This is our final article in a three-part series on desiging products for the Great Outdoors. In our first article, we covered water and dust resistant designs. In the second article, we covered how to account for temperature in a two-step process. This article will cover protecting products from exposure.
What do I mean about exposure? Exposure is any condition that can alter the reliability or functionality of a product if unprotected. This definition makes exposure very, very broad. To make this manageable, we will focus on the most common ones effecting our customers — mainly marine-air exposure, petroleum, fire, and power surges.
Marine air and airborne corrosion
The coastal marine layer is probably the most common exposure outdoor products can encounter. There is 12,000+ miles of coastline in the United States. The inland reach of marine layer can be quite surprising. NOAA says that the marine layer can effect Bakersfield, CA; which is about 100 miles inland. Marine air corrodes the electronic components and shortening the effective life of the products.
Beyond the marine layer; everyday airborne moisture and containaments have unintended effects on electronics. Nexsun recommends conformal coating the PCB and electrical assembly for all outdoor products. Conformal coating puts a protective layer of polymer that will prevent corrosion caused by sea-salt and other air-borne corrosive materials. However, there is a cost to conformal coating. Below are the three most common reasons to NOT do conformal coating:
- Product is deployed in an air-tight environment or in an air-tight enclosure. NOTE: be sure to paint or treat metal components used on the enclosure.
- The serviceability for the product was designed in a way that the rate of failure and cost of repair is lower than the cost of conformal coating all the units.
- Life-span for the product is not meant to be that long (< 2 years).
Sulfur and the “Achilles Heel” of electronics
High sulfur environments used to be thought of just for oil/gas fields. Three trends have increased high sulfur exposure to outdoor products.
- Rubberized weather proofing – effecting server farms, datacenters, and base stations
- Lubrication used in Automation – effecting industrial control, robotic assembly, and marine/agricultural equipment.
- Rubberized and bitluminous treated Roads – effecting traffic control, lighting, and automotive equipment
Sulfurated environments are the “Achilles Heel” of electronic products. Surface mount chip resistors are one of the cheapest and most used components in electronics; yet very susceptible to failure in a sulfurated environment. Over time, a buildup of silver sulphide crystals at the edge of the resistor terminal results in a fail-open condition. Nexsun recommends using Precision Resistive Products (PRP)’s PR9556 Anti-sulfurated Thick Film Chip Resistors. This resistor was specially designed to prevent silver sulphide build up; a great protection for products that expect to be in the environments covered above.
Roughly 48% of the 1,345,500 fires reported annually in the US are unclassified outdoor fires. An outside unclassified fire is responded to every 52 seconds. Designers should survey the industry that their products are being sold in to; and determine the survivability expectations have already been set. For example, fire and safety equipment require more survivability than a casual outdoor consumer product.
Survivability to open flames have two different classifications to consider: Flame Retardant and Fire Resistant. Flame Retardant specifies to tendency for a product to quit burning after it has been exposed to an initial ignition. See Table 1. Most electronics are designed to UL94 V-0. Fire Resistant specifies the ability of a product to resist damage from an external open flame event over time. See Table 2. Fire Resistance is a high value but costly feature. This requires a very holistic design support – heavily related to our second article. Nexsun has enclosure partners like Ensto and component suppliers like PRP that can meet the highest levels of flame retardant and fire-resistant requirements.
Table 1: Flame Retardant Classification
|Flame Retardant Classification||Burn Rate||Conditions:|
|UL94 HB||<76mm/min or 100nn||Slow burn horizontal <3mm thickness|
|UL94 V-2||Burning stops within 30 sec||Drips of flaming particles allowed|
|UL94 V-1||Burning stops within 30 sec||Drips of flaming particles allowed not inflamed|
|UL94 V-0||Burning stops within 10 sec||Drips of flaming particles allowed not inflamed|
|UL94 5VB||Burning stops within 60 sec||No drips allowed. Holes allowed|
|UL94 5VA||Burning stops within 60 sec||No drips allowed. No holes allowed|
Table 2: Fire Resistant Classification
|Fire Resistant Classification||High Inside Temp||Conditions:|
|UL72, Fire Resistant Class 350||< 350°C||outside 2,000°F, 2 hours|
|UL72, Fire Resistant Class 150||< 150°C||outside 2,000°F, 2 hours|
|UL72, Fire Resistant Class 125||< 125°C||outside 2,000°F, 2 hours|
Hazardous Locations and Intrinsic Safety
There are times when a designer knows a product could be exposed to flammable materials. Hazardous Location states where a product can be used safely; and Intrinsic Safety looks at design that ensures the product is safe to use those hazardous conditions.
Hazardous Location as defined by the NEC/CEC covers type combustible material exposure (Class), the normal location/usage of that material (Division), and the frequency of the exposure (Zones). Some examples of hazardous locations are:
- Gas stations
- Manufacturing facility – industrial liquid pumps that are fuel-based
- Datacenters and IT equipment – fuel-based backup electrical generators
- Lighting – anywhere lighting can be exposed to fumes and dust — ex. vehicle painting facilities, industrial production facilities
Depending on the industry a product is sold into, designers may be required to have components and products certified for one or more of these classes.
Table 3: Hazardous Classes
|Hazardous Classes||Type of flammable substance|
|Class I||Flammable vapors and gases may be present|
|Class II||Combustible dust may be found|
|Class III||Ignitable fibers and flyings|
Table 4: Hazardous Division
|Hazardous Division||Location Type|
|Division 1||Hazardous conditions are present during normal operations|
|Division 2||Location a volatile flammable liquids or gases are handled, processed or used.|
Table 5: Hazardous Zones
|Hazardous Zones||Exposure Frequency|
|Zone 1||Intermittent Periodically|
|Zone 2||Abnormal Conditions (infrequent)|
Instrinsic Safety is both a certification and a design requirement for products. Once a Hazardous Condition(s) is known, designers need to take steps to make sure their product design to operate safely within those conditions. AllAboutCircuits has great article “Ten Ways to Design For Intrinsic Safety”.
Exposure to Power Surges
An energy surge can occur right before a power outage or when equipment is near a lightning strike. Just in power outages, 145 million customers are effected by an average of 1.2 times a year according to the U.S. Energy Information Administration. In an outdoor deployment, designers should know that their products will not necessarily be protected by a circuit breaker and in-line fuse like many indoor products.
Nexsun has suppliers like BrightKing and World Products that provide circuit protection components meant to respond to over voltage and short circuit events. Detailed circuit protection design is beyond the scope of this article. Gas Discharge Tubes (GDT) in series with Metal Oxide Varistors (MOVs) can handle the surge event as well as the surge subsides. And for smaller voltage/current applications, it may make sense to use a TVS diode instead.
In some cases, like with MEAN WELL LED light drivers and industrial power supplies will have short circuit, over voltage and over temperature protection. Depending on the condition, the supply will auto-recover or need to be reset.
Unlike Article 1 on Waterproof/Dust and Article 2 on Temperature; exposure has the widest breadth of consideration for outdoor products. We included the most common ones in this article. Nexsun has a trained staff ready to support your designs. Go on. Get out there. The Great Outdoor awaits innovative products !!!