ELECTRIC SHOWER - 10.5KW

[JohnW2]

No it can't, as in a domestic premises isolation requires the breaking of all live conductors.

Not in the UK, it doesn't, other than in TT installations. I can't speak for other parts of the British Isles.

 

[securespark]

I take the view that local isolation is required, TT or not.
Why?
Because to isolate a fault, you may well need DP isolation. If that can only be achieved by using a DP device in the board, that could mean some (or all) of the rest of the circuits in the building have to be isolated at the same time, contravening 314.

 

[Risteard]

Not in the UK, it doesn't, other than in TT installations. I can't speak for other parts of the British Isles.

BS7671 clearly states that domestic single phase isolation requires all live conductors to be interrupted.

So if it doesn't break both live conductors then it's not suitable as an isolator. Hence why the double pole switch is required in the DB.

 

[securespark]

Only for the main switch.

What does 537.1.2 say?

 

[JohnW2]

BS7671 clearly states that domestic single phase isolation requires all live conductors to be interrupted.

Hmmm. I guess I must have missed it 'clearly stating' that ...

 

[securespark]

What you have posted says switching of neutral (assuming conditions are met) is NOT required!

EDIT: My sarcasm detector is broken.....

BUT: The 17th says main switch must break N. What does 18th say?

 

[bernardgreen]

The 17th says main switch must break N. What does 18th say?

The 19th will probably say

If there is an R in the month then N should be broken by the isolator,

If there is not an R in the month then N should be not be broken by the isolator

 

[securespark]

Time for electrical equipment manufacturers to build a timeswitch that mechanically disconnects the N side of a DP switch for that third of the year!

 

[JohnW2]

What you have posted says switching of neutral (assuming conditions are met) is NOT required!

Exactly my point ....

EDIT: My sarcasm detector is broken.....

Indeed it was, but you seem to have mended it!

BUT: The 17th says main switch must break N. What does 18th say?

The relevant note to Table 53.4 of 17th says:

... and the note in the corresponding Table of 18th (Table 537.4) says ...

I won't risk challenging your sarcasm detector again, but do you see a difference (other than the change in reg and Table numbers)?

If one thinks about it, it would be a bit daft to require DP isolation in a TN-C-S installation - since, unless one also isolated the CPC (which is definitely not allowed), the CPC at any point in the installation (including downstream of a DP 'isolator') would be connected to neutral at the cutout, regardless of any intervening 'isolator (DP or otherwise).

Kind Regards, John

 

[securespark]

What is the equivalent in the 18th of 537.1.4?

"A main linked switch....shall be provided....as a means of switching....and as a means of isolation."

"A main switch intended for operatio by ordinary persons....eg household....shall interrupt both live conductors of a single-phase supply."

 

[securespark]

the CPC at any point in the installation (including downstream of a DP 'isolator') would be connected to neutral at the cutout

What if the incoming supply neutral went open-circuit?

 

[bernardgreen]

would be connected to neutral at the cutout,

Yes of course the CPC is connected to the incoming Neutral at all times.... irrespective of the switches / isolators in the system.....

This is a cheap way of getting an "Earth" to the premises by avoiding the cost of a third wire in the supply network. This became necessary when the reliable metal water pipe Earths dis-appeared with the advent of plastic water pipes.

What if the incoming supply neutral went open-circuit?

When that happens the Neutral at the Cut out gets pulled up to some voltage and with it the CPC. All extraneous metal that has been correctly bonded to the MET then becomes hazardous to anyone outside the equipotential zone of the CPC.

With a TT system the CPC does not get pulled up above Ground potential when the network Neutral is not connecte to the Neutral in the Cut out.

 

[JohnW2]

What is the equivalent in the 18th of 537.1.4?
"A main linked switch....shall be provided....as a means of switching....and as a means of isolation."
"A main switch intended for operatio by ordinary persons....eg household....shall interrupt both live conductors of a single-phase supply."

Exactly the same, apart from the reg number ...

HOWEVER, as someone already observed above, this only relates to a switch at the origin of the installation (i.e. the main switch in a CU) - NOT to the requirement for isolation (e.g. of circuits and appliances) in general - which may be SP in TN installations.

Kind Regards, John

 

[JohnW2]

Yes of course the CPC is connected to the incoming Neutral at all times.... irrespective of the switches / isolators in the system.....

Exactly, so anyone who believed that anything would be achieved (in terms of the safety of people working on the wiring or anything connected to it) by requiring 'isolation' of the neutral, as well as the line, in a TN-C-S installation would be plain daft. So, for once, the regs are being sensible (non-daft).

However, the argument that the regs are 'being sensible' seems to be less strong for TN-S (for which they also don't require DP isolation), since there is then no local connection between CPCs and the neutral.

Kind Regards, John