fibre light readings

[guitarman3]

just as a matter of interest does anyone know what the fibre light readings test mean when checking the install of fttp? for example, apparently my readings read 19 at the telegraph pole but 21 at my property. just interested to know if this is good or bad or just ok. thanks

[guitarman3]

Thank you for this interesting information Gondola. My connection seems to be fine at the moment and it's good to know that the tolerances are within acceptable range. You've been very in-depth and concise. I've learned something new today and it's much appreciated.

[Gondola]

A perfectly made fibre optic connector cable (ignoring length loss that I'll assume is negligible in this case) with optical connectors at each end and single splice at the Customer Splice Point / Customer Service Point would be typically 0.3dB for each connector and each splice. So that's 0.9dB. But that answers your question about perfection.  If I assumed a fairly average connector loss of 0.75dB and a single splice loss of 0.3dB then that's 1.8dB. So, we're almost at the quoted 2dB that would be assumed to be "within tolerance" given that connector tolerance alone can allow up to 1dB per connector.

From your description of the works, the individuals involved weren't striving for perfection but just being "within tolerance". 

[guitarman3]

i didn't actually see the measurments being done but i'm pretty sure they did them via the green fibre optic connectors. the engineers hands were not the cleanest i've seen though. what, in your opinion would be an accepted loss of db in a perfect install?

[Gondola]

Did you see the measurements being made? Were they made via the green coloured fibre optic connectors? The connectors themselves could introduce loss of up to 1dB per connector if cleaned in the accepted way (not with bare hands). So if your setup is via connectorised cable then up to 2dB loss might be expected.

[guitarman3]

ok thanks for the info. so you think a 2db loss is quite extreme? could the cause be dirt on the fibre strand at the splicing point maybe? i noticed the engineer had quite dirty fingers when doing the splice.  cheers

[Gondola]

Hi guitarman3 

The light reading will be in dB (decibels) which is a relative measurement of light strength. So -19dB at the pole and -21dB at the ONT. The difference is going to be due to light loss at the fibre splice point(s) or any severe bending of the fibre. My only comment is that I wouldn't expect to see as much as 2dB loss even if the actual light level is perfectly good for the ONT to work.  i.e. If the ONT works reliably I doubt you'll get the fibre re-spliced.

[KeithFrench]

When asking such a specific question as this, I think you should be quoting the units involved and whether these are positive or negative values? 19 & 21 mean nothing at all without knowing the units