Measuring cable resistance

There is always the possibility that one or other of the five cores that make up the five-core cable has a different resistance to the rest. This can indicate a fault, and will certainly influence the performance of the device that the cable is feeding. We describe here a simple way to carry out the measurement. Attached is a spreadsheet that has been set up to calculate the resistance values of each of the cores. You will also need an earth bonding tester.

1. Open this Excel spreadsheet (22Kb).
2. Disconnect the five-core cable from the distribution board.
3. Before proceeding further check that the tails entering the room in which you are to do the measurements really do not have any voltage on them.
4. Connect L1 and L2 together at the distribution board end by means of a brass nut, bolt and washers.
5. Using the earth bonding tester, measure the resistance across the tails of L1 and L2 at the installation end, and enter the result in the first box on the spreadsheet.
6. Remove the nut and bolt and then connect L1 and L3 together. Measure the resistance and enter the result in the second box on the spreadsheet.
7. Remove the nut and bolt and then connect L1 and neutral together, and measure the resistance entering the result into the next box on the spreadsheet.
8. Remove the nut and bolt and then connect L1 and earth together at the distribution board end, then measure the resistance between L1 and earth at the installation end and enter the result into the spreadsheet.
9. Finally remove the nut and bolt and then connect L2 and neutral together, and enter the result in the spreadsheet.

The resistance values of each of the cores will now appear in the table on the right of the spreadsheet. There will always be some small variation in the value, but anything greater than +/-10% should be investigated.