Selecting a 3 or 4 pole main switch

[TJC1]

Evening all

Pricing up a consumer unit change in a large industrial unit

When it comes to selecting a 3Pole main switch for a 24 way distribution board, are there any factors that will change the decision between a 125A 3pole or 4pole main switch?

 

[Pete999]

Evening all

Pricing up a consumer unit change in a large industrial unit

When it comes to selecting a 3Pole main switch for a 24 way distribution board, are there any factors that will change the decision between a 125A 3pole or 4pole main switch?

Yes is there a requirement to switch the N?

 

[bill01803]

I alway fit 4 pole to make testing easier

 

[westward10]

There is a requirement for a means to disconnect the neutral, not necessarily switch it on a TN system.

 

[Ian1981]

If a TT installation then provision must be provided to isolate all live conductors so a 4 pole switch would be required.
For TN installations where the Neutral is considered reasonably connected to earth by a low impedance,then 3 pole isolation is all that is required.
A removable link for the neutral for testing purposes is still required

 

[Strima]

It is easier for IR testing on a TN system to have a four pole main switch.

 

[Ian1981]

As long as the neutral doesn’t break first before the line conductors when being switched

 

[Darkwood]

You also must be careful on 3-phase and switching the N with the 3-phases, if the N breaks slightly early you could damage expensive equipment, I would only get a 4pole if the N is 'emlb' ... early make, late break.

 

[James]

I have, rightly or wrongly always fitted a 3 pole switch, but most of my work is industrial and TN, you probably only have to worry about it on farms and other remote installations.

 

[Risteard]

As long as the neutral doesn’t break first before the line conductors when being switched

As above it's a potential mode of failure. Triple pole is better where the neutral doesn't need to be switched IMO.

 

[davesparks]

I alway fit 4 pole to make testing easier

It is easier for IR testing on a TN system to have a four pole main switch.

Is ease of testing really the most important consideration here? How much easier is it than dropping out the neutral test link and is that saving worth the potential risks?

If a four pole switch is used then it has the potential for the neutral contact to fail or not close in the correct sequence leading to a dangerous situation and significant damage to equipment. Far better to leave the neutral solid/unswitched wherever possible.

 

[davesparks]

When it comes to selecting a 3Pole main switch for a 24 way distribution board, are there any factors that will change the decision between a 125A 3pole or 4pole main switch?

Yes, fit a 3 pole switch unless there is a specific requirement to fit a linked switch in the neutral.

 

[TJC1]

Thanks for all the messages. Spoke to NICEIC today who said only 3P required on a TN system.