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Author Topic: Shunt fault?  (Read 2683 times)

konrado5

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Shunt fault?
« on: July 28, 2016, 11:23:21 PM »

Some day (year 2014) I've had reconnection when there was pavement resurfacing work. Then my attenuation there was 23.5/20.2 dB instead of standard 25.0/14.9 (standard on D-Link DSL-2740B, now I have Netgear DGND 3700v2). On the next day I've had reconnection again when there was pavement resurfacing work and attenunation came back to standard 24.0/14.9 dB.

I attach two Hlogs on one graph Double_Hlog1.png. The red graph is standard Hlog. The green graph is abnormal Hlog. The undulations and lazy roll around the tone 55 is the effect of measurements errors. The proof is green graph on Double_Hlog2.png which was generated when my target SNR margin was risded from standard 6.0 dB to 15.0/17.0 dB. It is noticable that on green graph there are less undulations.

Why do I think that on my circuit there is capacitive fault? On the red graph on the Double_Hlog1.png there is significant attenuation increase around the tone 210, on the green graph there is significant attenuation increase around the tone 150.

Quote
In other words, downstream attenuation tends to increase relative to the upstream attenuation, resulting in an increase in the attenuation ratio.
http://www.google.com/patents/EP2747401A1?cl=en

I think the pavement resurfacing work caused that the shunt fault was slightly and temporary modified (the attenuation increase on the earlier downstream frequency and increasement on upstream (20.2 dB instead of standard 14.8 dB).

What do you think about my hypothesis?

Best regards
konrado5
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burakkucat

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Re: Shunt fault?
« Reply #1 on: July 28, 2016, 11:59:44 PM »

It is rather difficult to either propose a cause or to analyse an observed change, for one incident, on one circuit, retrospectively.

However if the cable in question is not ducted but is buried directly in the ground, then any resurfacing work might impose an additional (physical) loading on the cable. Such crushing-like action could conceivably perturb the normal capacitive coupling between the two legs of a pair and, thus, appear as a temporary capacitive shunt across the pair.

So in answer to the question --

Quote from: konrado5
What do you think about my hypothesis?

 -- I would say that it appears to fit the observations. Unless there is some other more compelling suggestion, then I would be prepared to accept it as the most likely cause.
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konrado5

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Re: Shunt fault?
« Reply #2 on: July 29, 2016, 12:16:08 AM »

There must have been some misunderstanding. I don't think that pavement resurfacing work caused temporary shunt fault. I think that shaunt fault is permanent but pavement resurfacing work slightly and temporary modified the shunt fault effect. I think the shunt fault is permanent because on the standard Hlog graph (the red graph) there is significant attenuation increase around the tone 210 and this increase remain up to the end of graph. Pavement resurfacing work caused to the attenuation increase movement, this increase was around the tone 150 instead of tone 210.

But my observations does not reflect accurately expected result of shunt fault (higher attenuation on higher frequiences). On standard Hlog (red graph) there is also increase around the tones 33-50. On the abnormal graph (green graph) there was significantly lower attenuation on these frequiences. Unfortunately I have not any graph for upstream but the upstream attenuation was 20.2 dB instead of standard 14.8 dB. I conclude that on that day increase around the tones 33-50 was moved to the upstream.

To sum up, I see two frequency ranges with too much attenuation related to the remaining frequencies but pavement resurfacing caused movement these ranges to the lower frequiences. :)

Best regards
konrado5
« Last Edit: July 29, 2016, 12:21:27 AM by konrado5 »
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burakkucat

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Re: Shunt fault?
« Reply #3 on: July 29, 2016, 01:41:00 AM »

I shall have to give your observations and hypothesis some further thought.

What I am finding a little difficult to rationalise is how you can say, with any degree of certainty, that the circuit has a capacitive shunt fault when you are only considering a sample size of one! If you had the equivalent data for, say, one thousand circuits then it would be fairly straightforward to offer up the results of an analysis of all those circuits and, thus, support the claim for a shunt fault. Obviously every circuit has a natural capacitance; a function of the product of the intrinsic capacitance per unit length and the total installed length of the cable.
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konrado5

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Re: Shunt fault?
« Reply #4 on: July 29, 2016, 01:52:18 AM »

Sudden attenuation increase around tone 210 (around tone 150 on abnormal Hlog) is reason to think about shunt fault. In this patent there is that on shunt fault there is too high downstream attenuation relative to upstream attenuation. On my circuit there is too high 210-511 tone attenuation relative to other tones attenuation.

