Reliable Power Meters Supplementary Help-Desk: Using N-G as a 4th voltage

Using N-G as a
4th voltage channel

This article should be read in conjunction with "RPM-PR Input Structure".

If the Neutral-Ground voltage is not of concern then this channel is available as a 4th voltage channel. This means Voltage and Impulse Events (>130µS to 1000V), Voltage and Current RMS trends, and Voltage and Current Harmonics to the 63rd can be recorded.

What is not available is THD, Flicker, Power, VAr, Demand, Power Factor, and Frequency (although this last parameter is available on the phase channels). If merely wanting to keep watch on a 'control system supply voltage' or something similar, then this method is superb.

Note: Although high-speed transients (0.5-130µS to 6400V) can be captured, any transients that appear on this 4th channel could be reflected on the phase channels. It is advised that the high-speed captures be turned off for this type of monitoring.

As can be seen in the page on the Input Structure (referred to above), the recorder has the Neutral input as common for all channels. The only stipulation, therefore, is that the 4th voltage must me commoned to this same Neutral. Also, the recorder must be in '3-phase wye' mode for the 4th channel to be switched on - the input for the 4th channel is G.

Coupling the 4th voltage to the main input network can be accomplished through use of either a current choke (also called a 'common-mode choke') or isolation transformer. The former method is better at transferring high frequency components i.e. harmonics, but lacks the ability to provide isolation on floating systems that use Faraday effect to keep the voltages within safe regions. L and N are connected to the source being recorded, with L' and N' being connected to the G and N inputs on the recorder respectively.

If there is any doubt as to the voltages that could exist between the two systems then use an isolation transformer of required ratings. It is suggested that the VA rating be kept as low as possible so as to ensure the highest transfer accuracy of high frequency components.

Please ensure you upgrade to Power Analysis Software (PAS) version 5.01.006 or later for analysing Neutral voltages that exceed 150Vrms (earlier versions only allowed to 150Vrms). All versions of Scenario are usable.

If there are any questions please ask.

Note: This web page does not form part of any official documentation.
Any information contained herein is used at own risk.