Crackling on line - how does a BT engineer trace cause?


Prominent Member
When there's loud crackling audible from the phone - bad enough to cause frequent ADSL interruptions - and the fault is NOT inside the house, how does a BT engineer trace the cause?

It's often worse during rain and if I pick up the phone or receive an incoming call, it will kill the ADSL for certain. However, when it goes off, dialling a random 2 digit number will make the phone bleep after a few seconds and reestablish the ADSL. I assume this is additional current busting through a corroded connection somewhere. So how to trace it?

(I'm in Greece so this doesn't reflect badly on BT at all. I just want to know how a real engineer would trace the fault cause so that I can tell the muppets here. I'm fairly certain that sacrificing another sheep isn't going to help!)


Outstanding Member
They have equipment called a TDR tester (Time Domain Reflection tester ) which sends a pulse down the line and if the line is damaged some of the pulse is reflected back which can usually (but not always) be measured.
There are also noise test sets which are more complex and required specially trained engineers.


Prominent Member
Excellent, thanks! I knew there had to be something. I recall doing pulse reflection tests at university (a long time ago when 1 GHz was considered pushing the limits).


Distinguished Member
One other question occurs to me. How do they tell which wire pair is mine? Say in a cabinet 5 miles away?
It position in the wiring frame. If a pair was to get "lost" an engineer could put a signal on one end and listen at the other. (This has been dragged up from the memory of a class nearly forty years ago but I'm guessing that it hasn't changed.)


Prominent Member
Thanks. Yes, that's how we were taught at GEC Coventry but I thought they might have a magic box by now.

However, here in Greece, I suspect they simply connect 220 volts to the pair and watch for smoke! ;)

P.s. I was on the design team for the push-button Trimphone and for the RS22 telephone exchange - of which only one was ever made, and sold to the Duchy of Sark!
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Prominent Member
Our GEC PB version was lower at the front than the STC "brick" version. My instructions were to use the same cover moulding tool!
No problems with reliability and, unlike the dial version, it didn't contain radioactive tritium. (Somewhere in a field there's probably a large mound that glows menacingly in the dark. I never did find out how they "recycled" those circular, radioactive, glass tubes when the dial versions were scrapped.)


Established Member
Start at the exchange and work out towards the customer. Test at the cab, if thats fine, move to the dp etc. The cabs are all numbered up inside but if your pair is lost then all it needs is a tone to find it again.

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