17th edition question

[krishcanag]

Passed my 17th with flying colours the 1 question I got wrong, was abour an industrail lighting circuit.

A 3 phase is feeding 3 banks of lighting in a factory 1 for each phase. What switching mechnism would you also have;

1. switch live conductors.

2. One live

3. One live and netrual

4. Cannot rememeber


I went with 1 live (the phase feeding the lighting circuit we want to isloate) I think this is wrong and maybe it should be the live and netrual, but questionin a industrail place will the netrual be split into 3 from a common input so that both netrual and live for the correct lighting bank can be switched off?

KC

 

[telectrix]

answer is 1. switch live conductors.

 

[krishcanag]

Hi doesn't answer my question, what does that mean? I know there will be a main switch to disconnect all power, but if you have 1 light down in 1 bank of lights, you should only disconnect that 1 phase, so is the answer where the live and netrual is disconnected or just the live?

answer is 1. switch live conductors.

 

[telectrix]

generally you would have 3 phases going to a 3 gang switch. each gang would switch 1 phase. so the switching mechanism would switch 3 live conductors, 1 for each phase.

 

[krishcanag]

but that would suggest that all phases will be off, this cannot be right, it is a h&s issue if no lights are on while changing a light? Surely we must have a means of switching off just the 1 phase?

and is the N split into 3 in a phase system to a factory?

 

[brman]

I am assuming this is circuit isolation rather than just a light switch. I think I had a very similar question in my 17th.

Trouble is you have a common neutral between all 3 phases. So if you don't isolate all phases you can't guarantee safe isolation of the circuit (a fault on another phase could come back up the neutral).

 

[krishcanag]

Understand your point, so my thought was in a factory what I would do thinking from a design point of view is have the first full isloator, cabable of disconnected all pahses and netrual (so 4 way), then further downstream take the netrual and split it 3 ways (3 cables from a netrual terminal block), then take 1 phase and 1 netrual to a 2 way ganged isloator, and repeat from the other 2 phases, if you do this then you get full islolation for each phase, any netural fault on a other phase would not appear on the disconnected phase and there is a netrual isolation on that phase, hope this is clear, does no designer do this in the commerical world?

I am assuming this is circuit isolation rather than just a light switch. I think I had a very similar question in my 17th.

Trouble is you have a common neutral between all 3 phases. So if you don't isolate all phases you can't guarantee safe isolation of the circuit (a fault on another phase could come back up the neutral).

 

[gtownspark]

I think Telectrix just meant a 3 gang switch so you would switch each bank of lights separately not all at the same time with a 3 pole isolator

 

[SirKit Breaker]

The neutral is a live conductor, so as Tel says, the answer is switch live conductors.

Cheers..........Howard

 

[krishcanag]

damn, wish this forum had an ability to upload diagrams, a diagram speaks a 1000 words!!