Discuss Isolation for 3 phase domestic installations in the Electrical Forum area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

Ian1981

-
Mentor
Esteemed
Arms
Reaction score
3,818
4 pole isolation for domestic 3 phase installation.
Is it a requirement?
Regulation 537.1.4 requires a linked switch or circuit breaker to isolate both live conductors of a single phase supply for ordinary persons of a household or similar installation.
 
Do you mean like what was discussed in this thread?
 
Do you mean like what was discussed in this thread?
No.
I mean if a 3 phase DB for a 3 phase installation is installed does it require 4 pole isolation (forget about the fact the DNO will probably supply a 4 pole isolation switch after their meter) is it a requirement?
The regulations say that ordinary persons in a household or similar installation require isolation of the live conductors but only mentioned single phase.
Normally TN TPN installations do not require the neutral to be isolated however 537.1.4 requires all live conductors in a household for use by ordinary persons.
Only thing is it only mentions single phase perhaps because 3 phase domestic is rare maybe?
 
I think I would base the decision on the type of earthing that's installed.

I researched this for a farm job I was doing and the conclusion I drew based on what I read was if it's TT, install a 4 pole so the neutral is cut which avoids the possibility of a potential difference between the system earth and neutral, which is much less likely to be an issue with TNS or TNCS.

Edit:- Then again, having re-read the thread, I can see the quandry given neutral is classed as a live conductor. I think I would be inclined to go belt and braces and use a four pole isolator. You're guaranteed to be complying then.
 
Normally TN TPN installations do not require the neutral to be isolated however 537.1.4 requires all live conductors in a household for use by ordinary persons.

It does t say that, it says that a main switch intended for operation by ordinary persons shall interrupt both live conductors of a single phase supply.

It is not ‘mentioning’ single phase, it is clearly setting out the requirement for a single phase installation.
If this requirement applied to a polyphase installation then it would say something like ‘shall interrupt all live conductors.’ And wouldn’t specify single phase.
 
Edit:- Then again, having re-read the thread, I can see the quandry given neutral is classed as a live conductor. I think I would be inclined to go belt and braces and use a four pole isolator. You're guaranteed to be complying then.

What quandary? The regulations are very clear that switching of the neutral in any TNCS or TNS installation, single or polyphase, is not necessary if the supply is in accordance with ESQCR, except where regulation 537.1.4 applies.
 
I think I would base the decision on the type of earthing that's installed.

I researched this for a farm job I was doing and the conclusion I drew based on what I read was if it's TT, install a 4 pole so the neutral is cut which avoids the possibility of a potential difference between the system earth and neutral, which is much less likely to be an issue with TNS or TNCS.

Edit:- Then again, having re-read the thread, I can see the quandry given neutral is classed as a live conductor. I think I would be inclined to go belt and braces and use a four pole isolator. You're guaranteed to be complying then.
4 pole isolation for domestic 3 phase installation.
Is it a requirement?
Regulation 537.1.4 requires a linked switch or circuit breaker to isolate both live conductors of a single phase supply for ordinary persons of a household or similar installation.

Out of interest why ask?
 
It does t say that, it says that a main switch intended for operation by ordinary persons shall interrupt both live conductors of a single phase supply.

It is not ‘mentioning’ single phase, it is clearly setting out the requirement for a single phase installation.
If this requirement applied to a polyphase installation then it would say something like ‘shall interrupt all live conductors.’ And wouldn’t specify single phase.
Dave read the regulation it’s specifically mentions household and similar installations as an example as in a main switch intended for operation by ordinary persons.
But I understand what you are saying regarding the fact it only implies single phase and not 3 phase.
 
my mind wonders, or is it wanders, can't remember.
 
LOL. Not trying to be rude.

But it's clear that they have deliberately differentiated between single and three-phase installations. There's absolutely no reason to believe that this wasn't intended as they have gone to the trouble to make this differentiation.
 
Dave read the regulation it’s specifically mentions household and similar installations as an example as in a main switch intended for operation by ordinary persons.
But I understand what you are saying regarding the fact it only implies single phase and not 3 phase.

It doesn’t ‘imply’ single phase, it states categorically single phase.

Read exactly what is written in plain English, not what you want to see.
 
Imply was a poor choice of word on my part it does indeed categorically say single phase.
Just to note I did not once say it was a requirement, I simply asked people’s thoughts.
Should have expected the arrogant response I guess Dave.
 
Last edited:
I’m inclined to think that 537.1.4 was written with the presumption that in domestic premises, the supply will typically be single phase.
Where there is a three phase supply in domestic premises, I always provide 4 pole isolation to meet the spirit of this Reg.
I understand that in some parts of the UK, new build domestic properties are being provided with three phase supplies as standard.
 
I’m inclined to think that 537.1.4 was written with the presumption that in domestic premises, the supply will typically be single phase.
Where there is a three phase supply in domestic premises, I always provide 4 pole isolation to meet the spirit of this Reg.
But they have deliberately distinguished between single phase and polyphase for domestic installations. They could easily have simply written that domestic installations require it - clearly they did not intend this. So to suggest that a four pole isolator is within the "spirit of the Regulation" is frankly an incorrect conclusion to draw.

It's not really that surprising either, as it should be remembered that breaking the neutral in a polyphase installation is not without risk.
 

Reply to Isolation for 3 phase domestic installations in the Electrical Forum area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

Electrical Forum

Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical forum online. General electrical questions and answers can be found in the electrical forum.
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock