Locating a 'short'...

[Softus]

BT have a really good DMM that really can do just that , its acurate to within 1 foot. Thats how they know where to dig the road up

It doesn't sound very likely that they use a DMM, no matter how "good" one might be.

 

[Steve]

I saw 2 BT engineers in one of our shops once, trying to locate a JB in the ceiling void. They plugged in a large instrument, with a large backlit screen into the phone/BB socket in the office. It looked rather more sophisticated than a DMM. Then they had a distance, and based on the phone lines running in a straight line across the shop ceiling, they knew exactly where to lift the tiles.

 

[plugwash]

its called a time domain reflectometer, you fire off a very sharp transition or pulse and then listen for the reflections. If you know the propogation speed then first reflection will tell you the distance to the first discontinuity, after that it gets a little more complicated since you can have echos of echos but if you have an approximate idea of what should be there then you should be able to work out its positions.

Its a nice technique but the kit is expensive because the times involved are so short. Pulses in a wire typcially travel at a significant fraction (somewhere arround a third is fairly normal iirc) of the speed of light so you are talking measuring to nanosecond resoloution if you want a decent measurement.