But surely if it was as massive a problem as the op is worrying z out, it would present itself often as it's the nature of every supply cable in the land? Or is the cable in the street genuinely configured with a larger neutral? I know for certain that my current house (one of a pair of semis) has a same size neutral, as I can see the supply cable offcuts the DNO left behind. L1 feeds one house, L2 the other and L3 the common areas. All share the neutral and these are houses where everything is electric; heating, cooking...
3 phase question
- Thread starter athej
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I guess diversity kicks in at that point if it's just houses, it's not like one house is full of motors and the next with dimmers
As John has said, I think it's all a matter of 'diversity' ('balancing out').
I presume that the assumption (hope!) is that when multiple properties are being supplied, that things will 'balance out' across the three phases, such that the neutral current will usually be relatively low, almost always less than the highest current in any of the phase conductors.
As has already been said, if all the loads were resistive, then it would be mathematically impossible for the neutral current to be greater than greatest phase current. However, even when (as in practice), some of the loads are reactive, if they also (as is likely) were largely balanced across the phases then I think (need to so some sums!) that it would again be true that the neutral current could not exceed the largest phase current.
I therefore suspect that, in practice, the neutral current in the supply main will very rarely exceed the largest phase current, and that the 'normal' situation is probably for the neutral current to be considerably less than any of the phase currents.
Kind Regards, John
Part of the reason for the regs on power supplies and complex load currents is in fact down to the neutral problem. With SMPSs without any input current conditioning, the current is a short spike around the peak of the voltage - ie a very large third harmonic component. Discharge lighting (ie flouros) exhibit a similar current waveform. These harmonics do not cancel at all in a 3P system, so it's possible for the neutral current (in extreme) to be the sum of the single phase currents. I recall my father mentioning that there were some significant problems in the 60s/70s with large office buildings suffering from overloaded neutrals as electrical loads starting rising beyond the "occasional socket for the cleaner" level.
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If all the loads are linear and all have the same phase angle then the neutral current will never exceed the highest phase current.
So assuming all the loads are linear and the loads placed on each phase have similar characteristics it is very unlikely for the neutral to be singificantly overloaded.
Once you introduce nonlinear loads though that breaks down. Of particular concern is the third harmonic of the mains frequency. If you have a group of matched nonlinear loads connected to a three phase with neutral system then third harmonic currents will sum in the neutral (sixth and ninth harmonics will too but they tend to be much weaker)
Harmonics in the mains supply system were a sufficient concern that regulatory pressure was put on makers of larger DC power supplies (basically desktop PC size and up) to clean up their act.
There is no real technical reason that makers of lighting dimmers couldn't also clean up their act but I suspect they will not do so until/unless they are pressed by regulators.
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Indeed. That's obviously essentially what I said.
I understand that, but I confess that I did not think it would be a major issue in relation to domestic installations. However, it seems that I was wrong in thinking that, since you go onto say:
It's good to be educated
Kind Regards, John
In the domestic area it was probably not so much the neutral currents as the effect on transformers etc - after all, when this was coming to the fore and regs being drawn up, incandescent lighting was still the norm and non-linear loads were probably a relatively small part of the overall load.
In the commercial area things were probably much worse - widespread flouro lighting, rapid adoption of large quantities of IT gear, rapidly increasing proportion of motors on inverter drives, etc, etc. Just think what the load must be like in a large office block, with essentially no electric load other than flouro lighting and switch mode power supplies.
I vaguely recall that (during one particularly bored moment) I once worked out the rough load in the office I worked in - in one room I reckon there was something in the order of 80-100kW of 8ft flouro tubes It was a big office though - size of a football pitch or something like that - and it was horrible to work in. We had almost no IT stuff back then though.
In the commercial area things were probably much worse - widespread flouro lighting, rapid adoption of large quantities of IT gear, rapidly increasing proportion of motors on inverter drives, etc, etc. Just think what the load must be like in a large office block, with essentially no electric load other than flouro lighting and switch mode power supplies.
I vaguely recall that (during one particularly bored moment) I once worked out the rough load in the office I worked in - in one room I reckon there was something in the order of 80-100kW of 8ft flouro tubes
It was a big office though - size of a football pitch or something like that - and it was horrible to work in. We had almost no IT stuff back then though.
Running escalators continuously in an office tower block seemed wasteful until the services manager explained that the power factor of the motors cancelled the power factor of the flourescent lighting. Getting the over all power factor for the building closer to unity reduced the electricity bill.
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