Water in the Cable: Reading Copper Fault Tests on a VDSL2 Line
A VDSL2 customer 3.2 km from the cabinet reports sync drops every evening and a noise floor that rises when it rains. The line uses one pair of a 4-pair 0.5 mm aerial copper cable. Field tests give a loop resistance of 980 Ω, against a nominal 180 Ω/km for this cable, an insulation resistance of 0.4 MΩ between the pair and earth with the far end disconnected (the other pairs read above 200 MΩ), and measured insertion loss of 41 dB at 300 kHz. A TDR trace shows a 0.3 dB reflection at 1.1 km and no open circuit anywhere. The customer's modem is a VDSL2 profile 17a unit and the cabinet port has already been proved good on a test line. A second pair in the same cable carries a working voice-only subscriber. The operator has told the field team that a full re-sheath of this cable is not funded this year.
1. Interpret each measurement, then rank the faults by likely impact on the VDSL2 service.
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Loop resistance: 980 Ω against a nominal 3.2 × 180 = 576 Ω, which is 70% high. Series resistance adds directly to loss and makes the pair behave as if it were longer than it is. It does not vary with rain, so it points to corroded splices or poor joints along the route rather than to water.
Insulation resistance of 0.4 MΩ to earth is a fault. New cable should read in the hundreds of megohms; anything under about 1 MΩ means leakage, and leakage that worsens in wet weather means water inside a closure. It raises the noise floor and unbalances the pair, which is precisely what destroys a signal that runs up to 17.6 MHz.
TDR reflection of 0.3 dB at 1.1 km is a modest impedance discontinuity: a splice, a wet joint, or a bridge tap. Not catastrophic alone, but a bridge tap creates a notch that can remove part of the VDSL band, and a wet joint is consistent with the rain correlation.
Insertion loss of 41 dB at 300 kHz is high on its own; 17a needs usable bandwidth to 17.6 MHz, where loss is several times this figure, so the pair cannot sustain the profile.
Ranking: the insulation fault comes first (water ingress and safety), then the loop resistance, then the reflection. All three probably live in the same closure.
2. The customer asks to be moved onto the spare pair that is working for voice. Explain why that is not a repair, and state the correct action.
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The spare pair is in the same sheath, on the same route, and almost certainly through the same damaged closure. It passes voice tests because voice needs only 300–3400 Hz and tolerates leakage, loss, and unbalance that a 17 MHz signal cannot. Moving the customer would relocate the fault, not remove it, and it would break a working voice service in the process.
There is a second effect: energising another VDSL2 pair in a wet cable raises crosstalk into every other pair in the binder, so the move can degrade the neighbour as well as the customer.
Correct action: go to the closure near 1.1 km, open it, dry it, cut back to sound copper, re-splice with gel-filled connectors and re-close with a sealed, pressurisable closure. Then re-test insulation resistance, loop resistance, and insertion loss. If insulation recovers above about 20 MΩ and loop resistance comes back near 600 Ω, the pair is serviceable and the re-sheath can wait. If the insulation stays low after re-making, the water is in the sheath itself and the pair must be condemned; that is the point at which a fibre drop or a different pair becomes the rational answer, not before.
3. The operator proposes raising the target SNR margin by 3 dB to stop the evening dropouts while the budget is found. Evaluate that proposal.
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It will probably stop the dropouts, and it will do so by hiding the fault. A higher target margin forces the modem to load fewer bits per tone, which lowers the line rate and rides out a noisier channel. The customer gets a stable but slower service, and the water stays in the cable.
It also does nothing for the safety and asset side: a joint with 0.4 MΩ to earth is deteriorating, and the next wet week will defeat whatever margin you set.
Use it, if at all, as a 48-hour holding measure with a dated work order behind it, and tell the customer the speed will drop. Do not present it as a fix. The same applies to capping the profile or forcing a lower fallback rate: both mask a physical fault that the field tests have already localised to about 1.1 km.