Recording L-N & N-E

The Fluke 43B has two channels, one voltage, the other usually dedicated to current. However, there are times one wants to measure (and possibly log), especially on domestic supplies, the Live-Neutral and Neutral-Earth voltages simultaneously e.g. if there is a suspected Neutral, Earth, or Neutral-Earth fault.

The current probe input is capable of 5Vpk (3.54VACrms). At first, it may appear that all that is required is to bring the earth to Input 2. However, the problem is the voltage could rise to above what the input can accommodate. Please note that the input can tolerate 600VAC, but will merely show 'OL' (overload).

The answer lies in the use of a voltage divider probe. Depending on the level of fault, a 10:1 is more than sufficient, except it is personally preferred to use 100:1 at least (giving full measurement capability of a 230VAC domestic supply i.e. Live-Neutral swapped).


Operation is simple. Adjust the current probe sensitivity to what is shown in the table below, and readings are then simply interpreted as N-E volts = amps displayed. Please note that although Fluke only have 10:1 and 100:1 probes, direct connection and 1000:1 probes are also included as these may be self built.

- Ratio
Direct - 1V/A
10:1 - 100mV/A
100:1 - 10mV/A
1000:1 - 1mV/A

Please note: Both the L-N, and N-E (current) waveform polarity is with respect to Neutral (the N-E waveform appears inverted). This does not affect RMS recording. All other features, such as harmonics, are also available for inspection and recording.

A sample recording is shown alongside. Here the incoming L-N voltage has achieved a maximum of 237.1 and a minimum of 229.4V (all in ACrms). During the same time the N-E voltage reached a maximum of 157.4mV and a minimum of 86.3mV (all in ACrms).

The sharp spikes along the trace are typical of switching noise superimposed on the waveform. In this case these reached 40V on most occasions, and 150V on worst case (recorded later using the transient detection screen on the 43B).

If there are any questions please ask.

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Any information contained herein is used at own risk.

©  M.T.P. - 17.01.03