Electromagnetic effect - switch drop in metal conduit

[happysteve]

Daft question alert!

Is this ok?

It seems to go against 521.5.1, as the neutral does not follow the line down to the switch and back in the steel conduit. But I don't think it will cause any electromagnetic effects... when the switch is open, there is no current flow; when the switch is closed, the magnetic fields of the permanent line and the switched line oppose each other (which the neutral would do on a complete circuit) so it should be fine.

I have no practical industrial experience (to my shame) - is this standard practice, or a big no-no?

Thanks for any help (This isn't a homework question, it's purely to aid my understanding of the real world).

In other news, just done the 2394 and 2395 written exams, yesterday and this evening. Think I did enough to pass but we'll have to see (more confident about the '94 than the '95, I had a bit of a brain f@rt this evening), got the practical to do next week then a couple of months wait until the results, keep your fingers crossed for me...

 

[pennychew]

Whenever I've seen switch drops like that the neutral has been brought down and joined in connector strip... No idea what's right or wrong but just what I've seen when changing broken switch fronts. Fingers crossed for the exam results bud! You're well on your way now

 

[happysteve]

Whenever I've seen switch drops like that the neutral has been brought down and joined in connector strip... No idea what's right or wrong but just what I've seen when changing broken switch fronts.

Thanks for replying, Stu. I wondered if that would be the case (in practice). Anyone else got any thoughts on this?

Fingers crossed for the exam results bud! You're well on your way now

Cheers, mate! My head's still spinning, but at least I got a good night's sleep last night, after some much-needed alcohol!

 

[Bobster]

It's fine .

 

[Marvo]

In theory you're right, it's not a magnetically balanced circuit. Two problems that could in theory occur are eddie currents in the containment and RFI. If it was a light circuit that drew a large current then it would be a consideration but for a normal domestic light circuit it's not an issue.

Against the opinion of a lot of the sparkies on the forum it does however put a tick in the box for lighting circuits to be installed by running a L+N supply to each switch then switched live and neutral from there to the fittings.

 

[Bobster]

I disagree Marvo, live and neutral never separately leave the metal containment in that picture.

Look at it this way, its baisicly a metal box as its drawn.

 

[Marvo]

Yes and no. Eddy currents don't flow directly to earth and are more predominant in material that is perpendicular to the direction of the conductor, the fact the containment is electrically continuous doesn't prevent eddy currents from developing.

The leg of the circuit that doesn't have a neutral running in it (to the switch) has a live and a switched live which is basically a resistive neutral with an equal current flow to the live so, contrary to my original statement above, the circuit is fairly magnetically balanced.

This is getting a bit outside of the scope of general trainee electrical knowledge so here's a link for the trainees with a very basic explanation of eddy currents. http://www.voltimum.se/files/gb/others/A/201005299790eddy.pdf

 

[happysteve]

Ha! Thanks Andy! And there it is, on page 3 of the PDF you posted a link to, drawn a bit neater than my effort:



A reference like this (and knowledge about what's actually done in practice) was exactly the sort of thing I was after. Cheers all!

 

[Bobster]

Yes and no. Eddy currents don't flow directly to earth and are more predominant in material that is perpendicular to the direction of the conductor, the fact the containment is electrically continuous doesn't prevent eddy currents from developing.

The leg of the circuit that doesn't have a neutral running in it (to the switch) has a live and a switched live which is basically a resistive neutral with an equal current flow to the live so, contrary to my original statement above, the circuit is fairly magnetically balanced.

This is getting a bit outside of the scope of general trainee electrical knowledge so here's a link for the trainees with a very basic explanation of eddy currents. http://www.voltimum.se/files/gb/others/A/201005299790eddy.pdf

I do realise that it's more involved, having spent days calculating circulating currents in a HV network.

As you've shown though in this instance, there is nothing to worry about. I like you don't want to throw a huge amount of information, that most wont use unless they request it.

 

[Marvo]

Yeah Rob, much like yourself I'm sure, I love a good debate but in this forum I think keeping it on course and uncomplicated is more important

I'm glad the link was useful Happysteve, I think it's pitched at a sensible level for most trainees but contains all the relevant info you need at this stage.