Hard-wired E-stops with remote I/O racks?

PaulKraemer

Member
Join Date
Jan 2005
Posts
24
Hi,

I design control systems where I use 24 VDC I/O. I always try to make my E-Stop circuit hard-wired (not relying on logic inside the PLC to stop/remove power from the necessary components that have to be "taken out" by an E-Stop).

I have a hard-wired E-Stop circuit that includes all of my E-Stop "mushroom-head" pushbuttons in series with a momentary contact "E-Stop Reset" pushbutton. When none of the mushrooms are pushed in and the operator presses the "E-Stop Reset" pushbutton, this energizes my "Control Power" contactor. The "Control Power" contactor is latched on at this time (latching method is hard-wired as well). If one of the E-Stop mushrooms is then pushed in, the "Control Power" contactor is de-energized. Pulling out the E-Stop mushroom does not re-energize the "Control Power" contactor. The "Control Power" contactor is not re-energized until the operator presses the momentary "E-Stop Reset" pushbutton again.

In order to make sure an E-Stop "takes out" what I need it to, I feed my "+24 VDC" lead into one of the normally-open contacts of the "Control Power" contactor. Coming out of this contact, I have a wire I'll call "+24 VDC (E-Stopped)", which is only engergized when the "Control Power" contactor is energized.

I use this "+24 VDC (E-Stopped)" wire to power my discrete 24 VDC PLC outputs. This way, all of my PLC outputs will not be powered in the event of an E-Stop, even if the PLC was malfunctioning.

This works fine for me when I have everything in one panel. On larger machines, I have three control panels. My PLC / local I/O rack is in one panel, and I have remote I/O racks in the other two panels. My PLC communicates with these remote I/O racks over Ethernet/IP. Ideally, I would like my three panels to just be connected by an ethernet cable. However, I don't see how this is possible if still want my E-Stops to be hard-wired, and I need an E-Stop to "take out" my PLC outputs in my remote I/O racks,

Right now, I think that I will have to send my "+24 VDC (E-Stopped)" wire out to my remote I/O racks and use this wire to power my 24 VDC PLC outputs in the remote I/O racks, just like I do in my main panel (local rack).

I can do this, but I was just wondering if anyone else has any other suggestions that might be better. (I use Allen-Bradley ControlLogix PLC's for the most part, but I would be interested in hearing how any of you handle this sort of thing in other brands as well.)

Thanks in advance,
Paul
 
If you want hard wiring then you have to hard wire the estop chain panel to panel or do them in zones. I would make each panel a zone and maybe several zones in each panel depending on how many estop buttons you have and how large the machine is.

If you want networked safty then you need a guardlogix controller or a guard plc to do safety control over ethernet. There is no other legal and safe way. If you want to do safty over ethernet then there is no need to chin estops to each other as they should all be on their own input so you get the location and diagnosti benefits of networked safty. If you chain 30 estop to one input on ethernet safety it is kind of defeating the purpose of networked safety IMHO.
 
I agree with The PLC Kid on this one and think you should do what you stated here which is the best way to handle your problem you have.
Right now, I think that I will have to send my "+24 VDC (E-Stopped)" wire out to my remote I/O racks and use this wire to power my 24 VDC PLC outputs in the remote I/O racks, just like I do in my main panel (local rack).
 
Actually, nothing you described about how your e-stop is wired could be considered "safe".

First, what does your risk assessment state that you need for the SIL level of the safety devices?

Second, unless it is all a level 0, then you need safety rated pushbutton, and safety relays (dual redundant with force guided contacts).

I would suggest that you do a little more research and design the system better. Your way could lead to someone really getting hurt.

I have favored using the Banner SC22 safety controller with the associated relays. You could have mulitple zones just like PLC Kid describes, but I would still hard wire the e-stops and not use networked safety (I haven't made that leap yet).

There are several other sites that can help describe a better safety design. Please don't just use a contactor that latches in via one set of contacts on the estop and the "reset pushbutton".
 
But I would still hard wire the e-stops and not use networked safety (I haven't made that leap yet).


Bruce

I have 5 big boys here that have around 500 safety I/O between all the door switchs and estops and area guards, We also have safe off in the drives and safe speed on many secions of these lines.

They would have just been too expensive to do with hardwiring and provide diagnostics now pretty much each door is it's own input. You know where the problem is from the HMI no searching. In the past the baby brothers of these line which were almost half the size took hours even days sometime to find a faulty safety device because they were so many in each zone and a intermittant device would drive you nuts.

I like the safety I/O for thsi reason. I think if you do decide to try a safty I/O system you will really like it.
 
I use Allen-Bradley ControlLogix PLC's for the most part, but I would be interested in hearing how any of you handle this sort of thing in other brands as well.
Since you use ControlLogix, I would suggest you look at GuardLogix to handle safety functions. You can achieve SIL3, Cat. 4, PLe using distributed safety I/O.

Our design does not have a single hard-wired emergency stop button in it. Everything is PAC-controlled over EtherNet/IP. Once you get over the initial "You got to be kidding me, trust my life to a PLC and a network?" reaction, you will find it makes the design much less messy because you don't have to design long chains of emergency stop buttons and interlocks.
 
So use two wires in the cable for your Estop for example.
have a look at dold safetyrelais.
Just have all the estops in series only costing two wires. make them double switches so you can see where the button is pressed in your PLC.
 

Similar Topics

Hello! I have a control panel with two hard-wired push buttons. One button to switching on, and another one to switching off. And I have an HMI...
Replies
5
Views
2,022
Hello, Was wondering if anyone could improve on my "first up" circuit. I'm sure there is a way but cant quite get it! What I've got is 4...
Replies
17
Views
5,635
Now I know from hearing through the grap vine that hooking up a hardwired Motor overload with a AC VFD is no good. I am not sure why you would do...
Replies
7
Views
4,199
Hi - I'm a newbie to the PLC world but was recently asked if we could store more than the 2G that the CompactLogix 5370 allows by its max size SD...
Replies
13
Views
1,252
It's Saturday, so I thought it would be amusing to look at times gone by, imagine if you had to carry this to site "IBM 5mb Hard Disk 1950's.
Replies
16
Views
1,803
Back
Top Bottom