for years. We are a OEM builder so This is in reference to the NFPA79 Electrical standard for Industrial Machinery:
9.2.7.5 Use of more than one Operator Control Station, Where a machine has more than one operator station, measures shall be taken to ensure that only one control station shall be enabled at a given time.
In many cases, our machines have multiple control stations because conveyors in the system make access to each side of a machine time consuming to move around the machine. Going by the above code, we are struggling with how best to apply it or even if it applies to our machines at all since they are fully guarded.
Has anyone had any experiance with this and multiple control stations?
 
how hard is it to add an "enable control" PB at the station and a light indicating the station is the one in control.

personally, I have not run into this issue very often because it is rare that the same function would be controlled from multiple locations.

we do have one customer who has a main console for control and some remote PB stations as well. There is a selector at the console for each remote PB station to select if the console or the remote PB station is in control of that chunk.
 
Petersonra,
Thanks for replying. It's not hard to add the button. Just have to prove to our management team it's necessary, they feel it defeats the purpose of having two stations because if one station is under the control and they forget to remove that control, ie turn off the selector switch. Then the operator has to walk around the machine, turn that control off and then walk back to where they want control at.
Trying for cleaification of the code and it's intent.
Our customer wants to be able to turn the machine on in two places. These machines are fully guarded and operators can't access the moving parts. All guards have to be closed to reset the safety relay prior to starting so it's not like someone can turn on the machine and hurt another person.
I can see this type of control in areas where there are pinch points and rollers, like feeding a barrell roller with paper and jogging the rollers.
 
The argument about having to go back to the originally active station to release control is overcome by using latching relays and momentary pushbuttons to allow seizing control from the new station.
This design makes all panels unsuitable for lockout use, but it sounds like the safety relay or a hard disconnect is the right place for that type of function anyway.

Sent from my XT1585 using Tapatalk
 
More than one?

More than one?

I was taught as many stops as you want but only one start unless you had local/remote switchs.
We did what someone else suggested and used latching circuit with pushbuttons as a way of taking control of the start, but we had it so they had to be held in for x amount of time and sounded a horn at the other point of control before it took control.

I just got a machine in last year made in USA with start buttons at two points no interlock between them.
 
just the cowboy said:
I was taught as many stops as you want but only one start unless you had local/remote switchs.
We did what someone else suggested and used latching circuit with pushbuttons as a way of taking control of the start, but we had it so they had to be held in for x amount of time and sounded a horn at the other point of control before it took control.

I just got a machine in last year made in USA with start buttons at two points no interlock between them.
Click to expand...

This requirement had me puzzled, and I kept thinking back to printing presses that always had setups very similar to what you describe. Only one true "Run" button at the main control panel, but there were "Jog" buttons all along the presses. These turned everything at slow speed, and they had to be everywhere so you could use them to thread paper or to clean rollers. But there was always an estop by every jog button, and the jog button always rang a bell before starting the press.

Now that I think about it, if one person started the press with the jog button, the press better stop when they let go. If someone else pressed another jog button, and it kept running unexpectedly, it could be a really unpleasant surprise before they had a chance to hit the estop.

I'm wondering if it was even such that if two people hit different jog buttons at the same time, the bell gave a different sound, and the press wouldn't move. It seems logical to make sure one guy doesn't think his finger on the jog button is controlling the press when someone else at another station actually hit another button just an instant earlier.
 
steve66 said:
This requirement had me puzzled, and I kept thinking back to printing presses that always had setups very similar to what you describe. Only one true "Run" button at the main control panel, but there were "Jog" buttons all along the presses. These turned everything at slow speed, and they had to be everywhere so you could use them to thread paper or to clean rollers. But there was always an estop by every jog button, and the jog button always rang a bell before starting the press.

Now that I think about it, if one person started the press with the jog button, the press better stop when they let go. If someone else pressed another jog button, and it kept running unexpectedly, it could be a really unpleasant surprise before they had a chance to hit the estop.

I'm wondering if it was even such that if two people hit different jog buttons at the same time, the bell gave a different sound, and the press wouldn't move. It seems logical to make sure one guy doesn't think his finger on the jog button is controlling the press when someone else at another station actually hit another button just an instant earlier.
Click to expand...

is a printing press covered by NFPA79?
 
just the cowboy said:
I was taught as many stops as you want but only one start unless you had local/remote switchs.
We did what someone else suggested and used latching circuit with pushbuttons as a way of taking control of the start, but we had it so they had to be held in for x amount of time and sounded a horn at the other point of control before it took control.

I just got a machine in last year made in USA with start buttons at two points no interlock between them.
Click to expand...

Thanks everyone for the input.
We have E-stops and Stops at each station which always work regardless of the station in control, that's part of the Exception note on 9.2.7.5 "A stop command from any one of the control stations shall be effective where necessary for the safety requirements of the machine."
I like "Just the Cowboys" idea on having the annuciation at each point of control and having to hold the Control switch on for x amount of time.
At least that meets the requirements and notifys anyone around the other control station that something is happening.

I don't think a large number of Machine builders use the NFPA79, even though it is now in the NFPA70, article 670.1 as an informational note.
 
petersonra said:
is a printing press covered by NFPA79?
Click to expand...

Probably not, although I really have no idea since I don't have a copy.

Its been so long since I've worked in that field, I can't even remember for sure if that's how they worked. But it would make sense. They would probably be designed with similar safety standards or practices. In fact, I really wouldn't be surprised if the NFPA 79 requirement came from printing presses. After all, they were probably around before Ford's 1st assembly line.

Anyhow, my main point was just that I never realized how much thought could go into the controls just to jog a printing press. The requirement is starting to make sense to me.
 
I've done it like this, in a "tool" (machine) with multiple matching "cells" that were in a long robotic transfer system moving the product from one cell to the next for different processes (making the glass for touch screen monitors). In their case there was typically only 2-3 operators covering the tool with 30 cells and if they stopped one cell, the tool had to know and ignore that cell in the processing (every cell was redundant so as to not stop production). So that meant each cell could be individually restarted in mid stream, but that was potentially dangerous. So the Start buttons were set up on a timer circuit and to restart, the operator held the Start button down while the timer locked out any other Start buttons on cells that were down at the same time and set off a buzzer to warn other operators that a cell was restarting, then after 10 seconds or so, the cell started and he could release the Start button. This was also tied into a Safety Relay system watching all of the guarding, so if any Safety Relay was tripped on that cell or the robot transfer system, it couldn't restart. As per usual, all Stop buttons functioned always, and E-Stops shut down the entire tool. The E-Stops had thin plastic clear covers on them that broke when pressed, so they would know which one caused the shutdown. That stopped lazy operators from using the E-Stops at beak time...

After that experience, I used a similar concept on multi-segment conveyor lines in rock plants. Any Stop worked, but Starts had to take place in sequence to avoid having product pile up on non-moving belts. Control was usually done in the control tower, but each line had its own Start-Stop station near by, so the Start buttons has to be enabled in sequence (and tied into the Rope Pull switches).
 
Not sure how all of this relates to the very common use of distributed control systems in industrial plants where operations are controlled from multiple video touch screens, any one of which can, in theory, control any device in the system.
 
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