Best regards
konrado5
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burakkucat

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Re: Shunt fault?
« Reply #5 on: July 29, 2016, 05:40:01 PM »

Before the pavement resurfacing work the line exhibited a certain capacitance, Cbefore   (1)

During the pavement resurfacing work the line exhibited a different capacitance, Cduring   (2)

Upon completion of the pavement resurfacing work the line exhibited a different capacitance, Cafter   (3)

Are you proposing that the observed capacitance at event (2) is greater or lesser than at (1)? (Is Cbefore < Cduring or is Cbefore > Cduring ?)

Are you proposing that the observed capacitance at (3) is identical to that at (1)? (Is Cbefore = Cafter ?)   (4)

If the statement at (4) is invalid, are you proposing that the observed capacitance at (3) is greater or lesser than at (2)? (Is Cduring < Cafter or is Cduring > Cafter ?) Further, if the statement at (4) is invalid, are you proposing that the observed capacitance at (3) is grater or lesser that at (1)? (Is Cbefore < Cafter or is Cbefore > Cafter ?)

The major problem, as I see it, is that you are attempting to map observed results (of a set of one), whilst attempting to use that same set (of one) as a baseline reference for the observed result. A visual analogy would be the Penrose Stairs, where one may perpetually ascend (or descend) due to the visual baseline of reference being subtly modified.

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konrado5

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Re: Shunt fault?
« Reply #6 on: July 29, 2016, 07:03:10 PM »

Quote from: burakkucat
Are you proposing that the observed capacitance at (3) is identical to that at (1)? (Is Cbefore = Cafter ?)   (4)
Yes, the abnormal capitation lasted only one day. Next day pavement resurfacing working caused shock on circuit again and Hlog came back to usual.
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burakkucat

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Re: Shunt fault?
« Reply #7 on: July 29, 2016, 08:38:25 PM »

Ah, I see. Thank you. I am now prepared to accept your explanation as for the cause which then gave rise to the observed changes.  :)

So the work to resurface the pavement reduced the physical loading on the cable and the capacitance was ever so slightly changed from its earlier value. Then when the new surface was laid the physical loading on the cable increased once again. As a result, there was another ever so slight change in the capacitance.

You are making the (perfectly reasonable) assumption that no other characteristics of the cable has changed over the time-frame that elapsed whilst the work was taking place. Hence the variation in attenuation at specific frequencies has been correlated to the physical loading, first removed from and then restored onto the cable.

(There is one other environmental factor which might have had some relevance and that is the effect of temperature. But I would regard that as a somewhat slow acting effect and probably did not play a part during the couple of days that the cable was exposed.)

In Poland, is it normal for such cables to be directly buried in the ground? Perhaps it is just the individual service feeds that are buried direct and the larger, multiple-pair, distribution cables are deployed in ducts?
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konrado5

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Re: Shunt fault?
« Reply #8 on: July 29, 2016, 09:05:28 PM »

Still there is little misunderstanding. On 1 July 2014 I've had sudden reconnect and attenuation was changed from 25.0/14.8 dB to 23.5/20.2 dB. It is probably that reconnection had at that time more people because I've got SNR margin about 4.6 dB instead of standard about 6.0 dB immediately after connection. At the same time I've heard that workers was hitting the pavement. I also did reconnects on that day. It was continously 23.5 dB/20.2 dB for downstream (upstream slightly changing 19.8-20.2 dB). Next day (2 July 2014) I've had similar sudden reconnect and attenuation came back to usual 25.0/14.8 dB. It was therefore something else than physical loading. Something caused reconnect and change of circuit parameters.

Quote from: burakkucat
In Poland, is it normal for such cables to be directly buried in the ground? Perhaps it is just the individual service feeds that are buried direct and the larger, multiple-pair, distribution cables are deployed in ducts?
I'm not sure but I think that larger distribution cables are deployed in ducts. These cables are under the pavement.

