An 18th edition exam question

[timhoward]

Here's a question where nothing came to mind and I totally failed to find anything in the BBB. The post-exam chat afterwards established no other candidate did either and we all guessed.
Obviously from memory so may be misquoted:
"In which of these circumstances is it permitted for a single pole switch to be installed in the neutral conductor in a final circuit supplying a luminaire?"
and my memory of the options are:
1 - where there is additional protection
2 - where the circuit is supplying a 3 phase luminaire
3 - where the luminaire is controlled by an auxiliary controller
4 - where 3 plate switching is in use at the switch

Any thoughts from the wise people on here about what reg this was alluding to? (This may turn out to correct my memory of the question and answers)

 

[Dustydazzler]

4 - where a 3 POLE switch is used at the switch

 

[timhoward]

4 - where a 3 POLE switch is used at the switch

That makes sense - and would have made sense, but I distinctly remember the word "plate" as it was strange to me. Now I think of it, the question had the words "single pole" in which compounded the mystery. I've amended post now.

 

[i=p/u]

No 3 as in contactor

 

[i=p/u]

3ph luminaries in a tt system

 

[timhoward]

No 3 as in contactor

Ah - thank you . That was what I guessed in the exam in fact. (I'd got as far as thinking interrupting the coil of a transformer for dc lighting was a possibility in abstract).

 

[westward10]

You state single-pole switch.

 

[timhoward]

You state single-pole switch.

Yes. I originally didn't remember that bit and added that after the first answer above. I now clearly remember the words single pole in the question as it was the main point of confusion, and it being about switching a neutral conductor in a circuit supplying a luminaire.
And I clearly remember the words "three plate" in the answer that seemed most likely, though none of them jumped out.

As above I remember wondering if there was an obscure reg that allowed switching the neutral to a coil of a transformer if it was feeding extra-low-voltage DC lighting.
The whole thing didn't add up in my mind, and I couldn't see why you would ever want to in any case.
I would say I misread it, but other candidates were also wanting to talk about it.
Maybe I'll never know for sure.

 

[timhoward]

Ah - I think it relates to 463.1.2
"A single-pole switching device shall not be placed in the neutral conductor except for the connection of the control device for lighting circuits as shown in Fig 46.1"

Most likely answer 3 was in fact "where the luminaire is controlled by a controlling device" or similar

 

[i=p/u]

Sounds better

 

[timhoward]

It's a bit interesting -
463.1.2 has "A single-pole switching device shall not be placed in the neutral conductor except for the connection of the control device for lighting circuits as shown in Fig 46.1.
Switching of the neutral shall be in compliance with 530.3.3"

The last sentence is not worded brilliantly to my mind - it HAS to mean that doing the above meets the requirement, and that could be expressed better e.g. Switching of the neutral in these circumstances is deemed to meet the requirements of 530.3.3

[530.3.3 says "A switching device shall not be inserted in the neutral conductor alone".
This is of course the commonly understood principle.]