Interesting observation of SNR margin during a power cut

[#1]

I recently had a short power cut at home.
My PC/Router is on an APC “Smart UPS” system so this is not really an issue – I just continue working.
What was interesting however was the SNR margin details.
Post and prior the power cut the value was a fairly steady 6.5 dB. (sync 8096)
During the power cut however the SNR margin rose to peak at 9…as all the noise on the line generated from other peoples’ electric appliances/internet connections/BT cable crosstalk and whatever etc etc in the entire area suddenly vanished.
Following power restoration you could see the exponential decay of the SNR back to 6.5 as all this stuff started up again and spewed its interference out.
(Just in case you are about to suggest most of it is my own electrical systems – I’ve tested my PC on the UPS with the house electric off to check for this and I'm using shielded cat5 as the internal telephone wiring).
Obviously I’ve no way of knowing how much of this noise is badly suppressed electrical items radiating RFI and how much is interference from other lines within the BT cable bundle back to the exchange.
Interesting nevertheless………
On a related note I’m quite close to the street box but comparing internet speeds with two houses further away both fed from the same street box but on a different cable run showed a huge reduction in speeds obtained compared to mine. This does indeed suggest that the condition of the “last mile” of copper is a major determinate of line performance – at least in my (rural) location – and that the (mostly) better cabling from the street box(s) to the exchange is less of a determinate. This in turn has implications for the success of improving speeds at the consumer endpoint by running fibre as far as the street boxes only.

[#2]

You are assuming that the cables from the cabinet to the exchange all take the same route, this may not be the case.

[#3]

Thanks for the info, I'm surprised it was only a difference of 3dB tbh!

[#4]

Quote from: itsme

You are assuming that the cables from the cabinet to the exchange all take the same route, this may not be the case.

Indeed, that is a fair point which I had not thought about.
I do know that the cable bundles from the street boxes back to the exchange are in ducts while those from the boxes to the customers houses were laid direct in the ground many many years ago

[#5]

Just out of interest can you clarify one thing, you said that your PC and router are on the UPS, but you didn't  mention whether ALL your telephones remained powered as well.
I'm thinking if a telephone or other device on your internal phone wiring had lost or reduced power, that could be causing interference on your line.