Category 3 (EN954-1) E-Stop of conveyor / motor starter

[soulman]

We have a conveyor application. Safety is a big issue for our client. In the past we had always wired a pullcord switch in series with the motor starter coil.

After looking into the requirements of EN 954-1 it looks that we have to design our circuits in accordance with category-3. That means redundant E-Stop switch contacts, wired into a cat. 3 E-Stop relay (e.g. Pilz, OMRON).

Do you agree so far?

Now my question: Since redundancy is required for cat. 3: do we have to have a fail-safe motor starter with dual contactors, wired in series, in case the contacts weld on one unit? That can become quite expensice for larger NEMA seize motors.

EN954-1 is probably not applicable in the U.S. I looked into NFPA79, but that one is not very specific.

Thanks for your feedback.

Peter

P.S.: If you like, you can also comment on a different issue: The typical E-Stop scheme for a VFD. We prefer not to shut-off the power to the drive immediately; so we connect our E-Stop to a time-off delay relay, tripping an incoming contactor after the machine has coasted to a stop. But I doubt that this would be sufficient for cat. 3, specifically because of the electronic relay (not electro-mechanical).

 

[mc5w]

You can get self checking control relays form either Telemecanique or SquareD ( both are now the same company ), for this application. There is a version that has time delayed outputs that allow a VFD or soft start to be shut down correctly.

You do need double motor contactors each with a normally closed contact that confirms that the contactor is open before the start button is allowed to work. The contactor has to be built so that the normally open power contacts cannot be closed at the same time as the normally closed contral contact, what is known as mechanically linked. As far as I know, only Telemecanique and Klockner-Moeller care to make motor control contactors that meet this requirement but I could be wrong because I am completelu unfamiliar with Cutler-Hammer and Sprecher+Schu.

You can get just about any amount of safety for the right price. British coal mines have had ZERO electrocutions since 1963 and they have 1,100 volt and 3,300 volt extension cords. This is because someone figured out how to make extension cords up to 13,800 volts that are as safe as breakfast food. You still could choke to death on your breakfast.

Mike Cole, mc5w@earthlink.net