___ are odd harmonics divisible by 3 and cause problems on three-phase, four-wire circuits because they are not in phase with anything but themselves.

___ are odd harmonics divisible by 3 and cause problems on three-phase, four-wire circuits because they are not in phase with anything but themselves.

• A. Triplen harmonics
• B. Zero sequence harmonics
• C. Either A or B
• D. Neither A nor B

Answer: (C)

In three-phase four-wire systems, harmonics that appear in all three phases with the same polarity do not cancel in the neutral; they sum there and can cause overheating and voltage distortion. These are the triplen harmonics—the odd multiples of the third harmonic (3rd, 9th, 15th, …). They’re called zero-sequence harmonics because they have zero phase shift between phases, so they behave as a unit across the three lines and push current through the neutral. Since triplen harmonics are odd and divisible by 3, they are a type of zero-sequence harmonic. That’s why both terms describe the same phenomenon, making the option that combines them the best choice.

In three-phase four-wire systems, harmonics that appear in all three phases with the same polarity do not cancel in the neutral; they sum there and can cause overheating and voltage distortion. These are the triplen harmonics—the odd multiples of the third harmonic (3rd, 9th, 15th, …). They’re called zero-sequence harmonics because they have zero phase shift between phases, so they behave as a unit across the three lines and push current through the neutral. Since triplen harmonics are odd and divisible by 3, they are a type of zero-sequence harmonic. That’s why both terms describe the same phenomenon, making the option that combines them the best choice.