Is it possible that shunt fault causes high attenuation on two frequency ranges 33-50, 250-511 (on abnormal Hlog upstream frequiences and 150-511) relative to the other frequiences? I've read that shunt fault causes high attenuation on higher frequiences relative to lower frequiences. I see however two ranges: one low frequiences, second high frequiences.
« Last Edit: July 29, 2016, 09:15:31 PM by konrado5 »
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burakkucat

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Re: Shunt fault?
« Reply #9 on: July 29, 2016, 10:31:12 PM »

Something caused reconnect and change of circuit parameters.

Hmm . . . then I don't think we will be able to say what was the underlying cause. (A technician could have been working on a large joint-closure, for example.)

Quote

I'm not sure but I think that larger distribution cables are deployed in ducts. These cables are under the pavement.

Yes, I understand. Thank you.

Quote
Is it possible that shunt fault causes high attenuation on two frequency ranges 33-50, 250-511 (on abnormal Hlog upstream frequiences and 150-511) relative to the other frequiences? I've read that shunt fault causes high attenuation on higher frequiences relative to lower frequiences. I see however two ranges: one low frequiences, second high frequiences.

A series capacitive fault would behave as a high pass filter. Attenuating low frequencies but allowing higher frequencies to pass.

A shunt capacitive fault would show behaviour similar to a low pass filter. Attenuating high frequencies but allowing lower frequencies to pass.

You make reference to three bands of sub-carriers; 33 - 50, 250 - 511 & 150 - 511. Let's calculate the equivalent frequencies --

sub-carrier        frequency (kHz)
33                      142.3125
50                      215.625
150                    646.875
250                  1078.125
511                  2203.6875

Looking at the frequencies displayed in that table, I am having great difficulty in convincing myself that either of the two ranges (which you have mentioned) could both be simultaneously subject to increased attenuation as a result of a change in capacitive shunt effect.

I spent some time looking at the formulae on the RLC circuit Wikipedia page which eventually convinced me that what you have observed cannot be due to just an increase (or decrease) in shunting capacitive effect. Try as I might, I cannot find any other possible explanation.

My overall feeling is that this is another case where I have to say: "Sorry, I just do not know".  :(
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konrado5

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Re: Shunt fault?
« Reply #10 on: July 29, 2016, 11:40:03 PM »

Quote from: burakkucat
Hmm . . . then I don't think we will be able to say what was the underlying cause. (A technician could have been working on a large joint-closure, for example.)
Perhaps it coincided with works of technicans. But I've not observed similar changes except for July 2014 and pavement workings.

It is important also that reconnection during pavement works caused the higher attenuation moved from 33-50 tones to upstream and from 250-511 to 150-511. I have some kind of fault that can be modified that attenuated frequiences can be changed. Moreover, this fault can't cause CRC errors, SNR margin fluctuations. My circuit is stable on SNR margin about 1 dB. Any idea?
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burakkucat

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Re: Shunt fault?
« Reply #11 on: July 30, 2016, 12:14:26 AM »

I have some kind of fault that can be modified that attenuated frequiences can be changed. Moreover, this fault can't cause CRC errors, SNR margin fluctuations. My circuit is stable on SNR margin about 1 dB.

A very peculiar circuit when compared with those in the UK. I have tried to understand it but without success.

Quote
Any idea?

No, sorry. I am completely "out of ideas". I just do not know.  :no:
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konrado5

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Re: Shunt fault?
« Reply #12 on: July 30, 2016, 12:53:51 AM »

But do you see on my usual Hlog sudden and continuing attenuation risement above 250 tone?
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ejs

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Re: Shunt fault?
« Reply #13 on: July 30, 2016, 06:11:39 AM »

It's perfectly normal that higher frequencies have higher attenuation. Hlog is the channel transfer function, I think it's like the inverse of attenuation.
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William Grimsley

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Re: Shunt fault?
« Reply #14 on: July 30, 2016, 08:35:41 AM »

But do you see on my usual Hlog sudden and continuing attenuation risement above 250 tone?

burakkucat doesn't have any more ideas, as is evidently stated in his pervious post! ::)
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