One thing I don't understand is it's said that places with lots of electronic SMPSUs like data centres and that type of thing can have higher currents in the neutral than the phases how is this possible? I can't really understand it hope someone can explain it
The offender in this case is generally switched mode power supplies introducing 3rd order harmonics (i.e.150Hz)**. As Mike says the three phase currents are (approximately) balanced and sum to zero in the neutral but if you look at three phases of 150Hz offset by 8.33 ms in each phase then the peaks always coincide so they are additive in the neutral. The problem was so bad in the early days of office blocks (less filtering of the harmonics combined with computers requiring more power) that Terasaki actually produced a four pole circuit breaker with the neutral oversized compared to the phases, specifically to cater for this problem. These days the problem is better understood and such measures are unlikely to be needed.
Alasdair
**The order of the main harmonic is 2*n ± 1 times the fundamental frequency so for a single phase switched mode supply the biggest harmonic will be at 3 times, while for a three phase it will be at 5 and 7 times.
The offender in this case is generally switched mode power supplies introducing 3rd order harmonics (i.e.150Hz)**. As Mike says the three phase currents are (approximately) balanced and sum to zero in the neutral but if you look at three phases of 150Hz offset by 8.33 ms in each phase then the peaks always coincide so they are additive in the neutral. The problem was so bad in the early days of office blocks (less filtering of the harmonics combined with computers requiring more power) that Terasaki actually produced a four pole circuit breaker with the neutral oversized compared to the phases, specifically to cater for this problem. These days the problem is better understood and such measures are unlikely to be needed.
Alasdair
**The order of the main harmonic is 2*n ± 1 times the fundamental frequency so for a single phase switched mode supply the biggest harmonic will be at 3 times, while for a three phase it will be at 5 and 7 times.
