Q3

Neutral absent in two phase

Three lamps L1, L2 and L3 of 250 Volts 100 Watts are connected in star formation across three phase 433 Volts AC supply as in following diagram. Star point C of lamps is connected to neutral. Switch S1 is in phase B and Switch S2 is in Neutral as shown.

Circuit Diagram

 What happens when

Switch S1 is ON and switch S2 is ON

Switch S1 is ON and switch S2 is OFF

Switch S1 is OFF and switch S2 is ON

Switch S1 is OFF and switch S2 is OFF

Answer

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Quiz is a mental exercise to enhance reasoning power in the subject. Hence personal attempt for reply is expected before jumping to answer. Answer given here is just for confirmation.

Proceed

When switch S1 is ON, three phase supply is available. When switch S2 is ON, neutral is also available. Normal phase voltage i.e. 433 ÷ √3 = 250 is available across each lamp and therefore each lamp glow as normal at rated voltage.

When switch S1 is ON and switch S2 is OFF, three phase supply is available but neutral is not available. All the three lamps connected in star are identical and hence balanced load. So star point C of lamps is at neutral level even though neutral is not available. Each lamp get rated voltage and hence lamps glows as normal.

When switch S1 is OFF, only two phase supply is available but as switch S2 is ON, neutral is available. Lamps L1 and L2 get normal phase voltage and hence glow as normal. But lamp L3 does not get any voltage and hence remain OFF

When switch S1 is OFF and switch S2 is also OFF, only two phase supply is available and also neutral is not available. In this case, two lamps L1 and L2 will be in series across two live phases R and Y. As a result each lamp gets half the line voltage i.e. 433 ÷ 2 = 216 only. Lamps L1 and L2 get reduced voltage 216 volts against rated voltage of 250 volts. Therefore lamps L1 and L2 glow less bright than normal whereas lamp L3 remains OFF as obvious.