# Another optical cable question - where did it fail?

**URL:** <https://boards.straightdope.com/t/another-optical-cable-question-where-did-it-fail/946716>\
**Category:** Factual Questions\
**Created:** [July 17, 2021, 3:37pm UTC](https://boards.straightdope.com/t/another-optical-cable-question-where-did-it-fail/946716 "2021-07-17T15:37:03Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\
**Post date:** [July 17, 2021, 3:37pm UTC](https://boards.straightdope.com/t/another-optical-cable-question-where-did-it-fail/946716/1 "2021-07-17T15:37:03Z")

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So I’m replacing an optical cable between my stereo system and the tv. (see previous thread). I apparently broke it while trying to attach it to the baseboard, since it worked before that DIY project but failed to work after I was done hammering it into place with various clips. After removing the old one, I was curious to see where it had failed. I had cut it into 4 lengths to remove it. Each time I shined a light into one end of a segment, I could see the light in the other end. I couldn’t find the failed section. I was pretty sure I had cracked or somehow damaged the cable by being too heavy handed in my home improvement mode. Can an optical cable fail from simply being bent around a 90 degree corner? How else could it have failed?

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**Author:** ![racer72](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/racer72/32/3075_2.png) [@racer72](https://boards.straightdope.com/u/racer72)\
**Post date:** [July 17, 2021, 3:47pm UTC](https://boards.straightdope.com/t/another-optical-cable-question-where-did-it-fail/946716/2 "2021-07-17T15:47:31Z")

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I spent many years working with fiber optic cables at a big airplane company. Something that was big when dealing with these types of cables is called the bend radius. This is how tight of a loop or bend that could be made with the cable. The standard was 10 times the cable diameter. A cable .25 inches in diameter could only be looped or bent no less then 2.5 inches. Engineering designed the routing of these cables to have a much bigger radius. If you bent the cable to a 90 degree angle, you destroyed it.

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**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\
**Post date:** [July 17, 2021, 4:06pm UTC](https://boards.straightdope.com/t/another-optical-cable-question-where-did-it-fail/946716/3 "2021-07-17T16:06:25Z")

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A fractured fiber is an opportunity for attenuation of the signal. You wouldn’t be able to tell the attenuation with your naked eye’s sensitivity to light differences.

Also, the fracture could have been exacerbated by the curve stress it was under when it as routed and fastened, but not as bad when held in hand (especially if it was a short segment).

A tool called a fiber optic reflectometer is typically used to locate breaks and other signal affecting imperfections. Mark I eyeball has never been sufficient.

> **[Optical time-domain reflectometer](https://en.m.wikipedia.org/wiki/Optical_time-domain_reflectometer)**
>
> Pages for logged out editors learn more
> 
> An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber. 
> It is the optical equivalent of an electronic time domain reflectometer which measures the impedance of the cable or transmission line under test. 
> An OTDR injects a series of optical pulses into the fiber under test and extracts, from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back from po...