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November 18, 2008

Thoughts about ingress protection: eliminate potential points of failure

The most commonly used measure for protection against the elements is the IP rating, or Ingress Protection rating. The IP rating consists of two numbers where the first indicates protection against solids and the second protection against liquids. Solid ratings go from 1 to 6, with 6 meaning the best protection. Liquid ratings go from 1 to 8, with 8 meaning the highest protection. Essentially, the purpose of these ratings are the determination of how well a device can keep out dust and water. As far as liquids go, the purpose of the rating is not to signify waterproofing for underwater operation (though IP68 means a device is indeed waterproof) but how well a piece of equipment can keep out water during normal operation in the field. What could happen, for example, is that a device gets exposed to rain, or even strong driving rain during a storm. In a marine setting it is possible for a device to suddenly become exposed to heavy seas, and it may need to be protected against that.

All of this needs to be tested and certified, and the way it is usually done is by following standard procedures that describe a controlled lab testing setup, like document 60529 issued by the International Electrotechnical Commission (IEC).

The problem is that lab tests do not always accurately predict what may happen in real life. In that respect the ratings should really be considered guidelines rather than hard data. Consider, for example, two devices that both carry an IP67 rating. One of them has no external ports other than a single surface mount connector used to provide interfacing via a port replicator or dock. The other has a variety of commonly used ports, all protected by individual rubber plugs. One machine may also have an externally accessible expansion slot and an easily replaceable battery, each nicely sealed via o-rings and other high quality seals. Which device do you think is more at risk for leaking?

I'd say the second as it has multiple areas of entry as opposed to just one. No matter how well engineered the device may be, the probability of something going wrong is higher. A protective cover may not be pushed in all the way. A seal may have shrunk or gotten broken. A door was inadvertantly left open. It can happen.

A compromised seal may not necessarily mean a leak into the inside of the device. The port itself may carry enough sealing in addition to the protection provided by its cover to ward off damage. Then again, it may not. Bottomline is that the simplest and most foolproof protection is best.

Anything mission-critical should be failsafe. Failsafe means that if a system fails, it must fail in its safe state. A relay that snaps closed when it loses power is an example. The problem with protective rubber and other seals I'd that none are fail-safe. They are all fail-fail. So the best way to proceed is to have as few potential points of failure as possible.

What that means is that, all else being equal, a device with fewer possible points of failure will almost always be a better choice as far as protection us concerned.

Posted by conradb212 at November 18, 2008 11:12 PM

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