PLC E-Stop Feedback

Hello Guys,

I'm curious about what E-Stop/Control Power On signals are commonly used on your equipment as Feedback Signals into the PLC. We have a small conveyor system used for educational purposes with an e-stop circuit consisting of 2 E-Stop Pushbuttons, a Safety Relay and a contactor for control and motor Power. Currently we have an NO Signal through the contactor to the PLC to indicate that control power is on. We were also thinking of adding extra NO contacts to each of the E-Stop pushbuttons connected to 24V as inputs to the PLC. This way we can tell if an E-Stop button is still pressed, and which one rather than just if the control power is on. Is this a common solution, and do you think it is necessary? We were going to take a connection directly from the E-Stop Input circuit but it seems the safety circuit that checks the E-Stops uses something like 6V.

Thanks!

On the systems I build, the PLC is notified when the e-stop loop is broken. When the loop is broken, it opens the safety relay and drops control power to the motor drive supply contactors. The PLC isn't involved with opening the contactors, but it does know about the e-stop loop has been broken so the HMI can log the e-stop actuation.

In my case, my e-stops are all local, and usually within visual range of the operator, so this isn't a problem. My systems are also strange since they're reconfigurable and plug-and-play. I build theatrical automation equipment, and they need to be reconfigured on a show-by-show or at least a season-by-season basis, so everything from the power distro (also handles e-stop duties) to the drive cabinets to the e-stop stations are all cable connected and modular.

Having more info on which e-stop is tripped is always a good thing; it helps with getting the equipment back to normal operation and with troubleshooting. If it's easy to add that functionality, go for it. Especially on larger machines, knowing WHICH e-stop has been tripped is very helpful.

You're probably seeing ~6v on the e-stop loop as your safety relay may check the loop with a pulsed signal rather than simply the presence or absence of voltage. I presume this makes an e-stop button harder to wire past.

-rpoet

I at least use the estops with lights on them so you can tell at a glance if one is pushed. Feedback is good so that you can see why the machine is not running at a glance on the HMI.

Also, sometimes you may not want the machine to start up when the estop is reset because the machine may need to be cleared, or there is a particular startup order that needs to be followed. In this case the estop feedback could be used to reset running conditions and put the machine logic into a start from the beginning phase of the logic state.

jtn said:

I at least use the estops with lights on them so you can tell at a glance if one is pushed. Feedback is good so that you can see why the machine is not running at a glance on the HMI.

Click to expand...

+1 on lighted buttons. Easy to troubleshoot.

jtn said:

Also, sometimes you may not want the machine to start up when the estop is reset because the machine may need to be cleared, or there is a particular startup order that needs to be followed.

Click to expand...

OSHA regs state that resetting an w-stop CANNOT restart the machine. Resetting an e-stop can ONLY place the machine in a state whrere it can be reset by a separate action (press RESET button, for instance).

-rpoet

Hi

On the estop reset it can only be reset from one reset button and it should also be blue


Donnchadh

Donnchadh said:

On the estop reset it can only be reset from one reset button

Click to expand...

This is a requirement I'm unfamiliar with. Nothing in the US and even ISO and IEC standards, that I'm aware of, prohibit multiple reset buttons.

I'm not saying there aren't solid reasons for using just a single reset button, merely it isn't an absolute requirement.

I think that is a good idea.
If you have an HMI, then take a picture of the E-stop, and make it visible when open.
If you have more than one E-Stop, then they each get their own PLC input.
The more information that you give the operators, the easier it is on you.

jtn said:

Also, sometimes you may not want the machine to start up when the estop is reset because the machine may need to be cleared, or there is a particular startup order that needs to be followed. In this case the estop feedback could be used to reset running conditions and put the machine logic into a start from the beginning phase of the logic state.

Click to expand...

Somewhere, I thought there was code that said the machine could not auto re-start after E-Stop reset. IE.....motors are required to have Stop/Start switches, not a single On/Off switch, because when you reset E-Stop all the switches in "ON' would immediately initialize.
Using Start/Stop, after reset of E-Stop you would have to come back and re-start each motor to make a progressive start-up.

NetNathan said:

Somewhere, I thought there was code that said the machine could not auto re-start after E-Stop reset.

Click to expand...

Take your pick:

NFPA 79 section 9.2.5.4.1.1(3)
ISO 13850 section 4.4.5

are two I can cite chapter and verse.

I know CSA Z432, ISO 14118, ANSI B11 also have similar wording.

Timbert said:

This is a requirement I'm unfamiliar with. Nothing in the US and even ISO and IEC standards, that I'm aware of, prohibit multiple reset buttons.

I'm not saying there aren't solid reasons for using just a single reset button, merely it isn't an absolute requirement.

Click to expand...

Sorry Timbert,
I was unclear. What I meant to say (and thought I said, at least in my head ) is that the e-stop actuator itself cannot be remotely reset; it has to be twist-to-release, and cannot be momentary and reset-able elsewhere. I got a little lost in the weeds. Apologies.


-rpoet

Donnchadh said:

Hi

On the estop reset it can only be reset from one reset button and it should also be blue


Donnchadh

Click to expand...

The E-Stop can be zoned with each zone having its own E-Stop reset and safety relay. However a single blue master reset button to reset a master safety relay once all zones are reset. That is what I was recently advised to implement on a project for CE etc. Think of each zone safety relay wired 2 channel back to the master relay similar to e-stop buttons, and each zones e-stop buttons wired 2 channel back to their zones particular safety relay.

I would add I wished I had used a programmable unit in the end to simplify the wiring arrangment

Timbert said:

Take your pick:

NFPA 79 section 9.2.5.4.1.1(3)
ISO 13850 section 4.4.5

are two I can cite chapter and verse.

I know CSA Z432, ISO 14118, ANSI B11 also have similar wording.

Click to expand...

Here is where I found it before..
http://www.eng-tips.com/viewthread.cfm?qid=68965

Now to find my copy of NFPA 79: "ELECTRICAL STANDARD FOR INDUSTRIAL MACHINERY". Actually everyone that does control panels should have a copy.

If you register at NFPA you can read a lot of specs free.....including NFPA 79.

On the specific question about 'do you use contacts on the e-stop button for indication', I always include a normally closed contact right on the button whenever I have more than one button.

I do a lot of post-event investigations, and one of the things I find very helpful is to be able to tell for certain when an E-Stop button has been pressed. It helps distinguish between a momentary power loss that dropped out a seal-in circuit, or a loose wire on the safety relay, or the machine actually being turned off at the main disconnect.

I always also try to have enough power and safety system inputs so that I can tell the difference between a power cycle and a E-stop cycle or a PLC program download or HMI computer reset.

Of course, that's still not enough. This morning I had a system that was complaining of ControlNet connection failures, blown fuse detections, and drive network faults. It turns out all of those were symptoms of a loose main power lug. Fortunately you can find those just by looking for the big blue sparks.

rpoet said:

I was unclear. What I meant to say (and thought I said, at least in my head ) is that the e-stop actuator itself cannot be remotely reset; it has to be twist-to-release, and cannot be momentary and reset-able elsewhere.

Click to expand...

That is clear and correct. An E-Stop Device can only be reset manually at the device itself.

I agree all this can get confusing. Especially when you talk about machines that need the e-stop button reset, a safety relay reset, then a separate re-start. All of which I've seen, individually or collectively, described as 'reset'-ing a machine.

Great, thanks a lot for the info, guys. Looks like an extra contact on each E-Stop button is the way to go for PLC feedback.