clarification of cu upgrade on TT installation

[Stever_777]

Hi guys, after flat bashing for years, I'm now on my own and seem to only be working on TT installations. Can someone just clear this up for me please?When installing a new cu in a TT installation, do I need a 100mA time delayed main switch or can I just put a normal main switch in and have a split 30mA board.Steve

 

[white armour]

Normal main switch & 30mAmp protection for circuits should suffice.

Regards wa

 

[Stever_777]

Thanks white armour.

 

[telectrix]

agreed. but you need to get your Ze as low as possible.

 

[nickblake]

agree with Tel and white armour , all circuits need to be protected by an RCD on a TT circuit so a dual RCD board will cover it , possible additional RCD for the supply cables if installed in an area where they may be at risk but this RCD should be time delayed to eliminate .............brains gone what the hecks the word totally gone sorry will post when it gets to me

 

[nickblake]

Provide discrimination thats it got it sorry guys just went for a mo ... ... lol

 

[telectrix]

must have a swamp under the house to get 21ohms. realistic figure is more like 100ohms.

 

[Deleted member 26818]

as far as i know its 21 ohms max ze for tt systems. My own house was using the water pipe for main earthing when i bought it so i just rang yedl and they put a new service in for me with t n c s link. much better.
Regards discrimination.....i work for an electrician as an apprentice and we come across bs3036 rewirables a lot which have poor dicrimination but customers dont care about that its a case of if it works its right but i would always recommend changing them for rcd board (which is a requirement for tt on the 17th ). i think time delaiyed 100ma rcds were on the 16th.

21Ω is the maximum resistance for earth rods at the supply.
The requirements for RCD protection for TT systems is the same in the 17th, as it was in the 16th.
The changes for RCD protection between the 16th and the 17th, are for socket-outlets, special locations and cables concealed in walls. Those changes are applicable whatever system is in use, TT, TN-S or TN-C-S.

 

[keniff]

Between 21 and 200 ze on TT setup, but like tel said low as possible maybe 100 ish

 

[Spudnik]

1667 was the last time i looked with 30mA RCD protection.

 

[keniff]

Is that not zs? Sorry it's late

 

[brucelee]

Hi Mate
for TT its RA which is the sum of resistances of the earth electrode and the protective conductors connecting it to the exposed conductive part
RA = 50v divided by 0.03 (30mA) = 1667 ohms max rounded up but for TT anything over 200Ohms may be unstable
you can use ZS value instead as this will give you a worse case scenario if its a acceptable value with zs value then it will be with RA as this is just the protective conductor and ZS is the line and protective conductor

 

[IQ Electrical]

With 30mA protection, the maximum Ra is indeed 1667 Ohms but to ensure stability, BS7671:2008 and Guidance Note 8 suggest a measured maximum of 200 Ohms.Personally, I'd rather see a deep electrode with an Ra of 400 Ohms than a shallow electrode with an Ra of 100 Ohms, depth is king here to combat soil drying/freezing.Of course the lower the Ra the better, there is no minimum here!When using the '17th edition board' and hence no 100mA 'S' type RCD it should be noted that the consumer unit should be of an insulated type.

 

[Paul.M]

The 21 Ohms is the contracted maximum resistance guaranteed by the DNO at the supply just as Spinlondon said. Your max reading should be 200 but as Telectrix says, get it as low as possible and that does not include poring a bucket of water on it 5 mins before you test it infront of your asesser (i won't name any names, you know who you are)

 

[nickblake]

put the earth rod by a down spoute from the roof youll find it always stays wet then