3ph dis board main switches

[J.C.E]

just wanted advise- havn't really done much 3ph and will be putting a hager 8w 3ph dis board in a large house on a rewire

why do some people install a 3pole main switch instead of a 4pole?

heads saying do a 4pole so the N is isolated too like it would be with the 100a dp main switch in a single ph board.....just wondered why i see some people using only 3pole sometimes...

 

[J.C.E]

and why do people tend to use up the ways towards the bottom of the board and leave the ways towards the top spare- is this because if cable entry is from trunking at the top- it helps keep future additions neater? - i understand its good to balance your circuits across l1,l2,l3

 

[Andy78]

Look up the regs on isolation of supply. TN systems require only isolation of the line(s) whereas TT requires isolation of the N too.

A domestic installation requires isolation of both line and neutral though regardless of earthing system.

 

[J.C.E]

Look up the regs on isolation of supply. TN systems require only isolation of the line(s) whereas TT requires isolation of the N too.

A domestic installation requires isolation of both line and neutral though regardless of earthing system.

cheers for speedy reply- guessing to just use 4pole wouldnt hurt for all installs- unless your were really tight on price!

 

[Risteard]

cheers for speedy reply- guessing to just use 4pole wouldnt hurt for all installs- unless your were really tight on price!

Not really true. Breaking the neutral isn't without its own risks.

 

[Toneyz]

With work, I get supplied with three pole isolators but prefer if can to fit four pole as just easier to do installation testing.

 

[Lucien Nunes]

Breaking the neutral isn't without its own risks

This, because loss of neutral on a 3-phase system with any loads that need it (e.g any single-phase or unbalanced 3-phase loads) can result in damage due to over-voltage.

Although a 3P+N isolator should have sequenced contacts, if for any reason the neutral breaks contact first, or fails to make securely before the lines, or goes high resistance, the loads are at risk. I would always fit 3-pole unless there's a good reason for 3P+N.

 

[J.C.E]

This, because loss of neutral on a 3-phase system with any loads that need it (e.g any single-phase or unbalanced 3-phase loads) can result in damage due to over-voltage.

Although a 3P+N isolator should have sequenced contacts, if for any reason the neutral breaks contact first, or fails to make securely before the lines, or goes high resistance, the loads are at risk. I would always fit 3-pole unless there's a good reason for 3P+N.

even in a domestic house?

 

[davesparks]

With work, I get supplied with three pole isolators but prefer if can to fit four pole as just easier to do installation testing.

So you’d rather have ever so slightly easier testing than reduce the possibility of a dangerous situation and thousands of pounds worth of damage should the neutral contact fail?

 

[davesparks]

even in a domestic house?

As mentioned earlier the regulations require that the neutral be disconnected in domestic installs.

 

[westward10]

If the neutral is not switched the neutral bar will or should have a disconnecting link.

 

[Lucien Nunes]

even in a domestic house?

No, that statement was a general case and as per Davespark the regs' requirement for all poles to be isolated takes precedence here. Not a situation that applies in my work as I never touch domestics and very rarely TT.

 

[J.C.E]

mixed opinions as ever lol!
reading around 537. in the regs- all i am seeing is switching the N is not required in TNS or TNCS where is can be regarded as realiably connected to earth.

any reg pointers ref a domestic 3ph dis board HAVING to have 4pole main switch??

 

[westward10]

Reg 537.1.4 seems to assume domestic supplies are going to be single phase and suggests isolation for both poles. Personally I would also apply this criteria to a three phase supply in a domestic situation.