Commercial garage design

[01laverickd]

I'm looking at fitting a commercial garage out.

I plan to put a 4 pole 300MA S TYPE rcd as main switch on a 12 way 3 phase board providing I can get the Ra down low enough, should be fine though, otherwise it'll be a 100ma s type. Socket circuits and maybe lighting circuits will go on 30ma DP RCBO. There are a few circuits that don't need to be 30ma additionally protected. These circuits will have fault protection by the 300ma s type. And the ones on the 30ma will be fault protected with the 30ma rcbos. The final circuits that are non rcd protected, if a fault develops on them they will trip the 300ma rcd knocking the rest of the building off.

Two questions, the first;
I might be massively overlooking something here. But is there anyway I can design it better so that a fault of the non additionally protected circuits won't knock the 300ma s type out on a fault and cutting power to all other circuits?

I just can't think, forgive me.

The second question, and one I've not fully known for a long time, shamefully!

TT, use a 4 pole isolator and isolate the neutral, got it. TN don't have to isolate the neutral. Why?

Is it because the Neutral in a TN system is effectively tied down with earth either via the pme or at the substation so the potential difference would be much lower than between a neutral and earth fault on a TT system?

If the above is just boll***s, I'm sorry ?.

Cheers..

 

[01laverickd]

Any takers? ??

 

[jackhammerJIM]

The second question, and one I've not fully known for a long time, shamefully!

TT, use a 4 pole isolator and isolate the neutral, got it. TN don't have to isolate the neutral. Why?

Is it because the Neutral in a TN system is effectively tied down with earth either via the pme or at the substation so the potential difference would be much lower than between a neutral and earth fault on a TT system?

If the above is just boll***s, I'm sorry ?.

Cheers..

Yes in a TN system Neutral and earth are at the same potential .

In a TT system potential exists between neutral and earth .

Regarding the other problem

Can the electric board convert the earthing system ?

Install enough rods to reach a sub 1 ohm value .

RCBOS on all final circuits

 

[01laverickd]

Yes in a TN system Neutral and earth are at the same potential .

In a TT system potential exists between neutral and earth .

Regarding the other problem

Can the electric board convert the earthing system ?

Install enough rods to reach a sub 1 ohm value .

RCBOS on all final circuits

Yeah I thought that was the case.

Havent been down that route yet, it's not until the new year but it's certainly the route I want to go down.

If not...
Circuits that wouldn't normally have to be rcd protected in a TN system have to be offered fault protection in the way of an rcd, in this case a 300ma s type. If a fault appears it'll knock the whole building out, if I was to put a 30ma rcd in for the 3 phase stuff would I have to put a 4 pole one in? Or would the 4 pole 300ma s type suffice for the isolation of the neutral?

My understanding is that aslong as the circuit breaker isn't needed for isolating the circuit (ie there's an isolator further downstream near the electrical load) then the rcd in the dis board doesn't have to break the neutral?