ndustrial safety sensors come in all shapes, sizes and technologies, ranging from pressure sensitive mats, edges and bumpers, to mechanical and electrical interlock switches, to infrared light curtains and laser scanners. In most cases, these sensors have non-safety counterparts. So it's reasonable to wonder what are the important distinctions between non-safety and safety sensors.
"A cynic would say price and marketing," says Russ Wood, application engineering manager at Scientific Technologies Inc. (www.sti.com). "However, the real difference lies in design and approval to established standards."
The international community has well-established standards for the design and testing of safety sensors. There's EN574 for two-hand control devices, EN1088 for safety interlocks, EN1760 for pressure-sensitive devices and IEC61496 for electro-sensitive protective equipment such as light curtains. There's a standard that specifies exacting requirements for design and testing for all of them, says Wood.
You can take that a step further and be mindful of where and when the various standards don't have adequate recognition in North America. "One of the most important issues to watch out for before selecting safety components relates to approvals," adds Helge Hornis, intelligent systems manager at Pepperl+Fuchs (www.am.pepperl-fuchs.com). "For example, since AS-i Safety at Work was developed by a German-based consortium, this group of companies initially was most familiar with European approvals. Since then, U.S. approvals have been considered, but not all vendors can currently show that their hardware has been fully tested." So, says Hornis, customers should be very careful accepting "yes, we have approvals" statements from vendors. He says to consider the following issues:
In late 2002, the National Fire Protection Agency (NFPA), which is part of OSHA, rewrote its standard NFPA 79:2002 to allow safety devices on I/O networks. In this standard, says Hornis, NFPA specified that several constituent approvals are necessary to fully meet OSHA's needs. The approvals include UL for ordinary location; IEC61508:2000, a standard that evaluates microprocessor-driven devices and determines which Safety Integrity Level (SIL) they satisfy; and a NRTL-listing, where NRTL stands for Nationally Recognized Testing Lab, and the listing gives users assurance that the tests have been performed by a lab recognized as competent.
"Once all three conditions have been met, a device satisfies OSHA," says Hornis. "Additional tests exist--and they are useful--but are not sufficient to satisfy the NFPA 79:2002."
Hornis opines about several other standards, as well. "CE Mark is a very good test, giving users confidence in the field of EMC, but OSHA does not care about CE."
Then there's EN954-1:1997,an older standard that was used historically to evaluate E-stops and such, says Hornis. This standard does not allow the evaluation of microprocessor-driven devices, but looks at the electronic components and the construction. "Again, useful but not sufficient for NFPA 79:2002," states Hornis.
Wood noted above that EN61496-1:1997 is a standard for light curtains. "The reason we comply with it is that EN61496 demands higher EMC limits and since these nodes may be used with light curtains, we can state that our modules do not 'downgrade' the light curtains," says Hornis. "As before, this is useful but does not matter to OSHA."
Almost every type of device has a design standard that calls for devices to be robust and control-reliable. "In addition, design concepts such as tamper resistance, derated switching capacity, predictable failure modes, redundancy, force-guided contacts, self-monitoring, detection of external and internal faults, or requiring preset conditions that must be satisfied for operation, are important to overall system integrity," says Wood.
In addition to design standards for various safety sensors, adds Wood, application standards exist that require the use of safety devices to guard specific types of machines. An example of this, he says, would be ANSI B11.2-1995, which deals with hydraulic presses and the requirements for such equipment's safe construction, care and use.