RCD at the Head

[Hawca]

I seem to remember in under the 16th Edition, the minimum size RCD at the head was 100ma, this was due to nuisance tripping. Has this been removed under the 17th or am I just struggling to find the regulation for this.
Any help is appreciated.

Thanks

 

[Dunc]

Under the 16th fitting a 100mA RCD front end was only necessary on a TT supply and would normally be a time delayed model for discrimination. Such an RCD would not be necessary on a 17th edition installation based on the assumption that all circuits would have 30mA protection in most domestic premises.

 

[Octopus]

Putting it simply isn't it down to the run of the tails and material of the CU? i.e. if your tails are buriedand/or you have a metal CU on a TT system you need the RCD?

Or conversely your tails are visible and you have a plastic CU you don't need the "head" RCD.

 

[malcolmsanford]

I haven't got my 16th so can't remember if the 100mA is still regarded in the same context as the 17th would.

It was always that the 30mA gave the person protection and the 100mA was for equipment. So for instance if you go to section 705 you will see in reg 705.411.4 that a 100mA RCD is needed for socket outlets rated over 32amp, and I think it was the same for the 16th.

In domestic situations especially TT systems the use of the 100mA RCD was deemed necessary because of the instability of the earthing rod, and therefore the high Zs.

With the advent of the 30mA RCD for socket outlet protection for the use of mobile equipment used outside in the 16th, it was still deemed acceptable to have the lights, cookers, immersion heaters etc covered by the 100mA and therefore have discrimination.

Of course the 17th now requires much more additional protection by 30mA RCD.

As for the Reg concerning the size of an RCd at the head I can't recall a specific one, but as I said I don't have my 16th with me.

 

[Hawca]

to give more info, Ive carried out a PIR on a house, 15 years old, wire Red and Black, so we know its under the 16th.

TN-C-S with a 30ma at the head.

do i code this as a 4, or even a 2 as under 16th 100ma was the smallest at the head or leave it?

 

[malcolmsanford]

If you wanted to code it a 4 under reg 314.1 then you could. Personally I would not code it anything, but inform the client that it is not ideal in a covering letter.

Certainly not a 2 as that would give it an unsatisfactory.

 

[Hawca]

thanks for the quick feed back

 

[Deleted member 26818]

Using a 30mA RCD for up front protection would not comply with Regulation 314.1, in so far as avoiding hazards and minimizing inconvenience in the event of a fault.
You would have to decide whether having all the circuits switch off in the event of a single fault on a single circuit is particuarly hazardous, when deciding which code to apply.
If you though that the risk is negligible, then 4, if however you thought there may be some risk for instance due to all the lights going off when someone tries to use the cooker, or when the compressor on a fridge cuts in. Then 2 might be more appropriate.