How to check continuity over long distance


When I began my career as an electrical technician my first challenge was the problem of checking the continuity of a wire that spans a long distance. My problem was compounded due to the physical layout of the wire under test.
Let me use a simple picture to layout the nature of my problem.




A bunch of Armoured cables outgoing from the  PCC (Power Control Center) in Building 1 feeds a group of distribution panels in Building 2 over 100ft away. Ideally, the cables should be tagged but due to aging and poor maintenance, most of the tags were not visible. 
The problem was that some Distribution boards in building 2 had power while some others didn't.  The DB of interest didn't have power and thorough checks showed that there was no power on the cable feeding that DB. 

  All outgoing cables from Building one PCC had power so I suspected that the cable was no longer continuous. Anyway in the initial design of the facility 3 spare cables was laid underground but without tags, I couldn't know which cable was which.

Anyway, the simple and obvious solution was to twist the wires together at one end and test for continuity from the other end. If the wire was continuous I would tag it for proper identification. 
After this, I would untwist the wires and perform an Insulation Resistance check on the cable to confirm the reliability of the cable. If the cable passes IR test, then it can be connected and energized safely.

You can also use your multimeter to check for a short circuit in a similar scenario. Set your multimeter to measure resistance. Unhook both ends of all the wires you suspect to be shorted then with your probe check the wires using all possible combinations. There should be a large resistance (signified by an OL, 1, MΩ or GΩ 1 depending on the meter).
If it shows zero or some resistance in just ohms, then there is short somewhere between the two wires.
It is also possible to check the continuity of a single core able using a wire tracer. One end is a tone generator, the other end is a detector that can hear those tones. If the wire is continuous you will get a tone at the other end

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