NFPA E-Stop Requirements 1

[Jeffersonian]

I'm not entirely certain I'm in the right forum for this question, but it's NFPA-related, so here goes.

What are the NFPA 79 requirements for emergency stops? I know they have to be hard-wired and remove power from all motion-producing components in a control system. Specifically, my question is:

* After an E-stop, can simply pulling out the depressed E-stop button or resetting the pull cord re-energize power to the motion-producing components, or is a control power restart required?

I've always used a self-sealed MCR, dropped by an E-stop and manually re-energized, as my motion output power disconnector. This requires a conscious decision on the part of an authorized person to re-start a machine or process. I'm now reviewing a drawing set where the designer wants to re-energize the motion outputs of the PLC when the E-stop device is reset and I'm uneasy about allowing it, given that it depends on the PLC program to prevent the re-initiation of motion once power is restored.

Thanks in advance.

 

[MotionDave]

Jeffersonian,
This NFPA stuff involves pretty complex issues, so I would recommend you consult someone for more detailed explanation.
Basically, for certain catagory safety stops, power cannot be reapplied automatically after pulling out an estop button in my knowledge. It depends on how much injury can be caused by your machine, but if it is more than a scratch, then you need to press a hardwired reset button to restore power after pressing Estop.

Most importantly, your MCR relay you use MUST BE safety rated so it fails to safe. The Estops must be wired into a safety relay, such as Pilz, STI or Schliecher safety relays, which are designed to never fail to the on position. If you wire Estops into a regular MCR relay, that relay could fail to the closed position, in which case pressing estop would not stop the machine and would cause injury. I recommend talking to a representative from Pilz or STI to get more information on NFPA requirements.

If someone is hurt on a machine not in compliance in the US, your company will be liable for damages.

 

[BradJ]

Jeffersonian,

My experience is similar to yours. We've always done a separate reset button.

NFPA 79 says this:

10.7.3 Restoration of Normal Function After Emergency Stop.
It shall not be possible to restore an emergency stop circuit until the emergency stop device has been manually reset. Where several emergency stop devices are provided in a circuit, it shall not be possible to restore that circuit until all emergency stop devices that have been operated have been reset.

Clear as mud? Wouldn't it be nice if they just came out and said exactly what was required? Maybe they should create a Handbook with examples like NEC.

There's obviously much more in 79 about E-Stops, but nothing I could see that would directly answer your question. Still, it's a good resource to have. You can download the .PDF from

http://www.nfpa.org

for a mere $38.25.

If you can't get an answer, trust your gut and make him add the reset PB.

-Brad

 

[wylde]

Many moons ago I called the NFPA to clarify this. The answer from them was that the system must not be able to auto-start after the e-stop circuit is reset. They do not care if just the individual buttons are reset. The overall must be reset after the individual e-stops are reset and then you may restart the equipment.

 

[gwooster]

NFPA 79:2002 9.2.5.4.1.1(3) states "Reset of an emergency stop circuit shall not initiate a restart."

The question is what constitutes "initiating a restart".

It is ok for the PLC to control the re-energization of the power. But no power to the motors shold be restored until the user has pressed "Start". The motors should not even be enabled until the user has pressed start.

Automatic restarts after resetting emergency stop are an absolute no-no. NFPA 79 says it, numerous ansi machine standards say it and CE (European) standards say it.

Its all about safety.