E-STOP used as ON/OFF?

[Guest Bear]

Hello, I have a simple machine (singe ac motor with ac inverter) where a previous employee has retrofit a maintained estop button. It is wired to break the supply voltage(120 vac.) There is also a fused disconnect to shut down the machine. I'm not quite sure I'm comfortable with this for 2 reasons: 1) there may be a risk of shocking the operator if the button breaks, and 2) there is a risk of button malfuction in which case it won't stop the motor. Although I'm a little uncomfartable with the setup, I can't really see any difference between the E-STOP and a ON/OFF selector switch. The E-STOP is rated for the voltage and amperage of the machine. So, are my concerns valid? Do E-STOP buttons malfuction/fail? Is this a common use of E-STOP buttons? Is this an acceptable use of an E-STOP? Is it a recommended use? Thanks.

[KinK 0]

Ummm I wouldent worry about the voltage issue as long as it has a CSA or other safety stamp on it (for your local area) then it has been designed and tested at the maximum rated voltage and amperage.... Its hard to say with out all the info ie how is the drive programmed if there is a loss of power. Normaly I like to have a momentary pb for estop and then a seperate start button with a contactor to supply the power to the motor and inverter. that way the operator knows exactly what he is doing when he pushes the button. IMO. Edited 19 May 2004 by KinK

[JimRowell 1]

I agree with Kink. It's not acceptable if the motor will restart when the E-stop is released. I realize that a switch would also allow this but an e-stop, by definition, indicates to people that they can release it without startup occuring. If your invertor requires a manual restart action then I suppose you could argue that the setup is ok (I still don't like it). The other thing you need to concern yourself with is the HP rating of the e-stop contacts. Most e-stop contacts will have a very low rating. It may not even have a HP rating (just a current rating which is meaningless) in which case you shouldn't use it. No fun if the contacts weld on the button. You are also encouraging constant use of the button since it sounds like the only easy way of turning off the motor. And everytime you operate it, the e-stop is handling the full current of the motor. Worse, if the motor burns out and blows the fuses, someone will change the fuses and then operate the e-stop to see if it's fixed. They'll blow the fuses again before changing out the motor. Meanwhile, because of that last test, they've really done a number on that poor little e-stop, possibly even welded its contacts closed. So now, when the new motor is put in, you no longer have a stop button. Or worse, you have one which will fail a little bit down the road. I'd suggest you install a convenient on/off setup using a properly sized contactor with an e-stop breaking the contactor. Contactors are made for this sort of thing. Edited 20 May 2004 by JimRowell

[Sleepy Wombat 27]

Has a safety analysis of the machine been carried to determin the category type safety circuit required. If Cat 3 or 4 then you need a saftey monitoring relay for the E-sTop circuit, then the Safety Monitoring realy contacts cut the control circuit to the contactor and the contactor provides feed back to the safety relay to make sure that indeed it did realease (not weld) and also two contactors sould be used..... Do not try to take a minmilistic approach when it comes to safety. ...... As suggested, do not use the E-stop for standard stop operation....it is what the name implies Emergency Stop.....

[Guest Doug C.]

Estops should never be momentary; if the start or enable is stuck down, then the Estop would allow restart after it is released. -Doug C

[KinK 0]

The only reason I have push and hold estop buttons is so that I know which one was pushed, I always use a latched circuit that is held on(energysed) that way if a wire is cut or a button is pushed the Estop goes and you have enable the Estop that was pushed hit the Ack button on a diff pannel and then hit the restart button on whatever was stopped. Peace

[Pierre 1]

On the question of E-Stop failling You better darn be ready for it to fail!!!!! Look at how it is built. You have a NC contact block screwed or clipped to the back side of the operator (in this case a mushroom PB). Hold only the contact block in your hand when its not installed. Its contact are closed. Soooo the juice goes through. Imagine that you have your hand inside the machine and this clip or screw get loose. You are dead! This is why you need a reset circuit so any reset is intentional. I have experience of a coworker (operator) who lost 2 fingers cleaning the exit of a screw conveyor. The button failed, the system started, he had his hand like hamberger meat. That was before I went into PLC programming but I still remember.

[b2barker 0]

Most E-Stops I come across have two contacts. One is hardwired to break the supply voltage, and the other goes to PLC. This means either contacts opening will stop the drive, and it won't restart until the fault is acknowledged from the HMI. Some sites use rising edge of a E-Stop reset to clear faults on the drive when it is in Local to facilitate field testing without continual contact with remote HMI operators, but you would still have to push a Local start PB after you reset it. If your drive is not PLC control maybe consider getting a double NC contact E-Stop and connecting them in series. At least then if one weld you have a fallback. As I mainly work in the Mining industry here in Western Australia, using E-Stops for isolating a drive to stick yours hands in the moving bits is banned. You have to isolate the drive at the MCC an apply a personal padlock to which only you have key before working on the nasty stuff. Operating an E-Stop and sticking my hand in would see escorted to the gate and never to return on every site I have worked on in this state for the last ten years. Edited 9 Oct 2004 by b2barker

[Pierre 1]

That makes a lot of sense. Injuries are not always a results of idiots operators. What messages are transmitted from top to bottom in particular industries sometimes results in someone loseing a member. I have installed some systems which where operated by a single operator and the length of the production line was about 100 feet. When I revisited this place they had replaced ALL my Push-Pull type EM push button with momentary ones. There reason was that the operator had to walk to each EM PB to release them when he wanted to start the line. Wow! What a waste of time! So he would be at his post and Start the line. The message here was what? One of the thing I tought was that at this place there EM-PBs where used as Stop signal. That was why they had to re-Start them line often. Is this a good enough reason? No so. Sometimes there is a mechanic working on the line. Maybee under a conveyor...

[KinK 0]

ALWAYS I REPEAT ALWAYS LOCK IT OUT, I will not work on something unless the power has been removed compleatly or it is locked out and I have the key in my possesion

[Pierre 1]

Of course but there is times when you have no choice. Like when troubleshooting or adjusting or testing products or just going on coffe breack. Edited 13 Oct 2004 by Pierre