In the past... we have used two hand anti-tie down palm button logic in the programming of standard PLCs... we are now questioning the validity of that practice.

Is there a definitive statement on the acceptability of using the inputs of standard PLCs as safety inputs? If the proper two hand redundant safety relays are used in conjunction with the palm buttons, and the rest of the machine is properly guarded, is that acceptable practice?

 

I don't know were you come from, but in Europe this is absolutely forbidden. The production line will be closed if inspectors find this kind of safety features, and you'll be fined.

For more info try the Pilz website.

success,

Gerrit Jan

 

Check out NFPA 79 (Electrical Standard for Industrial Machinery 1997 Edition) 9.6.3.

"Where a Category 0 stop is used for emergency stop function, it shall have only hardwired electromechanical components. In addition, its operation shall not depend on electronic logic (hardware or software) or the transmission of commands over a communications network or link.
Where a Category 1 stop is used for the emergency stop function, final removal of power to the machine actuators shall be ensured and shall be by means of electromechanical components."

 

The answer depends upon the machine, the hazard, and the laws of the country you are in. The typical approach is for the two hand control relay to be in series with the output of the PLC (between the PLC and the device being controlled).

 

As far as machine start with two hand operator buttons I don't think there would be a problem using the PLC inputs. But all Estop and door safeties MUST use a safety relay.

Someone will correct me if im wrong!

Regards,

James

 

You might want to look at a more up to date document. Safety PLCs and Safety Networks are available.

 

Two hand control is typically a safety function, and may be at least as important as e-stop and guard interlocks. For example, in a press control the two hand control is being relied upon to protect the operator each machine cycle. In this application the two hand control is being used instead of a movable guard (primarily to reduce cycle time).

However, when evaluating the requirements, it should be kept in mind that not all machine elements constitute a hazard. Each application has to be evaluated independently.

 

If your palm buttons are used only for passage f step and are not used for the operators immediate safety then they would be acceptable. But it seems like they are being used to protect the operator from getting their hands in the machine process. In this case I have always been under the impression that a category 3 or 4 has to be satisfied. These require hardwiring with electro mechanical redundancy by using an intrinsically safety relay. I would say that PLCs cannot and should not be used for this type of safety application. You can’t be too safety conscience.

 

Safety PLCs do exist; they can be networked over Ethernet and DevicNet. A hardwired safety circuit is preferred, but it is questionable if it is truly the safest. I have seen many instances were hardwired circuits have been bypassed (usaully jumpers). Such as a case when muting or timing circuits exist. A safety PLC can change its logic in seconds. A fully designed and installed circuit can take days, weeks, indifinite periods of time to be redone. Plant managers will be more likely to contract a PLC software engineer for day @ $150/hr rather than rewire an entire line. Most involved in safety designs know this all too well.