Electronic Products & Technology

How to survive hostile control environments

March 24, 2015  EP&T Magazine

You have two possible problems when you make a hole in an enclosure to mount/fix anything (could even be as simple as a mounting bracket): contaminants could seep into the enclosure, damaging internal electrical and mechanical parts and/or collect on the outside and eventually prevent equipment operation. It is important, therefore, that two tasks be accomplished: seal the hole and protect what is outside.

Exposure to moisture, ice, dust, dirt, fungus, lubricants, cleaning solvents, food, salt air and other materials that can corrode or bind up mechanical devices, will eventually lead to damage of the internal workings of the product and/or cause external parts to become difficult to operate or become fully non-functional. Electromechanical toggle, pushbutton and rotary switches are often the preferred choice for equipment control. They provide good tactile feedback and minimize accidental actuation. Cost is reasonable and the result is a rugged, easy-to use and repair control. Circuit breakers accessible from outside the enclosure allow immediate troubleshooting and fast corrective action.

However, unsealed, electromechanical switches and circuit breakers, mounted through an enclosure, offer limited environmental protection against harsh contaminant penetration. These devices might be on an oil platform, a cement truck, a speeding train, a robot, a commercial floor cleaner, a racing snowmobile, or any of the myriad of products subject to attack by man or Mother Nature. Sealed switches are available, though their cost is relatively high. However, they are not without certain problems.

Many sealed switches come with no effective sealing of the mounting hole. Plus, if the equipment will be exposed to dirt, food residue or icing, the external switch operator may get jammed or frozen. The internals of the switch will not be damaged, but you may not be able to operate the switch. What is the solution? During World War II, the U.S. Military was made painfully aware of these types of control device reliability issues, and, shortly thereafter, a seal company joined forces with the Department of Defense to develop a series of switch sealing accessories, which led to the creation of the military standard MIL-DTL-5423, which is still being used today.

These switch accessories, commonly called ‘boots’ or ‘caps’, comprised a threaded piece, to which was molded a form-fitting, elastomeric cover (silicone rubber was the standard sealing material). The ‘boot’ included a rib molded under the nut to seal the mounting hole. The result was a cost effective sealing of the switch and mounting hole, with a life expectancy of over 25 years and over 50,000 operations and that could easily withstand temperatures from -70C (-94F) to plus +204C (+400F). See Figure 1 for a typical high quality design.

Over the past 65 plus years, the range of sealing boots has expanded to include not only bushing mounted toggle switches, but also pushbutton and rocker switches, bushing-mounted rotary devices (potentiometers, selector switches, etc.), panel-mounted circuit breakers, as well as a multitude of custom engineered solutions. (*See Figure1)

Specifying sealing boots

The sealing boot should be specified at the same time as the switch or breaker is being selected. This will insure proper fit and operation. Look for a distributor who offers both the switch/breaker and the corresponding boot. They can easily match the two devices. Care should be taken to specify the expected environmental exposure.

Enclosure protection level required

A good quality boot will be rated to provide IP66/68 (Type 4X) protection. If the overall equipment will be evaluated by UL or CSA, look for a boot that is already a UL Recognized Component. This will save you time and money.

Exposure details

State what the boot will be exposed to. Will it be just outdoor weather? Dust? Salty air? The standard material for a good quality boot is high performance silicone rubber. Be sure that the rubber is virgin and not recycled. If the equipment will be exposed to petroleum products or other materials, the boot may need to be made from fluorosilicone or some other resistant material. See Table 1 for a list of available rubbers and their characteristics.

Level of abuse

If the equipment will be taking lots of abuse, consider a half boot instead of a full boot. There are also boots available with stainless steel cladding.

Colour

Do you need the boot to be a certain colour to indicate use or brand? A reputable boot manufacturer will offer standard and custom colours for most rubber materials.

Operating force

Some of the newer and less expensive switches/breakers have a light spring action (minimum force required to move the switch or breaker handle). Some boots may try to move the handle of these switches back to the ‘upright’ position. The boot may then have to be made using a softer rubber (lower Durometer). A new design has just been patented that inherently minimizes the return force and is ideal for light action toggle switches and breakers.

Export

In today’s ‘flat’ world, the equipment must meet international specifications. This not only includes the enclosure rating per IEC 60529 (e.g. IP66 for equipment subject to wash downs), but also material restrictions specified in legislation like RoHS and REACH. Be sure that the boot is in compliance with legislative and regulatory requirements for the end use countries.

Dimensions and thread size(s)

If the switch/ breaker has already been selected and you need a boot, the easiest solution is to contact a reputable boot manufacturer. They have had years of experience matching the boot to the right switch or breaker. Provide the manufacturer and catalog number or a cut sheet. Also, the boot manufacturer’s website should have boot cross references to most popular switches and breakers.

Sealed switches

As mentioned earlier, sealed switches are becoming popular. They seal the internal workings of the switch from entry by liquid or solid particles. However, they do not inherently seal the mounting hole and are still subject to mechanical problems when exposed to elements that can solidify over the switch (ice, cement or coal dust, airborne food residue, etc.). If the exposure is mostly to liquids, a high performance bushing seal is available that consists of a stainless steel ring and a silicone rubber molded seal that will positively seal the switch mounting hole. (*See Figure 2)

If the equipment is also exposed to the above elements that could jam the switch operator, consideration should be given to using a half boot that will seal the mounting hole and shrug off ice and jamming particles like a winter wiper blade repels ice and snow.

Circuit breakers

Circuit breaker boots present another challenge. Protection from the environment is not enough. What is the breaker status? ON? OFF/TRIPPED? As the breaker handle normally shows the condition of the breaker, it is important that the boot does not hide this information. That is why most breaker boots employ ‘clear’ silicone rubber.

However, when subjected to UV light from the sun and ozone in the air, standard silicone rubber will, over time, yellow and become almo
st opaque. One boot manufacturer adds a proprietary agent to the silicone rubber to produce RUBRGLAS, which acts as ‘sunglasses’ for the boot, keeping it clear for years.

For lever style breakers, the breakers can be mounted behind or on the front of the panel. This latter design employs a frame to hold the rubber boot to the panel. For both types, the boot and breaker are held together by stainless steel sealing screws.

The rubber boot has been designed to allow easy breaker mechanical operation. For certain breakers with long handle lever travel, a patented, molded-in ‘trigger’ was developed, giving a mechanical advantage and minimizing rubber stretching.

Sealing hardware

After sealing the electrical components, don’t neglect to seal any other items that may be attached to the control box and/or the process equipment. As a compliment to the sealing boots, a line of sealing hardware has been developed that offers self-sealing screws, nuts, bolts and washers. All use stainless steel base hardware and silicone rubber seals. There is no need for application of sealing paste or tape and the seal is far superior to any other solution. (*See Figure 3)

Exposing your equipment to attack by the environment does not have to be a traumatic experience. By carefully choosing the protection for switches, breakers and equipment leak points, the equipment will be both reliable and have a long life, increasing brand reputation and loyalty. Here are just a few examples of applications of sealing boots and hardware.


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