In a 3-phase, 4-wire, 400/230 V system, a lamp (L1) of 100 W is connected to one phase and neutral and a lamp (L2) of 150 W is connected to the second pahse and neutral. If the neutral wire is disconnected accidently, what will be the voltage across a 100 W lamp (L1)?

• A. 160 V
• B. 180V
• C. 240V
• D. 120V

Answer: (C)

When neutral disconnected Lamp are in series connection,
So, for series connection current is same for both Lamp,
Resistance of Lamp (L1) = (230)²/P
= (230)²/100
= 529 ohm
Resistance of lamp (L2) = (230)²/150
= 352.67 ohm
So, current through the lamp = E/(R1 + R2)
= 400/(529 + 352.67)
= 0.454 A
Voltage across lamp (L1) = I*R1
= 0.454*230
= 240 V