best spot for E-stop button in a circuit?

Hi,

Should a E-stop button connect to the input of a PLC or directly on the output side of the coil?

That is


--E-STOP(Input)-----PLC------Coil
If E-STOP is pressed, PLC detects it and turn off coil.

OR

----------------PLC----E-STOP----Coil?
E-STOP is pressed and breaks the circuit, and hence stops the coil.


I read a lot of articles
for example , page 11 at
http://w3.siemens.com/mcms/sce/de/f..._EN_010-030_R1209_S7-1200_Timers_Counters.pdf


They all pictured that E-STOP was connected to the an input of plc. But isn't the second approach better?

Thanks

Those examples are really showing an auxiliary contact from the emergency stop button being used in an ordinary PLC program.

In practice, the actual emergency stop button acts through its own contacts or through a safety relay to remove power from the load directly.

Unless you are using a controller specifically designed for safety applications, you should never rely on the controller to perform a safety-related function.

I would say Yes your second approach would be better.

I never use an estop as an input to the PLC to Emergency Stop equipment.
I make the E-stop drop out control power to the contactor if a standalone piece of equipment or make it drop out the coil of a MCR if controlling several pieces of equipment.

I will often use a second contact on a E-stop into the PLC input but only to monitor and display which E-stop has been activated if I have multiple E-stops in series.

BCS

EDIT: Ken types faster

Thank you very much guys.

Just cant shake a nagging feeling about your thread here.

I am not a safety controls expert (or expert at anything for that matter) but lets push your discussion a bit further and maybe with any luck an expert on the subject (like in post #2) may jump in and take it to the next level.

In your example I would and did say the example # 2 is more correct than example #1.

But I don't really feel that your example #2 is 100% correct either.

Reason for this is dependent on many things but in general from the way you portray your example:

You show voltage coming out of a PLC Output > Then through a N/C E-stop contact > directly to a coil.

What's giving me a nagging feeling about this is what is going to happen when you pull that E-stop back out and close its contact again?......your coil is going to energize right away.

I personally would not want my equipment to start up right away after an E-stop has been implemented and then cleared.

I would want the E-stop to break a "Seal In' Circuit, (or control to a safety relay if you are working on an application that would require that).

This way once you close your E-stop circuit, an additional Run/Start command or action (pushing start button) will have to take place in order for the application to start running again.

This would be a much more acceptable approach to this in my opinion.

If you are not sure what I am talking about let me know and I can make you a simple drawing that may explain this better.

BCS

Can you give us more information, or what you're actually looking at doing?

In my mind, if this is a safety function (Especially relating to AS4024), then nothing is correct, and what's being described is questionable. I'm hoping this is some form of control feedback or signalling on an E-Stop.

An e-stop should be used with a safety relay and a master relay.
When the e-stop is pressed, the safety relay trips out which in turn de-energizes the master relay.
The master relay contacts typically kills the inputs, outputs, ac / dc power depending on your application.

We typically do the following.
Press the e-stop reset pb to reset the safety relay.
Press the power on pb to energize the master relay.
The master relay will then energize the ac / dc power to the inputs and outputs. A master input is designated as power on.

When the e-stop is pressed, the plc input will turn off.
The plc will then turn off the plc output logic.

You MUST research your local requirements.

Remember, the plc program CANNOT resume when the e-stop power is restored. There must be manual controls to return the machine to a safe condition. When the system is restored to a safe condition, start the automatic cycle.


regards,
james

The best way I have seen to explain how an e-stop should function was posted by Ron Beaufort awhile back in a similar post. I liked it so much I "borrowed" it as my signature for here as well as my work email.

Hitting the E-Stop button should "shut down" the machinery
WITH – or WITHOUT – or IN SPITE OF – whatever the PLC decides to do – or NOT to do - about your "emergency" signal.

The e-stop circuit, in my opinion, should to be indepentent of the PLC program, and used in conjuction with properly rated contactors. The resetting of the e-stop circuit needs to be independent of the PLC as James explained above. I know there are now Safety rated PLC's on the market, but I for one will still use a seperate e-stop circuit that only provides notification to the PLC. The actual safety side of things will remain a seperate system.

I don't really have a thing to achieve. I am just in an early stage of learning PLC.

