FEATURE SWITCHES, DISPLAYS & UIS RAISE THE ALARM DESIGNING CUSTOM SWITCHES THAT CANNOT FAIL
When considering fire alarm pull stations, any failure is potentially life threatening. In this case study, Mike Bolduc, global segment manager of industrial at C&K, looks at the steps taken when working with the leading manufacturers of these systems, to develop a switch solution that is rugged, reliable and cost-effective
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ngineers can face many complex design challenges when collaborating on a primary switch for a fire pull station. Such intricacies were met when C&K were approached by one well-known fire alarm maker, to develop a paddle switch for mounting in a pull station housing enclosure. Its basic function: as the handle is pulled, the switch operates, sounding the alarm. The simple function was to be an on-none-off operation at 0.4 VA max, with a current value of 20V, AC or DC. Additionally, it was required for it to be able perform properly in the presence of corrosive gases: in real-world scenarios, such environmental hazards could affect the system’s reliability. Consequently, a customised paddle switch assembly arose from an adapted version of an existing C&K product. The switch was modified with new terminals to fit the existing electrical footprint. Furthermore, it was built into a tailored, sealed housing enclosure, for protection against exposure to any ecological agents. The result was a more robust and cost-effective design, fulfilling the requirements of the application and entailing no additional change to the external features and footprint of the customer’s end product.
FORM, FIT AND FUNCTION On a different design project, the demands were rather more elaborate: the design necessitated two switches, one single pole and one double pole. This multi-pole plunger-based push switch had particular specifications, such as the prerequisite of the spring-loaded plunger being pre-loaded during the manufacturing process.
16 JUNE 2019 | ELECTRONICS
The customer was also looking to redesign the internal mechanism to improve assembly efficiency, reduce component costs and improve quality and reliability. Apart from this necessary consistency, the other strict design constraints involved the electrical properties, the switch configuration and the footprint. The key to success with this switch was yielding the force required to actuate the plunger. The specification called for a very narrow range of actuation force, in order to avoid nuisance tripping and comply with the Americans with Disabilities Act (ADA): this ensures that handicapped and older individuals can manage the device appropriately. Using computer aided design (CAD) and detailed 3D-printed models, the time taken to reach a solution was significantly shortened.
Rapidly generated, 3D-printed prototypes were delivered to allow the customer to test the solution before the design was sent for qualification testing. In both examples described here, prototypes were delivered within a few weeks and across a number of versions, and were finalised in a few months after testing.
This C&K solution includes an existing paddle switch assembly and housing, with additional sealing
TESTING THE PRODUCT There are three main areas of testing to consider in this regard: basic electrical and mechanical, endurance and environmental. Electrical testing was performed on a number of units, checking parameters such as contact resistance, insulation resistance and ESD resilience. Mechanical testing focused on the actuation force and actuation angle. Endurance testing covered both mechanical endurance and electrically loaded endurance. Altogether, these sequential tests were performed for up to 200,000 cycles, double the rated life of the switch. Above all other aspects of the switch’s development, this is perhaps the most vital: guaranteeing infallibility for the technology’s implementation, application and longevity. But it isn’t of isolated importance. Testing is part of a thorough process, needed where lives are concerned, dependent on its success. Because if successful, residents in buildings can have the confidence that the fire pull station they may need to use will work correctly the first time, every time.
C&K
www.ckswitches.com / ELECTRONICS
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