Simplified safety: taking safety beyond the MCC

When integrating safety into a motor control center, be sure to think outside the box to the components and devices in the plant that are under its control.

06/14/2011


How safe is your motor control center (MCC)? From equipment design parameters to arc flash protection measures, most companies rigorously implement practices and procedures to keep those working in and around MCCs out of harm’s way. But that’s only part of the story. A more important question, perhaps, is how safe are those same employees when they are working out on the shop floor in and around the components operated by the MCC?

A typical MCC may control numerous motors, raising the specter of multiple problems at multiple locations. If a safety incident occurs, said David Blauw, consulting applications engineer at Siemens Industry, mechanisms must be in place so that a motor won’t start up without appropriate guards in place. “How,” he asked, “does a facility ensure that its MCC is controlling its equipment adequately and maintaining safety out in process areas?” When thinking about safety, many U.S. manufacturers haven’t thought beyond the enclosure, observed Blauw. “When asked if their MCC is safe, most say yes, because they have met the requirements for those working on the MCC. But that thinking,” he insisted, “doesn’t go far enough.”

Integration, integration, integration

Comprehensive MCC safety must extend beyond the unit, and state-of-the-art technology today makes it possible through the incorporation of a safety PLC and the use of modern fieldbuses such as ProfiNet or Profibus. However, said Blauw, “the safety PLC plays an integral part in its integration into the MCC. Most manufacturers using an MCC have some kind of safety system, but an integrated safety PLC makes possible many benefits, such as the flexibility to develop a safety system that can reach into the plant and that can be reconfigured easily, if and when changes occur.”

For example, he went on: “A process area may be equipped with guard interlock doors or an emergency stop pushbutton. The safety controller inside the MCC monitors the button or the guard over the network and reacts to an event. If a worker hits an emergency stop, it receives a signal that an incident has occurred, say in room 1, area 3. There are three motors in that area, therefore the safety PLC has been programmed to shut down the motors safely and quickly,” explained Blauw. “This technology puts safety firmly in the hands of the safety controller within the MCC. Its action supersedes anything other process controllers may do. If the safety PLC in the MCC detects an emergency stop has been engaged on a motor at the far end of a process line, regardless of what any other controller says, it will not allow that motor to start.”

An integrated safety PLC provides more than safe shut down. Take a plant with 28 motors and upcoming expansion that specifies an MCC with the capacity to control 40 motors. How can it design a safety system if it cannot know where or how it will use the additional motors? “An integrated safety PLC,” said Blauw, “provides flexibility to add and modify. No rewiring, no new contactors are needed because all communication happens over the fieldbus network. Modification simply requires a connection and a program update.”

Tallying the benefits

Only a few years ago, “state-of-the-art” meant smart MCCs as intelligence was introduced into these systems. Once diagnostics data were incorporated and brought back to the control panel, the logical next step was to make smart MCCs safe. “We saw the potential in adding aspects of control to the MCC,” said Blauw. “Now when an emergency occurs out where the field devices reside, where the motors are, we can ensure that when an E-stop is pressed, the motor will shut down. And if changes need to be made in the future, it will not cost thousands of dollars and hundreds of hours of time to reconfigure the system.” Admittedly, the safety requirements of processes vary widely. A refinery differs markedly from a grain elevator. “Different safety aspects definitely need to be considered,” said Blauw. “What happens when a worker has to maintain a pump handling hazardous waste? How do you guarantee no one can inadvertently turn on a pump that could spray a hazardous material on the mechanic working on it? When the human factor is part of the equation, too often lock out/tag out does not happen. Electronic safety systems that monitor equipment conditions ensure that when something is not supposed to turn on, it won’t.”

In the realm of shorter production times and faster product changeovers, manufacturers need to look at getting every job done most efficiently and without risk to the employee. Integrated safety systems are another tool in the toolbox that, under certain conditions, can enable the plant to shut down certain areas, but maybe not others, saving time and production. Even saving a few hours can make a major difference in efficiency.

Safety PLCs provide a repeatability that cannot be achieved from the human perspective. Those programming the safety PLC can determine which conditions will trigger a shutdown and what actions the unit should take when those conditions occur. “A worker who needs to inspect or replace a bucket conveyor in an elevator must open an access door that should have interlocks, or set a brake to ensure the buckets don’t back up onto the worker,” said Blauw. “That door can be configured so that as soon as it is opened, the safety PLC detects the action and engages the required safety systems. There is no option to forget or ignore something. It will happen because it has been programmed to happen. The decision is out of the operator’s hands.”

Think outside the box

When considering an MCC upgrade or replacement, “thinking beyond the box is mandatory,” stressed Blauw. “Safety PLCs can be retrofit into older systems,” he pointed out. “However, it may be more economical to change out an existing MCC for a more up-to-date unit—especially a smart MCC—than to modify a very old one. For companies with a smart MCC, adding safety capabilities may not be as complicated. In any event, don’t decide your MCC is safe until you have looked at what’s beyond it.” A safety PLC is only as safe as the code directing it, added Blauw. “And the code is only as safe as the ratings of the hardware in the system allow it to be. All components, from the motor to the drive to the CPU must able to perform the functions being asked of them. Look at the total system.” Finally, adding safety to a system invariably costs less than the expense of an incident. “An accident or injury always costs exponentially more,” said Blauw. “If you think about safety—even a little bit—upfront and in advance, it will save a lot of headaches down the road. An MCC with an integrated safety PLC will give you the required flexibility—and the safety. Additional benefits are immeasurable.”

More information about motor control centers and safety PLCs is available from the Siemens Industry website at www.sea.siemens.com/us/Products/Controls/Motor-Control-Center/Pages/Motor-Control-Centers.aspx; or contact John D’Silva at john.dsilva@siemens.com for further details.

For more information on safety automation products, visit the Siemens Industry website at

www.siemens.com.

www.usa.siemens.com/safety

This article was submitted for the Siemens Simplified Safety custom newsletter. See other articles in the Siemens Simplified Safety newsletter.

www.controleng.com/newsletter/siemens-simplified-safety.html


At a glance: Safety beyond the MCC

Key points follow from David Blauw, consulting applications engineer at Siemens Industry, on safety beyond the motor control center.

· How does a facility ensure that its MCC is controlling its equipment adequately and maintaining safety out in process areas?

· If the safety PLC in the MCC detects an emergency stop has been engaged on a motor at the far end of a process line, regardless of what any other controller says, it will not allow that motor to start.

· An integrated safety PLC provides flexibility to add and modify, without rewiring or new contactors, because all communication happens over the fieldbus network.

· Electronic safety systems that monitor equipment conditions ensure that when something is not supposed to turn on, it won’t.

· Think about safety in advance to save a lot of headaches down the road.



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