Electrical Wall of Defects for Home Inspectors

[Homer J Simpson]

Interesting....  2+ hours

[CopperCone]

why is neutral allowed to be smaller?

[Brumby]

For a 3 phase supply the neutral only has to carry imbalance currents.  If the load has perfectly balanced currents across all the phases, there would be no net current flow through the neutral.

In this situation you could, in fact, disconnect the neutral feed from the supply and things would still keep running.


Worst case scenario would be if two phases had no load and the third phase was drawing maximum current.  Then, the neutral would NEED to be as big as the phase or it would be overloaded.  ANY increase in currents from the other two phases will result in a reduction of the neutral current, reaching a minimum when all 3 are equal.

[dmills]

Note however that that is only true for LINEAR loads...

There is a particularly nasty case with light dimmers (and also some arc lamp ballasts) where you can actually have the neutral current significantly exceed the phase current, and that occurs when the load current is largely third harmonic and multiples as these will sum in phase in the neutral rather then cancelling, seeing neutral current 150% of the largest phase current is not uncommon.

Granted that sort of thing is more a concern in large entertainment lighting situations (But can also occur in datacentres) then domestic installs, but it pays to at least be aware of the edge cases. 

Regards, Dan.

[capt bullshot]

Note however that that is only true for LINEAR loads...

There is a particularly nasty case with light dimmers (and also some arc lamp ballasts) where you can actually have the neutral current significantly exceed the phase current, and that occurs when the load current is largely third harmonic and multiples as these will sum in phase in the neutral rather then cancelling, seeing neutral current 150% of the largest phase current is not uncommon.

Granted that sort of thing is more a concern in large entertainment lighting situations (But can also occur in datacentres) then domestic installs, but it pays to at least be aware of the edge cases. 

Regards, Dan.

I've heard of greenhouses burning down because of an overloaded neutral. They used some kind of discharge lamps producing third harmonic to grow tomatoes. As a cure, special resonant harmonic block filters got installed. I've seen these filters, basically a L/C resonant transformer circuit, the primary is in series with the neutral, the secondary forms the resonant tank with an external capacitor. These filters effectively block the third harmonic current in the neutral - now you've got to watch for the filters not to overheat.

[dmills]

One would expect that raising the neutral impedance at harmonics like that would do nothing good for the voltage waveform, but I suppose for a discharge lamp load it may not much matter?

The entertainment industry usually solves it by doubling up the neutral conductors and hoping to hell that the star point on the alternators and transformers can take the abuse, that and cursing at the cheap ass lighting company that brought the old junk rather then the modern good stuff.

I can tell a story involving been stood in a services room with a BIG cylinder of CO2 and instructions to 'dump a bit into the lighting busbar cabinet anytime it starts to smell'....  Late 80s RR tour, they tended to be like that. 

Regards, Dan.