MEASUREMENT TECHNIQUES:
Creating a Phantom Neutral
 

This is dedicated to my Dad.

With 3 phase delta systems no Neutral exists but there are times that single phase devices need to be hooked up to such systems, especially if tracing imbalance etc. This is also a handy method of reducing the measured voltage should an instrument not be capable of the full voltages of a delta system.

There are two methods of creating such a phantom Neutral being resistors or transformers. Both circuits operate on one golden rule, when the voltages of a (balanced) 3 phase supply are summed the result is zero.

The transformer model has two purposes being it can either generate a phantom Neutral or create a third phase if only two are supplied (the latter situation requiring a Neutral to be present). The transformer based unit has one further advantage being a few watts of power can be drawn from it ('SUP' terminals) without adversely affecting the readings.

In both models a "safety lamp" is included to warn any unsuspecting electrical maintenance person the Neutral from the unit is not at a safe potential.

OPERATING

Phantom Neutral:
Connect the ground connection to any suitable ground point. This only feeds the one side of the safety lamp and does not perform any function other than to help the user stay alive. Connect the phases one by one onto the inputs. As the third phase is connected the safety lamp should extinguish - if not, check all phase connections, maybe one has been duplicated.

Only when all the inputs have been connected and the safety lamp is extinguished should you proceed with connecting the test instruments. First connect a digital multimeter (DMM) to the Ground-Neutral reading points and take a reading. Then connect the test instrument, first Ground (if fitted), then Neutral, and finally the phases. Check the Ground-Neutral voltage for any sharp increase. If a single phase instrument is fitted this is likely to load the Phantom Neutral point, the degree of loading being measured on the Ground-Neutral reading on the DMM.

Creating a 3rd phase (transformer model only):
Connect the ground. Then connect the Neutral. Connect the red and blue phases to the two available phases. Once connected to the power then only connect the instrument to the unit. The reason for this is high voltages can exist during connecting up, especially on the Neutral, that could damage an instrument.

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