Standard wiring technique

[mr_electrician 9]

Allrighty then, here is my new question to all those great people who don't mind teaching a new guy. My question is, what is common practice when wiring start and stop buttons to a plc. Is it common to hard wire more than one start button in parallel to and input, as well as more than one stop button in series to an input. I have never seen this done nor have I done it, but it would sure help on freeing up inputs on your plc. Also, why or why not to the above question. Thanks guys!

[paulengr 44]

First off, I don't use NC's for stop buttons. So wiring in series doesn't make any sense. A button is a button, regardless of what it represents from the PLC's perspective. Wire all buttons (at least those which form PLC inputs) as NO's because it is consistent and much safer. NC was used in the past for say a hard wired motor starter circuit simply because that was best for that application. The rules change with a PLC. Second, digital inputs are generally cheap. Wiring isn't. So I would typically put a remote input block at the panel and handle the logic at the PLC. Pulling a single communication cable and power cable back from a panel is far less costly than running several multiconductor cables to the same panel. And it speeds up troubleshooting and minimizes electrical noise since everything is all in one place. Finally, having multiple live start buttons at different panels is generally considered a bad idea for safety reasons. If you read NFPA 79 carefully (or the equivalent IEC codes), it specifically precludes you from doing this. It IS legal to have some sort of "which panel is active" control. But you should NOT have multiple start buttons active at any given time. The reason is simple safety. If say one person presses start while another presses stop, what do you expect to happen? The usual problem comes in the inevitable emergency situation. Two different people will simultaneously jump to the panel and try to control the equipment...and chaos breaks out. With multiple parallel input panels (except for HMI's where trying to pull something like this off is often difficult at best), I have generally made it a practice to have a "this panel is in control" type selector or button. This avoids the above issue. The alternate approach is to do this with "start" buttons only. All stop buttons would then be live all the time. This works but it generally tends to have variations of the same "two panels in control" issue. So in the end, I tend to take the approach of simply leaving only a single panel "active" at a given time. It avoids any possibility of confusion or who did what. "Emergency stop" buttons, whether as a general stop or a true E-Stop are different. Obviously E-Stops are hardwired together (well...not any more depending on the circuit design if the buttons are self-checking) and by Code, must trigger everything dead from a single action (pushing one button does it). Other types of stops may be more generalized and you have the freedom to be more creative with them. You may want a general "cycle stop" or "process stop" (one notch down from an E-Stop) active but leave everything else "dead". I did that recently for a pair of panels. That is the only button on the panel which is ALWAYS active. Another example: There are stop buttons on a large annealing furnace which is almost 300 feet long and 30 feet wide. There are 8 of them. In the event that somebody hits one or more, it is frequently difficult to figure out just which of the 8 buttons was triggered. Rather than wiring them in parallel, they are all wired to individual PLC inputs. Then it is a trivial matter for the HMI to display exactly which of the 8 buttons was triggered. And if you run into practical jokes and/or games and/or sabotage, it is also useful to be able to detect which buttons are pressed and when...again, only really practical with separate inputs. Another example: JOA switches. These are extremely common in the chemical industry. Each pump or other motor device has a "JOA" switch nearby...Jog/off/auto. Normally the switch is in the "auto" position. The off and jog positions (usually jog is spring loaded) are used for maintenance and lock/tag/try procedures (but not the lockout itself). That way you can conveniently test/run a motor manually directly at the motor and then release it back to PLC control as needed. There's nothing wrong with making the 3-position selector switch a "soft" (PLC inputs only) switch at all instead of hard wiring it to the starter. Again...it makes troubleshooting and maintenance easier. For instance, a hard wired JOA cannot be rigged to check for various interlock conditions unless you add a bunch more relays and other hardwired logic (which mostly just increases the number of potential electrical faults). Anyways...this may be a bit long and rambling but in general, wire all PLC inputs as NO (normally open) unless there is a specific reason to do otherwise (just because it's a stop button doesn't make it an exception). Wire the buttons with separate inputs. And do NOT make multiple operator control panels live simultaneously.

[Sleepy Wombat 27]

Thanks for summarising what i do also Pauleng... Excellent response

[ECSI 17]

I agree with paulengr on most things he said, except I always wire stop buttons normally closed to the PLC for two reasons: 1. If a problem does occur, it will be less confusing to an electrician who isn't that familiar with PLC's when he is checking the wiring. 2. If an operator presses start then the machine will start. What if a wire somehow gets disconnected from the contact block on the stop button? If its wired normally open, the machine will not be able to be stopped. If it is wired normally closed, the machine will stop. There are a lot of cheap buttons out there that have contact failures all the time. OK, so now you say that the E-Stop should take care of this issue, but you gotta admit that there are many many machines out there that don't even have an E-Stop, or if there is one its not readily available where the start/stop buttons are.

[mr_electrician 9]

I agree with using NC contacts for stop buttons for the same reason of the possibility of bad contacts, loose wires, and so on. I also agree that the plc doesn't care if the stop button is NO or NC. I will probably continue to use NC for my stop buttons.

[beegee 7]

I stopped using NC contacts (We only use them in E-stop circuits) to keep the logic simple. If you go to the PLC and look at the leds on the inputs you can easily see what inputs are activated. If you cable them NC because you are afraid of bad contacts, be aware that the unexpected and uncontrolled stop a machine can be as dangerous as not being able to stop with the normal stop switch. In case of Emergency you still have E-stops + we have stops located on different locations and on the touchscreen.

[gravitar 3]

We are still talking about a PLC input, right? I would hope that this stop button input would only be something along the lines of a "cycle hold", not something that would cause an uncontrolled, possibly dangerous stop!

[mr_electrician 9]

I am using stop buttons on my conveyors, and use NC contacts on the stop button. I obviously use NC on an estop cct as well which is hard wired to an MCR. My MCR is what controls my power feeding my output cards so that if there was an emergency and the E Stop was pushed the all my control voltage for the output cards are disconnected.

[paulengr 44]

That's what I do. E-Stops are hardwired separately as an all-NC circuit. When any E-Stop activates (they are wired in series), it drops the power to all output cards. It is also important to always provide an input to the PLC so that the PLC can reset any internal state it maintains. Otherwise deactivating an E-Stop can be even more hazardous than tripping one. Regardless of implementation though, this is one area where making everything as absolutely simple as possible is critical.