Floating Neutral in Building
with 3-phase Power Supply
and 1-phase Loads

   By TK NG  

            3-phase supply is usually provided in a building where power demand is high.  There may be both 1-phase and 3-phase loads connected to the power supply.  Figure 1 below shows a typical connection arrangement.  The neutral connection for 1-phase loads is grounded.  With a phase-to-phase voltage of 380V, the phase-to-neutral voltage should normally be 220V.

Figure 1

            If, for whatever reason, the neutral wire is disconnected as shown in Figure 2, the 1-phase loads are in effect forming a star-connection.

Figure 2

            In Figure 2, if the aggregate loads in individual prongs of the star are the same, i.e. forming a balanced load, the voltage across each prong would be 220V.  Nevertheless, individual loads may vary or be switched on and off from time to time.  Load balance is hence established or disrupted as time goes by, resulting in changing voltage and current across each prong of the star due to variation in prong impedance.  The voltage across each prong may be less or more than 220V and this phenomenon is called a floating neutral.

            A floating neutral poses serious risks to both people and properties.  Appliances may be damaged due to over- or under-voltage and fire may also be caused.  People inadvertently touches the neutral wire may get a bad shock as the wire is no longer grounded.  Such condition must be rectified as soon as possible.

            Loose or disconnected neutral wire is usually due to human error during installation, maintenance or inspection services.  Care must be taken to ensure a secure neutral connection after work is carried out at the power supply source.

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