Q What is the best way to locate a break in an Ethernet cable, when it runs under a floor for a distance of about 20 metres?
A Simply pinging each way along the length of the cable should give you reasonable grounds for suspicion.
If you can disconnect the cable at both ends, and have no better means, you could short two of the wires together at one end, and check the resistance across their terminal contacts at the other, but that is error-prone and cumbersome.
If you have significant amounts of network cabling and prefer to have your own ability to test it out, it is worth investing in a proper instrument. The best systems use time domain reflectometry to send signals down the cable and work out where the break or short is situated, and can thus demonstrate whether it is the cable that needs to be replaced, or just one of the connectors. An ideal tool for the job might be the Fluke Networks MicroScanner2 Cable Verifier. This is likely to cost around £400 + VAT, but should pay for itself over a number of similar jobs.
Updated from the original, which was first published in MacUser volume 26 issue 4, 2009.