The description “hostile environment” conjures up visions of battlefields, extreme climates such as deserts or polar regions, and even the conditions found in space or on other planets. However, for electrical and electronic equipment, hostile environments can be found much closer to home. We all know, for example, that electricity and water don’t mix and that doesn’t just apply to mains electricity, as anyone who has dropped a mobile phone into a washbasin or toilet bowl may have found to their cost!
Electromechanical products, like microswitches, are not only susceptible to water and other fluids but also to dust and solid particles that can impair their operation. Even in the relatively benign home environment, appliances in a steamy kitchen or bathroom can be affected by condensation with potentially dangerous consequences. Computers in offices are more prone to dust than in the home, which, even if this doesn’t cause problems with overheating due to restricted airflow, can result in unreliable operation if dust penetrates the power switch.
Far worse though are the environments encountered in factories and similar industrial locations where microswitches are potentially subject to the additional extremes of temperature and other harsh operating conditions. Here it becomes much more challenging and consequently water and dust proofing becomes an important consideration for the equipment designer.
Defining the Objective
Fortunately help is at hand, starting with the International Electrotechnical Commission (IEC) standard EN60529, which has established ingress protection (IP) ratings that define the degree of protection that mechanical cases or electrical enclosures afford against intrusion by solid objects and liquids. So, for example, with an electrical wall socket rated as IP21 the first digit “2” indicates protection against solid objects >12.5 mm across, such as a finger, while the “1” signifies protection from dripping water.
For a microswitch, protecting the internal mechanism from somewhat smaller objects that could prevent the switch from operating would seem more appropriate. A basic rating of IP40 will exclude solid objects >1 mm but provides no protection against water. To be dust-tight however the switch would need to be rated as IP6x. The second digit, representing the level of protection from water, can range from 0 to 8, i.e. from nothing through defined levels of dripping, splashing and spraying with water jets, to different degrees of immersion.
The table below shows the possible permutations of IP codes. Clearly, sealing a microswitch to achieve a rating of IP65 or greater requires the use of effective techniques to exclude dust and water. Achieving a seal around the moving pin, which actuates the internal switch mechanism, presents the greatest challenge – the typical use of a rubber membrane needs to ensure a tight fit while remaining flexible across a wide temperature range and resisting external pressure changes.
Engineering the Solution
Aside from meeting EN60529 sealing requirements, microswitches used in industrial applications will probably also need to comply with further standards, such as UL61058-1, which primarily relates to safety but also specifies an operating temperature range of 40°C to +125°C. Consequently, to avoid the mismatch that can arise due to the thermal properties of different materials, companies like Microprecision assemble its microswitch housings using an ultrasonic welding process rather than applying an adhesive or using a gasket.
Ultrasonic welding is also used when it comes to attaching the compression ring, which secures the sealing membrane around the actuator pin, to the switch housing. To guarantee the seal to the pin itself, Microprecision uses a two-part pin that sandwiches the membrane between the two parts to hold it securely in place. This approach allows for the optimum choice of flexible membrane material with the reassurance of a tight seal to the actuator pin and switch housing.
So far we’ve considered ingress protection for the switch, preventing dust and water from reaching the internal mechanism and electrical contacts. However in a real application it can be just as important to protect the switch terminals and the electrical connections to them. Often this is left to the equipment designer and may be accomplished using sealing boots, the application of sealant compound or methods such as potting. Alternatively manufacturers may offer microswitches with cables already attached that have been potted to meet the required IP rating.
Delivering the Goods
As an experienced manufacturer specializing in precision electro-mechanical components for demanding industrial applications, Microprecision provides a comprehensive line of microswitches. This includes a number of switch formats that combine housings with potted cable connections to deliver IP67 protection. For more versatile system design approach, its IP67-rated MP110 series of microswitches has conventional solder lug terminals but is supported with the option of either an IP64-rated protective terminal kit for field wiring or a plug-in cable assembly that provides full IP67 protection. Finally, should your application demand even greater ruggedness and continuous underwater operation, Microprecision’s MP215/225 series features a strengthened cable outlet and increased actuating force, and meets IP68.