Choose the right sealed switch for harsh environments
For design engineers, every harsh environment presents its own set of challenges stemming from a host of factors, such as extreme temperatures, water, chemicals, dust, oil and other types of contaminants. Selecting the right electromechanical switch for use in such environments is a critical step.
Whether selecting a switch for an oil drilling rig in the tundra or an in-cab heavy equipment application, improperly installing a switch that is not designed for harsh environmental conditions on an interface that will be used in such surroundings can result in dramatically decreased switch life expectancy and even critical switch failure.
This article describes the primary sealed switch options available and also which options are best suited for three of the most common harsh application environments: medical, industrial control and automotive/transportation/heavy equipment. There is also an outline of several other switch selection factors to consider beyond environmental considerations.
Sealed switch options
There are three primary options to take into account when considering a sealed switch:
- Splash proof boots
- IP-rated devices
- IP-rated devices with Splash proof boots for maximum protection
Splash proof boots
Though they do not provide a complete seal, splash proof boots offer sufficient protection against a variety of contaminating factors. If an application requires a splash proof boot, however, further decisions need made. Boots come in a variety of materials, each best suited to specific applications. Six of the most common boot materials include:
- Nitrile nutadiene rubber (NBR): This material offers an excellent balance between protecting against oil and cold temperatures, good elasticity and prolonged performance. However, it provides inferior ozone resistance.
- Ethylene propylene rubber (EPR): While this material offers good hot, cold, dust, ozone and water proofing, it provides mediocre oil resistance.
- Natural rubber: Natural rubber is a great choice for panel seals and gaskets.
- Polyvinyl chloride (PVC): PVC is typically only used for dust proof covers as other materials provide superior resistance against all other types of contaminants.
- Silicone rubber: This material provides very good hot, cold, ozone, aging and ultra-violet light resistance. It also offers very good flexibility, resilience and tensile strength over a wide temperature range.
However, it is less tear resistant than some other materials, so proper care must be used during installation.
Regardless of the type of boot material, the common pitfalls to avoid in boot installations include improper panel material or panel thickness, the wrong boot for the switch, improper toque sequence or assembly process and insufficient thread engagement.
Switch manufacturers can often retrofit most switch types, including pushbutton, rocker, rotary and toggle switches, with splash proof boots. The only exceptions are typically heavy-duty power rotary and slide switches. Half-boot options are also often available for toggle switches.
IP-rated switches are guaranteed to be resistant to certain environmental factors based on a ratings classification system. The IP code is part of the International Organization for Standardization’s (ISO) IEC60529 standard. This directive specifies the degree of protection of enclosures for low-voltage switches; specifically, concerned with protection of persons against contact with live or moving parts and the prevention of ingress of solid foreign bodies and liquid. The IP code is a specification used internationally and is similar to the National Electrical Manufacturers Association (NEMA) standard.
Common IP ratings include IP60, IP64, IP65 and IP67. Each of these ratings is guaranteed to protect against:
- IP60: Dust tight, but not protected against water.
- IP64: Dust tight and protected against splashing water from any direction.
- IP65: Dust tight and protected against low-pressure water jets from any direction.
- IP67: Dust tight and protected against effects of temporary immersion (up to 1 meter).
For designs requiring absolute precision, it is possible to rate to a specific IP rating and test according to a specific application’s requirements. A few examples of testing include cycle tests, environmental tests and shock and vibration tests.
IP-rated devices with splash proof boots
As the heading implies, IP-rated devices with splash proof boots combine the process sealed characteristics of IP-rated devices with an added splash proof boot made of one of the materials previously outlined. In many cases, using an IP-rated switch with a splash proof boot can be considered over design. However, if an application requiring an IP-rated switch impacts safety, either of equipment or human life, designing in such redundancy may be recommended to ensure the highest level of protection against contamination and disastrous switch failure.
Selecting the right sealed switch
Three of the most common harsh environments where switches are commonly required are medical, industrial control and automotive/transportation/heavy equipment. In all of these environments the need for a sealed switch is likely; however, specific needs for each environment exist and call for differing applications. Here is an overview of each environment and the ideal sealed switch option for them:
A key concern in medical environments is the harsh chemicals often used for sterilization, which can impact the performance of the switch. Key specifications that must be considered include the chemicals the switch is subjected to, frequency of actuation and wear and tear. In many cases, IP-rated devices are not required for medical applications.
