tt system

[welsh spark]

Excuse me while i have a blond moment but i am installing a new cu in a house with twin 30ma rcs to split up circuits on a tt system all circuits will be protected by a 30ma rcd dose the main sw need to be a 100ma type s like we used to use. having a mind blanc sorry for the silly question

 

[KFORDHUNTER]

No requirement for the main switch to be 100mA as long as you provide 30mA protection for all circuits.

 

[welsh spark]

Thanks mate didnt think i needed to have a 100ma main sw as long as all circuits are covered by 30ma rcd just wanted to clarify

 

[lewlec]

hi

main switch is fine unless you want to run sub main from dis board and you need discrimination then a 100ma would be needed.

hope this helps

 

[Spudnik]

The 100 mA RCD was used to protect the tails in the old days.

So, providing the new CU is all insulated or Class II then there is no need for the 100mA

Not sure why a 100mA would be needed for a sub main though.

 

[lewlec]

just thought if you were running a swa sub main to a outbuilding or something and you needed discrimination.
or am i being silly???

 

[Spudnik]

If you are running a submain then you just use a 40A mcb or whatever from the RCD side, to a non RCD CU as the main CU has the RCD.

Phew.

 

[lewlec]

what if a fault occurs at the outbuilding it will take out the 30ma at the house therefore loosing power to one half of the board. which could be a pain.

 

[Spudnik]

Then the out building can be isolated, the same as you would if any other circuit develops a fault.

 

[sparkdog]

I am doing the same type of install with twin 30ma RCDs on a TT system and although the 100ma s may not be required I think I will fit one as it would be a back up if one of the 30ma RCDs did go faulty and failed to operate.Unlikely I know but I think it would make a safer system.