The use of Earth Loop Impedance tester and clamp ammeter to identify the neutral conductor where at least one core is live.
Refer to HSS 40 059 for further information on the procedure
This procedure demonstrates how to identify the neutral conductor on a multi-cored cable that has lost one or more live phases.
1. Earth loop impedance test process
Identify cable in line with DSR 01 018 identification of LV cables
Prepare cable ready for testing at the location in which the neutral requires identification. The test location will need to be long enough so that the wedges can be inserted between the cores to allow enough gap for the clamp meter to be used later on in this procedure.
Confirm status of cable by completing full test using approved test lamps. This will indicate the live and dead cores. In this example core 0 is live and cores 1,2 & 3 are all dead.
Using an Earth loop impedance tester, test between a live core and all of the dead cores; make a note of the readings observed.
An example of the process is detailed below:
Figure 1 - Test between core 0 and core 2 - Result obtained 988Ω
Figure 2 - Test between core 0 and core 3 – result obtained 0.19Ω
Figure 3 - Test between core 0 and core 1 – result obtained 20.27Ω
From the results obtained core 3 has the lowest Earth fault loop impedance reading of 0.19Ω. This would indicate that cor3 I the Neutral core from this test.
Note
The results from earth fault loop impedance tests may vary considerably and if there is a complete open circuit on the cores, there may be no reading at all. The neutral will have a very low reading generally between 0.13 and 0.23 ohms, discretion will need to be applied and if in doubt seek advice from a suitably competent and experienced person.
2. Current clamp confirmation process
Using a clamp, Ammeter now test each dead core to detect Neutral current.
Figure 4 - Test core 2 – Result obtained 0.1 Amps
Figure 5 - Test core 3 – Result obtained 11.3 Amps
Figure 6 - Test core 1 – Result obtained 0.1 Amp
From the results obtained from the above process it is observed current flow was detected in core 3. This therefore confirms the Neutral conductor as core 3
Note
Where there is a dead core on a multicore cable there will be current flow in the neutral conductor because of the out of balance of phases. There may be a very low reading of current flow on open circuit cores but this will be considerably lower than the current flow in the neutral conductor.
Summary
From the results obtained from the testing completed in section 1 and section 2, Core 3 had the lowest Earth Loop Impedance reading of 0.19Ω and there was current flow detected in core 3. This would indicate that core 3 is the neutral core. Once the neutral conductor has been identified, the other dead cores can be cut to assist in fault location or supply restoration.
On completion of the work it is essential to carry out testing in accordance with DSR 01 016 Process on how to ensure customers have correct polarity and phase rotation
Figure 1 - Test between core 0 and core 2 - Result obtained 988Ω 1
Figure 2 - Test between core 0 and core 3 – result obtained 0.19Ω 2
Figure 3 - Test between core 0 and core 1 – result obtained 20.27Ω 2
Figure 4 - Test core 2 – Result obtained 0.1 Amps 3
Figure 5 - Test core 3 – Result obtained 11.3 Amps 3
Figure 6 - Test core 1 – Result obtained 0.1 Amp 4
This printed document is valid at 08/01/18, check after this date for validity.
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