Instead, splash proof boots are ideal for medical environments, since constant exposure to contaminants isn’t likely, but semi-frequent routine exposure is expected. As mentioned, the harsh chemicals often used for sterilization in medical environments can impact switch performance. However, the exposure to these chemicals is only during cleaning and typically not when the equipment is in use.
Several questions must be asked when selecting a switch for industrial control environments:
- What types of processes are being used in the facility?
- What is being made in the facility?
- Are there any contaminants in the air that could potentially affect switch performance?
All these factors are critical in the decision. In many cases, switches in industrial control environments will be subjected to fluids such as oils, cleaners or materials used in manufacturing processes. In such environments, where heavy machinery is often present, the consistent performance of a switch can become a safety issue with dire consequences.
Typically, industrial control environments require IP-rated switches. However, when safety is a factor in such environments, the redundancy of an IP-rated device with a splash proof boot is recommended. For example, consider the possibility that in such an environment, the switch housing could be compromised through accidental, but dramatic impact with a piece of machinery; thus, breaking the housing’s seal. The protective boot would thus provide a second, but necessary barrier of defense against contaminants.
A key characteristic of transportation/automotive/heavy equipment environments is that prolonged or particularly obtrusive exposure to contaminants is a certainty. These contaminants include dust, dirt and liquids. Such environments also often present temperature and vibration concerns, which could affect the fit and performance of a splash proof boot.
As such, IP-rated switches are the best choice for transportation/automotive/heavy equipment applications. For most designs, an IP64 rated device, which is guaranteed dust tight and protected against splashing water from any direction, is sufficient.
It is important to remember that there is much more than environmental conditions and the necessity of a sealed switch to take into account when selecting a switch for harsh environments. Here are several other considerations to take into account:
Meeting the needs of the end user
How the user will interact with the machine - often called the “man-machine interface,” is a crucial consideration. If a designer does a poor job with the man-machine interface, the machine will be difficult to use and ultimately fail commercially.
Designers should ponder questions relating to the interface and conditions surrounding switch use such as:
- What types of feedback does the user require?
- Is illumination needed to show status?
- Are legends needed to indicate functions?
Designers should also consider the ease of use of different switch sizes and designs. Engineers should select a switch that not only fits the size parameters, but is also sufficiently user-friendly.
Actuator style must also be considered. Options include pushbutton, slide, rotary, keylock, toggle, rocker and paddle. Each actuator style also often comes with a wide range of options such as illumination and legends.
Several engineering characteristics need to be evaluated when selecting a switch to be sure that it can perform the desired functions. When determining specifications, identifying the load characteristics is important.
Size and mounting constraints also factor into switch selection. Is a standard, miniature, sub-miniature or ultra-subminiature switch required? What mounting configuration is needed – surface mount, PC mount, solder lug or quick connect?
Contact material is another item to think about when choosing switches. Due to excellent conductive qualities and low electrical resistance, silver contact material is most common. In situations where any power rating is required, silver is the best choice. Gold contacts are typically necessary when switching at logic level, generally defined as covering a microamp to 100ma.
Whether a circuit is momentary or maintained will also influence the switch selection process. A maintained circuit remains open (or closed) after the switch is actuated, where a momentary circuit only remains open (or closed) while the switch is held in the appropriate position.
Finally, designers must remember to consider a switch’s life expectancy relative to the application. As a general rule, switches using momentary circuits have longer life expectancies than switches on maintained circuits.
Rules and regulations
Another important aspect of switch selection is compliance with government regulations and industry ratings. Such regulations include The Resource Conservation and Recovery Act (RCRA), The Clean Air Act (CAA) and The Clean Water Act. European countries have also produced regulations limiting the use of chemicals in manufactured goods and components, such as RoHS and Waste Electrical Electronic Equipment (WEEE).
In addition to government regulations, many switch applications require certification by UL, and CSA. For example, the UL94V rating measures the fire retardant properties of a switch, and TV-5 and TV-8 ratings measure arc-proof properties. These ratings ensure that a given switch will hold up to the demands of a particular application.
The right sealed switch – whether protected by a splash proof boot, IP-rated or a combination of the two – offers the peace of mind that no matter what the environmental conditions, actuation will occur every time without fail. Many designs do not require sealed switches, but for those that do, not using one can be a costly, and easily avoidable mistake.