I used to think E-stop just a NC button, until I saw you guys talking about safety/master relays which I have no ideas.

Sorry guys, without graphs I just couldn't follow all your ideas.

arad said:

I don't really have a thing to achieve. I am just in an early stage of learning PLC.

I used to think E-stop just a NC button, until I saw you guys talking about safety/master relays which I have no ideas.

Sorry guys, without graphs I just couldn't follow all your ideas.

Click to expand...

Not a problem, we all had to start somewhere. Start by making a distinction between "Control" and "Safety". These systems can work together, but are usually independent (As a simple example). Think a PLC and a Safety Relay.

Safety devices are rated, approved and certified for use in that type of application. Safety systems are designed to a set of rules or guidelines that ensure there is some level of monitoring and/or redundancy involved so that failures of the system don't result in a loss of safety function.

In Australia we have "Category", "SIL", or "Performance Level (PL)", for varying degrees of safety. Depending on the requirement and level; the safety system can vary in character. For example, if your E-Stop needs to be 1 channel or 2.

So, wiring from your E-Stop to your Safety Relay is the safety system, and wiring from your E-Stop to your PLC is your control system. As mentioned above, wiring from an E-Stop to a PLC is for the purposes of knowing when your E-Stop has been pressed. It is not responsible for actually stopping the machine, process, or removing energy from the system.

Have I confused you more yet?

arad said:

I don't really have a thing to achieve. I am just in an early stage of learning PLC.

I used to think E-stop just a NC button, until I saw you guys talking about safety/master relays which I have no ideas.

Sorry guys, without graphs I just couldn't follow all your ideas.

Click to expand...

Hopefully this will help. Attached is a quick drawing of a very simple e-stop and motor circuit. The safety portion of the circuit is not in any way controlled by a PLC, but it does provide an input to the PLC that can be used in the program. However the safety circuit itself will shut down the machine with, or without, or in spite of the PLC. One thing you will want to remember is that words like "safety" and "e-stop" have defined meanings in the control world, and the devices used need to be "safety rated". That is why you got so many responces so quickly trying to get you on the right path. Safety is serious business, and people will try very hard to make sure it is done correctly. Not only to protect the people using the machine, but also to protect the person who designs the system. Best of luck to you as you go forward with your learning!

Will.

An E-Stop is just a button that stops the machine from running and prevents it from running. How robust the circuit is completely depends on the safety level you are going for. There are official categories called SIL (Safety Integrity Level) and PL (Performance Level), as well as the older Category levels that are being phased out.

Your first example would not be considered a safety circuit, as in it is completely unsafe and you should just call it a stop button, not an Emergency Stop button.

Your second example would be in the lowest safety level, since it isn't redundant and you can't detect failure.

A proper E-Stop button has two contacts in it, one NO, one NC, so when you push the button, the signals trade (one loses voltage and the other gets voltage) and is wired to a safety device (Safe PLC or Safety Relay) that then measures the time it takes for the NO and NC signals to switch. Many of the better safety devices will also send pulses through the button and read them back to make sure the voltage is coming only from the safety device and there isn't a short.

The safety relay or safe PLC will drop power or safe enable to any devices that need to be stopped in an emergency when the button is pressed. The old school relays physically turn off the power, while Safe PLCs often rely on safe enable signals to drives and whatnot (which are just as reliable as physically turning off power).

2 PLC's

We just got a new machine in and it has 2 PLC's in it. One is a safety PLC that gets all the estops and guards in to it and it shuts off power to the second PLC outputs and gives a input to stop the logic. This way the safety PLC don't have all the control logic scan time in it so it can act faster. Also the HMI can read the guard and estop states from the safety PLC.

Do you guys have some E-stop button models for recommendation?

That way I can see what requirements are considered when choosing/implementing an emergency stop system.

Thanks

Just like everybody else I will say that an emergency stop button must be capable of making the device safe. This is not done through the PLC.

However we do have applications where we utilize multiple contacts on an emergency stop button. One set would go to a safety relay or a MCR. The other set goes to the PLC. In this way we can make the device safe but also communicate to the operator through an HMI. On the HMI there is an alarm message like "Load Zone E-Stop Hit"