Earth rod - rcd protection

[Jabbajaws]

Can anyone explain to me, if a 17th Edition Dual 30mA RCD board, is installed, with an earth rod (TT installation), why there is a need for a 100mA RCD to protect the whole installation?

Its something that l can't quite get my head round and would like someone to make things a little clearer.

Im aware that both 30mA RCDs are for electric shock protection, which would surely activate the event of an earth fault, so why have a 100mA RCD as well?

 

[telectrix]

it's not required, used to fit 100mA RCD on TT systems because the Ze was so high, but now fitting 30mA RCDs to all circuits negates the need for the 100mA.

 

[nickblake]

the only other reason for the installation of a 100 ma RCD or time delayed 30 ma RCD would be if the cable was installed in such a way it required additional protection by the RCD , but as Tel has said

 

[Sintra]

Up front 100mA required on tails if the cu is metallic. Can still use 100mA on certain final circuits depending on installation methods.

 

[nickblake]

Up front 100mA required on tails if the cu is metallic. Can still use 100mA on certain final circuits depending on installation methods.

Good point

 

[Jabbajaws]

Thank you everyone... :44:

 

[alarm man]

100ma rcd was used for the lighting circs 30ma for sockets,thers a diagram in the iee sparks guide to building regs tho theres a split 30ma diagram thats labelled for all installations,no real need for 100ma as tel and nick say

 

[Sintra]

100ma rcd was used for the lighting circs 30ma for sockets,thers a diagram in the iee sparks guide to building regs tho theres a split 30ma diagram thats labelled for all installations,no real need for 100ma as tel and nick say

Perhaps not used much in a domestic installation but 100mA RCD's are still commonly used in commercial and industrial installations.

 

[alarm man]

Perhaps not used much in a domestic installation but 100mA RCD's are still commonly used in commercial and industrial installations.

agree mate,every tt ive seen has a 100ma rcd main switch even those with a new split dual 30ma rcd board fitted tho all these 100ma rcds are stand alone units with the tails to the new cu,tho do you have to use a 100ma rcd main switch for metal cu's ,im not 100% sure to be honest

 

[brman]

Up front 100mA required on tails if the cu is metallic. Can still use 100mA on certain final circuits depending on installation methods.

can you point me at the reg for this one? (not doubting it, just can't find it....)

 

[alarm man]

can you point me at the reg for this one? (not doubting it, just can't find it....)

i havent the reg but sintra's right..

 

[brman]

Thanks, I have no doubt he is right. I am assuming it is to do with ensuring proper protection in case of an earth fault on on the input to the CU but I was wondering if there is a specific reg or whether it is just application of the normal fault protection rules.

EDIT: Don't worry, found it- 531.4.1