Do We Future-Guard Machine Designs?

Will Designing Redundant Switches Up Front Forgo Future Problems, Liabilities and Added Costs?

By Control Design Staff

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We install a number of different types of safety switches on our machines, all we believe in accordance with applicable standards that were in force at the time. We aren't called on to do regular upkeep or updating of the machines very often. We're a little concerned that, as time goes by and users change out components, the original safety design might be compromised. We're thinking about designing in redundant switches up front to forgo future, unforeseen problems, liabilities. We'd like some thoughts about whether the added costs provide more peace of mind for us as OEMs?

— From July '13 Control Design

SEE ALSO: Sensors, Switches Supply Safety

Answers

Two's Better
The topic of redundant switches is a controversial issue in the view of many. However, the simple fact is that if the switch is not properly maintained, it can be damaged and fail in an undetermined condition. Some manufacturers suggestions that their products will "fail-safe" damage can be wide and varied so there is no guarantee. Use of two switches in different operation modes or with different methods of actuation is the best and safest practice when monitored by a safety circuit for simultaneous operation; if the switches don't open or close together, the safety circuit will keep the machine from restarting until the fault is cleared. In this situation, the OEM can provide troubleshooting guidelines and guidance in the machine operation manuals on how to remedy the situation and keep machine operators safe.

Matt Dodds,
product marketing manager — safety,
Omron Automation and Safety

Identify and Mitigate
The peace of mind that comes from a well-designed safety system is indeed worth the upfront investment. The added time and cost in the design process is likely smaller than the potential financial burden associated with future questions or even litigation. More importantly, designing robust safety systems for machinery is an OEM's responsibility.

Conducting a risk assessment is the first step in any effective safety-system design process. OEMs should consider foreseeable use and misuse, as well as all individuals who might come in contact with the machine, including operators, engineers, maintenance and cleaning staff.

After completing a thorough risk assessment, OEMs should select and implement safety components that help mitigate the identified risks. Traditional electromechanical safety switches can be a cost-effective solution if installed properly and integrated to provide the required functional safety performance. If the system and its components are more robustly designed (which can include redundancy), then OEMs can expect longer life and higher diagnostics. If the safety switches are implemented such that they can be monitored and not easily defeated, then the suppliers can breathe easier.

The industry is moving toward safety switches that have a higher functional safety (safety integrity) within the switch itself, meaning Performance Level (PL) e per ISO 13849. This includes guard locking-type safety switches because folks are realizing that the locking part of the switch is also a safety-related function. Per the draft ISO 14119 standard, a single electromechanical safety switch can only achieve PL d when using fault exclusion. For new installations, this might or might not be appropriate.

Engineers and machine builders want to design and build once, and know that their machine is robust. This robust design includes more sophisticated and reliable safety components with diagnostics capabilities, all integrated in a way that allows users to easily monitor performance. By starting the design process with a risk assessment and adapting to changing standards and expectations, taking the next steps will be worth the potential added cost and effort.

Roberta Nelson Shea,
marketing manager,
Rockwell Automation

Built-In Redundancy
Industry continues to struggle with the cost and the possible loss of productivity in order to comply with the latest safety standards. In general, redundancy provides the highest level of safety, and this is very true when it comes to safety switch applications. A traditional design, to meet a SIL 3 rating, will use two safety-rated switches to monitor the position of a protective cover on a machine.

A relatively new concept for non-contact safety switches is to incorporate an RFID coding and built-in redundancy. This new approach provides several benefits to the customer. First, by having built-in redundancy, it's no longer required to have two sets of switches to achieve the highest safety rating. The second benefit in cross-circuit-detection is built-in as well, which allows multiple switches to be connected in series, and reduces the number of evaluation units (safety relays) required down to only one. Both of these benefits reduce the cost of the circuit.

John Burns,
product manager,
Siemens Industry

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