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What is the primary purpose of a fiber optic connector dust cap? To protect the connector endface from dust? No! In fact, a dust cap can actually introduce dust and other contaminants to the polished endface of a fiber optic connector.
Dust is everywhere. Smaller dust particles that have a diameter of 1um or less can remain suspended in air for very long periods of time, if not indefinitely. Dust can easily find its way into a dust cap, and stow away until it has the opportunity to jump aboard your fiber optic connector endface.
Even worse, dust caps often contain grease, gels or other compounds left over from when the dust cap was manufactured. For example, the plastic dust caps used on many fiber optic connectors may contain mold release residue. Even blasting air into the cap will not remove these tenacious compounds.
Unfortunately, many people believe that just because a fiber optic connector has a "dust cap" installed, it has been protected from contaminates and therefore does not need to be cleaned. As you now realize, nothing can be further from the truth. It is important to always clean a fiber optic connector endface immediately before mating it, regardless of whether or not it has been "protected" with a dust cap.
Your fiber optic system may contain the very best Corning fiber optics laser optimized for maximum performance. However, if you have dirty fiber optic connectors, you will experience sub-par performance. Proper connector cleaning is essential for the optimal maintenance of fiberoptic systems.
Why Use Fiber Optic Dust Caps?
Dust caps do a great job at protecting fiber optic connector endfaces from contact with objects that can scratch, chip, crack or otherwise physically damage the polished core of the fiber. Dust caps also provide ferrule protection and are ideal for protecting connectors from physical damage while in storage or in transit. If you accidentally drop a connector that's wearing a dust cap, chances are it will survive the fall. It advisable to install dust caps on any fiber optic connector that is not actively in use.
Dust caps can be purchased alone, and some fiber optic connectors come with protective caps already installed. Typically, the cap is a sleeve or boot that is closed at one end and which fits tightly over the ferrule of the fiber optic connector.
Which raises another issue. Before mating a fiber optic connector, don't just clean the endface. First, clean the body of the connector ferrule to remove any residual manufacturing compounds. Otherwise, the residue may cause the ferrule to bond permanently with the mating sleeve. This can make it impossible to avoid breaking the ferrule if there is any need to unmate the fiber optic connector.
In conclusion, the name "dust cap" is obviously a misnomer. A better term might be "protective cap." Old habits die hard, so it may be a long time before a new, more accurate phrase is adopted for popular use. In the meantime, just remain aware of what "dust caps" can and can't do in terms of protecting a fiber optic connector!